Actuator Volume Tank (Air Reservoir) Sizing Calculator
Size an air reservoir (volume tank) that lets a pneumatic actuator stroke a set number of times after the instrument air supply fails, as used for fail safe double acting actuators.
- Boyle: Vt(P₁ − P₂) = n × Va × P₂ (absolute)
- Vt = 2 × 10 × 4.513 ÷ (6.513 − 4.513) = 45.13 L
Volume Tank Formula (Boyle)
(P₁, P₂ absolute)
n is the number of strokes needed, Vact the actuator volume filled per stroke, P₁ the tank pressure when the supply fails and P₂ the minimum pressure that still strokes the valve against its load, both absolute. The tank must also cover tubing volume and leakage over the time it must hold.
Worked example
Two strokes of a 10 L actuator, with the tank at 5.5 barg and 3.5 barg minimum, need V = 2 × 10 × 4.51 ÷ 2 = 45.1 L, or about 56 L with a 1.25 factor.
Frequently Asked Questions
Why do double acting actuators need volume tanks?
They have no spring, so after air failure stored air is the only way to move the valve to its safe position.
Why is a check valve needed?
It stops the stored air flowing back into the failed supply header.
How many strokes should be allowed?
Usually set by the safety requirement, often one or two full strokes, sometimes a stroke and a return.
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