A double-acting actuator uses compressed air to drive the valve in both directions - open and close. It has no internal spring, so on loss of air supply it simply stops and holds its last position ("stay put").
A single-acting (spring-return) actuator uses air to drive the valve in one direction and an internal spring to return it automatically. On loss of air supply, it fails to a defined, predictable position - either fail-open or fail-closed, depending on how the spring is oriented.
What happens when the air fails
Double acting
Air both ends - no spring
- Now
- Held out by air
- On air failure
- Holds last position
Spring return
Air one end - spring the other
- Now
- Held out against spring
- On air failure
- Driven home by spring
At a glance
| Double Acting | Spring Return | |
|---|---|---|
| Air needed for | Both open and close strokes | One stroke only (spring does the return) |
| Behaviour on air loss | Holds last position | Moves to a defined fail-safe position |
| Torque output | Generally higher for the same actuator size | Reduced by the force needed to compress the spring |
| Typical choice when | Position on air loss doesn't matter, or a separate fail-safe system exists | The process requires a guaranteed safe position on air loss |
Frequently asked
Is spring-return always the "safer" choice?
Only when the process actually needs a defined fail position. For applications where holding the current position is safer than moving (e.g. some flow-control loops), double-acting with a separate air-supply backup may be preferred instead.
Can I convert a double-acting actuator to spring-return?
Not directly - spring-return actuators use a different internal design to house the return spring. You'd need to replace the actuator, not retrofit it.
Related
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