When selecting a valve actuator the correct type and size can be found utilizing the following selection criteria:

1.       Power source –The common sources of power for automated valves are electricity or fluid power. If electric power is selected, a three-phase supply is usually required for large valves; however, small valves can be operated on a single-phase supply. Usually an electric valve actuator can accommodate any of the common voltages. Sometimes a DC supply is available. This is often an emergency back-up power supply.
Variations of fluid power are much greater. First there is a variety of fluid media such as compressed air, nitrogen, hydraulic fluid or natural gas. Then, there are the variations in the available pressures of those media. With a variety of cylinder sizes, most of the variations can be accommodated for a particular valve size.

2.       The type of valve –Whenever sizing an actuator for a valve, the type of valve has to be known, so that the correct type of actuator can be selected. There are some valves that need multi-turn input, where as others need quarter-turn. This has a great impact on the type of actuator that is required. When combined with the available power supply, then the size and type of actuator quickly comes into focus.
Generally multi-turn fluid power actuators are more expensive than multi-turn electric actuators. However, for rising non-rotating stem valves a linear fluid power actuator may be less expensive. A definitive selection cannot be made until the power requirements of the valve are determined. After that decision has been made, then the torque requirement of the valve is the next selection criterion.

3.       Calculating the torque required by the valve –For a quarter-turn valve, the best way of determining the torque required is by obtaining the valve maker’s torque data. Most valve makers have measured the torque required to operate their valves over the range of operating line pressures. They make this information available for customers.
The situation is different for multi-turn valves. These can be subdivided into several groups: the rising rotating, rising non-rotating, and non-rising rotating valves. In each of these cases the measurement of the stem diameter together with the lead and pitch of the valve stem thread is required in order to size the automation for the valve. This information coupled with the size of the valve and the differential pressure across the valve can be used to calculate torque demand.
The type and size of the actuator can be determined after the power supply, the type of valve, and the torque demand of that valve have been defined.

4.       Sizing the actuator –Once the actuator type has been selected and the torque requirement of the valve has been determined, then the actuator can be sized using one of the actuator manufacturer’s sizing programs or tables. A further consideration in sizing the actuator is the required speed of operation of the valve. As speed has a direct relationship to the power required from the actuator, more horsepower would be needed to operate a valve at a faster speed.
Fluid power actuators can adjust speed of operation using fluid control valves. However, electric motor operators of the three-phase type have a fixed speed of operation. Smaller, quarter-turn actuators utilize DC motors, and may have adjustable speed of operation.