Steam Turbine Valve Actuator Diagnostics for Predictive Maintenance

M&DA Control Systems recently analyzed Steam Turbine Valve Actuator Diagnostics to establish a plan for Predictive Maintenance.  Hysteresis Test data was gathered and utilized to establish an in-service predictive maintenance practice.

This practice has allowed the customer to optimize their unit’s operational availability while reducing their maintenance costs. The predictive approach allows savings to be realized by avoiding unnecessary periodic maintenance while assuring component integrity, thereby avoiding episodic maintenance due to in-service failures which often result in forced outages. This service is available for all types of steam turbine servo controlled actuators on fossil-fired, industrial, and combined cycle units.

Currently, no regulatory maintenance practices or intervals have been specified in the United States for non-nuclear steam turbine valves and actuators. Typically, we have seen OEM’s, Non-OEM’s and other industry organizations recommend minimum maintenance intervals for steam turbine valve inspection and repairs every three (3) to five (5) years. Some say these inspections and repairs should occur, as a minimum, every 25,000 Effective Operating Hours (EOH).

Adopting a data-driven approach to condition analysis will produce the following beneficial results when compared to strictly periodic maintenance:

• Understanding of, and insight into, the asset’s current operating performance

• Awareness of life expectancy predictions for individual system components

• The time and confidence to plan and optimize downtime

• The ability to focus on and manage costly repairs before they are required

To further describe the steam turbine valve actuator diagnostics service performed, we started off by recording valve motion data from each of the customer’s Control Valves, Main Stop Valves, and Intercept Valves. Each of these valves had servo modulated position control capabilities and were opened and closed at a rate of 3% per second. Data was gathered for servo coil voltage and demodulated position feedback voltage.

Individual valve hysteresis test results for each valve were then graphed.  There were no concerns of mechanical problems with the hydraulic cylinders on the control valves and intercept valves.  As shown in the graph for one of the main stop valves below, observations made by MD&A’s controls engineer indicated that the measured servo coil voltage ramp rate showed anomalies. The slope changes indicate the actuator cylinder is either leaking or sticking.

Our recommendations included further investigation of main stop valve 2 within the next year, and the hysteresis test should be performed again to confirm the anomaly is repeatable.

The cost savings associated with this type of predictive analysis can be substantial. Maintenance on a high-pressure positioning actuator typically ranges from $20,000 to $30,000, including removal and re-installation costs. If the actuators on an averaged size turbine are found to be in good condition as a result of the analysis, savings could be well over $200,000.

MD&A’s Control Systems division maintains a staff of OEM experienced field engineers who provide installation, technical support, advanced troubleshooting, training, and consulting services for gas and steam turbine controls, generator excitation, and auxiliary systems. Call our MD&A Control Systems Division today at +1 (970) 224-2223 or use our Contact form.

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