Valve Actuator, based on the power source, falls into the three categories. Each power source of valve actuator has unique characteristic which provide different advantage to make it suitable for specific application.
- Pneumatic Actuator
- Hydraulic Actuator
- Electric Actuator
Pneumatic actuator is the most commonly used type due to the cheapest price and availability of its power source i.e. compressed air. The use of natural gas is also applicable if it is not feasible to install air compressor system. It is used both for control valve which operates in throttling mode or actuated valve which intermittently operates.
The superiority of hydraulic actuators is its capability of supplying very high torques. Since hydraullic is considered as non-compressible liquid it can provide fast stroking speed. A typical application of hydraullic actuators is for wellhead valves since very high torques and stroking speed become a concern.
Electric actuators has smooth operation, provide very high torque values, and retain thrust. In process platform, motor operated valve is used in sea water intake line where large valve which requires high torque is operating in on-off mode.
The selection of valve actuator type by its power source shall consider several issues as follow:
- Service in process system
- Remote valve location
- Fail-safe requirement
- Valve operating torque
Mode of operation such as on-off or throttling operation. The requirement of valve closing or opening time.
For pipeline in the middle of nowhere, pneumatic actuated valve with supplementary air or nitrogen bottle which periodically inspected is the only posibility.
Process requirement of valve to fail open, fail close or fail last. Standard motor operated valve will be fail last when loosing power supply.
The actuator torque shall be larger than the required torque by valve in specified operating process and condition. Safety factor shall be taken into account in selecting the size of the actuator.