Valve are defined as devices to control or regulate the commencement, termination and direction and also the pressure or rate of flow of a fluid under pressure which is delivered by a compressor or vacuum pump or is stored in a vessel.
Values of one sort or another, perform three main function in pneumatic installation
- They control the supply of air to power units, example cylinders
- They provide signal which govern the sequence of operation
- They act as interlock and safety devices.
The type of valve used is of little importance in a pneumatic control for most part. What is important is the function that can be initiated with the valves, its mode of actuation and line connection size, the last named characteristics also determining the flow size of the valve. Valves used in pneumatics mainly have a control function that is when they act on some process, operation or quantity to be stopped.
A control function requires control energy, it being desirable to achieve the greatest possible effect with the least effort. The form of control energy will be dictated by the valve’s mode of actuation and may be manual, mechanical, electrical hydraulic or pneumatic.
Valve available for pneumatic control can be classified into four principal groups according to their function:
- Direction control valve
- Non return valves
- Flow control valves
- Pressure control valves