I wanted to know if anyone had experance a failure of an ABEX 425 servo valve where the actuator closing seems to be working fine, but the opening flow is limited, i.e. opening too slow even with excessive current.

I'm thinking possible cause could be the fail safe spring has broken and the pieces stopping the spool from going full stroke.

Again, mainly looking for know failures, but will still like to hear any thoughts related to servo valves

 

We have 27 Abex 425 on site on 35 year old turbines. The cause of your problem is likely a plugged servo strainer. A plugged strainer will cause a loss of pressure in the nozzle and eventually the nozzle will not be able to overcome the biasing spring, and the valve will drift closed. The servo strainers should be cleaned periodically.

 

Thanks, That is the most common problem and is being considered. The customer has a very aggressive maintenance program where this servo was replace last year.

>"...a plugged servo strainer will cause a loss of pressure in the nozzle and eventually the nozzle will not be able to overcome the biasing spring"

 

Servo strainers can plug very quickly, in minutes if there is enough contamination. We once had a series of plugged servo strainers caused by an unauthorized weld repair to servo jet supply line that cause carburizing of the EHC fluid. By the time the control system could be set up to try open the valve, the strainer was already plugged.

It is possible to ultrasonically clean the servo strainers with the proper setup. Also it may be possible to purchase a servo strainer block with a 3 way valve so that a clean strainer can be valved in on line.

You could track the resistance of your servo coils for signs of degradation, shorted turns, grounding, etc. We have not done that due to the lack of problems.

There are also 9V battery devices with a current adjusting potentiometer that you can use to plug in your own variable servo current to see how the valve responds throughout the stroke to a fixed servo current. Varying the amount of current changes the speed of the stroke and it should be influenced very predictably throughout the range. It could be that the operating cylinder has some worn spots where leakage is high. This device might help diagnose. The valve position feedback device could also be checked for linearity also.

There are also servo refurbishment contractors that will function test and repair servos. We have had very good luck with our Abex 425-1607 servos.

 

the big problem is there is actually lack of evidence there is a problem. the slow opening was observed during CV testing as this valve opening response was slower than the other two. (4 valve, single admission). for normal modulation during operation, there is no abnormalities. Given valve test is quarterly, there is only a few archieved plots and they have been reduced to 1 sec sampling so alot of squewing. with this limited data, it just looks like the servo is responding properly (i vs Q) for closing up to about 8 gpm opening.

the recomendation is to make plans to replace this servo during next valve test.