A potential problem in one of our quick close valves was brought to my attention. During normal operation, the actuator to the main valve is held closed by an 80 psi instrument air line. Upon loss of air, the main valve is meant to close quickly by way of a quick exhaust valve (QEV). The potential problem seems to arise on cycling the main valve. There appears to be no vent to atmosphere, thus when the main valve loses air supply, the QEV vents air back into the actuator, equalizing the pressure and increasing the closing speed of the main valve (spring return closes the main valve). When the main valve is opened again, that air remains in the line. Thus when the actuator closes, it has to overcome a greater pressure in the line than it did initially. For example, equalizing pressure occurs at 50 psi, then 50 psi must be overcome to close the actuator. On the next cycle, however, the air will still be in the exhaust line and the equalizing pressure when the air supply is removed will be higher, say 60 psi. Thus as the valve is cycled, it appears that it will eventually reach a state where the air supply pressure will not overcome the pressure in the exhaust line of the actuator. I don't like the needle valves shown on the Rotork detail. These are easily left nearly closed, thus the valve not functional. Control the air flow rate with the tubing size.Look at the Versa VSP-3501 series normally closed pilot relay valve. The VSP-3301 should be OK for valves up to about NPS 3. Use a check valve to the atmosphere that is as large or larger than the tubing to minimize the pressure drop.

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