Duke Energy’s Buck Combined Cycle Plant in Rowan County, North Carolina, is a 630-MW facility that burns gas in two combustion turbines and uses their exhaust to generate steam for additional turbine generators. Ever since its startup in 2011, the safety isolation valve on the spray attemperator has been a bad actor. “We replaced it three times between 2011 and 2014, each time at a cost of $10,000,” said Jim Webb, account manager, R.E. Mason, which assisted Duke with the problem.
Together with Mark Nymeyer, global marketing communications manager forFisher flow controls, Emerson Automation Solutions.Webb presented the session, “Emerson Isolation Solution at Buck Combined Cycle,” at Emerson Global Users Exchange in Austin, Texas. “The valve cost $6,000, and installation cost $4,000,” Webb said. Worse, “when leaking, the valves were costing $2,000 per month in thermal performance degradation.”
The spray attemperator adds water to high-pressure (HP) steam to cool it to the right temperature to feed it to the turbines. The problem valve is on the line that supplies boiler feedwater to the attemperator, which operates at 350 °F and 3,500 psi. The isolation valve cycles several times each day when the unit is going in or out of load and was failing by destruction of its internal trim parts.
Together with Duke Energy, Buck Combined Cycle and Emerson specified and supplied a replacement valve that appears to be solving the problem. The replacement is based on the Fisher Z500 two-piece floating ball valve, which meets leak test criteria of standard AP 598 under both high and low pressure and is available in ½- to 36-in sizes with full or reduced bores in classes 150-4500 (also limited classes). Forged materials are standard.
The specified Z500 Severe Service replacement valve includes Inconel 718 internal components, spray-and-fused overlay coating and a double-d shaft. “A pressed-in primary seat was chosen, despite its potential for a leak path because the specifiers wanted it to be able to expand and shrink with temperature without fretting the other internal parts,” said Nymeyer. With a blowout-proof shaft and graphoil packing rings, it is capable of API 598 shutoff in the primary flow direction, and Class 5 under reverse flow.
The valve is fitted with a Bettis G Series actuator with two sealed single-pole, double-throw (SPDT) GO proximity switches. The actuator is rated to 176 °F. “It’s heavy and requires some extra support,” Webb said.
The new valve was installed on Nov. 1, 2014. As of Jan. 13, it showed a count of 3,166 cycles and no sign of leak-through. “At an ambient temperature of 28 °F, I measured the pipe temperature as 30 °F. That’s complete shutoff,” Webb said. The cumulative cycle count indicates an average of seven cycles per day. “I was shocked that it was still working when it cycles so much.”
Along with saving $24,000 per year in thermal performance, the new valve promises to save the plant $10,000 per year in replacement costs. “We have reliable, tight, metal-sealed shutoff,” Webb added. “Emerson provided a quality solution with improved thermal performance that saved a lot of money for the plant.”