LCC Quick Notes, Main Feed Pump Turbine HP Governor Valves, What's the Closed Position?
HP Governor Valves on most Main Feed Pump Turbines are rigidly coupled to operators over fulcrum linkages. In calibrating valve stroke the closed valve or minimum operator position must be determined, however, the absolute position changes significantly with the thermal expansion growth of the valve stem in the steam flow. If set for contact while "hot" the valve will leak through at this position when cold. If set for contact when "cold" the valve will be over-driven with high closing force if closed when hot which can distort linkages and damage the valve seat.
A Source of Trouble over the Years
Original equipment HP Governor Valve operators, or Servomotors, have been a source of maintenance trouble for years with the primary difficulty centering around returning a Servomotor to factory design stroke of minimum and maximum positions over control signal range. The valve thermal growth is often ignored in calibration attempts, but shouldn't be.
The full effective stroke of these valves is usually not long, falling in the 0.625-inch to 1.0-inch range. Even a small error in seating position can result in a significant steam flow which will be uncontrolled by any governor or upstream system. Likewise, distorted linkage arms and pivot pin holes are routinely detected in outage inspection efforts indicative of excess closing force.
Several users have found they needed to replace entire linkage arms to repeat stroke ranges after suffering over-travel damage.
Replacing the servomotor with an electric actuator configured with a Closing Zone feature eliminates the thermal offset linkage stress and steam pass problems permanently and relieves maintenance personnel from a difficult compromising calibration. With the controlling Servo Drive implementing a special control algorithm that treats the last 2% of travel as a Closing Zone, both positive valve closure and low linkage closing loads are achieved. Within the Closing Zone position commands to decrease the valve position are processed with a fixed current drive value to the actuator motor, applying enough force to seat the valve at whatever stage of thermal growth it might be in. Similarly, a Opening Zone prevents excess opening travel stress. The advanced tuning capabilities of the servo drive operated electric actuator permit this level of sophistication which is impossible with cruder hydraulic operators.
Electric Actuator Link
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