Integrity of double acting air actuators

[MarkkraM]

Due to the cost and reliability of using spring packs on some particular valves I'm looking at changing to double acting air actuators.
The valves have a required fail position. This then leads to the valves needing a reservoir of instrument air to ensure they can move to the fail position if we lose the main instrument air header.

I would like to know if there are any standard requirements to ensure the integrity of this reservoir (eg. are block valves allowed d/s of reservoir, how close does the reservoir need to be to the actuator, does a check valve suffice in preventing loss of reservoir to the main instrument air header, does the reservoir require its own pressure indication etc)

 

[ihg]

The answer depends on the required integrity (SIL)of the actuator. The 'traditional' solution is an accumulator (LPG bottles are common), at minimum sized to stroke the actuator fully without losing more than half pressure (ie, the same size as the actuator volume, preferably twice that) with two soft-seated check valves in series and a pressure indicator. A pressure switch or 'inexpensive' transmitter to tell that the supply pressure is there and the accumulator is available if you want security. Some folk would program the logic to trip the valve shut if the accumulator pressure drops near the minimum starting pressure to fully stroke the valve.

 

[johngerhart]

One thing you can do to a spring return actuator is add a quick exhaust ( $20) to vent the air in the diaphram so they close faster . We did not recognize the fact closure speed is not a function of how big the air cylinder /spring is but how fast you get the air out . We oversized the cylinders to the point of nearly driving the gate thru the bottom seat . Double acting devices with an air cylinder do work but have more components tha could fail .