ASME Conference Presenter Attendance Policy and Archival Proceedings

PVP2011-NS3 pp. i; (1 page)

This online compilation of papers from the ASME 2011 Pressure Vessels and Piping Conference (PVP2011) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in the ASME Digital Library and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Design and Analysis

PVP2011-57692 pp. 3-11; (9 pages)

In this paper, experimental and numerical methods are used to study the deformation and interfacial failure behavior of an adhesively-bonded thick joint made of multi-layer S2 glass/SC-15 epoxy resin composite material. The adhesive material is 3M Scotch-Weld Epoxy Adhesive DP405 Black. Continuum damage mechanics models are used to describe the damage initiation at or near the interface and final failure process. The effect of adhesive overlap length, thickness and plasticity on the interfacial shear stress and normal stress are studied. Experimental and analytical data are used to validate the proposed damage models.

Commentary by Dr. Valentin Fuster
PVP2011-57870 pp. 13-19; (7 pages)

Fiber-reinforced polymer composite piping is known to provide excellent corrosion resistance and high specific properties such as strength-to-weight ratio. However, the propensity for micro-cracking of the polymer matrix and associated potential for fluid leakage, and/or insufficient capacity to endure damage from abrasive fluids transported in the piping, often requires composite piping to be equipped with internal polymeric liners. It has been shown that polyurethane liners provide good abrasion resistance and reliable fluid containment. A durable bond between polyurethane liner and structural composite pipe can be achieved employing suitable manufacturing protocols. This was demonstrated by earlier research for laboratory-scale pipe structures. In the present work, manufacturing procedures for large-scale piping structures are described along with the involved chemical and mechanical processes.

Commentary by Dr. Valentin Fuster
PVP2011-58084 pp. 21-29; (9 pages)

Interfacial toughness and interfacial strength, as two critical parameters in an interfacial traction-separation law, have important effect on the fracture behaviors of bonded joints. In this work, the global and local tests are employed to investigate the effect of adhesive thickness on interfacial energy release rate, interfacial strength, and shapes of the interfacial traction-separation laws. Basically, the measured laws in this work reflect the equivalent and lumped interfacial fracture behaviors which include the cohesive fracture, damage and plasticity. The experimentally determined interfacial traction-separation laws may provide valuable baseline data for the parameter calibrations in numerical models. The current experimental results may also facilitate the understanding of adhesive thickness-dependent interface fracture of bonded joints.

Commentary by Dr. Valentin Fuster
PVP2011-57625 pp. 33-42; (10 pages)

A series of computational fluid dynamics (CFD) and numerical analyses were performed to investigate operational characteristics in a sulfur recovery unit waste heat boiler (WHB). Similar analyses of WHBs have been reported by the authors (Porter et. al. [1, 2]). The initial focus for the current investigation was to determine the reason for metal loss on the inside of the tube. This required extending the focus of the previous analyses that concerned a) the departure from nucleate boiling (DNB) leading to critical tube temperatures, and b) the downstream fluxes and temperatures from the inlet ferrule, to also investigate high inside surface temperatures of the tubes caused by shell-side tube outer diameter (OD) fouling. The results of the investigations were combined to provide future operational guidance for the boiler. As in the previously reported analyses, CFD submodels of the WHB process-side inlet were constructed and analyzed to determine the fluxes and temperatures that occur during several operational conditions. Queried results of these analyses were combined with the WHB’s historical operational data to predict the nominal operational temperatures, and associated corrosion rates on the inner diameter (ID) of the tube. A second set of submodels was used to determine inside tube operating temperatures resulting from external fouling. The queried results of these analyses were combined, using an expansion of standard thermodynamic analysis techniques, to study possible fouling regimes based on the standard fouling growth equation. Additionally, a 3-dimensional CFD analysis was conducted on the shell-side of the boiler. This analysis allowed the determination of the margin of safety (MOS) from a fall-off-the-cliff (FOC) event [1]. The results of the submodels, numerical analyses and the 3D shell-side analysis of the boiler were combined to determine operational limit curves for the boiler that were based on measurable process parameters including mass flow rate and thermal reactor temperature. It should be noted that the procedures and analyses detailed in this paper do not comprise the complete analyses performed to qualify the past performance of the boiler and to determine future boiler operational limits. Additionally, due to the proprietary nature of the investigations, the specific numerical values related to the boiler’s operation are not presented. Only the derivation of the equations and logic associated with the investigation and the derivation of operational guidance are given. Complete engineering to determine these limits requires additional analyses not detailed in this paper.

Topics: Boilers , Waste heat
Commentary by Dr. Valentin Fuster
PVP2011-57650 pp. 43-49; (7 pages)

A Computational Fluid Dynamics (CFD) model of the steam dome region of a Boiling Water Reactor (BWR) vessel was created to investigate anticipated flow patterns in the outlet plenum. Steady state and transient simulations were performed to investigate the hypothesized existence of stationary vortices at the entrance of each Main Steam Nozzle. These hypothesized stationary vortices are believed to be the cause of wear patterns observed on some BWR steam dryers and to contribute a low frequency (∼15–25 Hz) fluctuating pressure load on the outer hoods of steam dryers. Results of the simulations and available in-vessel visual inspection data support the hypothesis of the presence of the stationary vortices in each Main Steam Nozzle.

Commentary by Dr. Valentin Fuster
PVP2011-57817 pp. 51-60; (10 pages)

The present study makes a comparative assessment of different turbulence models in simulating the flow-assisted corrosion (FAC) process for pipes with noncircular cross sections and bends, features regularly encountered in heat exchangers and other pipeline networks. The case study investigates material damage due to corrosion caused by dissolved oxygen (O2 ) in a stainless steel pipe carrying an aqueous solution. A discrete solid phase is also present in the solution, but the transport of the solid particles is not explicitly modeled. It is assumed that the volume fraction of the solid phase is low, so it does not affect the continuous phase. Traditional two-equation models are compared, such as isotropic eddy viscosity, standard k-ε and k-ω models, shear stress transport (SST) k-ω models, and the anisotropic Reynolds Stress Model (RSM). Computed axial and radial velocities, and turbulent kinetic energy profiles predicted by the turbulence models are compared with available experimental data. Results show that all the turbulence models provide comparable results, though the RSM model provided better predictions in certain locations. The convective and diffusive motion of dissolved O2 is calculated by solving the species transport equations. The study assumes that solid particle impingement on the pipe wall will completely remove the protective film formed by corrosion products. It is also assumed that the rate of corrosion is controlled by diffusion of O2 through the mass transfer boundary layer. Based on these assumptions, corrosion rate is calculated at the internal pipe walls. Results indicate that the predicted O2 corrosion rate along the walls varies for different turbulence models but show the same general trend and pattern.

Commentary by Dr. Valentin Fuster
PVP2011-58007 pp. 61-67; (7 pages)

This study discusses on the implementation of an upwind method for a one-dimensional two-fluid model including the surface tension effect in the momentum equations. This model consists of a complete set of six equations including two-mass, two-momentum, and two-internal energy conservation equations having all real eigenvalues. Based on this equation system with upwind numerical method, the present authors first make a pilot code and then solve some benchmark problems to verify whether this model and numerical method is able to properly solve some fundamental one-dimensional two-phase flow problems or not.

Commentary by Dr. Valentin Fuster
PVP2011-57053 pp. 71-76; (6 pages)

If a pressure boundary bolted joint leaks during operation, then it is commonplace to re-tighten the joint either during swoop-down (lower pressure) or while the unit is online (full operating pressure). If engineers are involved, and often they are not, then it is common place to specify a torque to which the bolts should be re-tightened. The problem with that approach is that after even just a few days of operation, the nut factor for the bolt is completely unknown and so the torque specified has little relevance to the obtained load. In addition, the break-out torque for an operational stud can be in excess of twice the assembly torque, even at lower loads. To counteract these issues, an alternative approach is to use the Turn-of-Nut method to tighten the joint by a certain amount. This method is similar to that used for structural bolting, but is more complex, as the deflections of the flange and gasket must also be taken into account. This paper gives guidance on the factors that must be considered when determining the amount of nut turn to specify when using Turn-of-Nut and outlines practical guidelines for using the method on standard ASME B16.5 flanges.

Topics: Leakage
Commentary by Dr. Valentin Fuster
PVP2011-57139 pp. 77-82; (6 pages)

Leakage in Gasketed Flanged Joints (GFJs) has always been a great problem for the process industry. The sealing performance of a GFJ depends on its installation and applied loading conditions. This paper aims to finding the leak rate through ANSI class#150 flange joints using a compressed asbestos sheet (CAS) gasket under combined structural and thermal transient loading conditions using two different leak rate models and two different bolt-up levels. The first model is a Gasket Compressive Strain model in which strains are determined using finite element analysis. The other model is based on Porous Media Theory in which gasket is considered as porous media. Leakage rates are determined using both leak rate models and are compared against appropriate tightness classes and the effectiveness of each approach is presented.

Topics: Pressure , Leakage
Commentary by Dr. Valentin Fuster
PVP2011-57406 pp. 83-88; (6 pages)

There is a lot of misinformation published about anti-seize compounds around friction (or K factors) factors and temperature limits. Anti-seize materials exist to serve two functions, to help even out stud stresses/preloads during assembly, and most importantly, allow the components to be taken apart after they have been exposed to heat. This paper looks at how different anti-seize materials are affected after being exposed to enough heat to burn off the grease base.

Commentary by Dr. Valentin Fuster
PVP2011-57414 pp. 89-95; (7 pages)

In most cases, it is assumed that a manual or hydraulic torque wrench will deliver the requested torque value if set correctly. However, torque wrenches have moving parts that will be subject to wear. They are also subject to harsh operating conditions in the field, which elevates the risk of damage. It is common sense that calibration of wrenches on a periodic basis would be advisable. This activity is regulated and required in some industries, such as automotive and wind-power generation. However, in the Petrochemical and Oil & Gas industries there are relatively few companies that practice regular calibration. In part, this may be because there is not data available to justify the cost of such a calibration program. It is not until the calibration program is underway that the value can truly be determined. In this paper, the results of field calibration of several hundred torque wrenches is presented and an analysis of the data reveals exactly why calibration of torque wrenches should be an important part of any leak-free bolted joint program.

Commentary by Dr. Valentin Fuster
PVP2011-57453 pp. 97-102; (6 pages)

Appendix O “Determining Joint Assembly Bolt Load’ was added in the 2010 version of ASME PCC-1 “Guidelines for Pressure Boundary Bolted Joint Assembly”. It outlines a method of calculating an appropriate assembly load for pressure vessel and piping bolted joints. However, the possible uses and scope of the appendix go well beyond only the selection of assembly bolt load. For example, the method can be applied to gasket selection or assembly method selection. The calculations in the appendix can easily be applied to quantify when certain technology can be employed and when it shouldn’t and can also be used as a tool to contribute to the overall gasket selection or assembly method selection process. In fact, once an understanding of the basic behavior of the joint is obtained, then the method can be applied to the full life-cycle of the joint, including use as a tool to determine the root cause of leakage. Basic procedures for performing these types of assessments and some pertinent examples are outlined in this paper.

Commentary by Dr. Valentin Fuster
PVP2011-57556 pp. 103-111; (9 pages)

Spiral wound gaskets are used worldwide in piping and equipment flanges and can be manufactured in several combinations of materials, and in a wide range of dimensions, winding densities and shapes. This paper shows the sealability influence of winding densities, which are not specified by the current Spiral Wound B16.20 gasket standards, including flexible graphite filler thickness, height and number of windings. The effect of the flange rotation is also shown.

Commentary by Dr. Valentin Fuster
PVP2011-57738 pp. 113-122; (10 pages)

A design procedure for bolted gasketed pipe flange connections has been developed by the Sealing Technology Of Pressure-equipments (STOP) Committee organized in the High Pressure Institute of Technology Japan. This paper explains not only the design procedure itself but also the technical background of the leak tightness of gasket basis for the procedure. The point to be highlighted is that the design procedure has adopted a flexibility analysis of the flange connection based on the Oyama method, which is the modification of the Koumura method calculating the interaction among pipe, hub and flange ring including gasket stiffness, in order to determine the required bolt preload achieving the specified leak tightness. The Oyama method can calculate the variations of axial bolt force, the flange rotation, and the gasket stress distribution in the radial direction due to internal pressure using relatively simple formulas.

Commentary by Dr. Valentin Fuster
PVP2011-57813 pp. 123-127; (5 pages)

Leakage problems caused in large diameter gasketed flanged connections in piping systems are closely related to deformation of flanges caused by the high thrust force and rather low rigidity of the flanges. Therefore, it is necessary to understand the deformation characteristics of flanged connections when they are tightened and pressurized. In this study, experiments were carried out using a 16-inch gasketed flanged connection to examine the stress and strain in the flanges. In order to clarify the deformation characteristics of the gasketed flanged connection, a method to analyze stresses and deformations of a gasketed flanged connection was demonstrated using the classical theory by Timoshenko. Recently, finite element analysis (FEA) has widely been used in the analysis of gasketed flanged connections. However, analyses of flanged connection based on the analytical method using strength of materials are still important when parametric calculations of flanged connection are necessary. The experimental results and the analytical ones were compared and discussed to clarify the sealing behaviors of large diameter gasketed flanged connection.

Commentary by Dr. Valentin Fuster
PVP2011-57045 pp. 131-146; (16 pages)

Steam dryers in Boiling Water Reactors (BWRs), located in the upper steam dome of the reactor pressure vessel, are not pressure retaining components and are not designed and constructed to American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code Section III safety class. Although the steam dryers in BWRs perform no safety function, they must maintain the structural integrity in order to avoid the generation of loose parts that may adversely impact the capability of other plant equipment to perform their safety functions. The majority of licensees of many operating nuclear plants are applying for extended power uprate (EPU), which increases the thermal power output up to 20% above the original licensed thermal power (OLTP). Nuclear power plant components such as steam dryers can be subjected to strong fluctuating loads and can experience unexpected high cycle fatigue due to adverse flow effects while operating at EPU conditions. However, there are some unique challenges for operation of dryers at EPU conditions requiring special considerations to prevent fatigue damage from the effects of flow induced vibration (FIV). This paper examines the FIV considerations and margin recommendations for fatigue stresses due to many uncertainties in the prediction of the fluctuating pressure loading acting on the steam dryer and the structural analysis of the steam dryer, very limited bench marking of the methods, and operating experience with some dryer failures.

Commentary by Dr. Valentin Fuster
PVP2011-57056 pp. 147-152; (6 pages)

Local pit is the common volume defect in high temperature pressure pipe which is widely used in the fields of electric plant, nuclear power station, petrochemical plant and so on. In this paper finite element analysis code ABAQUS was used to simulate limit load of high temperature pressure pipe with an external pit defect which is in service for 105 hours. Four-point bending loading model was applied to calculate the limit load of the pipe. There are three dimensionless factors: relative depth, relative gradient and relative length which characterized the shape of an external pit defect. Orthogonal test of three factors at four different levels was carried out to analyze the sequence of the influence of these three parameters. In present paper when the maximum principal strain reaches 2%, the corresponding load is selected as the limit load. According to this strain criterion and isochronous stress strain data of P91 steel, limit load of high temperature pressure pipe with an external pit was determined by using ABAQUS. Firstly, isochronous stress strain data was generated and was inputted into ABAQUS as equivalent elastic-plastic constitutive relation. Then, sustained load versus cumulative strain curves at high temperature during service was obtained after the simulation. At last, limit loads of high temperature pipe during service time was determined based on 2% total strain criterion. In order to obtain the safety assessment curve of high temperature pipe, five types of limit loads for pressure pipe with an external pit were needed: ultimate limit bending moment, limit internal pressure, limit bending moments at the pressure of 0.25PL ,0.5PL and 0.75PL individually. 16 sets of data formed 16 groups of curves which expressed the relationship between the ratio of limit pressure and the ratio of limit bending moment for defective pipe and non-defective pipe. Based on the calculation results of limit load for pipe with 16 kinds of defects, a set of limit load formulae were established through multiple nonlinear regression of relative depth, relative gradient and relative length. So the equations of limit load and safety assessment for pressure pipe with an external pit under combined loading of pressure and bending moment were obtained. The results could provide a reference for safety assessment of high temperature creep pressure pipe with local pit defect.

