Spring Diaphragm Actuator Thrust Calculator
Check that a spring and diaphragm actuator can shut a globe valve tightly. Compare available seat load from the spring (or air) with the unbalanced force from shutoff pressure plus packing friction.
- Fail close: seat load = lower bench set × area = 0.4 bar × 450 cm²
- Unbalanced force = ΔP × π/4 × dseat² = 1,963.5 N
- Margin = available − (unbalance + friction) = −563.5 N
Actuator Force Balance
Fail open: Fseat = (Psupply − Pbench,high) × Ae
Frequired = ΔP × π/4 × dseat² + Ffriction
Assumes an unbalanced plug with flow tending to open the valve. Balanced trims greatly reduce the unbalanced force.
Worked example
A 450 cm² fail close actuator with 0.4 bar lower bench set gives 1,800 N. Shutting 10 bar across a 50 mm seat needs 1,963 N plus 400 N friction, so it is about 560 N short.
Frequently Asked Questions
What is bench set?
The air pressure range over which the actuator strokes the valve on the bench, with no process pressure; it is set by spring preload and rate.
Why does my valve leak at shutoff?
Often because seat load is too low for the shutoff pressure, or because the bench set was changed. Calculate the margin and check the seat.
What is a balanced trim?
A plug with balancing holes and a seal that equalise pressure above and below it, reducing the force needed to close.
Learn more on the blog
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