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Bull Run Fossil Plant: Technical Design Methods for Superheat Pendant Outlet Headers Replacement

[+] Author Affiliations
Salah E. Azzazy, Russell D. Cochran, Larry Sam Cox

Tennessee Valley Authority, Chattanooga, TN

Paper No. POWER2010-27007, pp. 295-308; 14 pages
doi:10.1115/POWER2010-27007
From:
  • ASME 2010 Power Conference
  • ASME 2010 Power Conference
  • Chicago, Illinois, USA, July 13–15, 2010
  • Conference Sponsors: Power Division
  • ISBN: 978-0-7918-4935-4 | eISBN: 978-0-7918-3876-1

abstract

Bull Run Unit 1, rated at 950 MW, is the first of four fossil supercritical power plants at Tennessee Valley Authority (TVA). The unit went into commercial operation in 1967. The boiler (consists of two furnaces) built by Combustion Engineering (CE) has a radiant reheat twin divided furnace with tangential-fired coal burners. The unit’s maximum continuous rating (MCR) is 6,400,000 lbs/hr of main steam flow, with a design temperature of 1003°F and pressure of 3840 psig. Through the end of 2008, the unit had a total of approximately 670 cumulative starts and 333,185 operating hours. After years of numerous tube cracks at the Superheat Pendant Outlet Header/Tube Nozzles resulting in repetitive forced plant shutdowns, TVA decided to replace the two Outlet Headers (one for each furnace) in Fall 2008 during a reliability outage. Since the entire Main Steam piping system was installed with cold pull at almost every longitudinal pipe segment, the main challenge from the engineering mechanics point of view was how to restrain the piping system especially at the Crossover Outlet Links inside each furnace Penthouse. Further constructability reviews indicated that there were not enough adjacent steel frames inside each furnace to restrain the four Crossover Outlet Links in the three global directions during the Outlet Headers replacement inside each Penthouse. The only existing steel above the Crossover Outlet Links is embedded in asbestos insulation, and the removal of the insulation to provide access for the temporary restraints was determined to be costly and time consuming. The insulation removal would have also caused the scheduled outage to be extended significantly and unrealistically. After careful assessment, technical evaluation, and several constructability reviews; it was decided to take an unconventional approach for relieving the inherent cold pull in three global directions by cutting the four Mixing Headers outside each furnace. In addition, the concept of installing several temporary restraints was utilized for the vertical and lateral directions inside the furnace Penthouse, as well as several others outside the Boiler to control the piping configuration of the four Mixing Headers. This approach achieved two purposes: 1- relieving the inherent cold pulls in three global directions and 2- controlling the four Outlet Links pipe end positions with respect to the new Superheat Pendant Outlet Header nozzles. This unconventional method used to relieve the piping cold pull from outside the Boilers, to control the Outlet Links movements inside the Boiler Penthouses, and to restrain the entire Main Steam piping system was successfully developed and implemented in the Fall 2008 reliability outage to replace the two Superheat Pendant Outlet Headers. This unconventional method is described in this paper.

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