Moog servo valve

We found that the new moog valve installed in Frame5 GT with MarkVie control was showing more than 5% difference in feed back and command in the lower side.

Tried to adjust null bias current. but it has to be increased to 5.5 % to get the feed back and command nearly equal.

So we tried to adjust the mechanical null bias screw very slightly CCW direction after keeping the valve in 25% open position. Position and feed back become normal. Null bias current is kept as before 1.94 %. We started the unit and found normal.

I read in this forum that it is not advisable to adjust mechanical null bias??? Please comment.

You seem to have been very lucky with your adjustment; most sites that attempt this end up replacing the servo-valve.

It's entirely possible that the servo-valve null bias spring was not adjusted properly by the manufacturer, though not likely. What may be more likely is that the L.O. and Hydraulic system were not properly flushed. Did you try cleaning the servo filter (not the upstream filter but the internal filter/strainer)?

Did you perform the individual servo polarity checks before adjusting the null bias current? Sometimes the wrong polarity being applied by one processor will cause a higher-than-normal null bias current to seeming be required. And, many people have found that even when installing servos using the "proper" color codes or even connecting them exactly as was found on existing servos when replacing one the factory color-coding did not result in the proper polarity being applied to all three coils--from the manufacturer!

Usually when it's necessary to adjust the spring tension it means there is something wrong with the servo--not always, but mostly.

Again, you may have been extremely lucky, but if something does happen in the future to restore flow-rate through the servo and the spring tension is in excess of what's required there may be other follow-on issues as a result of the change that was made.

I would suggest the packager of the turbine provide a new servo, since this was found during commissioning of a new machine--if I understand the original post correctly. The standard null bias of -1.33% to -4.0% per processor (-0.133 mA to -0.4 mA per processor) on a TMR control panel has been proven more than adequate for hundreds of turbines over the years with Mark IV, Mark V, Mark VI and Mark VIe control systems.

Adjusting mechanical null bias properly requires measurement of hydraulic flow-rate, and that's not really possible in the field. It requires a test "bench" with proper instrumentation to do properly, per the manufacturer's instructions.

Please write back to let us know if you verified servo current polarity before adjusting the null bias current.