Technical Paper: High Pressure Feedwater Heater Restoration
In late 1988 and early 1989, two of the four high pressure feedwater heaters at each of the three generating units at the Tennessee Valley Authority's (TVA) Thomas H. Allen Fossil Plant were replaced. In the Fall of 1997, after 8½ years of service, a significant number of tubes failed suddenly in the two Allen unit 3 replacement heaters. Within a 14 month period, one of the unit 3 replacement heaters has 97 tube failures and the other had 171 failures. The mode of these tube failures was determined to be stress corrosion cracking (SCC) initiated at a tube roll transition located within the tubesheet. In order to prevent continued progression of tubing damage and to restore failed tubes to service, TVA employed CTI Industries to install stainless steel inserts in the inlet and outlet ends of each tube in the affected heaters.
This paper discusses the failures, investigative techniques, probable root causes, and corrective measures evaluated and implemented to extend the service life of high pressure feedwater heaters at TVA's Allen Fossil Plant.
Introduction
TVA is a federally owned and operated electric power company serving the Tennessee Valley. TVA's total net electric power generating capability is approximately 28,000 megawatts from a combination of fossil-fired, nuclear, hydroelectric, combustion turbine, and pumped storage plants.
TVA's Allen Fossil Plant, located in Memphis Tennessee, was designed and constructed by Burns and Row Inc. for Memphis Light Gas and Water. The plant was placed into service in 1956, was purchased by TVA in 1984 and is an integral part of the TVA system. The Allen plant has three identical subcritical generating units. Each unit has a B&W coal fired wet bottom steam generator which produces 2,000,000 pounds of steam per hour to drive a Westinghouse tandem compound triple exhaust turbogenerator set at a rated capacity of 250,000 kW. Figure 1 depicts the steam / water cycle.
The feedwater heating system for the Allen units includes a single string of three low pressure heaters, one deareating heater, and four high pressure heaters. The heaters are numbered #1 through #8. Heaters #1 through #3 are low pressure, heater #4 is a deaerator, and #5 through #8 are high pressure heaters with #8 being the highest pressure. The system design includes capability to isolate each high pressure heater individually from extraction steam and to bypass feedwater around each heater individually. However, as with many facilities of this vintage, good isolation is not always achievable and depends on the condition of isolation equipment at any given time.
The original #5 and #6 heaters, manufactured by the American Locomotive Co., were replaced on all 3 units between December 1988 and June1989, with replacement heaters designed and manufactured by Southwestern Engineering Company. The replacement heaters are closed, horizontal U tube, three zone, (desuperheating, condensing, and subcooling) heaters with full access channels. The U-tubes are SB-168-400 drawn and stress relieved Monel, 5/8" O.D. x .060" wall tubing. The tubesheets are 10" thick carbon steel with Monel overlay machined to ¼" thickness. The tubes are roller expanded specification sheet for feedwater heaters #5 and #6.