Commentary by Dr. Valentin Fuster
PVP2011-57101 pp. 153-162; (10 pages)

Fatigue evaluation in B31.1-2007 is currently done based on B31.1 Equations 1 & 2 and generally considers only the stress due to displacement load ranges as per Equation 13. Yet, fatigue damage is also occurring due to pressure cycling and thermal gradients. To exacerbate this, power plant design pressures and temperatures are rising, new materials are being introduced, pipes and attached components are becoming increasingly thick, and owners are requiring power plants to heat-up and cool-down at faster rates. Also, power plant owners are more and more interested in extending the life of power plants beyond their original design life. This paper addresses the pressing need in today’s power plant environment for additional fatigue evaluation by providing a procedure for assessing an as-welded Butt weld of dissimilar metals and a Weld End Transition (or TTJ) (B31.1-2007 Figure 127.4.2) of similar metals to include the effects of thermal gradients calculated as per ASME Section III-2007 Subarticle NB-3600. The disadvantage of this approach is that the conservatism in the calculation of these thermal gradient stress intensities may produce unacceptable results. In that case, the assessment is a warning that something else needs to be done by way of either monitoring or modifying the thermal operation or more rigorous evaluation. The advantage of this methodology is that it will ensure a fatigue failure does not occur any sooner due to the effects of thermal gradients than would otherwise occur due to other factors. It maintains the traditional B31.1 approach to fatigue with the same limit of SA except that there is now an additional term, STG , to account for the fatigue contribution due to thermal gradients. Considering the effects of these thermal gradients in this way will further help to preserve the integrity of the piping pressure boundary and consequently, the safety of personnel in today’s power plants and into the future.

Commentary by Dr. Valentin Fuster
PVP2011-57123 pp. 163-171; (9 pages)

For the repair of pipeline defects, repair sleeves are the most widely used method in petro-chemical industry. The objective of this work was to optimize the thickness of non-pressure containing repair sleeve, by refining the existing design practice. Laboratory studies involving instrumentation of small-scaled repair sleeve system coupled with finite element analysis were carried out to refine the design procedure and optimize the thickness of the sleeve. Using a unified approach for finite element modeling, including failure pressure estimation and simulation of sleeve installation pressure, twenty-four cases ranging from 6–60 inches nominal diameter were investigated. In this paper, the details of the optimization approach used in this investigation have been presented.

Commentary by Dr. Valentin Fuster
PVP2011-57131 pp. 173-179; (7 pages)

Socket welds in nuclear power plant piping systems have had a history of failure due to high cycle fatigue. Cyclic bending loads, typically from vibration, have caused cracking either at the weld root or at the toes. A testing program was conducted to measure the effectiveness of proposed changes in socket weld geometry and fabrication [3]. One of the significant results of that testing program was that if the socket weld profile was modified such that the weld leg length along the pipe side is at least double the Code minimum requirement the fatigue life of the weld becomes approximately as good as that of a butt weld. The test results demonstrated that the fatigue strength reduction factor (FSRF) is about 37% smaller for socket welds with the longer weld leg, than for a standard size weld. Many plants have been replacing or building up their socket welds in locations susceptible to vibration to the 2×1 geometry to take advantage of the improved fatigue strength. However, there have been situations where welds were built up from the Code minimum, but inspections determined that the leg length was somewhat less than twice the minimum required and the weld was not accepted as meeting the 2×1 geometry. It is not known how the fatigue strength reduction factor would vary as a function of leg length for intermediate sizes. It is also of interest to determine the effect of increasing the weld leg length on the fitting side. For example, is it possible that increased fatigue resistance can be provided by adding additional leg length on the fitting side instead of the pipe side, or can a greater benefit be achieved if they were provided on both sides?

Commentary by Dr. Valentin Fuster
PVP2011-57297 pp. 181-186; (6 pages)

A steel reinforced plastic pipe (PSP), which is composed of two layers of high density polyethylene (HDPE) matrix and a high strength steel wire mesh skeleton, has wide applications in many industrial areas, such as gas and petroleum transportation, etc. In order to achieve higher efficency and lower costs, a large diameter PSP has been developed. However, requirements of the large diameter PSP in safety and economy are much higher, compared with those small diameter PSPs, and some potential problems should be taken into account. In this paper, relevant structural parameters of the large diameter PSP are determined, based on a previously proposed model, and a short-term burst test is carried out. The experiment results agree with the theoretical results quite well. Subsequently, the resistance of vertical pressure and uniform external pressure are evaluated by using experiment investigation and finite element method, respectively. And corresponding results indicate the large diameter PSP with determined structural parameters is qualified to use.

Topics: Steel , Plastic pipes , Design
Commentary by Dr. Valentin Fuster
PVP2011-57360 pp. 187-192; (6 pages)

In this paper, experience and results of the Pipe integrity design project applied to Unit 2 of Armenian NPP in Metsamor are presented. The feedwater and steam piping lines, which were covered by the Project, are located in the turbine hall on the elevation of +14.7 m, outside the containment. Piping was assessed from point of view of both pipe rupture postulation criteria defined in SRP 3.6.2 [2] and in BTP 3.4 [4] and seismic margin requirements [5] based on the international practice for the NPPs in operation. Concerning hydrodynamic calculations, possible causes of steam and water hammer were assessed. To lower the stresses in critical locations and to solve the effects of steam/feedwater/emergency feedwater pipe whip, some measures have been proposed — installations of whip restraints, hydraulic snubbers, etc. Detailed design files have been prepared for the proposed measures. Within this project, ISI program was conducted, with application to the most loaded locations. Proposal of the first stage of inspections was given, taking into account the possibility that the inspections will be performed within one outage.

Commentary by Dr. Valentin Fuster
PVP2011-57370 pp. 193-202; (10 pages)

The purpose of this study is to determine the Pipe Whip Restraint (PWR) location that would prevent the formation of a plastic hinge due to secondary effects of a postulated pipe break load in a high energy line(1) . The prevention of a plastic hinge formation at the PWR location is important since its secondary effects could lead to additional interactions with safety related equipment, structure, and component that are essential to safely shutdown the nuclear power plants. The proper location of the PWR can be found by using the relationship between bending moment-carrying capacity of the pipe and the applied thrust force. Several closed-form solutions obtained from several literatures were studied and used to calculate bending moment-carrying capacities of a piping system and ultimately used to determine a plastic hinge length. The plastic hinge formation is also determined analytically by using the Finite Element Analysis (FEA) method. ANSYS LS-DYNA® [8] Explicit Finite Element code is used in modeling the pipe whip models, which includes the piping system and pipe whip restraint. Comparisons are made between the analytical (FEA) results and the results from several closed-form solutions.

Commentary by Dr. Valentin Fuster
PVP2011-57371 pp. 203-214; (12 pages)

High levels of acoustic energy can be produced at the downstream of pressure-reducing valves, pressure safety valves and control valves in piping systems. The presence of acoustic pressure waves and their coupling with the piping wall flexural modes of vibration can result in high levels of dynamic stresses, which cause acoustic fatigue failure at points of discontinuities on the pipe wall. This work presents a procedure for the assessment of the acoustic fatigue of different piping components by the application of finite element analysis. The piping system process data is used to generate the acoustic pressure and acoustic power spectrum at the downstream of the pressure-reducing valve. This acoustic power spectrum is taken to act at a finite element model of the piping system. Dynamic analysis, by use of power spectrum and harmonic analysis, is performed to obtain the response dynamic stresses, which are used for the fatigue evaluation of the piping component. The methodology presented can be applied during the engineering phase in the design and stress analysis of piping components in critical services subjected to acoustic fatigue as well as in the detailed evaluation of the different proposed acoustic fatigue design solutions.

Commentary by Dr. Valentin Fuster
PVP2011-57512 pp. 215-221; (7 pages)

Bulletin WRC107 is most commonly used in attachment design, but still some uncertainties make it difficult to ensure safety in recent use. Two problems in fatigue evaluation were addressed here, first the bulletin is based on shell theory and some other assumption, and for various condition the calculation error is unknown; second pressure is not considered in calculation. To the first problem, an assessment was performed by comparing the total equivalent stress results of WRC107 and of finite element method (FEM). To the second problem, a method from Chinese standard HG20582-1998 Specification for Stress Calculation of Steel Chemical Vessels (HG method) was introduced as a supplement, and the reliability was studied. The results show that, total equivalent stress amplitude calculated by WRC107 may be underestimated, and its error mainly depends on parameters β and γ. Complemented by HG method, WRC107 could be used in calculation of shell under pressure and external loading.

Topics: Pressure , Stress , Errors
Commentary by Dr. Valentin Fuster
PVP2011-57521 pp. 223-232; (10 pages)

Primary loop recirculation (PLR) piping weld joints are more susceptible to stress corrosion cracking (SCC). But it is difficult to accurately predict SCC growth rate in PLR piping weld joints because the material and mechanical properties in weld joints are quite complicated. Especially, it is provided that hardening in the weld heat-affected zone (HAZ) might play an important role in promoting SCC growth. Considering welded mechanical heterogeneity, the local stress and plastic strain fields ahead of growing crack tip in 316L PLR piping weld joints are analyzed, the effect of constant stress intensity factor (KI ) and constant loading on SCC behaviors of PLR piping weld joints is investigated in this study. The results show that the mechanical fields of SCC tips behave quite differently under constant KI and constant loading because of welded mechanical heterogeneity and advanced crack length, which demonstrate that the effect of constant loading on the stress and strain ahead of the growing crack tip is bigger than that of constant KI .

Commentary by Dr. Valentin Fuster
PVP2011-57544 pp. 233-241; (9 pages)

In Fast Breeder Reactor, dominant stresses in the piping system are secondary, which are induced by constraint of the thermal expansion of components and pipes. Therefore, the structural design rule should essentially prevent the buckling caused by displacement controlled loads. In this study, the evaluation method of the buckling criterion of cantilever type pipes subjected to lateral displacements is proposed. We define the criterion of the deformation controlled buckling based on bending strain at local buckling portion. Then finite element analysis (FEA) is performed for estimating the displacement and bending strain at local buckling portion during the displacement controlled buckling. Those defined criterion and results of FEA are availed to investigate the evaluation method of the displacement controlled buckling criterion. In those FEA, material properties of Mod.9Cr-1Mo steel are applied, because the material is a candidate for primary and secondary heat transport system components of JSFR (Japan Sodium cooled Fast Reactor), and those of 316FR (type 316 stainless steel modified for FR) are also used to compare the FEA results of Mod.9Cr-1Mo steel. Results of FEA suggest that buckling behaviors strongly depend on the distribution of bending moment and strain hardening behavior of the material. Therefore, those features must be considered in construction of the evaluation method of displacement controlled buckling. To consider the distribution of bending moment, two indexes are defined. One indicates nonlinear bending displacement of a straight pipe. The other indicates the configuration of the pipe and material properties. Relationship between those indexes, which including criterion of displacement controlled buckling, is formulated by the enveloped line considering the effect of the strain hardening behavior of the material. As a result, an equation which represents the criterion for displacement controlled buckling of cantilever type pipes was proposed. This equation consists of two indexes as mentioned above, and can estimate criterion of displacement controlled buckling by material properties and pipe configuration. The proposed equation can be applied to the pipes made of Mod.9Cr-1Mo steel and 316FR.

Commentary by Dr. Valentin Fuster
PVP2011-57548 pp. 243-250; (8 pages)

In Fast Breeder Reactors, dominant stresses in the system are secondary, which are induced by constraint of the thermal expansion of components and pipes. Therefore, the structural design rule should essentially prevent the buckling caused by displacement-controlled loads. In this paper the applicability of the estimation method of buckling criterion, which was proposed by the authors on the basis of a series of Finite Element Analysis (FEA), was confirmed by carrying out a series of buckling tests. The displacement-controlled buckling tests were performed with cantilever type pipes made of Mod.9Cr-1Mo steel at room temperature and at 550°C. A visual measurement using image processing was applied to clarify both deformation profile and strain distribution at the local buckling portion of pipes. FEA was used in advance for estimating the buckling behavior of tests, such as load-displacement relationship, deformation profile and longitudinal strain distribution. Comparison between test and FEA results showed that buckling behaviors in the tests are predictable by FEA. The estimation method of buckling criterion on the basis of FEA was found to conservatively cover the test results, and we can conclude that this estimation method can be adopted for designing the piping systems of Fast Breeder Reactors.

Commentary by Dr. Valentin Fuster
PVP2011-57566 pp. 251-256; (6 pages)

Several piping failures caused by thermal stratification have been reported in some nuclear power plants since the early 1980s. However, this kind of thermal effect was not considered when the old vintage nuclear power plants were designed. Thermal stratification is usually generated by turbulent penetration from the RCS to branch line or leakage through damaged part of valve in branch line. In this paper, using the CFD analysis, characteristics of thermal stratification in a safety injection system of PWR plant were investigated and thermal stress evaluation was also conducted. First, CFD analyses were carried out on in-leakage model and out-leakage model according to operating condition. The case of out-leakage, the thermal stratification based on temperature distribution was generated a little at the rear of 1st valve. In contrast, significant thermal stratification was generated in front of 1st valve in in-leakage model because the effect of rapid flow velocity from RCS.

Commentary by Dr. Valentin Fuster
PVP2011-57648 pp. 257-266; (10 pages)

This paper provides examples of the application of repair procedures described in ASME Post Construction Committee’s PCC-2, Repair of Pressure Equipment and Piping [1] in an oil refinery environment. In one case, the use of a freeze plug is described. This plug was created on a medium diameter water line in order to replace a stuck valve. Another case deals with leaks on a small bore connection off a large high pressure process pipe. This leak was temporarily repaired using a welded sleeve. Another situation involves the patching of a critical water pipe with a through wall leak. This large diameter line is critical to the operation of two refinery units. Thus, its rapid repair was very beneficial to refinery operations. This repair also consisted of a welded sleeve.

Commentary by Dr. Valentin Fuster
PVP2011-57661 pp. 267-275; (9 pages)

Appendix P Alternative Rules for Stress Range Evaluation was first introduced into the 2004 edition of ASME B31.3 Process Piping Code [1] as an alternative method that was conceived to be more computer friendly and possibly a more nearly theoretically defensible method of protecting piping systems individually against fatigue failure and dimensional ratchet than the singular stress range requirements of the Code’s base text (“base Code”). This paper describes the progression of events that led up to the inception of the Appendix and the subsequent revisions to same and detailing the reason for the 2010’s newly coined Allowable Operating Stress to protect against dimensional ratchet and Allowable Operating Stress Range to protect against fatigue failure.

Commentary by Dr. Valentin Fuster
PVP2011-57930 pp. 277-281; (5 pages)

Feeder pipes that connect the inlet and outlet headers with the in-reactor fuel channels in CANDU nuclear power plants are considered as safety Class 1 piping items. Therefore, fatigue of the feeder pipes should be assessed at design stage in order to verify structural integrity during design lifetime. In accordance with the fatigue assessment result, cumulative usage factors of some feeder pipes have significant values. An active degradation mechanism for the outlet feeder piping made of SA-106 Grade B carbon steel is local wall thinning due to flow-accelerated corrosion. This local wall thinning can cause increase of peak stress due to stress concentration by notch effect. The increase of peak stress results in increase of the cumulative usage factor. However, present fatigue assessment doesn’t consider the stress concentration due to the local wall thinning. Therefore, it is necessary to assess the effect of local wall thinning on the stress concentration. This study developed the engineering formulae for stress concentration factors of the local wall thinning in the CANDU feeder pipe under internal pressure by using the engineering procedure of Kinectrics Co.. Finally, the developed formulae were applied to the elbow feeder pipes and compared with the finite element analysis results. As a result of comparison, it is identified that the engineering formulae is valid.

