We have made a couple attempts to calibrate our Moog servos after having experienced problems on both startup and shutdown of the ST. (MCV - Moog 761-4288B, ICV - Moog 761-4290B) The feedback overshoots the demand by 1 or 2% which starts oscillating the ST (sometimes an MCV and sometimes and ICV will go first), which then starts the governor oscillating as the ST rpm starts swinging with the steam flow changing.

This problem has been getting worse over the last year or so, so we tried calibrating the servos using the null adjust. The shutdown is now perfect, but we are still experiencing oscillations on the startup as the valves aren't coming off of the seat quick enough and end up overshooting the demand when the finally start moving at 5% demand.

We did have the startup looking good at one point, but then we were experiencing oscillations on the shutdown which were worse, so left them calibrated in the state where the shutdown is solid.

We would take another stab at calibrating them again, but we would like to know the proper procedure before we do this again.

Procedure that produced a good shutdown (oscillations on startup):

- disconnect the servo control cable and let the valve fall from 100% to zero%

- adjust the null adjust such that the time for the valve to fall to 0% is 10 seconds/inch of valve travel

Procedure that produced a good startup (oscillations on shutdown):

- adjust the null adjust such that the valve lifts fairly quickly off the seat (ie. 1 or 2% demand)

- not worrying about the fall time from 100%

If anyone knows the correct procedure that Moog uses to calibrate these servos, it would be greatly appreciated.

 

thegroovyguru,

What control system is driving the servo-valve?

Who is the steam turbine manufacturer?

Do the instructions provided with the steam turbine and control system say to adjust the null for stability?

 

Also, have you seem this document:

http://www.moog.com/literature/ICD/g761seriesvalves.pdf

 

Siemens T3000

Fuji valves

There are no instructions to say we should adjust the null adjust, but we have found it helps. Just need to get clarification on what exactly the procedure is for adjustment.

Read the manual excerpt, but does not give a procedure for adjustment, just tells you how to adjust it.

Thanks.