What Is A Control Valve?

A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller.This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.

In automatic control terminology, a control valve is termed a "final control element".

Operation

The opening or closing of automatic control valves is usually done by electrical, hydraulic or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening.

Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically-operated valves require additional cabling and switch gear, and hydraulically-actuated valves required high pressure supply and return lines for the hydraulic fluid.

An automatic control valve consists of three main parts in which each part exist in several types and designs:

• Valve actuator – which moves the valve's modulating element, such as ball or butterfly.
• Valve positioner – which ensures the valve has reached the desired degree of opening. This overcomes the problems of friction and wear.
• Valve body – in which the modulating element, a plug, globe, ball or butterfly, is contained.

Control action

Taking the example of an air-operated valve, there are two control actions possible:

• "Air or current to open" – The flow restriction decreases with increased control signal value.
• "Air or current to close" – The flow restriction increases with increased control signal value.

There can also be failure to safety modes:

• ''Air or control signal failure to close" – On failure of compressed air to the actuator, the valve closes under spring pressure or by backup power.
• ''Air or control signal failure to open" – On failure of compressed air to actuator, the valve opens under spring pressure or by backup power.

The modes of failure operation are requirements of the failure to safety process control specification of the plant. In the case of cooling water it may be to fail open, and the case of delivering a chemical it may be to fail closed.