Off-the-shelf electrodeposited-nickel bellows replace costly and stroke-limited custom bellows in gas-turbine fuel-control valves.
Director of Engineering
Cedar Grove, N.J.
Fast-acting fuel valves let industrial gas turbines run at near-constant speed, despite surges in demand (load) that could slow the machines, important for power generation and other applications. Dampers attached to the sprung-load poppet valves act like automobile shock absorbers to smooth valve action, squelching gas-flow spikes and droops that would otherwise trigger unwanted speed changes, explains Kris Yates, a design engineer with Continental Controls Corp., San Diego (www.continentalcontrols.com), a maker of gas-turbine controls.
Continental first looked at a dashpot and sliding-piston arrangement for the damping job. But piston O-rings introduce unpredictable friction and hysteresis that equates to jerky poppet action and erratic fuel flow. Gas-filled dampers were also considered. But they suffer compressibility effects that diminish damping action, especially for small displacements. Dampers filled with oil work much better, which prompted Continental to install the oil-filled units on some of its control valves. But the costly, welded diaphragms had limited stroke. And small position errors and contamination in valves with a 0.030-in. stroke can cause significant flow variations. However, increase the stroke to 0.25 in. and these same factors produce much smaller fuel-flow changes. That's the idea behind Continental's AGV-10 fuel-control valve.
Making the longer stroke possible are standard electrodeposited-nickel bellows from Servometer. AGV-10s use two 1-in.-diameter, 1.23-in.-long, silicone-oil-filled nickel bellows connected by a central orifice.
Though exact details are proprietary, here's basically how it works:
With the turbine shut down, the poppet return spring compresses the second bellows about 0.25 in. (full stroke) and forces oil into the first. On start-up, fuel pressure on the actuator diaphragm compresses the first bellows, displacing oil into the second. With the turbine at speed, oil shuttles from one bellows to the other to damp out minute fuel-flow variations. This dithering displaces the bellows about 0.010 in., typically. Because the bellows act as spring-loaded pistons in which pressure multiplied by an effective area determines force, the design spec called for effective area to be held within 1%.
Servometer's use of precision-machined mandrels results in bellows with consistent cross sections and therefore repeatable dynamic response. The electrodepositing process makes walls just 0.0035-in. thick or about one-quarter that of mechanical hydroformed bellows, yet it retains the mechanical properties of the bulk metal. The thinner walls cut spring rate tenfold compared with hydroformed brass bellows of the same size. In the control valve a low spring rate means poppet return springs do most of the work while the thin-walled bellows introduce near-zero hysteresis and negligible friction, key to precise valve control. Moreover, unlike hydroforming that builds in mechanical stresses, electrodepositing introduces no fabrication stresses so bellows have long fatigue life. This is important because Continental engineers specify 3,000 full-length strokes and 10 million strokes to ±5% full length over the control's five-year service life.
Maintaining structural integrity under external pressures to 240 psi was another requirement. At start-up with all gas pressure applied on only one bellows, structural rigidity alone maintains the second damper's cylindrical shape. Elastomeric bellows were rejected because pressure caused them to collapse and change internal volume. And under reverse pressure their rubber convolutions would catch and tear. Metal bellows, in contrast, have high radial rigidity to retain their shape under outside pressure. The metal bellows also withstand corrosion should "sour" natural gas introduce hydrogen sulfide into the fuel flow. A conformal polymer coating applied after soldering and assembly gives further corrosion resistance.
All told, standard electrodeposited bellows have the right mix of mechanical attributes, meeting the parallel goals of low friction, no measurable hysteresis, and long life. "Not to mention, these off-the-shelf bellows cost about 1/15th that of custom units," says Servometer Director of Engineering Paul Hazlitt.