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." Electraulic actuator proved an "expensive yet very effective solution."
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.
Tradeoffs between price, speed and accuracy
Typically plant operators have had two options for valve controls – pneumatic and hydraulic actuators – both of which involve tradeoffs between price, speed and accuracy. Pneumatic actuators are the choice when price is the main consideration, but not wen speed and accuracy are needed.
Since air is compressible, it takes a while for the pressure to build up the strength required to overcome the static friction (striction) of the actuator and valve and start moving. Once they do start moving, they tend to overshoot the desired set point.
“Smart” actuators reduce the overshoot by slowing down the actuator before it hits the set point, but this also slows down the response time. Hydraulic actuators, since hydraulic fluid is not compressible, don’t have those particular problems, but they are far more expensive and the pumps, high pressure tubine and fittings need frequent maintenance.
“The problem with the gas valve was due to the vibrations caused by the gas passing through the butterfly control valve,” the plant manager said. To address this issue, his company replaced the original true butterfly with a ‘modified’ butterfly valve. This reduced some of the high frequency noise and harmonics, but the pneumatic actuator was still having a hard time staying calibrated.
Electraulic actuator - expensive yet effective
Ultimately he chose to replace it with a Rexa electraulic actuator, which has two components – a power module and a control mode, which can be mounted together separately. No additional pump or tubing is needed.
Inside the module is a motor, a gear pump a flow match valve, make-up oil reservoir, a heater and a thermostat. Upon receipt of a signal from the control unit, the gear pump moves the oil from one side of a double acting cylinder to the other side, moving the valve precisely to the desired set point. Once in the right position, the pump shuts down and check valves lock the hydraulic fluid in the cylinder, holding the piston in position without the motor having to keep running.
The hydraulic cylinder can be either linear, such as the one the plant manager we spoke to uses for the scoop tube, or have a rotary, rack and pinion design, such as the unit he used for the main gas valve.
“These actuators are more expensive than pneumatic drives,” the plant manager said, so he cannot justify using the on all valves.
Yet, when precise control is need, or when a particular valve is proving troublesome – such as that with one main gas valve – replacing the pneumatic drive with a more advanced actuator can solve the problem and lead to more efficiency steam and overall combined-cycle operation.