When deciding to actuate a valve, whether for precision process control, inaccessibility to remote areas, for emergency fail-safe requirements or because of excessive valve operating torque, it is important to choose the proper actuator for the specific application.
Valve Actuator Selection
Valve actuation can be linear, part-turn or multi-turn and can be powered by a hydraulic, electric or pneumatic power source. When choosing the proper actuator for your application keep the following factors in mind.
- The chosen actuator must deliver enough torque to move and seat the valve with the minimum power supply available while maintaining the necessary position (open, closed or intermediate)
- The chosen actuator must be capable of completing the travel (90°, 180°, multi-turn) at the required speed.
- The chosen actuator must meet the area classification requirements.(For electric actuators NEMA area classification has to be taken into consideration (Nema 4x-weatherproof, Nema 7 explosion proof)
Valve operating forces are critical in choosing valve actuation. For Linear valves (gate, globe, diaphragm) that open and close in a reciprocating or up and down manner; electric, pneumatic or hydraulic actuators can be used, keeping in mind that it must provide thrust to overcome the surface friction caused between the actuator gland and the valve stem that enters the body.
Gate valves shut off the process flow via a gate traveling across the bore of the valve. This action creates a throttling effect and causes an increase in differential pressure across the gate. This effect forces the gate onto the downstream seat and the actuator will require additional thrust in order to overcome this friction.
Screw down stop globe valves rely on metal to metal contact to effect a tight shut-off and an additional thrust allowance is necessary to achieve a satisfactory sealing between the disc and the seat. The shape of the closing plug, the flow rate and the differential pressure must be taken into consideration when choosing an actuator for a globe control valve.
Part-turn valves typically only have to travel 90° and can be closed using relatively low power consumption. When choosing an actuator for a ball valve the coefficient of friction between the ball and the seat has to be considered as well as the process temperature, pressure and fluid velocity and viscosity.
Both the velocity of flow and the hydrodynamic forces generated by the valve closure come into play when choosing an actuator for butterfly valves. Actuators for butterfly valves are often fitted with self locking devices to ensure that flow forces do not move the valve closure plate.
When selecting and sizing an actuator consult the manufacturers maximum torque requirements needed to operate their valves under “normal service conditions”, keeping in mind that in applications involving high or low temperatures, dry gas, slurry or high velocity additional torque may be required.
Part 1 of 3 –Assured Automation Actuator Series