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Gas turbine user experience: Tightening up boiler control in CCPPs
At two US-based gas-fired combined cycle plants, controllability at the plants was a major issue, the responsible plant manager told Gas to Power Journal on condition of anonymity. Large natural gas swings caused large boiler pressure swings, which "created a situation where the load was swinging around a lot more than it needed to be". Steam turbines typically operate best when fed a steady supply of steam, but getting that constant pressure requires continuous and rapid adjustment of boiler input parameters. Gas flow changes depend on the turbine's demand for steam flow from the boiler and every time the gas flow changes, oxygen flow must also change to ensure complete combustion, he explained, adding that the main feedwater pump, recirculation valves and numerous other components must also be kept in continuous balance. Overshots, pressure fluctuations caused by unstable gas flows However, on one of the steam units, he manages, poor response and control by an old pneumatic actuator was causing unstable gas flow, leading to overshoot and pressure fluctuations. By replacing the Bailey pneumatic actuator on the main gas valve with a Rexa R5000 electraulic (electro-hydraulic) actuator, he was able to bring the balky boiler back under control.While the old actuator had sloppy control of the gas valve, leading them to overfire and underfi...
