Pneumatic Valves and Their Types

What Is a Pneumatic Valve?

The components that allow or block pressurized air produced by the pressure source from reaching the actuators, in the desired direction, pressure, speed, and timing, under specific logical conditions, are called pneumatic valves.

Valve Naming

Pneumatic valves are named by their number of ports and positions; for example, a “5/2” valve means 5 ports and 2 positions.

Operating Pressure

Standard industrial pneumatic valves generally have an operating pressure range of 2-10 bar.

Spring-Return Valves

Valves used with single-acting cylinders automatically return to their initial position via a spring when the control signal is removed.

Types of Pneumatic Valves

Based on their intended use, the types of pneumatic valves are as follows:

Directional control valves
Flow control (shut-off) valves
Pressure control valves
Time-controlled valves (time relay)
Logic valves
Quick exhaust valves

1- Directional Control Valves

Pneumatic directional control valves are valves used to permit, block, or change the flow direction of pressurized air, as needed. We can compare these to the switch/points system on railway tracks.

Video of a switch/points system on railway tracks

Video of piston motion control with a directional control valve

Railway switches obviously don’t have a function of blocking the train’s passage, but they can change the train’s direction of travel. This direction-changing operation can be controlled by hand, as well as by electrical signals. We can also think of pressurized air circulating in a pneumatic system as a train moving along tracks.

2- Flow Control (Shut-Off) Valves

Check Valve

One of the flow control valves is the check valve. Check valves are valves that allow pressurized air to pass in one direction. Flow in the other direction is blocked.

Operation of a check valve

The operating systems of check valves resemble the valves that allow one-way air entry into vehicle tires. When we stop pumping air into a tire, we see that the air doesn’t come back out — the valve only allows one-way flow. Check valves work with a similar system.

Various types of pneumatic check valves (above)

Flow Throttle Valve

In pneumatic circuits, it’s used to adjust the speed of actuators such as cylinders and motors, in both directions. Turning the adjustment screw enlarges or shrinks the cross-section through which the air flows. This affects the amount of air passing through.

operation of a pneumatic flow throttle valve

Uses of Throttle Valves: They’re used to control cylinder speeds. However, the throttling happens both on the intake and the outlet side. If we consider that cylinders generally only do work in one direction, this can be a disadvantage.

Adjustable Flow Throttle Valve with Check Valve

Used in pneumatic circuits to adjust the speed of actuators in one direction only. It lets air pass freely in one direction and throttled in the other. The check valve and the flow throttle valve are integrated into a single body. It’s used to shorten what’s called the “dead time” — the portion of a cylinder’s stroke where it doesn’t do useful work.

Check valve, flow throttle valve, and their combined symbols

3- Pressure Control Valves

Pressure Relief (Safety) Valve

Safety valves are used in pneumatic systems to protect circuit components from high pressures.

For example, they can be used on air receivers/tanks.

These valves stay closed at the pressure considered normal for the system, and open to vent air to atmosphere whenever the pressure rises for any reason.

When the pressure drops, the valve returns to the closed position.

safety valve internal structure and symbol — safety valve, its internal structure and symbol

Pressure Reducing (Regulating) Valve

In a system, more than one cylinder or pneumatic motor may need to operate at different pressures. In such cases, it’s used to reduce a high pressure down to a specific lower pressure. In other words, it reduces the inlet pressure to a desired, adjustable outlet pressure.

Pressure regulating valves are used quite often in control problems. For example, when clamping a package containing fragile parts, if a certain force must not be exceeded.

The gauge shows the outlet pressure (P2). The outlet pressure is increased or decreased using the adjustment screw at the bottom.

Pressure Sequence Valve

Used in pneumatic circuits to save time and to bring specific components online once a preset pressure value is reached.

Depending on the pressure value of an input signal (Z), flow is allowed from “P” to “A” in the directional control valve. This way, another operation or step is permitted based on pressure. The required pressure value can be adjusted. These are used quite frequently in pneumatic controls. For example, if a drilling operation needs to start only after sufficient clamping force has been reached.

Pressure sequence valve and its symbolic representation

4- Pneumatic Time Relay

Used to delay pneumatic signals. The time relay’s job is to respond to a start signal after a set amount of time. The response time can be adjusted.

A time relay consists of three components. These are a 3/2 directional control valve, an air reservoir, and a flow throttle valve with a check valve.

We can build a pneumatic time relay ourselves. We can get a ready-made 3/2 directional control valve and a flow control valve with check valve, and use a long or large-diameter hose as the air reservoir.

pneumatic time relay / time delay valve — Normally-open and normally-closed time relay symbol

The adjustment screw on the flow control valve is used to set the time. Precision varies depending on the cleanliness and pressure of the air used.

5- Logic Valves

OR Valve

There are two inputs and one output. If pressurized air arrives at either or both of the inputs, an output signal is produced.

Where it’s used: Used in applications requiring two or more controls, or where “OR” logic is needed. For example, if a cylinder needs to be controlled from two separate locations.

In the example shown in the figure, a cylinder is moved either by a pedal or by a button, using an OR valve.

OR valve cylinder piston 3/2 directional control valve control circuit diagram — Circuit diagram of an example use of an OR valve

AND Valve

There are three connection ports, two for input signals and one for output. When both inputs receive a “1” signal at the same time, a “1” signal is produced at the output.

Where it’s used: In interlocking and safety requirements, especially for occupational safety, on machines that must not operate unless two buttons are pressed at the same time.

The circuit shown in the figure belongs to a pneumatic press. To prevent a worker from leaving their hand under the press, the circuit is operated with two valves. If only one of the valves is triggered, the press won’t operate. Both buttons need to be pressed for the press to work. One button is placed to the worker’s left, the other to their right.

Circuit diagram of controlling a cylinder with an AND valve

6- Quick Exhaust Valves

Its primary job is to vent the air inside a cylinder to atmosphere as quickly as possible, thereby increasing cylinder speed, but it also allows smaller directional valves and longer control lines to be used.

Where it’s used: Especially used on large-volume cylinders to increase speed and eliminate the effect of various restrictions along the hoses. It should be mounted as close to the cylinder as possible.

In a typical application, an exhaust valve is fitted to the inlet of a spring-return or double-acting pneumatic cylinder. Supply air coming from a control valve is directed to the exhaust valve’s inlet port.