The directional control valve or spool valve can be seen in pneumatic or hydraulic systems. Several well-known control valves are available in the industry, such as Globe Valves.
This article discusses different types of control valves known as Directional Control Valves or DCVs. Spool valves and solenoid valves are also common names for them.
Watch this video or read this article to find out how directional control valves work and how their symbols should be read and interpreted.
Directional Control Valves vs. Modulating Control Valves
Most control valves are known for their throttling capabilities and adjustability. DCVs, however, control the flow of liquid within a pipe by controlling the “direction”.
Flow control systems using pneumatic and hydraulic directional valves are both common.
There are two types of directional control valves: pneumatic and hydraulic. Hydraulic directional control valves are sometimes referred to as spool valves.
Directional Control Valve Application Example
Our example is based on a pneumatic valve system since we are learning about the basics of DCVs in this article and there is not much difference in their working principles and symbols. The good news is that if you learn one of them, you can easily learn the other as well.
First, let’s look at a simple pneumatic circuit. An air compressor supplies the compressed air that powers this pneumatic system. This slide gate valve is opened and closed by the directional control valve using its actuator, which is a double-acting cylinder, that directs compressed air in two directions.
In one direction, the cylinder expands and closes the slide gate valve to block the material coming out of the silo, while in the other, the cylinder retracts. Slide gate valves are therefore normally closed valves.
Directional Control Valve Components
The first part of the valve is its body or housing, which contains the paths through which air flows. An integral part of the valve’s body is a series of holes, known as ports.
Within the housing, there’s a moving part that directs air toward different valve ports and blocks others. Spools are commonly used to describe this moving part. An electric solenoid is responsible for moving the spool within the housing.
How Directional Control Valve Works (5/2 Solenoid Valve)
As the DCV spool is in its neutral or rest position, when the solenoid is energized by a PLC card command, the coil will push the spool and squeeze the spring on the other side. Therefore, the piston of the cylinder retracts and moves to the right.
A PLC’s command will remain on the solenoid as long as the piston remains at its last position and the spool stays stationary.
Once the PLC removes the command, the spring will return the spool to its original position and the air path will change.
In this way, the air behind the piston will be expelled into the atmosphere through the exhaust port of the valve. The next step is to learn how to name and read the graphic symbols associated with the directional control valves.
Why do They Call it a 5/2 Directional Control Valve?
This valve is known as a 5 by 2 solenoid valve or a 5 by 2 directional control valve. What is the meaning of 5 by 2?
The first digit represents the number of ports on the valve. A second digit indicates the number of states the spool can be in.
How to Read a Directional Control Valve Symbol
Sometimes it can be challenging to read and interpret directional control valve symbols. As an example, let’s look at the symbol for a pneumatic directional control valve with 5/2 openings. For each state, some arrows illustrate the paths air can take through them.
5 by 2 DCV has 5 parts, and these are the ISO designation of the ports, which is more common practice. There may be an alternative designation of Directional Control Valves by alphabets in which;
- The P stands for the “power” or “pressure” that comes from the air source.
- The exhaust ports are marked Ea and Eb.
- The A and B ports are the actuator’s output and input ports.
- On the schematic of the Directional Control Valve, there are signs for electric solenoids and spring returns.
In our circuit, the square beside the spring symbol is active when the valve is at rest.
The other square becomes active when the solenoid energizes and the spool moves. It is a rule of thumb for any directional control valve schematic that the square next to the solenoid sign becomes active when the solenoid is energized.