In brief
A valve actuator is an electric, pneumatic or hydraulic drive that automatically operates valves such as butterfly valves, ball valves or globe valves.
Contents
What is Valve actuator?
Valve actuators convert a control signal into a rotary or linear movement. They make isolating and control valves remotely operable and are thus the link between the control system and the pipeline. A distinction is made between quarter-turn actuators for quarter-turn valves, linear actuators for globe valves and multi-turn actuators for gate valves.
Pneumatic actuators work as diaphragm or piston actuators, for example with rack and pinion or scotch yoke, and are single-acting with spring return or double-acting. Electric actuators use a motor with gearbox and offer limit switches, torque cut-off and position feedback. Important parameters are torque or thrust, operating time, fail-safe position on loss of power, control pressure or voltage, degree of protection, explosion protection where applicable, and the valve interface, for quarter-turn actuators often to ISO 5211.
Failures affect the seals and springs of pneumatic actuators, the motors, gearboxes and limit switches of electric actuators, and accessories such as solenoid valves and positioners. Seal kits and accessories are often available separately, so the complete actuator does not have to be replaced. For valves with a safety function, the fail-safe position should be checked after every replacement.
In practice: maintenance and procurement
For a replacement actuator, the actuator type, required torque with a safety margin, fail-safe position, control pressure or voltage, flange pattern and shaft connection of the valve and the accessories needed are specified. If the flange and square drive do not match, an adapter is possible, but it changes the overall height.
Synonyms and abbreviations: quarter-turn actuator · rotary actuator · valve drive
Frequently asked questions
What is the difference between a single-acting and a double-acting valve actuator?
A double-acting pneumatic actuator is moved in both directions by compressed air and usually stays in position if the air supply fails. A single-acting actuator is operated by compressed air in one direction only, with springs providing the return. If the compressed air fails, it automatically moves to a defined fail-safe position, i.e. open or closed.
What information is needed to replace a valve actuator?
Required are the actuator type, torque or thrust, angle of rotation or stroke, operating time, fail-safe position, control pressure or supply voltage, degree of protection and, where applicable, explosion protection. For mounting, the flange pattern and shaft connection of the valve matter. It should also be stated which accessories, such as limit switches, positioners or solenoid valves, are fitted.
How is a valve actuator sized?
The basis is the torque or thrust that the valve requires under unfavourable conditions, for example on breakaway after a long period at standstill. A safety margin is added to this. For spring-return actuators, the torque must be sufficient at both the spring end and the air end. Valve manufacturers state the required torques in their documentation.
What does ISO 5211 mean for valve actuators?
ISO 5211 specifies the connection dimensions for attaching part-turn actuators, i.e. actuators with a quarter turn, to the valve. These include flange sizes with pitch circle and holes. If the flange and shaft connection match, actuators from different sources can be interchanged more easily. The exact shaft form must nevertheless be checked separately.