Commentary by Dr. Valentin Fuster
PVP2011-57001 pp. 285-293; (9 pages)

Two dimensional (2-D) axisymmetric finite element models (FEMs) are often used as a simplification to modeling cylindrical nozzles that intersect a cylindrical pressure vessel. However, an axisymmetric model has the effect of representing the vessel as a spherical shell rather than a cylindrical shell. Previous work has been done to determine 2-D axisymmetric to three dimensional (3-D) stress correction factors (CFs) for the total stress at the nozzle blend radius to account for this inconsistency. The present paper expands on that work to investigate the effects of the 2-D axisymmetric modeling simplification on the through wall stress distribution at the nozzle corner. The through-wall stress distribution is necessary for some fracture mechanics analyses performed for corner cracked nozzles and for using the simplified elastic-plastic analysis given in NB-3228.5. A simplified method is proposed which can be used to obtain a nozzle specific correction factor, rather than a bounding correction factor, that can be applied to 2-D finite element analysis stress results to correct for the inaccuracy introduced by modeling the intersection as an axisymmetric section.

Commentary by Dr. Valentin Fuster
PVP2011-57100 pp. 295-303; (9 pages)

Every engineering project involving the design of pressure equipment, including pressure vessels, heat exchangers and the interconnecting piping requires that the interface loads between the equipment and piping be established for the pressure vessel nozzle design and the limitations on piping end reactions. The vessel or exchanger designer needs to know the external applied loads on nozzles and the piping designer needs to know the limiting end reactions on any connected equipment. However, the final loads are not known until the piping design is completed. This requires a very good estimate of the piping end loads prior to completing the vessel or piping design. The challenge is to develop a method of determining the optimum set of design loads prior to design. If the design loads are too low, the piping design may become too costly or impractical. If the design loads are too high the vessel nozzle designs will require unnecessary reinforcement and increased cost. The problem of the stresses at a nozzle to vessel intersection due to internal pressure and external forces and moments is one of the most complex problems in pressure vessel design. The problem has been studied extensively; however each study has its own limitations. Numerous analytical and numerical simulations have been performed providing guidance with associated limitations. The objective is to establish allowable nozzle load tables for the piping designer and the vessel designer. The loads and load combinations must be based on a technically accepted methodology and applicable to all nozzle sizes, pressure classes, schedules and vessel diameters and thicknesses and reinforcement designs within the scope of the tables. The internal design pressure must also be included along with the 3 forces and 3 moments that may be acting on the nozzle and the nozzle load tables must be adaptable to all materials of construction. The Tables must also be applicable for vessel heads. This paper presents the issues, including the limitations of some of the existing industry approaches, presents an approach to the problem, utilizing systematic Finite Element Analysis (FEA) methods and presents the results in the form of tables of allowable nozzle loads.

Topics: Stress , Nozzles
Commentary by Dr. Valentin Fuster
PVP2011-57314 pp. 305-310; (6 pages)

A skirt-to-shell attachment of a coke drum experience severe thermal cyclic stresses, which cause failures due to low cycle fatigue. Various skirt attachment designs, therefore, have been proposed and implemented. A design where the skirt is attached by a weld build-up is most commonly used. A design where the skirt is attached to the drum shell by utilizing an integral machined plate or forging has been utilized in several projects. One of the advantages of the integral skirt attachment is that a large inner radius can be formed which allows reducing stress concentration compared with the weld build-up design. This advantage can be confirmed easily by FE-analysis in recent years [1] [2] [3]. Another major advantage of the integral skirt attachment is that the area of highest stress intensity is located at the base metal section, not at the weld metal or the heat affected zone which are generally thought to have lower fatigue strength. The fatigue design curve from ASME Section VIII Division 2 [9] is based on fatigue tests for the base metal. It is necessary to reveal differences of fatigue strength among these metals. This paper describes a comparison of fatigue strength of three metals: i) base metal ii) weld metal iii) heat affected zone provided by the low cycle fatigue test for 1 1/4Cr-1/2Mo materials. Our results indicate that the fatigue life of the base metal is about twice as long as that of the weld metal and about three times as long as the heat affected zone. Accordingly, the integral skirt attachment is more resistant to cracking than its welded counterpart from a fatigue strength viewpoint.

Commentary by Dr. Valentin Fuster
PVP2011-57356 pp. 311-320; (10 pages)

The theory behind the pressure-area method that is incorporated in the ASME B&PV Code, Section VIII-2 is presented in this paper. Background and insight to the nozzle rules of ASME B&PV Code, Section VIII, Division 2, Part 4, paragraph 4.5 are also provided. Recommendations for modifying the current nozzles rules, those published in ASME B&PV Code, Section VIII, Division 2, 2010 Edition, is given based on continuing research and development efforts. A comparison between experimental results, results derived from detailed finite element analysis (FEA), the rules prior to the VIII-2 Rewrite (2004 Edition), and the rules in VIII-2 are provided in terms of a design margin and permissible maximum allowable working pressure (MAWP) computed with the design rules. A complete description of the theory including a commentary and comparison to experimental results is provided in WRC529 [1].

Commentary by Dr. Valentin Fuster
PVP2011-57407 pp. 321-327; (7 pages)

Various kinds of nozzles are attached to a pressure vessel including Steam Generator (SG) in a pressurized water reactor plant. The downcomer feedwater nozzle on the upper vessel shell and the economizer feedwater nozzle in the lower vessel shell of the SG are representative nozzles which have a non axi-symmetric shape. In most cases, external loads composed with forces and moments are imposed on those nozzles during the plant operation. In order to evaluate structural integrity of junctures between the nozzles and vessels in compliance with the ASME Boiler and Pressure Vessel Code, Section III, it is essential to find the maximum stress intensity resulting from those loads. Welding Research Council (WRC) Bulletin 297 has been used to find the maximum stress intensity since it is not straightforward to calculate the stress intensity with a non axi-symmetric two dimensional finite element model. However, the compatibility of adopting WRC Bulletin 297 to nozzles which have a variety of geometries shall be considered. Moreover, the applicability of the stress intensity resulting from the bulletin should be into consideration when interested lines where stress intensity linearization is to be performed are not exactly consistent with the line defined in the Bulletin. In this study, the nozzles in cylindrical vessel shells are developed as three dimensional finite element models, which are loaded with unit forces and moments. The stress intensities from finite element models are investigated through a comparison of WRC Bulletin 297. In addition, a methodology to apply the stress intensity results from WRC 297 to different lines is proposed.

Commentary by Dr. Valentin Fuster
PVP2011-57475 pp. 329-338; (10 pages)

Many investigations and practical experience show that the leakage failure of the tube-to-tubesheet joints is a common failure mechanism for the shell and tube heat exchangers. To systematically research the tightness of the hydraulically expanded tube-to-tubesheet joints, a three-dimensional finite element model was developed to consider the influence of such factors as the hydraulic expansion pressure, operating temperature and multi-tubes expansion. on the residual contact pressure between the tube and tubesheet-hole of the joints. The analytical results show that the hydraulic expansion can improve the tightness of the joints significantly. The leakage pressure of the joint first increases and then decreases with the increase of the expansion pressure, thus, an optimal expansion pressure exists. The leakage pressure improves at the beginning and lowers gradually with an increase in the temperature. The tightness of the joint is reduced very little after the temperature cycle. The optimized expansion pressures are quite different between the single-tube expansion model and multi-tube expansion model. Thus, the influence of expanded tubes on the expanding tube should be taken into consideration in the research of the tightness of the joints.

Commentary by Dr. Valentin Fuster
PVP2011-57601 pp. 339-344; (6 pages)

The simplified FEA method was proposed on tubesheet of fixed tubular heat exchanger with bellows expansion joints. Taking the waste heat recovery heat exchanger for example, the entire model and the simplified model were established using ANSYS software, and the calculation results of the key components of the two models were compared. The results show that distribution of TRESCA stress intensity and radial displacement of tubesheet for the simplified model are consistent with those of the whole model, but the stress of the simplified model is slightly larger than that of the entire model, so the result is conservative. It means that the simplified method is feasible. Meanwhile, application research on tubesheet of different kinds of heat exchanger was made using the simplified design method, it provides an efficient FEA method of the tubesheet design for heat exchanger with expansion joints.

Commentary by Dr. Valentin Fuster
PVP2011-57735 pp. 345-350; (6 pages)

For pressure vessels and other welded structures subjected to cyclic loading conditions, fatigue failure of welded joints has been a major concern in design and operation. ASME Boiler and Pressure Vessel Code Section VIII Division 2 [1] provides different methods for fatigue evaluation on welded joints with various geometries, which facilitates the design and evaluation of welded joints subjected to cyclic loading. The selection of fatigue evaluation method will affect accuracy and outcome. This study attempts to demonstrate the design and evaluation process of a welded joint. Two major different fatigue evaluation methods have been used and results are analyzed and compared against operation data. The intention of this study is simply to present a practical case, through which proper selection of fatigue evaluation method for the specific welded joint could be suggested to achieve a safe, and yet, economical design.

Commentary by Dr. Valentin Fuster
PVP2011-57988 pp. 351-359; (9 pages)

The paper examines the effect of increased bottom plate projection for tanks with rigid ring wall foundations. A theoretical model, based on the existing model from Denham is proposed to determine the maximum projection length. The behavior of the tank, specifically near the bottom is studied till failure using FEA. The formation of plastic hinges in the bottom plate on the inside and outside of this joint is discussed in detail. The validity of the assumptions made by Karcher with regard to plastic hinges for obtaining the expression for the design life cycle in elevated temperature tanks is briefly analyzed.

Commentary by Dr. Valentin Fuster
PVP2011-57049 pp. 363-372; (10 pages)

In the pressure vessel and piping and power industries, creep deformation has continued to be an important design consideration. Directionally-solidified components have become commonplace. Creep deformation and damage is a common source of component failure. A considerable effort has gone into the study and development of constitutive models to account for such behavior. Creep deformation can be separated into three distinct regimes: primary, secondary, and tertiary. Most creep damage constitutive models are designed to model only one or two of these regimes. In this paper, a multistage creep damage constitutive model is developed and designed to model all three regimes of creep for isotropic materials. A rupture and critical damage prediction method follows. This constitutive model is then extended for transversely-isotropic materials. In all cases, the influence of creep damage on general elasticity (elastic damage) is included. Methods to determine material constants from experimental data are detailed. Finally, the isotropic material model is exercised on tough pitch copper tube and the anisotropic model on a Ni-base superalloy.

Commentary by Dr. Valentin Fuster
PVP2011-57130 pp. 373-379; (7 pages)

The isochronous stress/strain curve is a long established method of representing creep data in a manner which, under certain circumstances, provides a quick and often surprisingly accurate approximate solution to time dependent structural problems. Despite criticisms of the foundations of the method, it has survived over the years because it has either been the only method feasible at the time, or it is capable of providing solutions which are often good enough for practical purposes. This paper plans to trace the evolution of structural analysis based on isochronous curves, examining its boundaries of application and the circumstances under which it might be expected to yield plausible answers. Different types of isochronous curves will be described, together with procedures for constructing them from different forms of material data such as constant strain rate tensile tests. Special attention will be given to the representation of tertiary creep in the form of isochronous curves, and how such curves might be used in carrying out simplified analyses of propagating creep damage in complex components. Recent extensions to the original methodology to include variable load and thermal histories will be examined. Possible applications in the emerging field of very high temperature applications, as are expected to be experienced in Gen IV nuclear plant in the future, will be reviewed, with special attention given to the problem of rate dependent short term properties, which looks to become a serious question in development of design allowables for very high temperature applications.

Commentary by Dr. Valentin Fuster
PVP2011-57618 pp. 383-391; (9 pages)

The paper provides details about buckling tests on six steel cones and the corresponding numerical estimates of failure load (asymmetric bifurcation and/or collapse). Test models were machined from 250 mm billet. The wall thickness was 2 mm, small-end radius was 74.0 mm and the large radius end was 100 mm. The semi-cone angle was 14 deg. Cones had substantial, and integral top and bottom flanges. Experimental failure loads were obtained for: (i) the first two cones subjected to axial compression, (ii) subsequent two cones subjected to external pressure, and (iii) the remaining two models subjected to combined action of external pressure and axial compression. The magnitude of test pressure was about 5 MPa, and the axial failure load was approximately 230 kN. Good repeatability of experimental failure loads was obtained. Numerical estimates of failure loads were obtained for elastic perfectly plastic, engineering stress-strain, and true stress–true strain modelling of steel. Apart from axisymmetric modelling of shells, true geometry with true wall thickness distribution was adopted in calculations. Some of the numerical estimates of buckling loads are close to test data but other are not. The reasons for these discrepancies are highlighted in the paper.

Commentary by Dr. Valentin Fuster
PVP2011-57887 pp. 393-397; (5 pages)

The unprecedented seismic event and tsunami that caused the prolonged power outage at the Fukushima Daiichi nuclear plant have highlighted the importance of establishing stable long term cooling of the nuclear fuel and the containment following a complete station blackout. This paper presents an overview of the advanced passive safety systems implemented in the NuScale nuclear power plant. During normal operation, each NuScale containment is fully immersed in a water-filled stainless steel lined concrete pool that resides underground. The pool, housed in a Seismic Category I building, is large enough to provide 30 days of core and containment cooling without adding additional water. After 30 days, the decay heat generation is sufficiently small that natural convection heat transfer to air on the outside surface of the containment coupled with thermal radiation heat transfer is completely adequate to remove core decay heat for an indefinite period of time. These passive safety systems can perform their function without requiring an external supply of water, power, or generators. A complete assessment of the NuScale passive safety systems is being performed through a comprehensive test program that includes the NuScale integral system test facility at Oregon State University.

Topics: Safety
Commentary by Dr. Valentin Fuster
PVP2011-57901 pp. 399-404; (6 pages)

NuScale Power, Inc. is commercializing a 45 Megawatt electric light water nuclear reactor NuScale Power Module (NPM). Each NPM includes a containment vessel, a reactor vessel, a nuclear reactor core, an integral steam generator, and an integral pressurizer. The NuScale Power Module is cooled by natural circulation. The primary coolant in the Reactor Pressure Vessel is heated in the nuclear core, it rises through a central riser, it spills over and encounters the helical coil steam generator, it is cooled as steam is generated inside the steam generator, and it is again heated in the nuclear core. The Steam Generator also must be designed to provide adequate heat transfer, to allow adequate primary reactor coolant flow, and to provide adequate steam flow to produce the required power output. This paper presents the CFD results that describe the transport phenomena on the heat transfer and fluid flow dynamics in helical coil steam generator tubes. The ultimate goal of the CFD modeling is to predict the steam outlet conditions associated with the chosen helical coil tube geometries, solving the primary and secondary flow region together coupled with the helical coil tube. However, current studies are focused on the primary side with the heat flux boundary condition assigned on the outer surface of the helical coil steam generator. In this study, the ANSYS CFX v. 12.1 [1] was used to solve the three-dimensional mass, momentum and energy equations. The helical coil steam generator has complex geometry and modeling entire geometry requires the enormous memory that is beyond our hardware capability and is not practical. Therefore, geometry was limited to 1 degree of the wedge and 5% of the total length in the middle. Only external flow, single phase flow around the helical coils, is simulated using the standard k-ε model and shear stress transport model. From the results of the numerical simulation, the pressure drop and temperature profiles were determined. It is important to understand thermal hydraulic phenomena for the design and performance prediction of the reactor internal.

