faulty proportional directional valve

[chumet1]

the governor valve controlled by the vickers proportional directional valve with feedback was closing on its own. the controller opened it to the required position and it closed again causing the unit to trip.
at first the module was changed and the problem re-occured then the proportional directional valve was changed and the problem disappeared meaning it was the root cause.
my question is, basing on knowledge and experience, what is the failure mode/ functional failure of the proportional directional valve with feedback that causes it to behave like this....
THANK YOU

 

[hydroman247]

Without more information, at a guess the feedback loop was faulty within the DCV. Possibly damaged sliding contacts but no way of knowing without a strip down to investigate or if someone else has had a similar issue.