Control Valve Positioner and Control Valve Actuator Basics

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Control valves regulate the flow and direction of fluid such as gas, water, steam or chemical compounds. It controls the rate of fluid flow as the valve plug or disk position is changed by an actuator. Control valves are used in gas centrifugal compressors to avoid damages to the compression machine. They are controlled in order for the load disturbance can be compensated.  

Automatic control of modern plants is done with the help of control valves; they can exchange energy and reduce pressure. The control valve is the final control element. It is the major part of the automatic control. They provide the required energy to the controllers. 

Controller setpoints typically include flow rate, pressure, and temperature. Such valves can also control product parameters, such as density, concentration, liquid level and others.  

Control valves have three main parts: actuator, positioner and body. This article explains the various control valve actuators and positioner types. 

Actuators 

The purpose of a control valve actuator is to provide the motive force to operate a valve mechanism. Both sliding-stem and rotary control valves enjoy the same selection of actuators: pneumatic, hydraulic, electric motor, and electric-hydraulic. 

There are four commonly used control valve actuator types: 

1. Pneumatic control valve actuator 

This type has a flexible diaphragm with pressure applied against the force of the actuator spring. When the control system sends its signal, the actuator creates a force that overcomes the power of the spring, moving the actuator stem. 

There are two types of action in a pneumatic actuator: direct and reverse. Direct action involves the actuator pushes the stem down and the spring pushes it up. Reverse action will cause the stem to move up, and the spring moves it down. 

2. Hydraulic control valve actuator 

This actuator works much like the pneumatic one, and one can use it in linear and rotating control valves. However, it uses fluid rather than air to create force in its system. 

3. Electric control valve actuator 

This actuator has a motor and gearbox to create a torque that moves the valve up and down. One can find this type in linear and rotating control valves. 

4. Electric-hydraulic control valve actuator 

This type mixes electric signals and hydraulic units to act on the valve. The signal will control oil flow to open and close the valve, using a flapper-nozzle system similar to a pneumatic system. 

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Positioners 

Control valve positioners translate signals from the control system to send the right pressure to an actuator. In a nutshell, it tells the actuator to move the valve to a specific position based on the input data, allowing remote, digital control of this mechanical device. 

Control valve positioners come in different types. Pneumatic control valve positioners use a pneumatic input signal to send pressure for opening or closing the valve. Others use analogue signals to do the same thing. Smart control valve positioners can even change analogue signals to digital to control the valves. 

There are three commonly used control valve positioner types: 

1. Pneumatic control valve positioner 

Pneumatic control valve positioners use the flapper/nozzle system. They require compressed air to make everything work correctly and use a signal of 3-15 pounds per square inch to receive the data from the controller.  

The input tells the bellows to expand or compress, depending on the signal. The bellows move the flapper assembly and the beam that measures the valve stem feedback through the cam. 

In all this movement, the flapper will move closer or further from the nozzle. Thus the relay will increase or decrease the air output to the valve actuator. When the valve stem moves, it sends feedback through the beam. Then the air output will decrease or increase again to position the valve adequately based on the input signal. 

2. Electro-pneumatic control valve positioner 

The working principle of the electro-pneumatic control valve positioner resembles that of the pneumatic control valve positioner. But rather than a pneumatic signal, an analogue signal is used, usually 4-20 milliamps.  

The signal creates an electromotive force (EMF) based on the electrical signal applied to a coil. This force moves the flapper and increases or decreases the air to the valve actuator. There is also feedback from the valve stem and positioner to reduce or increase the output to the valve actuator if needed. 

3. Smart control valve positioner 

Smart valve positioners numerous benefits to the end-user. They provide more flexibility for setup, and they make it easier to get diagnostics. Furthermore, these diagnostics can increase the lifespan of the valve, reducing maintenance and unscheduled downtime. 

Different types of working principles vary by vendor. So one can find Hall-effect positioners, devices with potentiometers, and more. Best of all, these smart positioners use loop power and offer a handful of field protocols to choose from. 

Conclusion 

Control valves are indeed essential to the operation of many process facilities. Equally important are the accessories that allow a control valve to perform optimally. This article has covered the most commonly used accessories – actuators and positioners.  

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