Like some other vintage Frame 6B-powered cogen plants, this facility in the California oil patch, recently visited by CCJ editors, was plagued with fuel-gas valve problems, usually requiring at least a man-shift’s worth of work each outage to disassemble, clean seats/plugs, and reassemble. Sometimes an outage would have to be taken if leakage was indicated by the P2 pressure between scheduled outages.
The original valve actually was two valves within one housing, according to the plant manager (PM)—a ratio valve and a stop valve. Both valves share the job of reducing gas pressure from the supply line to that required by the turbine. The ratio valve takes the first pressure drop, but in that process, the fuel gas drops below its dew point, allowing sulfur-bearing liquid mercaptans to drop out in the second valve passage.
Over many operating hours, this leads to deposition which impairs operation and has to be removed at every planned (and sometimes unplanned) outage. The deposits get in the way of passing startup permissives, especially what’s known as the P2 test, which ensures that the ratio valve will not pass too much fuel into the turbine while it’s closed and create an explosion hazard upon ignition. This occurs because the deposition leads to improper valve seating and potentially allows leakage. P2 is a leak-detection permissive in the control logic. Before light-off, the startup controls monitor fuel supply manifold pressures.
One solution offered by the marketplace is to reheat the fuel gas to above the dew point (120-130F) in between the valves, but this is considered far from ideal. The better solution proved to be replacing the original valve with a “smart” electric control valve (Fig 1) in the same location as the original and a separate solenoid-operated stop/isolation valve about 6 ft upstream—just outside and below the turbine enclosure (Fig 2). The control valve acts as a ratio valve and a control valve. The modifications were made in spring 2013.
Since then, reports the PM, the facility has avoided at least half a dozen trips. The deposition problem hasn’t been completely solved—borescope inspection shows that it still is occurring in the control valve—but, critically, not in the isolation valve. Additionally, the new control valve self-calibrates and is able to reset to a new starting position, usually few millimeters at most, based on any deposition. Thus, if valve stroke is off by one tenth of one percent, notes the PM, that position is used as a new “zero point.”
Finally, new startup curves are used at the plant based on the far superior sensitivity and flexibility of the new valve. The logic can recognize a 1% valve opening, far better than the 30% crude opening sensitivity of the original, and provide accurate, repeatable readings to the control system at small openings.
According to Young & Franklin, the valve supplier, the position transmitter has greater than 300:1 position turndown. The stop/ratio function can now be handled in the software. Previously, the plant would have to call a GE field service engineer out to make the modifications, create new control cards, and capture new instructions in the read only memory (ROM).
