Types of actuators
- Spring and diaphragm actuators
- Piston actuators
- Electric actuators
- Hydraulic actuators
9.1 Spring and Diaphragm Actuators.
Examples of the most common type of actuators are the Fisher 657 or Fisher 667.
These are used for what are called sliding stem valves.
9.1.1 Direct-acting :
in the 657 actuator, air pushes down on the diaphragm and the opposing spring tends to lift the diaphragm.
9.1.2 Reverse-acting :
In the 667 actuator, air acts on the bottom surface of the diaphragm to lift it and the spring pushes or pulls it down.
10.0 Cylinder and Yoke (BETTIS)
These control valve actuators are in two types :
- Air to open or close with spring return
10.2 Air to open and air to close (Double Acting)
11.0 Air to open and Air to close(Sliding Stem)
In this case the valve is either opened or closed by a pneumatic actuator and the action is reversed by the action of a powerful spring, this means the valve will be “Air Fail open” or “Air Fail Close” depending on the type of valve fitted to the actuator.
11.1 A.F.O and A.F.C
In the diagram below both actuators are Direct-Acting but because different valves are fitted to the actuators, the valves have the opposite action on Air Failure.
11.2 Air to open and Air to close – Air Fail Steady
However, in this case, where air is required to both open and close the valve, one side of the piston is pressurised to open and at the same time the other side is vented, and Vice -Versa to close the valve, as in the Bettis Double-Acting Actuator.
Loss of air supply results in a “Fail Steady” situation. However, sometimes with extra pneumatic supplies and devices the valve can be made to fail as required, this of course depends on process requirements.
12.0 HYDRAULIC ACTUATORS
These operate in very much the same way as double-acting pneumatic actuators, and are used where more power and greater accuracy are required in the control system.
Very often used as final control elements for the speed control of Gas turbines.
Almost all the flight controls of modern aircraft use this form of final control element, to control the flying surfaces whether under automatic or manual control.
13.0 ELECTRIC ACTUATORS
Electric actuators are usually used on rotary control valves, where the rotary motion of the electric motor is easily converted to the rotary motion of the ball valve or butterfly valve.
They are frequently used on gate valves also, where a ‘Worm and Wheel’ type of drive is used which converts the ‘Rotary’ motion of the electric motor into the ‘Linear Motion’ required by the Gate Valve.
Two of the most well known manufacturers of this type of actuator are the companies of “Rotork” and “Limitork”.
13.1 The LIMITORK Electric Actuator.
13.2 The ROTORK Electric Actuator