Woodward Hot Valves Increase Wärtsilä Gas Engines’ Wastegate Reliability
Just eight miles from Denver, Colorado stands the Plains End natural gas-fired peaking power plant. When originally commissioned, it was the largest power plant of its kind in the world. Plains End is a peaking plant, as well as a non-spinning reserve plant available to fill-in for wind and other non-renewable, volatile energy sources on the local power grid. One key feature of the plant is a quick start and stop process enabling it to take on a 25% load within two minutes of engine start-up and reach full output capacity within 10 minutes.
The Plains End project was developed in two phases. The first phase, known as Plains End 1, was commissioned in 2002. Its 20 Wärtsilä 18V34SG (18 cylinder) natural gas combustion engines generate approximately 113 MW. Phase 2 commissioned 14 Wärtsilä 20V34SG (20 cylinder) engines in 2008 providing about 115 MW.
Power generated at Plains End is controlled and dispatched remotely through an automated generation control (AGC) system located in Denver. This AGC system initiates load and allows the plant to ramp up and down as necessary. The engines operate at just 65 psig (pound-force per square inch gauge) of natural gas pressure at full load which is considerably lower than gas turbines. Plains End can take individual engines out of commission for repair or maintenance even during peak demand periods.
Overall reliability and cost of operation of the plant improved by replacing one of the original 125 mm pneumatic hot valves used for the wastegate with a Woodward electric hot valve on one engine. With pneumatic valves, everything must work almost perfectly (i.e., no pneumatic system leaks). But they require service intervals as short as 500 hours of operation (approximately every six months). These valves required extensive piping, valves and vents, etc. to control them. Pneumatic failures result in hours of troubleshooting, searching for leaks, identifying failed components, etc. Other reliability issues include loose butterfly valve fittings, torn diaphragm seals and broken tubing. Every two years, Plains End replaced leaking or sticking pilot valves. Up to one-third of the 80 – 90 lbs instrument air supply (not air starters) were used for the original pneumatic systems.
However, after replacing the pneumatic hot valve with the Woodward electric hot valve, there have been no failures in over two years of operation. This resulted in considerable savings in labor for maintenance and replacement parts costs.
Woodward’s replacement hot valves provide a steadier and more robust manifold pressure–resulting in increased reliability, reduced maintenance costs, and better load acceptance. Load instability resulted in fluctuations in manifold charge air pressure. These fluctuations in turn caused excessive engine vibration, shortened spark plug life, and even bearing life. The 450 ms slew rate of Woodward’s valve is considerably faster than the pneumatic valves (approximately two seconds).
Retrofitting the hot valves was easy. The hot valves are mounted under the engines in perhaps the worst possible location considering the heat and vibration they encounter. Woodward’s replacement valve was virtually a drop-in replacement for the old pneumatic valves. No piping or flange modifications were required. The only thing needed was a 24 VDC supply which was provided by the auxiliary pump module. While the pneumatic valves struggle in this harsh environment, Woodward’s electric valves had no problems. They are designed and tested to operate reliably in even more severe temperatures and vibration.
Although actual maintenance costs are difficult to quantify, the Plains End Operations and Maintenance Manager believes that they have had virtually zero down time with the Woodward hot valve. “Basically,” he said, “we just haven’t had to worry about it.”
This article originally appeared in Woodward’s Industrial Controls News, April 2016.
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