An actuator is an assembly fitted to the control valve to provide power for moving the move-able parts – plug, ball or vane. The operation of the control valve essentially involves positioning these move-able parts(plug, ball or vane) relative to the stationary seat of the valve. In view of this, the valve actuator performs two basic functions:
(1) Drive the move-able parts to a position required by the control signal
(2) Provide default positioning when there is no drive power available i.e close or open the valve or hold it steady in the present position.
In the valve control loop, the actuator accepts a signal from the control system and in response, moves the valve to either a fully open or fully close position or an intermediate position depending on whether ON/OFF or continuous control is used.
Types of Actuators for Control Valves
The actuator provides the drive power for the operation of the valve parts under all flow conditions of the control valve as well as hold it firmly shut when needed. Three principal types of actuator drive powers are in use today:
- They are the most widely used method of actuation
- They are capable of delivering the required forces with adequate speed of travel for most applications
- They provide fail-safe responses on loss of drive power by including spring sets in their design
- They are inherently safe against explosion hazards
- Common designs include piston, diaphragm or rotary vane types.
- The electric actuator uses an electric motor and a gear reduction drive to move the valve in linear or rotational travel.
- They can be used for ON-OFF applications for large isolating valves
- They are increasingly being used to provide continuous positioning control by providing position feedback to a controller providing variable speed reversible drive action
- They widely applied in the power generation industry, water supply/treatment plants, pharmaceutical industry etc.
- They use liquid pressure to move the valve mechanism
- Almost all hydraulic actuator designs use a piston rather than a diaphragm to convert fluid pressure into mechanical force.
- They generate an enormous amount of mechanical force hence they are used in large isolating valves on gas pipelines
- They exhibit very stable positioning due to the non-compressibility of hydraulic oil
- Some actuator designs combine hydraulic and electric power resulting in what is called a Electro Hydraulic valve
- A typical Electro Hydraulic valve consist of an electrically driven hydraulic unit supplying pressurized fluid, an actuator mounted suitably and linkage to valve stem with a hand wheel for manual override.
- Electro Hydraulic actuators are ideal for isolated locations where pneumatic supply pressure is not available but where precise control of valve plug position is needed.
These types of pneumatic actuators are very common and they use a flexible diaphragm to seal the pressure chamber and are capable of delivering a substantial drive force for a relatively low pressure due to the large cross sectional area that can be achieved. Recall that:
Pressure = Force/Area
Therefore Force = Pressure * Area
Hence with a relatively small pressure and a large area, a large actuating force can be generated from a diaphragm actuator.
Diaphragm actuators are broadly classified into:
(1) Linear diaphragm actuators
(2) Rotary diaphragm actuators
Both types of actuators use the diaphragm chamber principle but the rotary type simply has a crank arm or a rack and pinion gear set to convert linear motion to a rotation range of 90 degree. The basic parts of a diaphragm actuator are shown in the schematic below:
Direct Acting and Reverse Acting Actuators
Pneumatically operated diaphragm actuators use air supply from a controller, a positioner or any other source. They are mainly used with sliding stem control valves – globe style valves. Pneumatic actuation can be accomplished in two ways:
(1) Direct Acting
(2) Reverse Acting
Direct Acting Diaphragm Actuators
In this type of actuator, air pressure pushes down the diaphragm and extends the actuator stem which in turn closes the valve. When air pressure is released, the stem is retracted by the compression spring. Hence the valve is designated an Air To Open, Fail Close (ATC – FO) valve:
Reverse Acting Diaphragm Actuators
Here, air pressure pushes up the diaphragm and retracts or pull up actuator stem which in turn opens the valve. When the air pressure is released, the stem is pushed down by the compression springs. Hence the valve is designated an Air To Open, Fail Close valve.
Field – Reversible Multi-Spring Actuators
These types of actuators can be assembled for either direct or reverse action within the plant area by changing the air pressure connection point to the top or bottom of the valve actuator (see diagrams for double acting piston actuators below to understand what is meant by FIELD REVERSIBLE).
In piston actuators, compressed air is supplied to a solid piston in a cylindrical chamber to drive a piston rod and the piston rod is in turn coupled to the valve stem by a coupling mechanism. A schematic of the basic design of a piston actuator is shown below:
There are single acting(see schematic diagram for piston actuator above) the and double acting piston actuators. Single acting designs drive the piston against a spring while the double acting designs have two opposed cylinders with the air supplied to the drive side while the opposing side is vented or relieved to the air return circuit. The schematic for double acting piston actuators are shown below:
Basic Facts About Piston Actuators:
- They can be designed to allow high air pressures to be applied to a relatively small diameter pistons
- Piston actuators can use high-pressure instrument grade air supplies of 6 -10barg
- They are well suited to ON-OFF applications where high speed is required
- The most widely used design of piston actuators are those providing 90-degree rotational drives to quarter turn valves.