Topics: Boilers
Commentary by Dr. Valentin Fuster
PVP2011-57979 pp. 405-410; (6 pages)

With growing worries about climate change and greenhouse gas emissions, issues and risks associated with hydrocarbon supplies, many government agencies are working to push the development of licensing for Small Modular Reactors (SMRs). SMRs are part of a new generation of nuclear power plants being designed all over the world. The objective of these SMRs is to provide a flexible and cost-effective energy alternative. Small reactors are defined by the International Atomic Energy Agency as those with an electricity output of less than 300 MWe, although general opinion is that anything with an output of less than 500 MWe counts as a small reactor. Modular reactors are manufactured at a plant and brought to the site fully constructed. They allow for less on-site construction, increased containment efficiency, and heightened nuclear materials security. The NRC’s licensing process (NUREG-800) up to this point has focused mainly on large commercial reactors. The design and safety specifications, staffing requirements and licensing fees have all been geared toward reactors with an electrical output of more than 700MWe. Licensing for SMRs has been an ongoing discussion, with workshops in October 2009 and June 2010 about licensing difficulties, and a congressional hearing in May 2010. A discussion of these design and licensing aspects and relative importance as it pertains to the NuScale Small Modular Reactor design is provided.

Commentary by Dr. Valentin Fuster
PVP2011-57004 pp. 413-422; (10 pages)

For the investigation of cracked problems in thick-walled pressurized cylindrical vessels, the displacement-based finite element method has become one of the main computational tools to extract stress intensity results for their fatigue life predictions. The process of autofrettage, practically from partial autofrettage level of 30% to full autofrettage level of 100%, is known to introduce favorable compressive residual hoop stresses at the cylinder bore in order to increase its service life. In order to extract the fatigue life, stress intensity factors (SIFs) need to be obtained a priori. The necessity for determining SIFs and their practical importance are well understood. However, it is usually not a trivial task to obtain the SIFs required since the SIFs largely depend on not only the external loading scenarios, but also the geometrical configurations of the cylinder. Our recent work has shown that the Bauschinger Effect (BE) may come into play and affect the effective SIFs significantly for an eroded fully autofrettaged thick-walled cylinder. In this study, we further investigate the SIFs for the Bauschinger effect dependent autofrettage (BEDA) and the Bauschinger effect independent autofrettage (BEIA) at various autofrettage levels. The crack is considered to emanate from the erosion’s deepest point in a multiply eroded cylinder. The commercial finite element package, ANSYS v12, was employed to perform the necessary analysis. A two-dimensional model, analogous to the authors’ previous studies, has been adopted for this investigation. The residual stress field of autofrettage process, based on von Mises yield criterion, is simulated by thermal loading. The combined SIFs are evaluated for a variety of relative crack lengths with cracks emanating from the tip of erosions with various geometrical configurations and span angles. The effective SIFs for relatively short cracks are found to be increased by the presence of the erosion and further increased due to the BE at the same autofrettage level, which may result in a significant decrease in the vessel’s fatigue life. Deep cracks are found to be almost unaffected by the erosion, but may be considerably affected by BE as well as by the level of partial autofrettage.

Commentary by Dr. Valentin Fuster
PVP2011-57027 pp. 423-428; (6 pages)

In order to ensure the integrity of a seamless butt-welding elbow, both the central section and ends of the elbow need to be assessed, as the maximum stress is normally located at the central section of the elbow but there are no welding residual stresses. Furthermore, at the ends (welds) of the elbow, very high welding residual stresses exist if the welds have not been post weld heat treated but the primary stresses induced by the internal pressure and system moments are lower. For a 90 degree elbow welded to seamless straight pipe, both maximum axial and hoop stress components in the elbow can be calculated using ASME III NB-3685. At the ends of the elbow, axial and hoop stress components can be obtained using the stress equations presented in the paper of PVP2010-25055. In this paper, a series of limiting defect assessments have been carried out on an elbow assuming a postulated axial external defect as follows: • A number of assessments have been conducted directly using the axial and hoop stresses calculated based on ASME III NB-3685 for different system moments. • A series of assessments have been carried out using the axial and hoop stresses calculated using the stress equations presented in the paper of PVP2010-25055, a wide range of welding residual stresses and different system moments. A comparison of the assessment results in the elbow and at the ends of the elbow shows that when system moments are relatively low and the welding residual stress is high, the limiting defect size is located at the ends of the elbow; when the system moments are high and the welding residual stress is low the limiting defect size is located at the central section of the elbow. Therefore, it can be concluded that when assessing an elbow, the assessments should be carried out at both the central section and the ends of the elbow, in order to ensure the integrity of the elbow.

Commentary by Dr. Valentin Fuster
PVP2011-57030 pp. 429-438; (10 pages)

This work focuses on the evaluation procedure to determine the elastic-plastic J-integral and CTOD fracture toughness based upon the η-method for C(T) fracture specimens made of homogeneous and welded steels. The primary objective of this investigation is to enlarge on previous developments of J and CTOD estimation procedures for this crack configuration while, at the same time, addressing effects of strength mismatch on the plastic η-factors. The present analyses enable the introduction of a larger set of factor η for a wide range of crack sizes (as measured by the a/W-ratio) and material properties, including different levels of weld strength mismatch, applicable to structural, pipeline and pressure vessel steels. Very detailed non-linear finite element analyses for plane-strain models of square groove, center cracked C(T) fracture specimens provide the evolution of load with increased crack mouth opening displacement required for the estimation procedure. Overall, the present study, when taken together with previous investigations, provides a fairly extensive body of results to determine parameters J and CTOD for different materials using C(T) specimens with varying overmatch conditions.

Commentary by Dr. Valentin Fuster
PVP2011-57134 pp. 439-448; (10 pages)

We study ductile fracture using Reproducing Kernel Particle Interpolation and the Gurson-Tvergaard-Needleman (GTN) model. The meshless simulations are compared with the available experimental results and previous finite element simulations for crack propagation. The results agree well with experimental results, and it is confirmed that the proposed method provides a convenient and yet accurate means for simulation of ductile fracture.

Commentary by Dr. Valentin Fuster
PVP2011-57168 pp. 449-458; (10 pages)

The welded joints are particularly sensitive areas in the structures in terms of harmfulness of defects. Given the complexity of the problem (geometry poorly controlled, multi-material aspect, the potential influence of residual stresses), the tests are conducted based on pessimistic assumptions that can wrap all the uncertainties of the problem. In the case of a defect assessment, the considered toughness is deduced from conventional characterization tests with a crack in the welding, considering the current standards, ISO 12135 [1] or ASTM E-1820 [ 2] which are valid only for an homogeneous specimen. In 2010, a new standard ISO 16563 [3] was published to address the specificity of welded joints. If it covers some of the difficulties, it remains incomplete. In nuclear piping, welds have a mismatch M, ie the ratio between the yield strength of the weld metal and the base metal, usually greater than 1: this avoids any problem of strain localization at the junction and ensure that the stresses in the base metal are also easily supported by the welded joint. In this configuration, it turns out that for a given mechanical loading, a crack in the weld located generally has a solicitation, quantified by the parameter J, less (depending on the size of the junction) to those that would see the same crack located in the base metal. Unfortunately, this phenomenon exists also potentially for a characterization test, which would overestimate the true toughness of the welded joint. Plasticity that develops from the crack tip can quickly reach this interface and be affected. To evaluate this phenomenon, we considered two types of representative welded joint (PWR secondary loop ferritic weld and a 316 stainless steel weld) and performed a F.E. analysis of the multi-material CT specimen mechanical answer and on the η coefficient conventionally used to derive the plastic component of J from the area under the curve force-opening displacement.

Topics: Toughness
Commentary by Dr. Valentin Fuster
PVP2011-57171 pp. 459-471; (13 pages)

RCC-MRx code provides flaw assessment methodologies and related tools for Nuclear Power Plant cracked components. An important work has been made in particular to develop a large set of compendia for the calculation of the parameter J for various components (plates, pipes, elbows,[[ellipsis]]) and various defect geometries. Also, CEA in the frame of collaborations with IRSN, developed a methodology for J analytical calculation for cracked pipes and elbows submitted to thermal and combined mechanical and thermal loadings. This paper presents first the development of this methodology and an overview of the validation strategy, based on reference 2D and 3D F.E. calculations. The second part of the paper presents the last version of the MJSAM tools which is based on the 2010 version of the appendix A16 of the RCC-MRx code. All compendia (for KI, J and C* calculation) and all defect assessment procedures have been implemented in the tool: It covers crack initiation and propagation under fatigue, creep, creep-fatigue and ductile tearing situations. Sensitivity and probabilistic analyses can also been performed with this tool, directly linked to Microsoft Excel software for the results exploitation.

Topics: Pipes
Commentary by Dr. Valentin Fuster
PVP2011-57178 pp. 473-478; (6 pages)

Leak-before-break (LBB) assessment of nuclear piping involves ductile fracture analysis of pipes or elbows with postulated through-wall cracks. Due to the fact that the crown part of an elbow is one of the positions that crack initiation occurs in most frequently, the calculation of J-integrals to investigate fracture behavior are important research topics. This paper proposes a 3-D finite element model of an elbow embedded with an axial through-wall crack to estimate the J-integral parameters under bending moment. The J-integral values can be calculated by using ABAQUS and taking into account the effects of geometrical and model of material in non-linear analysis. The results show that the non-linear deformation and contact condition of crack surfaces play important roles for the J-integral values. In addition, the J values estimated by the proposed model are more conservative and realistic than previous studies.

Commentary by Dr. Valentin Fuster
PVP2011-57185 pp. 479-485; (7 pages)

This paper derives analytical solutions of the elastic follow-up factor for power-law creeping cruciform plates under bi-axial displacements to investigate the effect of multi-axial stress states on elastic follow-up behaviors. Validity of the proposed solutions is checked against the results from finite element analyses using power-law creep material. Based on proposed solutions, effects of the biaxiality, geometry, Poisson’s ratio and creep exponent on elastic follow-up factors are discussed. Present results show that the elastic follow-up factor for structure with structural discontinuity can be significantly affected by the multi-axial stress states.

Commentary by Dr. Valentin Fuster
PVP2011-57233 pp. 487-494; (8 pages)

Overlay cladding is classified to non-pressure boundary. Not only the ASME Boiler & Pressure Vessels Code Section III [1] but also the JSME Design and Construction Code [2] prescribe that no structural strength shall be attributed to cladding and the presence of the cladding shall be considered with respect to both the thermal analysis and the stress analysis. This means the codes do not require stress evaluation for overlay cladding itself. If overlay cladding has a fatigue crack, the crack may grow and extend to the base metal. Thus overlay cladding may give an influence on the integrity of base metal in the pressure boundary. The thermal expansion of stainless steel cladding is different from that of base metal made of low alloy steel, and this difference causes discontinuity of stress distribution between the cladding and the base metal. It is questionable that a stress evaluation line is set on such stress distribution including discontinuity between the cladding and the base metal. An evaluation method based on elastic-plastic analysis is preferable to evaluate such portion. ASME B&PV Sec.III and Sec.VIII, Div. 2 [3] have plastic analysis provisions. Also the JSME D&C Code issued a code case on alternative design methodology by using elastic-plastic finite element analysis for Class 1 vessels [4, 5]. In this paper, shakedown, fatigue and environmental fatigue evaluations are performed for the overlay cladding of direct vessel injection nozzle of Reactor Pressure Vessel by using the JSME Code Case on the alternative design methodology.

Commentary by Dr. Valentin Fuster
PVP2011-57234 pp. 495-503; (9 pages)

Thermal fatigue phenomena are long term deterioration mechanisms which become more and more important as the life-time of nuclear power plant increases. Some incidents as the incident of Civaux I and some works in thermal fatigue have disproved current methodologies and usual criteria to predict propagation of thermal fatigue cracks in nuclear power plants. This paper presents the results of the thermal fatigue tests, Fat3D, which are conducted on 304L austenitic stainless steel pipes. This experiment has been designed to study the problem of closure effect and fatigue crack growth under thermal fatigue conditions on quasi-structure specimens. The importance of the initiation and the propagation phases on a notched specimen and the evolution of the stress intensity factor according to the propagation are investigated as well. The use of different non destructive techniques to detect and follow crack propagation is also assessed. In parallel, a numerical interpretation is developed based on a material characterisation and using finite element analyses with the French Cast3M code. This combined experimental and numerical study enables to assess improvements of classical methods to accurately predict the crack growth propagation under thermal loads and to understand the influence of the main parameters concerning crack propagation in such components.

Commentary by Dr. Valentin Fuster
PVP2011-57294 pp. 505-513; (9 pages)

Ductile crack initiation behaviors were experimentally and analytically investigated using compressively prestrained notched round bar specimen (lower stress triaxiality condition) and four point bend specimen (higher stress triaxiality condition). The materials used were SM400B and HT780. It was observed that ductile crack initiation of notched round bar specimen occurred at the center of specimen and was caused by coalescence of micro voids. Ductile limit curves, which are the relationships between equivalent plastic strain and stress triaxiality factor at ductile crack initiation, were obtained using FE-analyses and experimental results. Ductile crack initiation of four point bend specimens as cracked specimens could be evaluated using ductile crack initiation limit curves obtained from notched round bar specimens if the positions of void nucleation at crack tip are properly considered. It has been found that ductile crack initiation limits can be evaluated using ductile limit curves under both low and high constraint conditions (stress triaxiality conditions.

Commentary by Dr. Valentin Fuster
PVP2011-57339 pp. 515-520; (6 pages)

Austenitic stainless steel components operated in the light-water reactor (LWR) environment are susceptible to intergranular corrosion cracking (IGSCC). It is well known that the environment and materials near the crack tip are the most important factors affecting the IGSCC crack growth rate in the high-temperature aqueous environments. An intergranular crack growth model based on the multi-scale method is proposed in this study to accurately capture the stress-strain state at the crack tip of IGSCC. This model is implemented in ABAQUS via a sub-model technique. The effect of the extension direction of the kinking cracks on IGSCC behavior is studied for better understanding the effect of stress-strain field at crack tip on the crack growth rate in LWR.

Commentary by Dr. Valentin Fuster
PVP2011-57362 pp. 521-528; (8 pages)

For the assessment of an under-clad defect in a vessel submitted to a pressurized thermal shock, the plasticity is considered through the amplification β of the elastic stress intensity factor KI in the ferritic part of the vessel. The current solution in the French RSE-M Code has been developed from a fitting of F.E. calculation results and specifies to keep constant the amplification (deduced from its value at the maximum loading point) when the stress intensity factor is decreasing. A more physical solution is proposed in this paper to deal with the initial increasing loading phase but also with the unloading phase. It takes into account two phenomena: the amplification of the elastic KI due to the plasticity in the cladding and a plastic zone correction in the ferritic part. The first amplification is determined assuming that the plasticity in the cladding could be represented as an imposed opening displacement on the crack lips. When the loading is decreasing, the model takes into account the initial elastic unloading of the cladding but also the plastic compression which leads to reduce the cladding influence on the loading of the crack tip located in the ferritic part.

Commentary by Dr. Valentin Fuster
PVP2011-57365 pp. 529-537; (9 pages)

This paper deals with the brittle fracture risk evaluation for a C-Mn piping component in the upper shelf of the brittle to ductile fracture transition temperature range, with the main objective to validate a predictive criteria, able to demonstrate the complete absence of brittle fracture risk. The criteria is based one a critical stress and the volume around the crack were the maximum principal stress exceed this critical stress. The model is calibrated on notched tensile specimens and CT specimens. A four-points bending pipe test has then been designed using this criterion to insure that no brittle fracture will occurs at a temperature that all CT specimens failed by cleavage. The material is a French secondary loop Tu42C ferritic steel and the pipe dimensions for the test are the same than the size of the principal secondary loop pipes. The results of the pipe test confirm the prediction with the model and the interpretation lead to define an equivalence between the loading conditions (based on the J parameter) of the pipe and the loading condition of a CT specimen.

