Switch to self-contained hydraulic valve operators improves turbine-bypass performance
Top performance from steam-turbine bypass systems during startups, shutdowns, and trips at combined-cycle plants is critical for achieving the operating-flexibility and availability goals critical to the plant’s financial success.
Lawrenceburg Power uses a triple-pressure cascading bypass system which helps in managing thermal imbalances between the gas turbines and heat-recovery steam generators in cycling scenarios. It includes high-pressure (HP), hot-reheat (HRH), and low-pressure (LP) bypass valves.
In this control scheme, the HP bypass valve maintains HP pressure to minimize thermal stresses on the drum. The HRH bypass, downstream of the reheater, maintains HRH header pressure, and reduces steam pressure/temperature to the condenser. Finally, the LP bypass valve maintains LP drum pressure and protects the condenser by reducing LP steam pressure/temperature to an acceptable exhaust condition.
The pneumatic actuators supplied with the turbine bypass valves were problematic, sometimes even failing to operate on a trip. Plant personnel noted that some of the actuators had multiple volume boosters and the oscillations were “ridiculous.” Before the unit was able to settle out, the control system was already calling for the valves to move to a new position. Think of the valves as being in perpetual motion.
Reheat pressure oscillations caused by pneumatic-actuator stiction, overshoot, or dead time cause significant fluctuations in HRH header pressure. Because of the sluggish performance with the TBS blending the lead and lag units it regularly took 3.5 hours for a warm startup.
After reviewing alternative actuators, staff decided to move away from pneumatic actuation in favor of REXA self-contained hydraulic operators. Personnel originally were skeptical about moving to a hydraulic medium because of issues experienced with oil cleanliness in the past, but they liked the compact/sealed design of the REXA product and were sold by the fact that there were no filters and no requirements for oil maintenance. The new actuators were installed on the existing valves as a drop-in-place solution. Performance improvement was noticed within minutes after the first startup.
Lawrenceburg effectively reduced its blending time by 80 minutes for a warm start. The blending scenario occurs between five and 50 days annually (or greater depending on the market), reducing fuel consumption and increasing operating time. Better control of steam pressures and temperatures also promote longer life for the HRSGs.
Trip events associated with the turbine bypass system, common with the original pneumatic actuators, have been eliminated completely. The new actuators operate with zero overshoot or hysteresis, and their response is virtually instantaneous after initiation of the command signal. An added benefit is extended trim life in all turbine bypass valves because the actuator is now driving the plug to the correct seated position.
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