Directional-control valves are one of the most basic components of any system that controls the flow of a fluid or gaseous medium and are used in numerous applications.
Directional control valves perform three basic functions:
- Stopping the flow of fluids and gasses
- Allowing The flow of fluids and gasses
- Changing the direction of the flow of fluids and gasses
The most basic example of a directional control valve is in a simple is the 2-way valve. 2-way control valves have two ports, these ports are normally referred to as the inlet port and the outlet port.
A 3-way valve has three ports. These ports are normally referred to as the inlet port, the outlet port and the exhaust port. A 3-way valve not supplies fluid or gas not only to to a destination, but allows fluid to return from it as well when the valve is in the second position, some three way valve have a third position that stops the flow entirely.
The job of an actuator that controls a directional control valves, is to automatically control the flow through the valve. This control can be exercised in any of the following:
- Fully closing the valve under normal conditions
- Fully opening the valve under normal conditions
- Modulating the valve’s position based off a control parameter (for example fluid pressure)
- Opening or closing the valve to a predesignated fail-safe position in the event of power or control signal loss
Valve actuators tend be one of three basic types types, pneumatic actuators, electric actuators and hydraulic actuators .
Each of these three basic types have different strengths and weaknesses, such as torque and speed of actuation, and the selection of the most appropriate one is one of the most important taken by the control system designer.
- Pneumatic valve actuators tend to have the highest reliability of operation at a relatively low cost.
- Electric valve actuators tend to have a higher resilience to changes on environmental conditions and can use a far smaller fail-safe system.
- Hydraulic valve actuators offer very precise control compared to the other two, and are generally far more powerful, however this comes with relatively high costs.