Topics: Steel , Brittleness , Pipes , Failure
Commentary by Dr. Valentin Fuster
PVP2011-57389 pp. 539-551; (13 pages)

There is now a large amount of small cylindrical specimen test data, which indicates that in a Light Water Reactor (LWR) environment, compared to that in air, the fatigue life of stainless steel is significantly reduced. The current ASME III design fatigue curve does not explicitly include factors to account for a LWR environment. Using the available cylindrical specimen test data, methods for accounting for a LWR environment in fatigue assessments have been proposed in NuReg/CR-6909 and in two American Society of Mechanical Engineers (ASME) code cases. One of the code cases (N-792) uses a penalty factor (Fen ) approach, similar to that in NuReg/CR-6909, another (N-761) utilizes a set of environmentally corrected fatigue curves. A third code case, which is still under development, uses a flaw tolerance approach. In this paper the background to the methods for correcting for a LWR environment in fatigue calculations is presented. The safety margin present in the ASME fatigue design methodology is discussed and a short review of civil nuclear plant operating and geometrical features testing experience provided. The NuReg/CR-6909 and ASME code case N-761 methods are applied to a number of ASME III Class 1 austenitic stainless steel components, and the cumulative usage factors calculated compared with those obtained using the ASME 2007 design fatigue curve. An objective of the paper is to highlight some of the issues arising out of applying the newly proposed methods to reactor plant components.

Commentary by Dr. Valentin Fuster
PVP2011-57467 pp. 553-558; (6 pages)

In this paper, creep crack growth simulations were performed based on finite element (FE) damage simulations for several types of test specimens such as C(T), CS(T), SEN(T), M(T) and SEN(B). The Graham-Walles creep law was adopted to describe all three phases of creep. The Creep parameters were obtained from uni-axial creep test results of SS316H at 550°C. Damage model was also included based on creep ductility exhaustion concept, which is defined as the ratio of creep strain to the multi-axial creep ductility. Creep crack growth rates resulting from FE simulations were compared with the experimental data.

Commentary by Dr. Valentin Fuster
PVP2011-57480 pp. 559-567; (9 pages)

This paper describes steady-state stress on welded branch components using detailed three dimensional elastic creep finite element analyses. In our previous paper [1], it was found that the mismatch effect in creep on steady-state stresses within the weld metal for a various branch junction could be uniquely quantified by the mis-match factor, defined as a function of creep exponent and constant. In actual branch components, the branch junction contains the heat-affected zone (HAZ) when the branch pipe is welded. Thus, additional mismatch factor for HAZ should be presented. This paper deals with not only the mismatch effect for weld metal but also that for HAZ. The creep exponent and constant for the base and weld metal as well as HAZ were systematically varied to analyze under-matching, even-matching and over-matching conditions in creep. In order to investigate the effect of the loading mode, FEA was carried out under internal pressure and in-plane bending to the branch pipe. Two geometries such as large bore branch and medium bore branch were considered. It was found that steady-state creep stresses within HAZ can be quantified as mismatch factor with specific characteristics.

Topics: Creep , Heat , Stress , Steady state
Commentary by Dr. Valentin Fuster
PVP2011-57488 pp. 569-575; (7 pages)

Charpy testing across a range of temperatures is a cost effective way to characterise the ductile-to-brittle transition region. It is often convenient to fit a curve to Charpy data through the transition region: a commonly used method is to use a continuous tan-h fit, a single mathematical expression that links lower shelf, transition region and ductile upper shelf behaviour in one continuous curve. Using this method, the temperature dependence of Charpy energy is a unique feature of each individual steel with some steels exhibiting steep transition curves and some shallow curves. In contrast to Charpy data, fracture toughness data are usually analysed by partitioning upper shelf and transition region data. The transition region data is generally accepted to fit a universal temperature dependence, the Master Curve, as proposed by Wallin [1] and standardised in ASTM E1921 [2]. Recent research on nuclear pressure vessel steels [3, 4] has indicated that when Charpy data is assessed using a similar method to that used for fracture toughness data, a common exponential temperature dependence is observed. This paper presents the current results from an on-going investigation aimed at assessing the effect of exponential curve fitting methods on a large dataset of Charpy V-notch energy data from Tata Steel. The Tata Steel data cover a wide range of parent plate steels. The results are compared to the recent studies on nuclear pressure vessel steels and a similar exponential temperature dependence is observed.

Topics: Steel , Databases , Fittings
Commentary by Dr. Valentin Fuster
PVP2011-57500 pp. 577-584; (8 pages)

In recent years we have seen a development of novel experimental techniques that enable one to non-destructively characterize polycrystalline microstructures. These techniques hold significant advantages over approaches like serial sectioning since the specimen is not destroyed in the characterization process. This is of immense value in advancing our understanding of materials and multiscale computational models. In particular, processes at the small length scales like the initiation and early development of grain boundary damage can now be measured more closely while the resulting simulations can now be directly compared to the experimental data. The task is, however, far from being simple as extremely complex geometry needs to be coupled with advance constitutive models for the bulk grain material and the grain boundaries themselves need to be combined. In this work a model, based on a X-ray diffraction contrast tomography data of a stainless steel wire with a diameter of 0.4 mm is presented. 3D topology and crystallographic orientation of individual grains are directly transferred into a finite element model. Grain boundary damage initialization and early development is then explored for a number of cases, ranging from isotropic elasticity up to crystal plasticity constitutive laws for the bulk grain material. In all cases the grain boundaries are modeled using the cohesive zone approach. Also, the stability of the simulations and measures aimed at improving it are reported upon.

Commentary by Dr. Valentin Fuster
PVP2011-57537 pp. 585-592; (8 pages)

For a better understanding of thermal fatigue in piping mixing zone, EDF, CEA and AREVA ANP have developed a mock-up named FATHER. Tests performed on the mock-up have given access to fluid temperature thermal fluctuations and external surface strains. Tests were completed by an endurance test. Examinations revealed the presence of numerous cracks at different locations. EDF R&D has performed a numerical interpretation of the test, in relation with different specimen fatigue tests (HCF tests on polished specimens, influence of surface roughness, influence of mean stress,[[ellipsis]]) and with the use of different fatigue criteria (RCC-M, Fatemi-Socie, Dang Van[[ellipsis]]). Numerical results are compared with the results of examinations. A comparison is done with results obtained by codified methodologies for nuclear plants.

Commentary by Dr. Valentin Fuster
PVP2011-57576 pp. 593-601; (9 pages)

The present paper deals with a structural integrity assessment of a seamless steel cylinder for transportation of HCNG blend for a 200 bar pressure application with 30% H2 content. First a fracture mechanics approach with an estimation of expected number of cycles and failure mode is applied to a typical cylinder for CNG transportation designed with a traditional approach. The same approach is then used for an assessment of the same cylinder working in a HCNG environment. Fracture toughness was experimentally measured in HCNG blend according to a stepwise procedure and used as input for cylinder failure mode prediction. Fatigue crack growth rate was estimated based on comparison of two interrupted full scale tests carried out in oil and in the investigated HCNG environment. Acceleration of crack propagation, when using HCNG blend, suggests that a dedicated structural integrity assessment of the cylinder is needed to determine the proper design parameters for a safe product life.

Topics: Steel , Design , Cylinders , Hydrogen
Commentary by Dr. Valentin Fuster
PVP2011-57582 pp. 603-608; (6 pages)

The primary goal of this paper is to demonstrate and validate the applicability of the failure assessment diagram (FAD) approach for predicting failure behavior of piping subjected to thermal loading caused by thermal stratification. The analysis is performed for an austenitic surge line of a PWR circuit where thermal gradients over both the pipe length and circumference produce high bending moments. For the most critical location, circumferential through-wall cracks of variable length are assumed and assessed with respect to their initiation and possible unstable propagation. The assessment result is shown to be decisively influenced by the modeling approach, which in effect depends on the categorization of the bending stresses as either primary or secondary loading. To achieve realistic fracture mechanics assessment, the whole surge line is described by a finite-element model, whereas thermal loading is imposed by mapping the temperature distribution measured under service conditions. Subsequently, elastic-plastic stress and fracture mechanics calculations are performed for the entire surge line model containing cracks of different size in the most stressed location. The analysis clearly confirms the LBB behavior. The numerical results are then compared to those based on the failure assessment diagram (FAD) approach, whereas the section bending moment determined from pipe calculations is categorized as either primary or secondary loading. It is concluded that most realistic analytical failure prediction is achieved when considering the bending moment as secondary loading. Even though the analytical approach is found in this case to produce conservative assessment results, its accuracy is concluded to be acceptable within a wide range of crack sizes considered.

Topics: Stress , Pipes , Failure
Commentary by Dr. Valentin Fuster
PVP2011-57623 pp. 609-616; (8 pages)

The emergence of fitness-for-service standards has created an opportunity to advance pipeline integrity assessment technology and methodologies. This paper describes modern approaches for assessing cracks and other planar flaws in pipelines. Many traditional approaches rely on simplistic characterizations and equations that have not evolved to take into account advancing technologies. For example, the NG-18 method dates from the early 1970s and is still widely used today. However, so-called conservative methods such as NG-18 can actually be unconservative in some instances, such as when used to determine hydrostatic testing intervals. This paper highlights today’s advanced approaches to pipeline integrity assessment. Case studies are presented illustrating the assessment of pipeline anomalies, such as crack assessment following hydrotesting and simulation of burst tests using a ductile fracture model.

Commentary by Dr. Valentin Fuster
PVP2011-57630 pp. 617-624; (8 pages)

Nuclear power plants necessarily include connections of branches conveying fluids at different temperatures. Thermo-hydraulic fluctuations arising from the turbulent mixing of the flows may affect the inner wall of the pipes and lead to fatigue damage. The FATHER experiment was carried out in order to better understand this phenomenon of High Cycle Thermal Fatigue (HCTF). The aim of this paper is to present an analysis of the FATHER experiment with a simplified engineering method which is used in French nuclear industry to identify and classify zones presenting a HCTF risk.

Topics: Fatigue , Cycles
Commentary by Dr. Valentin Fuster
PVP2011-57700 pp. 625-634; (10 pages)

Thermal stratification is a common phenomenon in the surge lines of Pressurized Water Reactors (PWR). The stratification temperature difference (ΔT) and cyclic action severities are most prevalent during the heatup and cooldown operations of a PWR, when the system ΔT between the pressurizer and the Reactor Coolant System (RCS) hot leg is the greatest and system inventory fluctuations are highest. This paper describes the computer simulation of thermal stratification loading in a surge line nozzle connected to the RCS hot leg to correlate to unusual behavior of plant sensor data in the hot leg and the subsequent development of a monitoring model to account for thermal stratification effects in the transient and fatigue evaluation performed in the online monitoring system. What makes this particular investigation unique is the geometry of the nozzle of interest. In many PWRs, the surge line and the surge line hot leg nozzle are horizontal at the hot leg connection. This particular nozzle is oriented at an upward angle before the attached surge line piping bends into a horizontal configuration. This orientation required a more detailed treatment of the stratification effects than has been typically developed for horizontal nozzles, with respect to both the orientation and the potentially detrimental effects of increased cyclic behavior indicated by nearby temperature sensors. This investigation combined Computational Fluid Dynamics (CFD) modeling of the system to correlate the plant data with a detailed stress model that will enable the fatigue usage factor calculation in the plant’s online transient and fatigue monitoring system.

Commentary by Dr. Valentin Fuster
PVP2011-57708 pp. 635-644; (10 pages)

This study investigates the fatigue performance of a preloaded threaded fastener under two scenarios of axial cyclic in-service loading. Since the fastener mean stress level would naturally affect the fatigue performance, various methods are investigated for their suitability for use to generate mean-stress adjusted Ś-Ń fatigue curves. The maximum in-service cyclic tensile load that may externally be applied to the joint for a desired fatigue performance is also investigated. Limited fatigue test data set-up is generated for comparison with various methodologies for generating the mean stress-adjusted fatigue strength curves of A286 fasteners. Cumulative damage analysis is also performed in accordance with commonly used damage theories.

Commentary by Dr. Valentin Fuster
PVP2011-57749 pp. 645-652; (8 pages)

In this paper the stress range versus the number of cycles to failure (S-N) curves has been developed for steels used in the pressure vessel and construction industries. These include SA 533 Grade B and C36 Steel. The method used to obtain the S-N curves involves a material parameter based on fatigue crack threshold stress intensity factor range and the endurance limit. This parameter has been used to study fatigue crack growth of short cracks as a means to study fatigue crack initiation in steels and is based on the concept that initiation occurs only when the material ahead of the crack tip is damaged enough by cyclic straining. A separate model has been explored that incorporates a microstructural variable, namely the grain size. For the two materials studied, the initiation and growth of small cracks have been investigated with consideration for crack closure. Using the material parameters, the number of cycles to initiate a fatigue crack has been determined.

Commentary by Dr. Valentin Fuster
PVP2011-57860 pp. 653-658; (6 pages)

Coke Drums are critical equipment in refineries due to variable temperature and pressure. The temperature is highly fluctuating which causes the thermal fatigue in coke drums. The phenomenon of this thermal fatigue plays a very crucial role in failure of coke drums. In the present study, a coke drum is subjected to pressure - temperature reversal with each cycle of 48 hours duration. Temperature and pressure varies from 338 to 738 K and 0 to 0.404 MPa, respectively. The material of construction is 1.25Cr-0.5Mo (SA-387 Gr. 11 Cl. 1). This coke drum is at higher temperature of 738 K for 24 hours in one full cycle. The skirt to shell and cone junction is a critical portion of coke drum because it is highly susceptible to fatigue at higher temperature. In addition to this, the skirt is provided with slots at specific pitch all around circumference to induce flexibility. This has great influence on fatigue life due to localized stress at higher temperature. API 579-1/ASME FFS-1 2007 is used for the fatigue life evaluation in the present case. Nonlinear transient thermal analysis is coupled with the elastic-plastic structural analysis for calculation of stresses and strains. These stresses are further used for deducing permissible cycles for fatigue as per the approach based on API 579-1. A Parametric study has been carried out in Finite Element Analysis tool ‘ANSYS-12.1’ for different configuration of skirt to cone junction specifically in terms of inside crotch radius. The effect on fatigue life has been studied with variation in inside crotch radius. It has been found that fatigue life at skirt to cone junction is increasing with increase in crotch radius. But, this increase in crotch radius is adversely effects on slot tip where the life is drastically reducing. Such parametric study can be considered for selection of inside crotch radius for given skirt thickness and diameter.

Commentary by Dr. Valentin Fuster
PVP2011-57893 pp. 659-663; (5 pages)

Socket welds are used extensively for small bore pipe branch connections (up to a two-inch nominal diameter). For more than 15 years, numerous failures due to fatigue cracking occured in French NPPs (EDF operates 58 PWRs), mainly on safeguard systems as regards nuclear island safety-related systems. Such failures have significant impacts, on costs, due to repairs or In-Service-Inspections and sometimes, on plants availability. Destructive examination of cracked small bore pipe branch connections have led to a better understanding of failure mechanisms and have allowed to predict crack initiation locations depending on specific features such as main and small bore pipes thicknesses. Modifications implemented, based on empirical approaches, have turned out to be much or less efficient, depending on nuclear units. A three-stage methodology was devised in the early 2000’s and applied on the 8 most important safety-related systems. Based namely on vibration measurements, this methodology has allowed to identify sensitive small bore pipe branch connections, define a relevant In-Service Inspection program, determine vibrations origins and possible solutions. New modifications are currently underway on the 2 most affected systems. So far, they have shown to be effective. Solutions for the other systems should be decided at the end of 2011 and implemented on the whole French nuclear fleet.

Topics: Fatigue , Pipes , Bifurcation
Commentary by Dr. Valentin Fuster
PVP2011-57898 pp. 665-674; (10 pages)

Dissimilar Metal Welds (DMW) are required in nuclear reactors to join low alloy steel components and stainless piping. The thermal and mechanical mismatch between the dissimilar material characteristics favors the stress concentration in the weld along the austenitic/ferritic interface. Assessing the ductile tearing resistance of DMWs is an issue even for crack initiation. But predicting large crack extensions is even more difficult for several reasons: crack path deviation, J-R curve determination, transferability from specimen to structures. This paper presents an approach to determine the resistance curve of a DMW on the basis of a ductile tearing simulation using a cavity growth model.

