Valve Actuators - Types and Specification
What is a Valve Actuator?
A valve actuator is a mechanical or electrical device that is fitted to a valve that converts motive power e.g. pneumatic power as used in compressed air actuators, into movement of the valve stem. The movement of the valve stem opens, closes or modulates the valve.
Types of Valve Actuator
Valve actuators are generally split into one of four types, dependant on the motive force they receive. The four actuator types are:
Manual actuators rely on a person supplying the motive force, either through a hand wheel, lever or chain block. A manual valve actuator assembly does not lend itself to control or fast acting shutoff applications and for this reason they are rarely, if ever, considered by the instrument engineer. The assembly of valve and actuator are most often specified by the piping engineer, and the valves are referred to simply as "piping valves".
Pneumatic Valve Actuators
Pneumatic actuators are the most commonly used actuator for valves in the process and oil industries. Compressed air is used to move either a diaphragm, or piston, which in turn moves the valve stem. Air operated valve actuators are usually equipped with a spring, and the air pressure overcomes the spring to provide movement. This allows the pneumatic actuator to be either configured as spring-close or spring-open. For spring-close valves, the valve will "fail" to the closed position and air is required to open it. The opposite applies to spring-open valves. Some pneumatic valve actuators are "double acting" which means that they don't rely on a spring for the return movement but instead compressed air is required to both open and close it. This allows thes valves to be "fail stay put" on loss of air supply.
Hydraulic valve actuators rely on a virtually non-compressible fluid, e.g. hydraulic oil to provide the motive force. Hydraulic actuators can provide greater force than pneumatic actuators and this leads to them often being used in high pressure piping systems.
See our page on Hydraulic Oil Cleanliness for further details on hydraulic systems.
Electric valve actuators use an electric motor to provide torque to operate a valve. Electric motor valve actuators are not equipped with springs, therefore on a loss of power the valve will fail in its current position unless there is a back up power supply to move it to the fully open or fully closed position. Electric motor valve actuators are commonly used in remote locations where it would be impractical to pipe hydraulics or compressed air.
Things to Consider when Specifying Valve Actuators
Valve and Actuator Compatability
An oversized actuator i.e. one that provides excessive power, can damage the valve stem therefore it is important that the strength of the valve stem is considered in relation to the actuator selected. This is often called the valve safety factor, and can be considered as the ratio between "torque produced by an actuator" to the "torque required to move the valve stem". Typical values of safety factor are between 1.5 and 2. Maximum torque to move the valve stem is dependant on the maximum differential pressure across the valve, i.e. when the valve is fully closed and has full line pressure upstream and low downstream pressure. The instrument engineer should select an actuator that will supply the necessary torque to move the stem in this worse case scenario (the so called breakout torque) but still be within the valve safety factor.
Actuator Motive Power Available
- For pneumatic valve actuators what pressure of air supply is available? How much air will be consumed by the air actuator, and does this have a detrimental effect on the air supply system? Do you need a pressure regulator? Air Pressure Regulators maintain constant output pressure despite variances in input pressure. These are invariably used in conjunction with pneumatic actuators. It is common for them to be fitted with a filter so to ensure that no contaminants pass into the air actuator, and in this case they are referred to as air filter regulators.
- For hydraulic valve actuators what pressure of hydraulic fluid is available? What volume of fluid will be required to be added to the system for this actuator? Is the hydraulic fluid clean? The small clearances within the ports of hydraulic actuators require the hydraulic fluid to be clean and free of contaminants - see our page on hydraulic oil cleanliness for more details.
- For electric valve actuators what voltage is available locally? If the valve is required to operate in shutdown conditions will the electric power supply be fed from a switchboard that will still be live in these conditions?
Will the actuator be located in a potentially hazardous area? If so the actuator assembly must not act as an ignition source therefore hot surfaces on the actuator and ignition sparks created by the actuator valve have to be avoided. This can be achieved through design and certification of all internal electric and electronic assemblies. See our pages on Hazardous Area Certification and surface temperature classification for more information on hazardous areas and temperature classification.
Enclosure Ingress Protection
Location of the valve actuator assembly, e.g. in a splash zone, may require a higher IP rating than normally specified for other actuators on the plant. See our page on IP ratings for more information on Ingress Protection.
As always, thought should be given to ambient temperature, humidity etc. Also will the actuator be located in saliniferous atmosphere? If so then consider what type of coating it should have e.g. perhaps a two part epoxy resin.
What size of electrical entry connections are required. Signal cables usually use M20x1.5 ISO. See our page on cable glands for further discussion on cable entries.
Limit switches are often fitted to valves and or actuators to provide positive indication to the control system that the valve is either fully open, partially open, or fully closed.
Solenoid valves are required for on/off valves, and may also be required for pneumatic control valves that are required to fully close or fully open in emergency situations.