● Large valves and valves required to be operated against high-fluid pressure
● When the time required to open, close, throttle, or regulate the valve manually is longer than that required by system-design criteria
● When the valve is required to be operated from a remote location
● When the valve must attain a position (open or close) in the event of an accident or in a particular mode of system operation
The valves to be furnished with an actuator utilizing external source(s) of energy, such as electricity, pneumatics, hydraulics, mechanical springs, or a combination of one or more of these energies, are called actuated valves. Upon failure of the external source of energy, a valve may not be in the required position for accomplishing the design function. One must be aware of failure modes prior to selecting a valve actuator.
Failure ModesFail-As-Is (FAI): The valve remains in its last position following upon an external power failure.
Fail-Closed: The valve is provided with external source(s) of energy to place it in the closed position, regardless of valve position before power failure.
Fail-Open: The valve is provided with external source(s) of energy to place it in the open position, regardless of valve position before power failure.
Types of Actuators
Electric Motor Actuators
The length and speed of stroke may vary considerably, depending upon the type of pneumatic actuator. These actuators are capable of providing either fail-open or fail-close failure mode upon loss of air. Piston-type air actuators are furnished with springs to open or close the valve upon air failure. Diaphragm air actuators have limited thrust and torque generation capabilities due to limits on air pressure because of diaphragm strength. In addition, the diaphragm actuators have limited stroke ranging from 1 to 4 in (25 to 100 mm).
Air-vane pneumatic actuators are used with quarter-turn valves, and they can be directly mounted on the valve stem.
Hydraulic actuators utilize pressurized liquids, usually oils but sometimes water, or the process liquid is used to provide the motive force for actuating the valve. Like pneumatic actuators, these actuators can help achieve fail open or fail-close failure modes.
Solenoid ActuatorsSolenoid actuators have short-stroke and low-thrust capabilities.Two types of actuating methods are used in solenoid valves: direct acting and pilot operated. In the direct acting solenoid valve, the disc is lifted off the seat to a fully open position by magnetic flux generated by energizing the coil, and the disc is returned to its seat by De-energizing the coil. The pilot-operated solenoid valve utilizes the system pressure to provide the actuating force.
Solenoid valves can accomplish all failure modes.