Topics: Welded joints
Commentary by Dr. Valentin Fuster
PVP2011-57942 pp. 675-683; (9 pages)

Fatigue of Niobium stabilized austenitic stainless steel (X6CrNiNb1810 mod) was studied using specimens extracted from a solution annealed and quenched primary piping material sample. This paper reports and discusses results of non-standard experiments to complement previously published test data. The NPP primary piping components spend long times in operation temperature between fatigue cycles originating from thermal transients. This was roughly simulated by fatigue tests periodically interrupted for intermediate annealing in elevated temperature. Fatigue endurance was notably increased when low strain amplitudes were used. The life extension is explained by the cyclic stress strain response. Hardening followed by slow cyclic softening was consistently observed after annealing. It is generally assumed that cumulative accumulation of fatigue damage occurs at a wide range of loading amplitudes. We performed two level and spectrum straining tests combining amplitudes above and below the (Nf > 107 ) endurance limit. The endurance limit seems to be effective also in variable amplitude loading. In terms of modified Miner rule, even “negative damage” was obtained in two level tests below and above the constant amplitude endurance limit. This behavior is linked to prominent secondary hardening of the steel.

Commentary by Dr. Valentin Fuster
PVP2011-57947 pp. 685-690; (6 pages)

Cohesive zone modeling has been shown to be a convenient and effective method to simulate and analyze the ductile crack growth behavior in fracture specimens and structures. However, the cohesive zone modeling has not been applied to simulate the ductile crack growth behavior of a circumferential through-wall cracked pipe. In this paper, cohesive zone modeling has been applied to simulate the ductile crack growth of a past through-wall-cracked pipe test that was conducted during Degraded Piping Program. The ABAQUS code was used for the three-dimensional finite element analysis. The bending moment at crack initiation, maximum bending moment, crack extension, and J-integral values were calculated from the finite element analysis. These results were compared with the experimental results. In addition, results obtained from an existing J-estimation scheme (LBB.ENG2) were provided for comparison. All results showed reasonable agreement. The results of the present study demonstrate that the cohesive zone modeling can be applied to simulate the ductile crack growth behavior of a through-wall cracked pipe.

Commentary by Dr. Valentin Fuster
PVP2011-57951 pp. 691-699; (9 pages)

In May 1998, a leak (30 m3 / h) occurred in the reactor heat removal system (RHRS) of the CIVAUX 1 power plant (PWR type N4 – 1400 MWe) which was then in a hot shutdown situation. A 180 mm through-wall crack was found in a 304 L austenitic stainless steel elbow in a mixing area of high and low temperature fluids [1, 2]. All mixing zones of main (∅10″ ) and minimum flow lines (∅4″ ) of the four N4 plants were affected by cracking [3]. After metallurgical examinations of these austenitic stainless steel components and an analytical damage evaluation, the major root cause for cracking was identified as high cycle thermal fatigue. The cracks were found in the mixing tees and at the roots of welds in mixing areas. The presence of ground surface finishes and geometrical discontinuities (weld roots and tapers) were identified as amplifier of fatigue damage. For the new RHRS mixing zones of N4 plants, decision was taken to suppress welds or locate them away from mixing area and to improve the surface condition (remove the weld root singularity, remove striations due to machining by polishing and reduce residual stresses). For the other 54 French PWRs (900 & 1300 MWe) with different design of RHRS mixing zones, the inspections showed that they were also all damaged by thermal fatigue with generally small cracks less than 3 mm excepted for the PWRs of Saint Alban 2 (5 mm) [4]. To reproduce the thermal fatigue phenomenon occurring in mixing zones, a representative endurance thermal fatigue test named “FATHER” was performed by CEA under an EDF, CEA and AREVA NP agreement [5, 6]. The test lasted 300 hours. It was performed on a 304L stainless steel mixing zone of 7 mm thick and 6″ diameter with a temperature difference of 160°C between cold and hot fluids. Different internal surface finishes were introduced in the test mock-up: coarse and fine grinding, industrial polishing, as extruded surfaces and as welded or flushed joints. Numerous NDE were performed during and after the endurance fatigue test like ultrasonic examinations or dye liquid penetrant inspections. They lead to the observation of many small thermal fatigue cracks located near as welded joints, on ground surfaces and on unpolished flushed welds. Cracks were not observed on industrially polished surfaces reproduced in straight piping sections or in flushed plus polished welds. After the test of 300 hours, the mock-up was axially cut in two symmetric half parts and sampling plates containing thermal fatigue cracks were machined from each of the half mock-up to perform detailed metallographic examinations. More than 50 thermal fatigue cracks with depths of 100 to 1000 μm were observed. Cracks initiate mainly on geometrical discontinuities like weld toes or grinding striations. Test results have also allowed to improve and to validate methods and tools for predicting crack initiation in mixing zones. The “FATHER” experiment can be seen as a significant contribution for preventing the risk of HCF in PWR equipment.

Commentary by Dr. Valentin Fuster
PVP2011-58020 pp. 701-708; (8 pages)

For western pressure vessel reactors, assessment of pressure vessel steels irradiation embrittlement due to neutron irradiation is based on a semi-empirical formulae which predicts the shift of a reference lower bound fracture toughness curve as a function of fluence and embrittlement-involved chemical elements. Periodically, in order to monitor the embrittlement of each RPV, the predictions of the formulae is confronted to experimental results obtained from Charpy specimens located in surveillance capsules irradiated with a higher fluence level than the pressure vessel itself. Historically only the shift of the temperature index defined for a given level of energy, e.g. 56J in the French surveillance program, is used. In support to the French surveillance program methodology, for some of the French RPVs, physical models of fracture (for both cleavage and ductile fracture) are used to analyse in details the whole experimental basis available at different levels of fluence. This study presents the methodology developed in order to analyse the experimental results of a RPV steel from the french surveillance program, including Charpy and fracture toughness tests at different levels of fluence i.e. of embrittlement. The methodology applied aims to use the numerous Charpy tests results available in order to assess, at the same fluence levels, the fracture toughness embrittlement. The results are then compared to available fracture toughness results for a given level of embrittlement.

Commentary by Dr. Valentin Fuster
PVP2011-58054 pp. 709-714; (6 pages)

For plant life extension, it is the regulatory requirement to assess reactor coolant environmental impacts on critical components of the nuclear power plant including at least those mentioned in NUREG/CR-6260[2]. The pressurizer surge line is the most easy-to-fail component in view of LWR (Light Water Reactor) environments when it comes to meeting the current ASME code limit of the fatigue evaluation. Cumulative Usage Factor (CUF) value could be increased to a maximum of 15.35 times due to the environmental effects, which makes it easy to exceed the allowable fatigue limit (1.0). This paper discusses the process of the environmental correction factor calculation described in NUREG/CR-5704[4], and five proposed schemes for reducing the environmental CUF value to the ASME code limit or below. This paper concludes that the proposed schemes are effective in lowering the environmental CUF value of the pressurizer surge line.

Topics: Fatigue , Surges
Commentary by Dr. Valentin Fuster
PVP2011-58062 pp. 715-726; (12 pages)

Crack-face closure occurs physically at the compressive edges when a shell is subjected to bending loads. However, in traditional shell theories, crack closure effects are not concerned when evaluating the stress intensity factor (SIF). In reality, crack closure effects influence significantly the SIF. This article presents the theoretical and numerical analyses of crack-face closure effects on the stress intensity factor of shells under bending. The theoretical formulation is based on the shallow shell theory of Delale and Erdogan, incorporating the effects of crack-face closure, which are modeled by a line contact at the compressive edges of the crack faces. It is shown that due to curvature effects crack closure in shells may not occur on the entire length of the crack, depending on the nature of the bending loading and the geometry of the shell. To validate the theoretical solution finite element analysis (FEA) is also performed; the two results agree well. As an example, the stress intensity factor for a pressurized cylinder containing an axial crack is determined based on the improved shell theory which takes into account the effects of crack-face closure.

Commentary by Dr. Valentin Fuster
PVP2011-58064 pp. 727-735; (9 pages)

The structural integrity of the RPV is an essential issue for the plant safety. At the design stage, the demonstration is required with material properties at end of life, to ensure the adequacy of the design with the expected operating transients in all conditions. During operation, the integrity assessment is updated every ten years with new existing knowledge and feedback of operating experience, in particular in service aged material data coming from Irradiation Surveillance Program ISP, fluence evaluations taking into account the effective in service core arrangements in each vessel, in service detected flaws plus a postulated subclad crack whose detection cannot be guaranteed by the qualified ISI program. The final assessment showed that the regulatory criteria are met until the fourth decennial outage for 900 MW RPV. The analysis is performed in accordance with French regulations (use of safety coefficients) and follows a deterministic approach in which the input parameters and uncertainties are taken into account conservatively. For the future demonstration beyond 40 years, a multidisciplinary effort is committed to improving knowledge in order to reduce uncertainties in data and in methods. This extensive program involves in particular: - Thermohydraulic analysis and description of transients: temperatures and heat exchange coefficients; - Mechanical analysis: warm pre-stress effect and crack arrest. In addition, a complementary study using a probabilistic approach to rationalize the level of conservatism of input data is launched. In this report, the French deterministic approach and the main results for 40 years duration are presented and the new developments for the future.

Commentary by Dr. Valentin Fuster
PVP2011-57280 pp. 739-747; (9 pages)

The metallurgical characteristics of the damage observed in both service and laboratory test samples indicate that creep rupture is the dominant failure mode for Dissimilar Metal Welds (DMW) in some high temperature service conditions. However, it has also been observed that temperature cycling contributes significantly to damage and can cause failure even when primary stress levels are relatively low. Therefore, a creep-fatigue assessment procedure is required as part of a remaining life calculation. API 579-1/ASME FFS-1 2007 Fitness-For-Service standard includes a compendium of consensus methods for reliable assessment of the structural integrity of equipment containing identified flaws or damage. Part 10 of this document includes a method for protection against failure from creep-fatigue. In the assessment of DMW, a creep-fatigue interaction equation is provided to evaluate damage caused by thermal mismatch, sustained primary stresses, and cyclic secondary loads. In this work, alternative methods based on the ductility exhaustion with creep-fatigue interaction R5 V2/3 and R5 V6 are compared to the API 579-1/ASME FFS-1 standard method. The validity of an elastic FEA with linear material behavior is evaluated based on comparing results from FEA with nonlinear material behavior. A 2.25Cr 1Mo to SS 347 dissimilar joint welded with alloy 625 in a hydroprocessing heat exchanger nozzle joint was selected for the analysis. A Finite Element (FEA) model is used to estimate the sustained and cyclic primary and secondary stresses and strains for this weld. The model includes details of the geometry, material properties, boundary conditions, and loads. The results from the FEA are post-processed using the fatigue methods described above. Lastly, a sensitivity study based on operating temperature is performed. The results of this work indicate that the predictions of the number of cycles and time in service to crack initiation and creep failure are not significantly different between various methods. However, the results of the R5 V2/3 method using linear elastic FEA become invalid at higher temperatures because of significant stress redistribution. The temperature sensitivity analysis clearly showed that the life of the weld is strongly influenced by the service temperature for this type of joint.

Commentary by Dr. Valentin Fuster
PVP2011-57615 pp. 749-758; (10 pages)

An assessment was completed to address the failure of internal thermal sleeve weld for reheat condensate nozzle of steam generators. The plant is operated by Ontario Power Generation (OPG) in Pickering, and has CANDU® 6 type reactors. The objective of assessment was to evaluate the effect of the failed weld on the overall structural integrity of the nozzle for the defined operating service conditions. The fitness for service of the steam generator nozzle was demonstrated by comparing the maximum stress ranges of the initial nozzle design with the failed weld nozzle configuration under the same service conditions. Two nozzle configurations were considered for this assessment. One configuration represents the original shape with no leakage at weld indicating as-designed condition. Transient heat transfer and the stress analyses were performed according to the defined service limits. Another configuration completed for the faulty condition in which weld is failed and thermal sleeve separated. The same transients as the first configuration were applied, but the leakage was introduced at the thermal sleeve weld. The effect of leakage was considered by changing the convection heat transfer coefficient in annulus area between the external side of sleeve and internal surface of the nozzle. Critical locations on the nozzle were identified for the whole transient cycles, and assigned different stress lines. The maximum and minimum stress intensity ranges of the initial nozzle design and the cracked weld nozzle design were compared for these stress lines. It was concluded that the thermal sleeve weld failure with the conservatively postulated leakage flow provides better results in terms of stress ranges compared to the as-design condition. The thermal shield was over constrained in as-design condition. And for the fitness for service evaluation in was decided to leave the failed weld in-service without repairing it.

Commentary by Dr. Valentin Fuster
PVP2011-57657 pp. 759-774; (16 pages)

Provided in this paper is a case history that highlights use of advanced numerical analysis techniques to assess the Fitness-for-Service of cyclic pressure vessels using API 579-1/ASME FFS-1 (API 579) [1]. The components were subject to repeated thermal and pressure loadings which resulted in cracking of the base material of these components. A detailed level 3 FFS assessment was performed to assess potential for brittle fracture, determine critical flaw dimensions and evaluate the structural integrity of engineered repairs. The case study demonstrates the value of advanced numerical analysis to substantiate critical run-repair-replace decision making for pressure containing equipment. Detailed simulation of Weld Residual Stress (WRS) and local Post-Weld Heat Treatment (PWHT) showed that the local PWHT performed in the field was ineffective at reducing residual stress and highlights the need for more engineering of PWHT layouts and procedures, particularly for cases where stress corrosion cracking or brittle fracture are limiting design/operating considerations. The results also show how advanced analysis can be used to gain further insight into structural behavior in order to validate more simple methods that support practical decision-making related to fitness-for-service assessments going forward. Fatigue life predictions from a strain-life approach are also compared to ASME Section VIII, Division 2 [2] (S8D2). In addition, a brief comparison of these fatigue analysis methodologies is provided. The results are framed in terms of practical field experience obtained by periodic UT shear wave inspection of 16 identical vessels over a 10 year operating history.

Commentary by Dr. Valentin Fuster
PVP2011-57660 pp. 775-821; (47 pages)

Reactors of cyclic catalytic reformers require catalyst regeneration about every 7 days (∼50 cycles per year). To facilitate the in situ regeneration, large diameter motor operated valves (MOV) are used (typically Class 600 NPS 8 to NPS 16 API 600 gate block valves). Double block-n-bleed arrangements of the MOVs are used to assure isolation of the 350 psig 960°F process and regeneration media. The regeneration MOVs will be closed and in isolation for up to 4 days and can reach temperatures close to ambient depending on their distance from the process flow. Once the MOVs are swung open, the valve body quickly transitions from ambient temperature to 960°F over a matter of minutes. Such an extreme thermal shock has historically led to a number of cracks in the MOV valve bodies. With each unit having over 50 MOVs, it becomes expensive and time consuming to open every MOV at a scheduled turnaround, inspect, and repair any cracking noticed.

Commentary by Dr. Valentin Fuster
PVP2011-57041 pp. 825-832; (8 pages)

In order to determine the ratchet and shakedown limit curves for even a simple component, such as a tube under a constant pressure load and cyclic thermal load, can be a daunting task when using conventional analysis methods (elasto-plastic cyclic finite element analysis) that require repeated iterative simulations to determine the state of the structure, elastic, shakedown, plastic or ratchet. In some cases, the process is further complicated by the difficulty in interpreting results of the cyclic loading to determine in which regime the structure is. Earlier work by Abou-Hanna and McGreevy was able to demonstrate limit load analysis of a structure whose yield strength is modified based on cyclic load, provided the ratchet limit [1]. The method, called Anisotropic Load Dependent Yield Modification (LDYM), was implemented by using a user subroutine with ABAQUS, a general commercial finite element code. The approach adopted provided ratchet limits for only one individual cyclic load value. The work presented here describes a process for analyzing the structure and determining the elastic, shakedown and ratchet boundaries all in one finite element simulation using only one analysis step. The approach manipulates the structure material behavior that enables the resetting of the material characteristics to their original values in order to be able to analyze the structure for different sets of cyclic and primary load combinations. The process was verified using problems available in the literature, such as the Bree tube and Ponter’s Holed Plate. Additionally, a tubular T-joint was used as an example of the effectiveness of the process for a three dimensional complex geometry. The tubular T-joint results are verified against baseline data from the iterative elastic-plastic simulations used to determine the elastic, shakedown, and ratchet limits. The work presented highlights the advantages and limitations of this numerical approach which requires little interaction with the analyst.

