Choosing the Appropriate Actuator for a Control Valve

By Chikezie Nwaoha

A control valve is the final control element in any process system because it is the final element that ultimately manipulates the value of the variable in the control process (see fig 1). The control valve usually has a control linkage, which is known as an actuator. The actuator accepts a signal from an independent source and in response positions (opening or closing) the valve to the required or designed position. Valve actuators provide remote operation of control valves, which is very essential for increased working safety and environment. Actuators can be moved into position by either hydraulic, air/gas or electric signals. And, they are used to position a valve to either more closed, more open, fully closed or fully open. There are different types of control valve actuators, and they are classified according to the type of supply that is required for activation. They are: pneumatic valve actuators, electric valve actuators and hydraulic valve actuators.

Pneumatic Valve Actuators: This is a type of control valve actuator that can adjust the position of the valve by converting air pressure into rotary motion or linear motion. Rotary motion actuators are used on butterfly valves, plug valves and ball valves, and they position from open to closed by a 90-degree turn. Meanwhile, linear motion actuators are used on globe valves, diaphragm valves, pinch valves, angle valves and gate valves, and they employ a sliding stem that controls the position of the element (closure). Pneumatic valve actuators can be single-acting, in that air actuates the valve in one direction and a compressed spring actuates the valve in the other direction. Single-acting devices can be either reverse-acting (spring-to-extend) or direct-acting (spring-to-react). The operating force is generated from the pressure of the compressed air. Choosing between reverse-acting and direct-acting is dependent on the safety requirements (in an event of compressed supply air failure), response/activation time, air supply pressure, etc. For example, for safety reasons, steam valves must close on failure of air supply.

Pneumatic valve actuators have the advantage of simple construction, requiring little maintenance and quick valve response time to changes in the control signal.

Electric Valve Actuators: This is another type of valve actuators that is compact with a large stem thrust. They are usually used in systems where pneumatic supply is not needed or available. The electric valve actuator is more complex than the pneumatically operating valve actuator. When the control valves are spread out over large distances, as in the situation with pipelines, then an electric valve actuator should be chosen for purely economic reasons. This is because electrical energy is cheaper and easier to transport than instrument air and hydraulic fluid. An electric valve actuator depends on an electrical power source for its signal, and in response to the signal automatically adjusts the valve to a desired position. It makes use of a single-phase or a three-phase AC/DC motor to move a combination of gears in order to produce the desired level of torque. Subsequently, the rotational motion is converted into a linear motion of the valve stem via a gear wheel and a worm transmission. Electric valve actuators are mostly used on linear motion valves, globe valves and gate valves. They also are allowed on quarter-turn valves — like butterfly valves, ball valves and other quarter valves. Linear electric valves are installed in systems where tight tolerances are required. While rotary electric valve actuators are suitable for use in packaging applications and electric power industries, etc., electric valve actuators have a disadvantage of valve response, which can be as low as 5 seconds/min — in some varying systems might be very slow.

Hydraulic Valve Actuators: Hydraulic valve actuators usually employ a simple design, with a minimum of mechanical parts. Hydraulic valve actuators convert fluid pressure into linear motion, rotary motion, or both. Like electric actuators, they are also used on both quarter-turn valves and linear valves. In the case of quarter-turn valves, the hydraulic fluid provides the thrust that is mechanically converted to rotary motion to adjust the valve. For linear valves, the pressure of the hydraulic fluid acts on the piston to provide the thrust in a linear motion, which is a good fit for gate or globe valves. Hydraulic valve actuators are used particularly in situations where a large stem thrust is required such as the steam supply in turbines or the movement of large valves in chimney flues. In a situation where very large valves are to be actuated, it is often advisable to install the actuators on mechanical gearboxes in order to provide an increased output (torque). There are different types of hydraulic valve actuators that convert linear motion to rotary motion. For example, while diaphragm actuators are generally used with linear motion valves, they also can be used for rotary motion valves if they are outfitted with linear-to-rotary motion linkage. Likewise, lever and link actuators transfer the linear motion of a piston cylinder or diaphragm to rotary motion. Rack-and-pinion actuators transfer the linear motion of a piston cylinder to rotary motion, and scotch yoke actuators convert linear motion to rotary motion as well. For safety reasons, most hydraulic actuators are provided with failsafe features of either Fail Open, Fail Close, or Fail Stay Put. For a control system to be effective, the control valve must adjust to its desired position as quickly and efficiently as possible. To achieve this, the right valve actuator must be selected for the application. Therefore, it could be said that the valve actuator specification process is more important than the selection of the control valve itself. To ensure the right valve actuator is chosen for a given process, critical site information, such as the availability of power supply, hydraulic fluid pressure and air pressure, must be considered. In addition, the stroke time of the valve, failsafe position, control signal input and safety factors also must be given due consideration.

For control to be very effective, it is imperative that a control valve adjusts to its desired position as quickly as possible. To achieve this, the right valve actuator must be selected for the right application. Therefore, when the various types of control valve actuators are available and their individual features given, the selection of the correct control valve actuator becomes as critical as selecting the control valve.

Finally, in selecting the right valve actuator for a given process, critical site information, like the availability of power supply, hydraulic fluid pressure and air pressure, must be considered. In addition, the stroke time of the valve, fail safe position, control signal input and safety factors, etc., also are taken into consideration.

Chikezie Nwaoha, B.Eng has a bachelors degree in Petroleum Engineering from Federal University of Technology, Owerri. He has also completed a term as an operator (student trainee) with Port Harcourt Refining Company (PHRC, www.nnpcgroup.com/phrc.htm) in Nigeria, and is currently working on several research projects involving flow systems design. As part of his research, Nwaoha has authored a number of engineering articles in leading international journals. Nwaoha is a member of SPE, ASME, AIChe, IMechE, ICE, IGEM, and the Nigerian Gas Association (NGA, www.nigeriangasassociation.org). He can be reached at chikezienwaoha@yahoo.com