Kind of an all encompassing question. I wont confuse the issue with more questions since control valve has a very distinct difference when relating to instrumentation and a true controlling valve as found in turbines and such.
There are historic problem areas with all valves;
Seats and Discs- Check for cracks (indications to the pure), Check contact and build up of process fluid.
Stems- Check that they run true and have good contact at back seats if back seating type. NDT? depends on application.
Gaskets and seating surfaces - Clean and inspect.
Welds - NDT
Hydro? Depends.
Balanced valves - Check stem lift to crack and travel.
Sequential lift valves - Check settings of camshafts, lift beams etc.
and on and on
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If you fail to follow through, you will fail.
Control Valve Inspection
Re: valve inspection
