A pnuematic
valve actuator turns or moves valves by use of air. The basic design is a
cylinder where the
piston is connected either directly to the valve stem or through a
mechanism that converts the linear motion to rotary.
The actuators are categorized as single acting or double acting. A single acting actuator uses air to actuate in one direction while a spring acts in the opposite direction. In this case an important selection criteria is the fail position of the actuator which is what position it reverts to when no air is supplied. Single acting or spring return actuators are used in safety related applications and the fail position is selected to cause the least amount of damage. For example, on a burner you would select the fail position of the fuel isolation valves to be closed while the coolant valves would be selected to fail open.
A double acting actuator uses air in both directions of the stroke. Although these actuators cost less than single acting ones, installation is more complicated since 2 air lines are required instead of one.
There are a number different types of pneumatic actuators:
- Spring Diaphragm: This actuator comes in a spring return version only. The presses against a diaphragm that is connected by means of a rod to the valve stem. The other side of the diaphragm is a spring which provides the return action. As the diaphragm moves up and down, the sides roll and this way have the least possible amount of friction. For this reason these actuators are typically used on control valves.
- Piston: These actuators are designed like a conventional cylinder and come in both singla and double acting variety.
- Double Opposed Piston: These actuator are also called rack and pinion actuators. They have 2 cylinders that have attached to each a rack which rotates a central pinion as they move. The spring return versions are pneumatically operated from the center with the springs on the outside. The double acting versions are pressurized from both the inside and outside.
- Vane: These actuators are usually double acting. They have a vane that looks like a rigid flag inside an enclosure which looks like a quarter of a circle. The air is applied to one side of the unit which then swings the flag over to the opposite side.
The actuators can be equipped with various accessories such as
solenoid valves,
limit switches,
valve positioners, or
filter regulators.