What is a Directional Control Valve?

In this article, you’ll learn about the working principles of the directional control valve in pneumatic system.

If you have ever come across a pneumatic or hydraulic system in the industry, you may have seen directional control valves or spool valves. There are a variety of well-known control valves in the industry such as Globe Valves.

But today’s article is about different types of control valves known as Directional Control Valves or DCVs for short. You may hear of them as solenoid valves or spool valves as well.

If you want to know how directional control valves work and how you should read and interpret their symbols, read this article to the end, or watch the following video.

Directional Control Valves vs. Modulating Control Valves

The control valves are mostly known by their adjustability and throttling capabilities.

Modulating Valves

But directional control valves or DCVs are the types that control the “direction” of the liquid flowing inside the pipe.

How Directional Control Valve Controls the Flow

Directional control valves are used both in pneumatic and hydraulic flow control systems.

So there are pneumatic directional control valves and hydraulic directional control valves. Sometimes the hydraulic directional control valves are called spool valves.

Pneumatic Directional Control Valves and Hydraulic Directional Control Valves

Directional Control Valve Application Example

As we’re learning about the basics of the DCVs in this article and there’s not much difference in their working principle and symbols, we develop our example based on a “pneumatic” system. And of course, if you learn one of them you can easily learn the other.

Let’s start with a simple pneumatic circuitry.

The main power of this pneumatic system is coming from the compressed air that is supplied by a compressor.

Directional Control Valve Application Example

The directional control valve is directing the flow of the compressed air in two different directions to open and close this slide gate valve using its actuator which is a double-acting cylinder.

Flow Direction in Directional Control Valves

A double-acting cylinder has an air chamber on each side to move the piston back and forth.

Pneumatic Double Acting Cylinder

One of the directions expands the cylinder and closes the slide gate valve to block the material coming out of the silo, and the other direction retracts the cylinder to its first position.

Therefore, the slide gate valve is normally closed valve.

How Pneumatic Directional Control Valve Works

Now, let’s take a look at the components and simplified mechanism of the directional control valves.

Directional Control Valve Components

First, is the body or housing of the control valve inside of which, there are the paths that air flows through them.

Housing of a Directional Control Valve

There are some machined holes in the body of the valve that is called Ports.

Ports of the Directional Control Valve

Within the housing, there’s a moving part that leads the air toward different ports of the valve and blocks the others. This moving part is commonly referred to as Spool.

The component that causes the spool to move within the housing, is an electric Solenoid.

Directional Control Valve Components

How Directional Control Valve Works (5/2 Solenoid Valve)

When the spool of the DCV is on its neutral or rest position and the solenoid gets energized by a command from the PLC card, the coil will push the spool and thus it will squeeze the spring on the other side.

As a result, the piston of the cylinder retracts and moves to the right side.

Directional Control Valve Working Principle

As long as the PLC’s command remains on the solenoid, the position of the spool will remain still and the compressed air will cause the piston to keep its last position.

As soon as the PLC removes the command, the spring will return the spool to its first position and the air path will be changed.

Therefore, the air behind the piston will be purged into the atmosphere via the valve exhaust port.

How Pneumatic Control Valve Works

Now, it’s time to learn how to name the directional control valves and how to read their graphic symbols.

Why do They Call it a 5/2 Directional Control Valve?

The valve in our example is known as a 5 by 2 solenoid valve or a 5 by 2 directional control valve.

5 2 Directional Control Valve

But why do we call it 5 by 2?

– The first digit is for the number of ports the valve has.

– The second digit is for the number of states the spool can be in.

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How to Read a Directional Control Valve Symbol

Reading and interpreting the directional control valve symbols may seem a little bit challenging sometimes.

For instance, let’s consider a 5/2 pneumatic directional control valve symbol.

5 by 2 Directional Control Valve Symbol

Each square in the symbol represents a position or state.

Directional Control Valve Schematic

In each state, some arrows show the paths that air can flow through them on that specific position.

How to Read a Directional Control Valve Symbol

We learned that a 5 by 2 DCV, has 5 ports and this is the ISO designation of the ports which is a more common practice.

ISO Symbol of Directional Control Valves

But sometimes you may see an alternative designation of the Directional Control Valves by alphabets in which;

– The P is for “power” or “pressure” that comes from the air source.

– The Ea and Eb are for the exhaust ports.

– The A and B are the output ports to/from the actuator.

5/2 Directional Control Valve Schematic

There are the signs for electric Solenoid and Spring Return in the Directional Control Valve schematic.

Pneumatic Directional Control Valve Symbol

In our circuit, when the valve is in its rest position, the square besides the spring symbol is active.

Directional Control Valve Symbol

When the solenoid gets energized and the spool changes position, the other square is active.

Generally, the square just beside the solenoid sign is active when the solenoid is energized and this is a rule of thumb for any directional control valve schematic.

Pneumatic Directional Control Valve Schematic

In the end, keep in mind that there are a variety of other directional control valves with more complex functions in different applications that we can check them in future articles.

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