Commentary by Dr. Valentin Fuster
PVP2011-57108 pp. 833-844; (12 pages)

In the current research, the shakedown limit loads for a cylindrical vessel–nozzle intersection is determined via a simplified technique. The cylindrical vessel–nozzle intersection is subjected to a spectrum of steady internal pressure magnitudes and cyclic in–plane bending moments on the nozzle. The determined shakedown limit loads are utilized to generate the Bree diagram of the cylindrical vessel–nozzle intersection. In addition, the maximum moment carrying capacity (limit moments) and the elastic limit loads are determined and imposed on the Bree diagram of the structure. The simplified technique outcomes showed excellent correlation with the results of full elastic–plastic cyclic loading finite element simulations.

Commentary by Dr. Valentin Fuster
PVP2011-57220 pp. 845-851; (7 pages)

Mod 9Cr-1Mo steel (T91) is a candidate material for steam generator of SFR (Sodium Fast Reactors). In order to validate this choice, it is necessary, firstly to verify that it is able to withstand the planned environmental and operating conditions, and secondly to check if it is covered by the existing design codes, concerning its procurement, fabrication, welding, examination methods and mechanical design rules. A large R&D program on mod 9Cr-1Mo steel has been undertaken at CEA in order to characterize the behavior of this material and of its welded junctions. In this program, the role of the Laboratory for structural Integrity and Standards (LISN) is to develop high temperature defect assessment procedures under fatigue and creep loadings. In this frame, complementary studies are conducted in order to validate the existing methods (developed for the fast reactors) and to get new experimental data on Mod 9Cr-1Mo steel. In particular, some new experiments are conducted on specimen with a weld joint and compared with classical experiments on base metal specimen. These results associated with finite element modeling allow to propose a weld joint coefficient at 550°C for the Mod9Cr 1Mo steel.

Commentary by Dr. Valentin Fuster
PVP2011-57477 pp. 853-861; (9 pages)

Recent computer hardware is greatly developed to make possible fatigue analysis and creep-fatigue analysis of structures, which takes much computational time in the past. The code and standard recommend the simplified method in the fatigue analysis and creep-fatigue analysis instead of the detailed inelastic finite element solutions. It is widely recognized that the employed constitutive model affects inelastic finite element solutions significantly. However, the inelastic finite element solutions can consider effects of geometry shape or boundary conditions easily compared to the simplified methods. The other advantage of the inelastic solutions can also assist the mechanism of inelastic deformations. Thus, the accurate inelastic finite element solution is still intense research subject in this area.. The present paper will study constitutive model and the determination method of the material constants for the fatigue analysis and creep-fatigue analysis in order to simulate inelastic behavior at saturated condition, which differs from those at the initial loading.

Commentary by Dr. Valentin Fuster
PVP2011-57486 pp. 863-871; (9 pages)

Finding plastic (limit) loads for elbows under various loading conditions such as in-plane bending and out-of-plane bending is not an easy task due to complexities involved in plastic analyses. Considering complexities involved in plastic limit analysis of elbow, deriving analytical solutions of plastic loads for elbows would be extremely difficult. So, recently the limit analysis using finite element program has been widely adopted. Based on extensive and systematic FE limit analyses using elastic-perfectly plastic materials, closed-form solutions of plastic loads for defect-free elbows under in-plane closing, in-plane opening and out-of-plane bending were presented. This paper summarizes the well-known criteria for finding plastic (limit) loads proposed by ASME BPVC Sec.III [1], Zahoor [4], Chattopadhyay et al. [17] and Kim et al. [19] The purpose of this paper is to integrate and improve the proposed solutions by Kim et al. Also, comparison results with published experimental data are presented. From these results, the pros and cons of each criterion for finding plastic (limit) loads for elbows are discussed.

Topics: Stress
Commentary by Dr. Valentin Fuster
PVP2011-57489 pp. 873-876; (4 pages)

Heat exchangers are widely used in power plants as well as petrochemical facilities. Typical heat exchangers consist of a number of thin-walled tubes and several support plates. Operating experience of the heat exchangers has shown that flaws of various morphologies frequently occur in the tubes. Therefore, an accurate integrity assessment of the cracked tube is quite important from the viewpoint of safety and reliability. The heat exchanger tubes are supported at regular intervals by the support plates, and rotations as well as translations of the tubes are partially restrained. It was reported that the burst pressures for the circumferential through-wall cracks are significantly affected by the support plates, and existing limit load solutions differ from the actual burst pressure. However, the effect of the support plate on the burst pressure of axially flawed tubes has not been fully investigated. This paper evaluates detailed burst pressures and crack opening displacements for the tubes with axial flaws considering the effect of the support plates through three-dimensional finite element analyses.

Topics: Pressure
Commentary by Dr. Valentin Fuster
PVP2011-57884 pp. 877-887; (11 pages)

An active part of a hermetically sealed oil transformer is a heat source increasing the temperature and the volume of insulation oil. The oil volume increase results in significant pressure variations to a tank of the transformer, and also can cause serious damage of the tank. For this reason, a corrugated type radiator is generally adopted to the transformer since it can provide prevention from the internal pressure rise by expansion of corrugated radiator fins. In this paper, design of a corrugated type radiator for the hermetically sealed oil transformer is suggested, and structural integrity of the radiator is evaluated by both numerical and experimental methods. The design of the corrugated type radiator has been proposed and variation of oil pressure inside the radiator is estimated for operating load case. The structural integrity of the proposed radiator is performed, and measured strain values are compared with the analyses results. In addition, a fatigue analysis of the radiator is conducted. From the results, it is concluded that the design of the radiator meets the requirements for structural integrity in the operating condition.

Commentary by Dr. Valentin Fuster
PVP2011-57916 pp. 889-896; (8 pages)

Stress and strain locus in inelastic deformation is important to evaluate the fatigue strength and creep-fatigue strength. The stress redistribution locus (SRL) of perforated plate under displacement-controlled condition has been studied so far by the present authors. The SRL curve under displacement-controlled loading is almost independent of the employed constitutive equation, and the SRL takes the similar curve as Neuber’s one, if the inelastic stress/strain is non-dimensionalized by elastic solutions. However, the SRL under force-controlled loading is not studied yet, which is closely related to collapse load and fatigue strength. The response under thermal loading is also important for fatigue strength and creep-fatigue strength. Based on the 3D FE solutions under force-controlled loading or thermal loading for the perforated plate, the nonlinear feature will be discussed.

Topics: Force , Stress , Displacement
Commentary by Dr. Valentin Fuster
PVP2011-57391 pp. 899-904; (6 pages)

While going through the startup process of a 600MMSCFD Gas Processing Plant, the piping downstream of a gas expander bypass valve and supporting structure was observed to be shaking abnormally. The shaking was significant enough that plant personnel limited the valve flow rate to well under the design capacity and at a level that limited the plant startup. The initial assumption was that the piping or the piping supports had been improperly designed. An investigation revealed no unusual looseness in the piping supports and no significant piping natural frequency at the observed vibration frequency. Further investigation revealed that the root cause of the problem was a flow-generated pulsation in the discharge of the bypass valve that excited the piping and structural supports. Changing the valve flow path and applied valve opening limits provided a temporary work-around that allowed the plant to operate at sufficient flow rates to complete the startup. Subsequent replacement of the valve with one using the same trim but with different gas flow path characteristics proved to be the ultimate solution to the problem.

Topics: Pipes , Valves , Vibration
Commentary by Dr. Valentin Fuster
PVP2011-57427 pp. 905-915; (11 pages)

Reaction force of safety valves acting to the piping system is one of key factors for the piping system design around the safety valves. In case of open discharge system, it is well known that a large reaction force acts to the piping corresponding to the fluid momentum force at the atmospheric discharge. On the other hand, reaction forces for closed discharge system may be relatively small since the forces acting to the adjacent two points with flow direction change such as elbows and tees are balanced within very short period. However, large reaction forces may act as a result of unsteady flow just after the initial activation of the safety valve. API RP520 mentioned that a complex time history analysis of the piping system around the safety valves may be required to obtain the transient forces. This paper explains a method of a comprehensive dynamic simulation of piping system around safety valves taking interaction among the valve disc motion, the fluid transient for compressible flow and the piping structural dynamics into account. The simulation results have good agreement with the experimental data. The effectiveness of this method is confirmed throughout an application to actual piping system around safety valves.

Commentary by Dr. Valentin Fuster
PVP2011-57441 pp. 917-926; (10 pages)

The most simple and effective method for destroying chemical weapons, and thus eliminating stockpiling problems, is to detonate the entire payload in a chamber without separating the chemical agent from the explosives. A double walled chamber was developed by Kobe Steel, Ltd. (KSL) and has destroyed more than 7000 chemical weapons throughout the world since 2000. Explosion analysis (by AUTODYN) and dynamic structural analysis (by LS-DYNA) performed during development of the chamber design are presented with the experimental results. Design of the latest version of the double walled chamber was evaluated in accordance with the ASME Code Section VIII, Division 3 and Code Case 2564 based on the results of dynamic analysis and measured strains.

Commentary by Dr. Valentin Fuster
PVP2011-58019 pp. 927-933; (7 pages)

In this study, the dynamic characteristics of a reactor coolant pump (RCP) which can be used for APR1400 nuclear power plant are investigated using an equivalent simplified solid model. An effective simplified solid mass model to preserve the behavior of the RCP is presented. The equivalent simplified solid model for the dynamic analysis is developed from the detailed FE model of RCP through iterative runs. The results show that the fundamental frequency of RCP resides around 11Hz in the bending mode.

Commentary by Dr. Valentin Fuster
PVP2011-57009 pp. 937-947; (11 pages)

The current regulations, as set forth by the United States Nuclear Regulatory Commission (NRC), to insure that light-water nuclear reactor pressure vessels (RPVs) maintain their structural integrity, throughout their operating life, when subjected to planned normal reactor startup (heat-up) and shutdown (cool-down) transients are specified in Appendix G to 10 CFR Part 50, which incorporates by reference Appendix G to Section XI of the American Society of Mechanical Engineers (ASME) Code. The technical basis for these regulations are generally considered to be conservative and some plants are finding it operationally difficult to heat-up and cool-down within the accepted limits. Consequently, the nuclear industry has developed, and submitted to the ASME Code for approval, an alternative risk-informed methodology that reduces the conservatism and is consistent with methods previously used to develop a risk-informed revision to the regulations for accidental transients such as pressurized thermal shock (PTS). The objective of the alternative methodology is to increase operational flexibility while continuing to provide reasonable assurance of adequate protection to public health and safety. The NRC and its contractor at Oak Ridge National Laboratory (ORNL) are reviewing the industry proposed risk-informed methodology. Previous results of this review, have been reported at PVP, and a NRC report summarizing all results is currently in preparation. The objective of this paper is to discuss and illustrate mechanistic insights into trends shown previously associated with normal cool-down.

Commentary by Dr. Valentin Fuster
PVP2011-57416 pp. 949-955; (7 pages)

One solution to improve over margin design for piping is to apply reliability based approach. By applying limit state function method in design stage, safety margin can be expressed explicitly as reliability index. From practical point of view, however, it is rather complicated for designer to perform reliability approach every time at design stage. Thus, this paper proposes a reliability based contour design curves which are derived from the uncertainty both for Markl’s best fit curve and for loads. Using this method, reliability based fatigue design can be done easily for the given reliability target. This paper shows the derivation of reliability based contour design curve together with some calculation results and finally proposes the simplified procedure.

Commentary by Dr. Valentin Fuster
PVP2011-57421 pp. 957-965; (9 pages)

This paper discusses the margins of the design fatigue curve in the ASME Boiler and Pressure Vessel Codes Section III from a reliability analysis point of view. It is reported that these margins were developed so as to cover uncertainties of fatigue data scatter, size effect, and surface condition[1], but the reasons for them remain unclear. In order to investigate the physical implications of the design margin, a probabilistic approach is taken for the collected fatigue data of carbon and low-alloy steels. In this approach, these three parameters are treated as random variables, and an applied stress is also taken into consideration as a random variable. For the analysis, to begin with, a limit state function for fatigue is proposed. Next, reliability index contours of the design fatigue curves for carbon and low-alloy steels are obtained based on the proposed limit state function. The contours indicate that the margins 2 on stress and 20 on life do not provide equal reliability. The margin 20 on life is more conservative and the margin became a minimum near intersections of the design curves with margins 2 on stress and 20 on life. For practical applications, the partial safety factors (PSF) for the target reliability are computed for all materials and several levels of coefficients of variation (COV) of the applied stress. A sensitivity analysis of the PSFs clarifies that only two parameters, the strength (or the life) and the applied stress, are predominant. Thus, the partial safety factors for these two parameters are proposed in a tabular form.

Commentary by Dr. Valentin Fuster
PVP2011-57528 pp. 967-974; (8 pages)

This work deals with the possibility of the life extension of nuclear power plants in France. The aim is to justify the resistance of the pressure vessel, which is non-replaceable. The brittle fracture deterministic integrity assessment of the nuclear Reactor Pressure Vessel (RPV) is based on the analysis of a flaw under the austenitic cladding of the RPV. The demonstration of the RPV resistance is controlled by the regulations. It is proposed here to use a probabilistic method by propagating uncertainties into the deterministic mechanical model in order to quantify conservatism of the deterministic method. The regulatory requirements must be respected and the purpose of the work presented here is thus to link the probabilistic result to the deterministic method.

Commentary by Dr. Valentin Fuster
PVP2011-57669 pp. 975-984; (10 pages)

In this work is presented the development of generic models that emulates the behavior of finite element models under cyclic loads, with the probabilistic representation based on samplings of base-model data for a variety of test cases. The base-model is a pipe with a notch subjected to pressure loading translated into hoop stress and the thermal loading is applied as a cyclic load through the pipe thickness. The probabilistic method takes variations of the nonlinear material properties, loading conditions, and geometrical dimensions, whereas the response variables are defined in terms of stress intensity for the static analyses, and the total accumulated strain as well as the strain ranges translated into the number of allowable load cycles by using the Manson’s common slope method define the response variables for the nonlinear calculations. Bree diagram converted into the Interaction Diagram is used to correlate the results of the nonlinear cyclic analyses and the ASME Code limits for primary and secondary loads from linear elastic analyses, whereas the definition of the shakedown towards of the steady cycle is identified in terms of the local and global components of strain. Furthermore, the Bayesian statistics expands the results of the nonlinear cyclic analysis by combining the interpretations of statistical results to scenarios either not accessible by the frequentist statistics or better served by complex stochastic models.

Commentary by Dr. Valentin Fuster
PVP2011-57149 pp. 985-991; (7 pages)

The Japanese standard “KHK-S-0220” (KHK-code) and the American standard “Boiler and Pressure Vessel Code Sec.8 Div.3” (ASME-code) concerning ultra-high-pressure gas equipment were applied to Hitachi’s ultra-high-pressure compressor, and a series of strength evaluations were carried out. Hitachi produces and maintains ultra-high-pressure reciprocating compressors with a design pressure over 200 MPa. In Japan, ultra-high pressure gas equipment over 100 MPa must be designed according to KHK-code established by the High Pressure Gas Safety Institute of Japan. This Japanese standard was applied to an ultra-high-pressure compressor, and design pressure limits, shakedown limits, required absorbed energy of materials, leak-before-break (LBB), and fatigue strength were evaluated. ASME-code was also applied to the compressor, and strength evaluations like the above were carried out. As a result, it was found that KHK-code and ASME-code gave conservative evaluation of fatigue strength for an ultra-high-pressure compressor.

