The actuator is an essential part of the control valve, which drives the control valve stem. In order to select an appropriate control valve, an engineer should know about the varieties in valve actuators and its specifications.
The actuator for a valve is selected based on one or all of the following factors:
- Reliable operation
- Control and process system performance
- Inaccessible or remote valve location
- Excessive valve torque
- Emergency response and whether it is fail-safe
Pneumatic and electric actuators:
Actuators serve a different purpose according to its action. There are basically two kinds of actuators available, pneumatic and electrical based on the power supply used.
All types have advantages and the availability of data is important in order to make the best choice.
The most practical pneumatic actuators use 40 to 120 psi (3 to 8 bar) of air pressure. They are generally sized for 60 to 80 psi (4 to 6 bar) supply pressure. For producing optimal operating torque, higher air pressure is usually difficult to guarantee and lower pressures require a very wide piston or diaphragm diameter.
Electrical actuators are often used with a power supply of 110 VAC but are available in single-phase and three-phase with a wide range of AC and DC motors.
Spring return or fail position is another important specification in the process industry. A spring-return actuator moves the valve to a fixed safe position when power or signal failure occurs. This is a practical and cheap choice with pneumatic actuators and is an important reason why pneumatic actuators are widely used throughout the industry.
Electric actuators are not widely available in a spring return version; however, a battery backup system is an elegant solution.
There is a 100 percent duty cycle for pneumatic actuators. The more they are actually working, the better they are doing. Electric actuators with 25% duty cycle motors are most commonly available.
This means that the motor must periodically rest to avoid overheating in high-cycle applications. Due to the fact that most on-off automated valves remain idle 95% of the time-duty cycle is not usually a problem.
An important advantage of the design is the ability to control the speed of a pneumatic actuator. The best way to control the speed is to fit the actuator at the air pilot’s exhaust port with a variable orifice (needle valve).
Since electrical actuators are geared motors, it is impossible to make them cycle faster unless a gear change is made. A pulsing circuit can be added as an alternative for slower service.
An electrical actuator interfaces well with current electronic control systems in the modulation of operation and removes the need for electro-pneumatic controls. With pneumatic actuators, a pneumatic or electro-pneumatic positioner is used to control the valve position.
The use of electrical actuators in a hazardous environment is sometimes difficult to justify, but if compressed air is not available or if a pneumatic actuator does not provide the necessary operating characteristics, then an electrical actuator with a properly marked enclosure may be used.