Gas turbines need smooth, precise fuel control for consistent starts and the prevention of troublesome speed changes. When Continental Controls Corp., a manufacturer of fuel controls for gas turbines and reciprocating engines, designed a new "smart" control valve for fast response and precise metering, engineers sought a damper with smooth, consistent response. Attached to the valve poppet, the damper had to make constant, gradual movements to prevent erratic speed changes. The perfect damper had to work with near-zero friction and negligible hysteresis. It also had to withstand constant flexing, resist corrosive gas, and be cost-effective.
Continental Controls Corp. devised a turbine gas valve damper with two electrodeposited nickel bellows in tandem.(photo above)
Continental's control valve designers found the solution in electrodeposited nickel bellows from Servometer. Continental designed the AGV-10 smart valve to interface programmable logic controls with industrial gas turbines. Located within the fuel line, the microprocessor-controlled smart valve matches the flow of natural gas to the rpm and power needs of the machine. In power generation applications, for example, the fast-responding valve enables the turbine to accommodate transient surges in electrical demand.
Early smart valves used direct-opening solenoids with hydraulic damping. Continental engineers looked for a reliable mechanical damper to double as a pressure balance for the poppet. "A return spring alone would be unstable," says Kris Yates, mechanical design engineer at Continental. "We needed a device similar to a car shock absorber for damping."
In addition to consistent performance, the valve damper had to have fatigue resistance to survive constant dithering over the control's projected five-year service life. It also had to withstand a corrosive environment should "sour" natural gas introduce hydrogen sulfide into the fuel flow.
Continental engineers devised a damping scheme with two thin-walled silicone oil-filled bellows in tandem, connected by a central orifice. With the turbine shut down, the return spring on the poppet compresses the second bellows and forces the oil into the first. On startup, fuel pressure on the actuator diaphragm compresses the first bellows, displacing oil into the second. Once the turbine is running at speed, oil constantly shuttles from one damping element to the other, correcting automatically for minute variations in fuel flow. After a full stroke at startup, the flexible bellows might extend and contract just 0.01 in. but would remain in constant motion for long periods.
Another requirement of the damper is the structural integrity to maintain shape when pressurized. Maximum external gas pressure on the bellows is 240 psi. At startup with all the gas pressure applied on only one of the bellows, the second damper must retain its cylindrical shape through structural rigidity alone.
More information on electrodeposited nickel bellows is available by contacting Servometer, 501 Little Falls Rd., Cedar Grove, NJ 07009, calling 973-785-4630, writing in 315 on our reader service card, visiting www.servometer.com, or replying online at www.pddnet.com.
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