O&M: When a quick disconnect makes sense during a turbine shutdown

Illustration courtesy Moog.

A safe, quick fix to preventing unplanned outages

The inlet guide vane actuators and fuel gas control valves on turbines regulate the ratio of air and fuel in varying loads for efficiency and minimal emissions. The fuel gas control valve on an actuator is the primary interface between a control system and the mechanical part of the plant. Maintaining that link produces power.

When there is an issue with the air-fuel metering equipment, the turbine may be forced to shut down for corrective measures. Any changes would have to happen in less than 60 minutes because any more time could cause the turbine shaft to cool unevenly, warp and make a restart potentially damaging. Properly done, the shut-down would allow the shaft to cool for about 12 hours, meaning lost power (and revenue) for up to half a day.

The OEM’s design perspective

International safety standards for areas with potentially flammable or explosive environments require the use of intrinsically safe, flame-proof or explosion proof electrical equipment, wiring, conduit and terminal boxes. Since turbine makers must balance safety, cost and ease of maintenance, the considerations around how easily technicians can maintain the system typically receive a lower priority. The ability to replace a failing servo valve within 60 minutes was likely not a consideration when designing the gas turbine engine.

When troubleshooting an actuation system, it’s common practice to swap out the servo valve early in the process. With traditional explosion-proof systems, this requires opening the terminal box, removing wires from terminal strips, detaching conduit from the servo valve, pulling the wires through the conduit, removing hydraulic connections and finally removing the servo valve from the manifold. The reverse process is required to install the replacement servo valve.

Often the wires are cut to reduce the time involved. Most people are not aware that the wires on these types of valves are continuous all the way to the torque motor. When technicians cut the wires, they must replace the torque motor at added cost. In many cases, the issue is not the result of a bad servo valve. But with the wires cut, workers would then have to send the servo back to its manufacturer for repair anyway.

This process requires skill and attention to detail. Servo valves are often located in tight areas or overhead, requiring a degree of physical dexterity. Since good instrument technicians are worth their weight in gold, it’s preferable not to tie up such a valuable resource for something as seemingly simple as replacing a servo valve.

Case in point

After experiencing more than one extended outage attributable to valve replacement, the manager at a southeastern U.S. electric utility’s 2,550-MW power plant – consisting of three combined cycle units – wanted a solution for its Mitsubishi combustion turbine.

“Mitsubishi and Moog were called by the plant’s engineering and I&C folks, since our organizations had a relationship,” said Steve Beddick,market development manager for Power Generation, Industrial Products & Services at East Aurora, N.Y.-headquartered Moog Inc.

“Our engineers and the plant’s engineers and technicians discussed a solution. The first system we drew up fell short of expectations and was more difficult to install in the field than the original design.”

With the second attempt, the team designed and manufactured a quick disconnect cabling system for explosion-proof servo valves, something that works on not only Mitsubishi turbines but also General Electric, Siemens and other types of units. With the quick disconnect kit, a power plant technician can swap out a servo valve in 20 minutes. The kit complies with National Electric Code Class I, Division 2 and ATEX Zone 2 locations.  

Retrofit kit makes servo valve replacement safe, easy

Technicians with varying levels of experience can deploy the kit to replace servo valves with no chance for wiring mistakes. The kit makes swapping out a servo valve easy because design engineers mounted a quick disconnect to the servo valve with a transition adapter, allowing the flexible conduit to connect the terminal box and quickly disconnect from the servo valve.

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