I am reviewing a case where an ESDV on the liquid outlet line of a glycol skimmer did not close. Butterfly ValvesThis line feeds into part of the glycol regeneration package (more specifically the glycol regenerator heater).

If such an event was to occur in the future (it has currently been ascertained that the problem was on the hydraulic oil return line to the ESDV which had a needle valve that had been locked closed), I was wondering what safeguards would be in place to detect a failure? Bellows Seal Valves

Ones I was thinking of were possibly the Level Alarm in the gylcol skimmer vessel and the downstream level control valve, but I also wondered whether the PSV on the glycol regenerator heater could potentially also act as protection against overpressurng the piping in between the glycol skimmer and glycol regenerator heater?

I do not have a lot of knowlege on glycol packages, so any thoughts would be appreciated.

If a valve is described as an emergency shutdown valve it should use a spring to go to the failure position.  By mentioning hydraulic oil my guess is that you have a gas over oil supply to a vane type actuator.  Such an arrangement is not fail-safe.Check Valves  The gas over oil actuator can be plumbed to close upon loss of the control signal.  However no double acting (force used to open and reverse force used to close) actuator is fail-safe as hydraulic or pneumatic force is required to change the valve position.

If you require a fail-safe actuator you need a spring that opposes the actuator force; typically provided by a pneumatic air supply but the force could be sweet process gas in the patch.Plug Valves  Most quarter turn ball valves used for emergency shutdown have a spring opposed piston actuator.  Rack and pinion actuators are used up to about NPS 2 and Scotch yoke actuators are normal NPS 2 and above.  Within process plants that have an instrument air supply the actuators are often sized for about 80 psig and require a safety factor of about 40%; large enough to assure that the actuator spring force can overcome the valve resistances  - and small enough not twist off the valve shaft.

Even with spring oposed piston actuators, speed control accessories such as the needle valve mentioned can be closed or can plug.Safety Valves  Design the actuator using the tubing size if you need to slow the actuator speed instead of throttling the supply medium.

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