Combined Cycle, Gas Turbines

Cold Start: Improving Turbine Bypass Valve Reliability

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 (Figure 1) due to malfunctions of the turbine bypass valves. This article describes how the problems were solved by replacing the actuation components on those valves.

Figure 1: This 620MW combined cycle power plant suffered problems with its bypass valves.

Plant Overview

The combined cycle power plant has two General Electric combustion turbines fueled by natural gas. Total output capacity is 620 Megawatts. Exhaust from the combustion turbines is sent through two Nooter/Eriksen heat recovery steam generators to produce steam which is sent to a Toshiba steam turbine/generator to produce additional electricity. The plant was originally built in 2002, went into operation in 2005, and was acquired by its present owner in 2013.

Valve Problem

The plant often missed startups in winter months because of problems with the turbine bypass valves (Figure 2). When there is an issue with the bypass system, the plant can’t start up, and it can’t shut down properly. The steam bypass system is also needed during steam turbine trip and no-load conditions. The plant continually had issues with these valves, including travel deviations, and damaged or frozen components.

Figure 2: Three bypass valve system.

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 valve actuators and accessories were very sensitive, and not being rated for cold weather use made problems worse, explaining why most of the problems occurred during the winter months.

The plant tried reinforcing and bracing travel indicators and feedback mechanisms to minimize travel deviations. They also spent tens of thousands of dollars on insulation improvements and heat tracing to keep components from freezing.

However, once the units were insulated, the issue became too much heat, which led to new component failures. The original valve manufacturer (OEM) was unable or unwilling to help. Missed starts were costing the plant revenue and fines, so the plant looked elsewhere for assistance.

Diagnosis

Many of the plant’s issues centered around valve tuning and calibration, specifically the derivative boosters and direct mount positioners.

A derivative booster is used with a positioner to increase stroking speed, but it is difficult to tune.

This is because it is mainly an on/off device that operates like a quick exhaust valve. For this application, two boosters need to be used. It is very difficult to get to the travel setpoint with tight controllability using this setup.

Figure 3: Step study of a quick exhaust valve. Black is the setpoint and green is the valve response.

As can be seen in Figure 3, when a quick exhaust is used, the valve response has a lot of overshoot and undershoot. This is problematic for the turbine bypass application because it requires not only high speed, but also high accuracy and tight controllability. In addition to this, derivative boosters are built with tight tolerances internally. If any debris were to enter the device, it would lock up and stop working. These units did not lend themselves well to the plant environment, and they frequently needed to be disassembled for cleaning.

The plant had difficulty finding an expert, even within the OEM’s service organization, who could accurately and consistently tune the valves. 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. Replacement parts also had long lead times and high costs. This led the site to start looking elsewhere for a solution.

In 2014 the plant began working with an Emerson local business 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.

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 to be tested. 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.

Since only the actuators were being replaced, Emerson engineering needed to be sure that there were no valve issues that could potentially carry over and negatively affect operation, even with the new actuation packages. Dimensional inspection was done to ensure that the new actuation packages could be retrofitted into the existing units without any issues.

The -40˚F Solution

As a result of the above analysis, the plant installed six Fisher Type 585C/685 piston actuators (Figure 4) 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.

Figure 4: Fisher Type 585C/685 piston actuators with DVC6200 digital valve controller.

The ODV diagnostics provide optimized tuning for specific applications, along with on-line condition monitoring via the plant’s asset management system. The ODV package is a fully engineered and tested solution that can meet fast stroking speed requirements while maintaining tight controllability of the valve. This is invaluable when operating large valves that need to stroke quickly in power plant applications such as turbine bypass.

The accessory selection in these packages is customized to meet the customer’s specific stroking speed requirements. The full actuation packages were installed on the bypass valves during a scheduled outage in 2016.

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 (Figure 5).

Figure 5: The Fisher SS-263 Volume Booster

The Fisher SS-263 Volume Booster works by amplifying the output from the valve positioner to increase actuator stroking speed. The dual-ported design of the booster amplifies both the supply and exhaust flow, which are both precisely controlled by the pneumatic signal from the valve positioner. The angled body design optimizes the center of gravity for the valve assembly, improving vibration resistance. The unit can also be directly nipple-mounted or flange-mounted for easy installation and removal in tight quarters. For increased reliability, the internals are built with a rugged design in order to withstand plant environments.

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.

Solution Verification and Testing

After installation, the valves were tested with Emerson’s ValveLink software to verify their performance. Step tests were performed and the results are shown in Figure 6. The blue line is the input signal and red line shows the valve’s travel response. As can be seen in the figure, the valve performed very well and followed the input signal with ease. The valves were able to stroke quickly as needed with tight control.

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Figure 6: Valve step response tests with the new Fisher actuation package. Blue is the signal given to the valve and red is the valve response.

Results

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.

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.

About the Author: Mary Barker is employed at Emerson as a Senior Sales Engineer for their flow controls products on the Global Industry Sales team. She holds a Bachelor’s degree from Kansas State University in Chemical Engineering, with a minor in German. She has been with Emerson since June 2014 in the Global Industry Sales department and started her career in the Pulp and Paper, Metals and Mining, and Life Sciences industry group. Most recently, she is working in the Power industry group, supporting various Emerson local business partners to grow the Emerson business with power customers