LCC Quick Notes, Feed Pump Turbine Governor Valve Closing Rate Requirements


Governor valve closure rates are often a major point of discussion when planning electric actuator upgrades.  It is important to understand the design criteria for governor valve closures before expending large sums for excess closing speed control.  


 

(1) Addressed Topic

In redundant steam feed mechanical drive turbines each steam supply source (HP main steam and LP reheat steam for Westinghouse and GE feedwater pump turbines) must have Stop Valve Operators capable of closing steam supply to the turbine within sufficient time to arrest speed excursions to less than 12% under rated operating conditions, or typically one second. Further, Governor Valve Operators are to close following the Stop Valve trip to prevent a reset run-off situation. No performance standard is assigned Governor Valve Operator closure rate, but it is generally accepted that positive predictable closures are better to assist in emergency shut down situations. Also, both the Stop Valve Operators and the Governor Valve Operators are supposed to have full closure under input positioning signal loss failure mode. These criteria are a homogilation of ASME Power Test Code and industry insurance underwriter's requirements established in the 1950's. European standards vary, and are exempt from this discussion.

The question addressed by this paper is how an electric actuator retrofit compares in meeting the trip performance of the Governor Valve Operators with the original equipment servomotor (Westinghouse) and secondary operating cylinder (GE) operators.

(2) FP Turbine Design Points

Feed pump turbines are mechanical drive prime movers which operate at speeds using one of two steam supplies. In normal operation, the available enthalpy of the throttled steam supply in service equals the retarding torque of the driven pump when controlled to a steady state speed by a governor. Unlike a generation turbine, the retarding torque load is only suddenly lost in a catastrophic failure incident like a coupling bolt shear failure or pump discharge line failure. A generation turbine can lose all load by a main disconnect breaker opening and is therefore treated as an entirely different case. For the purposes of failure analysis the feed pump turbine load loss is considered an extremely rare asynchronous event.

(3) Original Hydraulic Operator vs. Electric Actuator Operator Trip Times

Original hydraulic Governor Valve Operators connect to trip oil headers via check valves and close position upon hydraulic trip (autostop) oil pressure loss. Hydraulic Governor Valve Operator closure times vary between 1.5 and 2.5 seconds.

Electric Governor Valve Operators connect to electrical trip and trip oil header pressure switches and close position upon hydraulic trip (autostop) oil pressure loss. Electric Governor Valve Operator closure times are 0.4 to 0.7 seconds.

(4) Trip Response Under Failure Modes Comparisons

Original hydraulic Governor Valve Operators have the following possible failure scenarios and closure affects:

■    Main Oil Pump AC Loss induces 2.5 to 3.5 second closure.
■    Servo Pilot Valve Contamination slows closure rate.
■    Trip Solenoid Failure prevents trip closure on electrical trips.
■    Feedback Linkage Disconnect Failure induces uncontrolled opening.

Of these potential failures, a catastrophic turbine failure can only occur in the instances of the last two cases if combined with a simultaneous combination of pump mechanical load loss and Stop Valve Operator failure.

Electric actuator Governor Valve Operators have the following possible failure scenarios and closure affects:

■    AC Feed Loss induces 3.5 to 7.5 second closure by back roll.
■    Servo Drive Failure induces 3.5 to 7.5 second closure by back roll.
■    Distress to Roller Screw Threads Locks Stroke fails actuator in place.

Of these potential failures, a catastrophic turbine failure can only occur in the instance of the last case if combined with a simultaneous combination of pump mechanical load loss and Stop Valve Operator failure. Thus three asynchronous events are required.


(5) Conclusion

Electric actuator replacements of hydraulic operated governor valves improve trip response in normal operation and reduce potential catastrophic failure potential in the cases of rare simultaneous asynchronous events.
 

 

 

 

 

 

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