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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 underfire the boiler, with the new actuators, the boiler is much more controllable, he said.
Replacing pneumatic actuator
The plant manager's experience with the new actuators started when he was looking for a linear drive that would operate a main feed pump hydraulic coupling scoop tube. The hydraulic coupling uses the tube to scoop oil out of the coupling or allow oil to remain, and turn the coupling at full speed. With the scoop tube, there were three or four pivot points going to the Bailey pneumatic drive units and each one of them allowed a small amount of slop.
"Add together those multiple small points of slop, and you end up with a major control problem," he said. There were points in the travel of the scoop where the actuator tried to force its way out, and in other positions would try to pull the scoop in. By eliminating all those points of slop he was able to tighten up the controllability of the boiler feed pump speed.

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