What is a Valve Actuator?
A valve actuator is the device that is joined to the valve that converts motive power e.g. pneumatic power, into movement of the valve stem. This movement opens, closes or modulates the valve.
Types of Actuators
Valve actuators are generally split into one of four types, dependant on the motive force they receive, namely:
These rely on a person supplying the motive force, either through a hand wheel, lever or chain block. For this reason they are rarely, if ever, considered by the instrument engineer. The valve and actuator are most often specified by the piping engineer, and the valves are referred to as "piping valves".
These are the most commonly used in the process and oil industries. Compressed air is used to move either a diaphragm, or piston, which in turn moves the valve stem. Pneumatic actuators are usually equipped with a spring, and the air pressure overcomes the spring to provide movement. The actuator is either configured to be 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 actuators are "double acting" which means that they don't rely on a spring for the return movement but instead air is required to both open and close it.
These 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.
These uses an electric motor to provide torque to operate a valve. Electric actuaors 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.
Things to Consider when Specifying Actuators
Compatability with valve. An oversized actuator can damage the valve stem therefore it is important that the strength of the valve stem is considered in relation to the actuator selected.
Motive power available.
- For pneumatic actuators what pressure of air supply is available? How much air will be consumed by this actuator, and does this have a detremental 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 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 actuator, and in this case they are reffered to as air filter regulators.,br> - For hydraulic actuators what pressure of hydraulic fluid is available? What volume of fluid will be required to be added to the system for this actuator?
- For electric 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?
Hazardous Area. Will the actuator be located in a potentially hazardous area? If so all electric and electronic assemblies will need appropriate certification. See our page on Hazardous Area Certification for more information on hazardous areas.
Enclosure Ingress Protection. Location of the valve, e.g. in a splash zone, may require a higher IP rating than normally specified for other actuaors on the plant. See our page on IP ratings for more information on Ingress Protection.
Ambient conditions. 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.
Electric Connections. 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 to provide positive indication to the control system that the valve is either fully open, partially open, or fully closed.
Solenoid valve. Required for on/off vlaves, and may also be required for control valves that are required to fully close or fully open in emergency situations.