Control Valve Failure Actions


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How do Failure Last (FL)control valves
(pneumatic diaphragm actuated) respond in case of a total air supply failure as opposed to only a signal failure? For that matter how do the pneumatic piston/cylinder type FL control valves respond in case of a total pneumatic supply failure?
the fail last action is only applicable to the loss of air supply. Pneumatic relay will lock the air in the positioner stopping any movement of the valve, this normally lasts for a few hours, and until the leak from the lock assembly causes the valve to change position.