Supply pressure goes to one side of the piston to open and to the other side to close, so all the force is available for moving the valve and the unit is compact. On loss of air the valve simply stays where it was, called fail-last or fail-in-place, unless a volume tank or accumulator and check valve are added to give a defined fail position.
Those big pipeline ball valves are double-acting with a volume bottle, there's no room for a spring that size.
Also heard as
- DA
Do not mix it up with
- Spring-return: A spring-return actuator has a built-in fail position; a double-acting one does not unless extra hardware is added.
Related terms
Spring-return
Describes a single-acting actuator that uses air to move one way and a spring to move back and fail safe.
Actuator
The powered device that opens, closes or positions a valve using air, hydraulic oil or an electric motor.
Scotch yoke
A rotary actuator mechanism that turns the straight push of a piston into a 90 degree shaft rotation through a slotted yoke.