Topics: Pressure , Compressors
Commentary by Dr. Valentin Fuster
PVP2011-57457 pp. 993-1000; (8 pages)

The floating roofs are used in large aboveground storage tanks to prevent evaporation of the content. The single-deck floating roof, which is considered herein, consists of a thin circular plate called a deck attached to a buoyant ring of box-shaped cross section called a pontoon. Under the accumulated rain water condition, the deck is deflected largely, and both its edge part and the pontoon are compressed circumferentially. Since the load condition due to the rain water depends on the deflected deck shape, it is difficult to find the unique equilibrium condition. This paper describes the deformation analysis for the single-deck floating roofs under the accumulated rain water condition using the geometrically nonlinear axisymmetric shell finite element method. The load incremental method, in which the equivalent nodal forces due to the rain water converges to coincide with the rain water load derived from both the current rain water volume and the deflected deck shape, is used.

Commentary by Dr. Valentin Fuster
PVP2011-57599 pp. 1001-1008; (8 pages)

This paper presents finite element solutions for elastic-plastic J for circumferentially cracked pipes under combined mechanical and thermal loads in terms of the V/Vo factor used within a strain-based failure assessment diagram. In this study, 3-dimensional finite element analyses are conducted to calculate the V-factor under combined mechanical and thermal load. It is found that estimation of V/Vo is sensitive to the method used for its evaluation. For larger thermal stresses, currently proposed estimation methods are overly conservative.

Topics: Stress , Pipes , Failure
Commentary by Dr. Valentin Fuster
PVP2011-57052 pp. 1009-1014; (6 pages)

Ground effect is an aerodynamic phenomenon that occurs when moving bodies come in close proximity to the ground. A “cushion” of air is created underneath the moving body which provides additional lift by increasing the local pressure under the body surface. To experimentally test ground effect vehicles, a unique wind tunnel is currently being redesigned and constructed at West Virginia University. This wind tunnel incorporates a rotating belt as the ground plane and a centrifugal fan that generates the air flow through the test section in the same direction as the belt’s rotation. The combination of a rotating belt and airflow is used to mimic ground effect in that it is representative of a body moving through still air in close proximity to the ground. The test section and fan assembly sit on a platform that is connected to a movable base frame. The base and testing platform connect through a pivot point that is capable of being raised upward to a maximum angle of fifty degrees to account for gravitational vector alignment between modeled and real world conditions. When the platform is raised and the belt is spinning, the structure is less stable and has the potential to create errors in force readings due to these oscillations, as well as the potential to tip in extreme wind conditions. Thus, the evaluation of the original design and the subsequent redesign are addressed in this research effort. To stabilize the wind tunnel, additional structural elements have been added downstream of the test section. Two telescoping poles were added to the end of the platform that will connect onto outriggers attached to the base structure. These poles and outriggers will form an A-shape support system when the platform is raised to any degree between zero and fifty. The width of the outriggers was calculated and then modeled in conjunction with the existing base structure. The final design is presented in this paper.

Topics: Wind tunnels
Commentary by Dr. Valentin Fuster
PVP2011-57261 pp. 1015-1028; (14 pages)

This work provides an estimation procedure to determine the J-integral and CTOD for pipes with circumferential surface cracks subjected to combined bending load and internal pressure for a wide range of crack geometries and material (hardening) based upon fully-plastic solutions. The present investigation broadens the applicability of current evaluation procedures for J and CTOD which enter directly into structural integrity analyses and flaw tolerance criteria. Extensive 3-D nonlinear analyses of circumferentially cracked pipes with surface flaws having different crack depth (a) over pipe wall thickness (t) ratios and varying crack length for different strain hardening properties provide the dimensionless parameters relating the elastic-plastic crack-tip driving forces with the applied (remote) bending moment and internal pressure. The investigation provides a fairly comprehensive body of numerical solutions for J and CTOD in circumferentially cracked pipes subjected to biaxial loading.

Topics: Pressure , Pipes
Commentary by Dr. Valentin Fuster
PVP2011-57334 pp. 1029-1037; (9 pages)

In order to assess the structural integrity of the components in nuclear power plants, it is necessary to accurately evaluate the J-integral. EPRI J-integral estimation method has been widely used. However for the pipes with circumferential surface cracks, since the solutions of fully plastic factors H1 were few under the combined pressure and bending loading condition. In such a case, EPRI J-integral might be non-conservative in elastic-plastic transition zone. Considering the problems, detailed finite element analysis (FEA) were conducted in this work, and H1 , shape factors F were provided for the pure pressure, pure bending, and combined pressure and bending conditions. The reasons for non-conservatism of EPRI method were discussed, and a modified J estimation method was developed. For validation, failure assessment curves (FACs) were employed to compare the proposed J-integral estimation, EPRI J-integral and FEA method. The results showed that FAC based on the proposed J-integral method agreed well with FEA result, which provided good confidence in the use of this method.

Topics: Pressure , Stress , Pipes
Commentary by Dr. Valentin Fuster
PVP2011-57458 pp. 1039-1047; (9 pages)

Coke drums are vertical pressure vessels used in the delayed coking process in petroleum refineries and oil sands plants. Significant temperature variation during the delayed coking process causes damage in coke drums in the form of bulging and cracking. In order to better understand the damage mechanisms, an experimental investigation of coke drum material behavior under various thermal-mechanical loading conditions was performed. A thermal-mechanical material testing system is successfully designed and implemented. Six types of various thermal-mechanical cyclic tests were performed: 1. cyclic thermal loading under constant uniaxial stress; 2. in-phase thermal and mechanical stress cycling; 3. out-of-phase thermal and mechanical stress cycling; 4. fully-reversed uniaxial cyclic loading with in-phase thermal cycling; 5. in-phase thermal-axial stress cycling with constant circumferential stress; 6. in-phase thermal-axial stress cycling with mean stress. Some of theses tests are similar to the actual loading scenario experienced by the coke drums. The experimental findings lead to better understanding of the damage mechanisms occurring in coke drums such as bulging.

Topics: Coke
Commentary by Dr. Valentin Fuster
PVP2011-57587 pp. 1049-1058; (10 pages)

The quality of Natural Gas Piping Systems, NGPS, must be ensured against manufacturing defects. The main purpose of the present paper is to investigate the effect of loading mode and load angle (30°,45°, and 60°) on the limit load of miter pipe bends, MPB, under different crack depths a/W = 0 to 0.4 at a crosshead speed 500 mm/min. The geometry of cracked and un-cracked multi miter pipe bends are: pipe bend angle, α = 90°, pipe bend factor, h = 0.844, standard dimension ratio, SDR = 11, and three junctions, m = 3. The material of the investigated pipe is a high-density polyethylene, HDPE, which is commonly used in natural gas piping systems. The welds at the miter pipe junction are produced by butt-fusion welding. For all loading modes the limit load is obtained by the tangent intersection method, TI, from the load deflection curves produced by the specially designed and constructed testing machine at the laboratory. Tensile tests are conducted on specimens longitudinally extruded from the pipe with thickness, T = 10, 30 mm, at different crosshead speeds (5–500 mm/min), and different gauge lengths (G = 20, 25, and 50 mm) to determine the mechanical properties of welded and un-welded specimens. The fracture toughness is determined on the basis of elastic plastic fracture mechanics, EPFM. Curved three-point bend specimens, CTPB, are used. All specimens are provided with artificially pre-crack at the crack tip, a/W = 0.5. The effect of specimen thickness variation (B = 10, 15, 22.5, 30, 37.5, and 45mm) for welded and un-welded specimens is studied at room temperature (Ta = 23°C) and at different crosshead speeds, VC.H , ranging from 5 to 500 mm/min. The study reveals that increasing the crack depth leads to a decrease in the stiffness and limit load of MPB for both in-plane, and out-of-plane bending moment. In case of combined load (out-of-plane and in-plane opening; mode) higher load angles lead to an increase in the limit load. The highest limit load value occurs at a loading angle, φ = 60°. In case of combined load (out-of-plane and in-plane closing; mode) the limit load decreases with increasing load angles. On the other hand, higher limit load values are proved at a load angle, φ = 30°. For combined load opening case; higher values of limit load are obtained. The crosshead speed has a significant effect on the mechanical behavior of both welded and un-welded specimens. The fracture toughness, JIC , is greater for un-welded than welded specimen.

Topics: Stress , Pipe bends
Commentary by Dr. Valentin Fuster
PVP2011-57637 pp. 1059-1063; (5 pages)

This paper presents the residual vector method that is employed for improving the accuracy of transient or harmonic multi-degree of freedom dynamic analyses based on the mode-superposition method. In the transient analyses of mechanical systems it is often difficult, for a variety of reasons, to determine the number of natural frequencies that have to be extracted in the initial phase of the dynamic calculation that is the modal analysis. The natural modes of vibration in the higher frequency range are often neglected in order to improve the computational efficiency. The error due to this truncation of the frequency range, and the corresponding mass that depending on the mechanical system configuration may be ignored, can move the accuracy of the computational solution outside the acceptable levels. Typical examples include nuclear piping systems with non-linearities and non-uniform distributions of stiffness, and structures with heavily damped components such as those with viscoelastic layers. In this context, the number of natural frequencies extracted and the resulting accuracy of the computational analysis is dependent on both the spatial characteristic and the frequency content of the forcing function. Forcing functions characterized by impact or shock loading are noted to excite high frequency modes; although these loading conditions are a specific group of applications in which the residual vector method can be employed, they are not within the scope of this paper which instead focuses on the method that can be used when performing analyses involving arbitrarily chosen harmonic and transient loadings. The residual vector method improves the accuracy of the mode-superposition type of dynamic calculation by the application of orthogonalized residual vectors that account for the higher frequency modes not used in the initial solution. Numerical examples are employed to demonstrate the accuracy and the efficiency of this method applied for both harmonic and transient mode superposition calculations.

Commentary by Dr. Valentin Fuster
PVP2011-57654 pp. 1065-1072; (8 pages)

Girth-butt welds are used to join sections of stainless steel pipe in the primary circuit of Pressurised Water Reactors. The welding process creates residual stress fields across the weldment, which can contribute to the crack driving force when a defect is present. Assessment procedures account for such defects, enabling safety justifications to be made for continued operation of nuclear power plant. Such procedures require the size and nature of the residual stress field to be determined in order to make reliable structural integrity assessments. This paper describes the investigation of the residual stress field and fracture behaviour of a recently developed narrow-gap 304-stainless steel girth-butt weld in a primary circuit pipe. Two residual stress measurement techniques, Neutron Diffraction (ND) and incremental Deep Hole Drilling (iDHD), were used to measure the original residual stress field in the pipe weld. A second pipe weld specimen was used to fabricate tensile and fracture toughness specimens from which the mechanical properties of the weld material were determined. The residual stress and mechanical test data were used to develop numerical models of the pipe weld containing a postulated circumferential defect under an applied axial load. The numerical simulation results were applied within a failure assessment diagram, comparing different interaction parameters on the prediction of component failure load.

Commentary by Dr. Valentin Fuster
PVP2011-57896 pp. 1073-1082; (10 pages)

The use of relief valves is crucial for the safety of power plants. Indeed, these simple and robust in their design valves are the ultimate protection when all other systems are insufficient. This study is focused on valve opening characteristics which can be concluded through the determination of flowforces applied at the valve disk. A spring loaded safety relief valve (1 1/2″ G 3″ ) and transparent model of this valve are tested under static conditions. The spring is removed and the forces exerted at the valve disk for different inlet pressures and lift positions are measured. Compressible, incompressible and two-phase flow behaviors are investigated and compared. Inversed force of air and water is noticed above a certain value of valve lift. Numerical simulations are performed to verify experimental findings.

Commentary by Dr. Valentin Fuster
PVP2011-57275 pp. 1085-1094; (10 pages)

Coke drums are pressure vessels that are used in oil sands and conventional refinery processing industries for the thermal cracking of reduced bitumen to recover additional, saleable gas and liquid product streams. The drums are constructed to the requirements of ASME VIII Division 1 although they are in a low cycle thermal-mechanical service environment. Recent practice has been to introduce design by analysis considerations from ASME VIII Division 2 even though service temperatures of the actual equipment exceed the design limits of the Code. In this paper, simplified elastic-plastic analysis models are developed for assessment of the stress and strain levels in coke drums during specific operational phases. One model is applied for determination of the local stress caused by differences in the coefficients of thermal expansion between the clad liner and base materials. Because clad construction is used throughout the vessel, the impact is extensive. Consideration is given to the general stresses induced by the cyclical, progressive dilation and contraction action of the drum shell caused by axial thermal gradients. Another model estimates hot and cold spot formation and the development of localized stress / strain distributions in coke drums. The resulting thermal stresses include the local stress from suppression of differential expansion between clad and base material, thermal bending stresses and local thermal stresses from hot and cold spot formation. It is found that the evaluation of these loads on drum cracking aligns with industry survey results. Better understanding of these loads has impact on materials selection and fabrication procedures for new drums and repair of existing drums. Operational considerations can also be identified to help improve drum reliability.

Topics: Design
Commentary by Dr. Valentin Fuster
PVP2011-57530 pp. 1095-1100; (6 pages)

A long-term life prediction method for a compressed fiber sheet gasket under a high-temperature environment is studied. Non-asbestos compressed fiber sheet gaskets are now being used as a substitute for asbestos in the bolted flange joint, for instance chemical plants and oil plant facilities. Consequently, there is a real need for a technology to predict the lifetime of non-asbestos compressed fiber sheet gaskets quantitatively. We have previously reported that the facing surface which was formed the gasket and flange was visualized with scanning acoustic tomography (SAT). Voids were observed on the facing surface of the gasket and increased with the increase in exposure time at high temperature. If a leakage path for inner fluids was created by the increasing number of voids, the leak occurs on the facing surface of the gasket surface. The probability of a leak due to voids and the lifetime of this gasket were predicted by applying the percolation theory, which describes the connectedness of voids and clusters. In this report, the influence of internal pressure of test flange joint upon the rate of increase is studied.

Commentary by Dr. Valentin Fuster
PVP2011-57604 pp. 1101-1114; (14 pages)

The effects of severe thermal-transients on coated substrates with indentation-induced, blister type defects were analyzed using finite-element methods. Both explicit and implicit approaches were used depending on the timeframe under scrutiny. For the modeling, an axisymmetric mesh and cohesive zone elements was used to assess the transient thermal- and stress-states and the propensity for fracture related damage and evolution. For all calculations (and when available), temperature dependent thermophysical and elastic properties were used during the analysis. The model also utilized uniform heating across the surface imposed via convective coefficients and a piece-wise linear pressure function. Preliminary results indicated complex interactions between the boundary conditions and their timing and the resulting propensity for damage evolution. Given the need for robust coatings, the modeling procedures developed during this study will have important ramifications for coated tube designs and the evaluation of candidate materials.

Commentary by Dr. Valentin Fuster
PVP2011-57621 pp. 1115-1125; (11 pages)

In order to assess the high-cycle thermal fatigue damage risk of the nuclear power plants mixing zones, the knowledge of the temperature fluctuations effect on the structure surface is necessary. To advance the accuracy and reliability of thermal fatigue load determination, a combined experimental and numerical investigation has been conducted on cylindrical components of 316L stainless steel subject to cyclic thermal shocks of varying intensity. Slightly different experimental conditions were applied in each test to explore the effect of ΔTmax values of increasing severity, addressing also higher temperatures typical for fast reactors, the effect of a superimposed static axial load to study the effect of a constant pressure on the thermal fatigue damage and a reduced test piece wall thickness. A comparison between thermal down-shock tests with and without additional constant tensile load and with different maximum temperatures are analysed in details here below.

Commentary by Dr. Valentin Fuster

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