|Figure 1. A typical control loop assembly.
The control valve, typically outfitted with an actuator, provides the final control element in many process systems. The actuator accepts a signal from an external source and, in response, positions (opens or closes) the valve to the required or designed position. Valve actuators enable remote operation of control valves, which is essential for worker safety in many application environments.
Actuators can be moved into position by either hydraulic, air/gas, or electric signals. Typical control valve position commands include More Closed, More Open, Fully Closed, and Fully Open. There are different types of control valve actuators, and they are classified according to the power supply required for activation. Types of valve actuators include 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 actuator that is compact with a large stem thrust. Electric valve actuators are typically employed 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 is often the case in pipeline applications – then an electric valve actuator should be chosen for purely economic reasons (i.e., because electrical energy is cheaper and easier to transport than instrument air and/or hydraulic fluid).
Electric valve actuators rely on an electrical power source for their position signal. They employ single-phase or three-phase AC/DC motors 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 are also used on quarter-turn valves, such as butterfly valves, ball valves, etc. Linear electric valve actuators are installed in systems where tight tolerances are required, while rotary electric valve actuators are suitable for use in packaging, electric power, etc.
Electric valve actuators have the disadvantage of valve response, which can be as low as five seconds/min.
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 must also be given due consideration.
Chikezie Nwaoha has previously worked 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, including an initiative with the Caribbean African Student Exchange Initiative (CASEI). As part of his research, Mr. Chikezie has authored a number of engineering articles in leading international journals. Mr. Chikezie is a member of SPE, ASME, AIChe, IMechE, ICE, IGEM and Nigerian Gas Association (NGA). He can be reached at +234-703-135-3749, or firstname.lastname@example.org