We have frequent hunting of SRV in our GT#2 refurbished MAKE-JOHN BROWN, SIZE-FRAME 5,TYPE-SINGLE SHAFT PG 5341 P. i have changed the old servo moog valve with a new one. Still SRV is hunting but little lesser.

The feedback and command i.e 96SR is not matching. what would be the causes for hunting.

vv_kumar78 [at] rediffmail.com

 

Dear Vasanth,

you can check the following

1. Is hydraulic pressure stable?

2. Is hydarulic accumulator pressure normal?

3. Hydraulic cylinder

Servo filter won't be a problem as you changed the servo valve.

G.Rajesh

 

1. After replacing the servo valve, did you calibrate (tune) per the control specification?

2. Have you checked the health of the interstage pressure transmitter (96FG-2, I think). Is it stable or is it drifting?

3. What is the control (Mark II, Mark IV Simplex, Mark IV triple redundant, or other)?

4. Is grid frequency stable? (SRV responds to speed change.)

 

We have no idea of the type of turbine control system in use on the turbine, and whether it's a TMR or a SIMPLEX system, or what.

There's just too much we don't know about the conditions and what you've done to troubleshoot them. We could make a lot of recommendations only to have you respond and say you've already done that, and not provide any data, only that the problems still persisted. We don't know what was changed during the troubleshooting, other than replacing the servo-valve. We don't know if you verified the polarity of the current being applied to the servo coils when you changed the servo-valve. We don't know what the difference is between feedback and reference. We don't know if this condition of feedback being not equal to reference is during running or during off-line "stroking" of the valve. We don't know if this condition of feedback being not equal to reference is during running or during off-line "stroking" of the valve. We don't know what how much current is being applied to the servos from the unknown control system when it's trying to make the feedback equal to the reference.

So, you need to remember that the SRV regulator loop is supposed to be a pressure control loop with position feedback. When the turbine is running the reference for the valve is P2 pressure and the regulator is going to move the valve to whatever position it needs to be at in order to make the actual P2 pressure equal to the P2 pressure reference.

If during manual stroking of the SRV the servo-valve and actuator can't make the actual position equal to the reference, then it might be a problem with the actuator. We don't know how long it's been since the actuator has been serviced; the cylinder walls and the rings do wear out. Also, if this is happening only at rated speed or the SRV position around where it would normally be when at rated speed it might be scored valve stem or sticking valve stem seal.

 

We are not connected with Grid. We have a captive power plant.

 

The hydrallic pressure is stable.