As often as four times a year during winter months, a power plant in Wisconsin would miss a scheduled start of its combined cycle power plant due to malfunctions of the turbine bypass valves.
When there is an issue with the bypass system, the plant can’t start up, and it can’t shut down properly.
Mary Barker, Senior Sales Engineer at Emerson and the Global Midstream Oil & Gas/LNG Industry Manager for Emerson, says the plant continually had issues with these bypass valves, including travel deviations, and damaged or frozen components. Her article, Cold Start: Improving Turbine Bypass Valve Reliability in the August 2018 issue of Power Engineering magazine, describes how the problems were solved by replacing the actuation components on those valves.
Mary says the valves that gave them the most frequent problems were the hot reheat turbine bypass valves.
Over time, the plant identified that many of the accessories—including volume boosters, trip valves and solenoid valves—on the original actuator packages were not rated for use in cold weather. The plant tried changing to cold weather accessories rated for -40˚F ambient temperatures, but this did not solve the reliability issues.
The original equipment manufacturer (OEM) of the valves was unable or unwilling to help. Missed starts were costing the plant revenue and fines. The plant had difficulty finding an expert, even within the OEM’s service organization, who could accurately and consistently tune the valves. Mary says this was a real problem:
There was no local service, so when support was required, it was only available via telephone. It was very difficult and frustrating to work with a remote after-hours service center in the midst of a missed start-up. And when a valve engineer from the OEM did fly in from another part of the United States to tune the derivative boosters or consult with the plant engineers, it cost the plant in excess of $10,000.
In 2014 the plant began working with an Emerson Impact Partner, Novaspect in Appleton, WI (www.novaspect.com), to upgrade to Fisher FIELDVUE™ DVC6200 positioners with remote mount in an effort to eliminate some of the temperature-related problems. This improved performance and solved some of the direct mount heat-related failures, but not all, Mary pointed out:
Many of the plant’s issues centered around valve tuning and calibration, specifically the derivative boosters and direct mount positioners. However, the plant still experienced problems with travel deviations on the hot reheat valves due to insufficient travel feedback mounting and bracing on the 12-inch stroke. After installing the remote-mount DVCs, Novaspect was back on-site several times in an attempt to come up with better travel feedback bracing.
The plant authorized flow scanning and dimensional inspection of existing actuators in 2015, both done with the plant online.
Dimensional inspection was needed so that Emerson and Novaspect engineers could determine the exact specifications for new actuators to fit the OEM valves. Ideally, the measurements should be taken while the plant is down and actuators removed from the valve bonnet. This was not possible at the time, so Emerson engineers performed tests using a FlowScanner™ valve diagnostic system to make sure that the valves themselves were in good condition.
The Fisher FlowScanner system is a valve diagnostic tool that can evaluate the performance of all makes and models of control valves. A portable field test instrument, the FlowScanner 6000 does not require removing or disassembling the valves for testing. FlowScanner 6000 evaluates the valve’s operating condition and identifies any required corrective actions. The diagnostic system allows verification and check of the complete valve assembly while the valve is still in-line and operating.As a result of the above analysis, the plant installed six Fisher Type 585C/685 piston actuators with yoke adapters, remote-mounted type DVC6200 positioners with optimized digital valve (ODV) diagnostics, Fisher SS-263 Volume Boosters, and Model 377 trip valves. These were all part of the ODV actuation package that was fully engineered and tested at the Emerson factory.
All of the new components were rated for operation in -40˚F temperatures. The new systems eliminated the need for the troublesome derivative boosters; instead, the positioners use more reliable volume boosters. Because the same volume boosters were used elsewhere on the site, the plant’s maintenance engineers were familiar with their operation, and the volume boosters also provided more consistency and standardization of components and equipment.
The plant has experienced no missed starts due to actuator freeze up since installation—the first time without such a mishap since the plant was built in 2002. During the winter of 2016/2017, the plant went through every possible startup sequence scenario and did not have a single issue with performance of any of the turbine bypass valves. And service is much more convenient these days, Mary says:
Novaspect can now service and calibrate complete valve assemblies, including actuators and positioners. Since they are local to the plant, the site no longer has to fly in service crews or rely on over the phone troubleshooting. Response time has been reduced to hours rather than days or weeks, the cost of on-site service went from $10,000+ to under $5,000, and the frequency of required service was greatly reduced.
The plant can now rely on power production as needed, without worrying about startup issues caused by the turbine bypass valves.
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