BEST PRACTICES: Walton County Power
Drain-valve automation improves safety, reliability
Challenge. During shutdown, Walton County Power’s V84.3A2 gas turbines would trip when fuel gas transitioned into “diffusion” mode. During full-load operation, the units rely on the primary “premix” fuel supply, leaving the diffusion header, used only during startup and shutdown, with no flow.
Plant personnel learned that condensation accumulates in the gas piping and subsequently blows out the flame when the diffusion valve opens during shutdown. Heat tracing was installed on all the fuel-gas headers to reduce the accumulation of condensate. This helped to reduce trips during transition from premix to diffusion but did not completely solve the problem.
Operations staff improved results by blowing down the diffusion header. This was accomplished by opening the manual blowdown valve for a minimum of five minutes prior to the transition. The procedure proved effective in minimizing unit trips. Safety, however, was a concern: To access the valve (circled in Fig 1), operators had to crawl on their hands and knees under the running turbine and be exposed to very high noise levels, potential hot gases, and trip hazards.
Additional hazards included risks associated with hot piping, poor lighting, and potential eye injury from debris blowing under high pressure. In addition, the failure of an operator to close this valve prior to gas transition could result in high-pressure fuel gas being dumped into the turbine enclosure through the ½-in. valve.
Solution. The solution was established in two steps. First step was to immediately address the safety concern by installing piping to the outside of the turbine compartment and placing a manual blowdown valve outside the package for ease of access. The drain was put in a location away from heat sources, such as the turbine itself, and in a well-ventilated area so any gas that might be released would dissipate quickly.
The second step (Fig 2) was to install an air-operated ball valve and solenoid (normally spring closed). The DCS was configured with automatic control logic to cycle the valve at approximately 10 minutes prior to transition for a duration of five minutes.
The control room operator (CRO) also was given manual control of the valve via the HMI if the duration had to change because of ambient conditions. As a safety backup, DCS logic fails the valve closed at 100 MW under all conditions, providing a safe margin before diffusion gas enters the header.
Results. This approach allowed an immediate safe atmosphere for all personnel during the blowdown procedure. It produced a standard blowdown time which further reduced unit trips attributed to flame out. The CRO can now monitor the operation from the control room and release the outside operator to perform the numerous other duties required during unit shut down.
Unit trips have been reduced dramatically, decreasing thermal and mechanical stresses on the gas turbines and thereby reducing equivalent operating hours and extending the time between major maintenance intervals. The total installation cost of this solution was $228 per unit for a total of $684 for the plant.
Project participants: Wes McMillian, Michael Gilbert, Charles Gibson, Scott Hobbs, Chris Harris
Walton County Power LLC
Owned by Southeast PowerGen LLC
Operated by Consolidated Asset Management Services
440-MW, gas fired, three-unit, simple-cycle peaking facility located in Monroe, Ga. Plant now owned and operated by Cogentrix.
Plant manager: Mike Spranger
