O&M, Major Equipment – Athens Generating Plant

Challenge.

Athens has three 1 × 1 combined cycles powered by 501G gas turbines, which use steam for cooling transition pieces. Since commissioning in 2004, Athens has experienced sticking of transition steam outlet valves. When valves stick, the unit unloads and late startups occur, which has caused a loss of over18,000 MWh of production between2004 and 2008. During startups, when no steam is available, the transitions are cooled with air provided from the last stage of the compressor.

At 20% load, the engines are transferred from air to steam cooling. This step is required to increase load. During the transition of cooling from air to steam, a series of valves open and close in a very critical sequence (Fig 110). If a valve sticks for more than 15 seconds, the unit will be unloaded to below 20% of rated output. Unit load must then be stabilized, and the unit allowed to heat back up, before the transition from air to steam cooling can be attempted again. During this period, load cannot be raised and the startup is delayed. The designed arrangement presents a very difficult service for the transition steam outlet control and block valves, which sit with ambient temperature on one side and 800F steam on the other (Fig 111). The valves have a tendency to stick at random times under all startup conditions.

Solution.

Plant staff made multiple attempts to eliminate the valve sticking problem, all of which achieved limited success:

• Contacting the valve manufacturers, who recommended trim changes to eliminate the problem.
• Modifying the valve positioners to increase their operating speed.
• Cycling the valves prior to each startup.
• Leaving the valves partially open to pre-warm the valves prior to use.
• Modifying the piping to reduce pipe stress in the area of the valves.

In late 2008, plant staff proposed installing a valve warming system to pre-warm the valves, thus eliminating the temperature differential. Working with Team Industrial Services Inc, the plant designed a heating system that consists of custom-braided heating blankets, Kduplex thermocouples, a 480/220-V transformer, and a two-zone control console. The valve insulation was also replaced as part of the project to improve insulating capabilities and maintain the valve temperatures over longer periods. The first warming system was installed on Unit 2 during a scheduled outage in November 2008, and then commissioned that December.

Prior to unit startup, the heaters are turned on to slowly raise the valve body temperatures to 600F. The control console (Fig 112) maintains a specified ramp rate and has dual thermocouple inputs in case there is a thermocouple failure. A failed thermocouple will shut down the heaters as a safety feature to prevent overheating of the valve. The console controls the temperature ramp by varying the output voltage between 0 and 85 Vac. The supply power (480 V) for the unit was provided via a new welding outlet in a dual effort to improve reliabilitywhile adding an additional welding outlet for use during outages.