# 3/2-Way Pneumatic Valve - How They Work

Figure 1: A 3/2 way pneumatic valve

A 3/2-way valve has three ports and two positions that can be driven pneumatically, mechanically, manually or electrically via a solenoid valve. They are used, for example, to control a single-action cylinder, driving pneumatic actuators, blow-off, pressure release and vacuum applications. A valve is used to fill the cylinder, and also to exhaust it afterwards, so that a new working stroke can be realized. Therefore, a valve with two ports would not be adequate. Venting requires a third port. There are two kinds of 3/2 valves: mono-stable and bi-stable. Mono-stable 3/2-way valves can also be normally closed or normally open, just like 2/2-way valves.

## Circuit function of 3 way air valves

The 3/2-way pneumatic valve has three connection ports and two states. The three ports are:

• inlet (P, 1),
• outlet (A, 2)
• exhaust (R, 3)

The two states of the valve are open and closed. When the valve is open, air flows from the inlet (P, 1) to the outlet (A, 2). When the valve is closed, air flows from the outlet (A, 2) to the exhaust (R, 3). A valve that is closed in non-actuated state is normally closed (N.C.), the opposite is called normally open (N.O.).

Figure 2: Circuit function of a mono stable, normally closed, 3/2-way valve

Most valves are mono-stable and return to their default position when not actuated, this is achieved with a spring mechanism. Bi-stable 3/2-way valves retain their position during power loss, and require a separate action to switch the valve state. Therefore, they cannot be designated as Normally Closed or Normally Open. Bi-stable pneumatic solenoid valves typically have a coil at each position and are pulse operated. Summarized, the different functions of the 3/2-way valve are:

• 3/2-way mono-stable NC
• 3/2-way mono-stable NO
• 3/2-way bi-stable

The circuit functions can be shown with valve symbols. For the three above mentioned functions, the symbol of an indirect operated solenoid valve is shown below. You can find detailed information about other pneumatic valve symbols and their explanation in our valve symbol article.

Figure 3: Symbols of 3/2-way pneumatic solenoid valves, from left to right: normally open, mono stable (left), normally closed, mono stable (centre), bi-stable (right).

3/2-way valves can be actuated by different means such as:

• pneumatically
• manually
• mechanically
• electrically (solenoid valve)

Furthermore, the valves can be direct operated or indirect operated. With indirect operation, the valve uses the inlet pressure to help switching the valve state.

## Design

3/2-way valves are available in several designs. The sealing mechanism of the valves can be a poppet or a spool. The valve’s main parts are the following: housing, seals, poppet (or spool) and an actuator

With direct operated valves, the spool or poppet is moved directly by the actuator. Several types of actuators are possible:

• Solenoid (coil)
• Push button
• Lever
• Foot pedal, etc.

The valve closes or opens by moving the spool or poppet. Mono-stable valves return to their default position, which is accomplished by spring force. In case of indirect operation, the spool is not directly actuated by the solenoid.The valve uses the system pressure to move the spool. In order to do this an additional pilot valve is used. This pilot valve is a small direct operated 3/2-way valve. The pilot valve delivers compressed air to a small air cylinder inside the valve. The compressed air in this cylinder pushes against the piston and actuates the solenoid to switch the valve. This way, a relatively small solenoid can be used to switch the valve.Mono-stable valves are built with one coil, bi-stable valves with two coils. With a NAMUR housing (housing with standardized footprint), the valve can be fixed directly to an actuator which also meets the NAMUR standard. Manifolds can be used to save space and to group valves. Not only can several 3/2-way valves be built up into one manifold, there is a possibility to mix valves. For example, you can mount a 5/2-way valve next to a 3/2-way valve. The possible combinations depend on the type and design of the manifold.

You always have to keep the environment of a pneumatic system in mind. When there are aggressive substances in the environment, the valve body and the seals have to be resistant against corrosion. In clean room areas, ATEX environments and the food industry special valves are applicable.

A 5/2-way valve can be used as a 3/2-way valve by only using only one inlet and the corresponding outlet port. With two 2/2-way valves the function of a 3/2-way valve can be mimicked.

## Typical applications

3/2-Way valves are suitable for several tasks: driving pneumatic actuators, blow-off, pressure release and vacuum applications.

### Controlling a single acting cylinder

Operating a single acting cylinder is a typical application of 3/2-way valves. A single acting cylinder has one pneumatic port to fill and empty the air chamber. The cylinder moves in one direction by filling the air chamber, and returns by spring force. The 3/2-way valve either fills the air chamber or vents the chamber to the atmosphere. A basic pneumatic circuit for a single acting cylinder can found on the picture below.

Figure 4: Schematic representation of a single-acting cylinder drive with a 3/2-way valve

### Controlling a double acting cylinder

A double acting cylinder has two air chambers. The cylinder is moved by filling one air chamber while venting the other. Each chamber has its own connection port, so in most cases a 5/2-way valve is used to operate a double acting cylinder. However, it can also be done with two 3/2-way valves that each connect to a port of the cylinder. One valve drives the piston rod to the extended position (a1), the other valve drives the piston back to its initial position (a0) (see the picture below). One advantage of these circuits is that two different pressures can be applied to the cylinder ports, without installing a pressure regulator between the valve and the cylinder. Another advantage is that the two air chambers can be vented simultaneously, resulting in free movement of the piston rod. with a 5/2-way valve this is not possible.

To drive the piston with two NC valves from position a0 to a1, one valve needs to be energized (switched on, 1) while the other needs to be de-energized (switched off, 0). The valve that is switched off lets compressed air exhaust at the port (R,3). Therefore, the piston moves to the required direction. More states are given in the tables below.

At least one of the two valves should be in the "exhaust" position, to not pressurize both cylinder ports at the same time. When both cylinder ports are pressurized, the piston movement depends on the previous state of the piston, the amount of pressure, the cylinder type, etc.

#### One NC and one NO pneumatic solenoid valve controlling a double acting cylinder

 NC Valve ’left’ NO Valve ’right’ Piston position 0 0 a0 1 0 No stable state 0 1 Free movement between a0-a1 1 1 a1

Figure 5: double-acting cylinder drive with 1 NC and one NO valve

#### Two NC pneumatic solenoid valves controlling a double acting cylinder

 NC valve ’left’ NC valve ’right’ Piston position 0 0 Free movement between a0-a1 1 0 a1 0 1 a0 1 1 No stable state

In the tables above the figures mean the following:

• 0 = valve not actuated
• 1 = valve actuated

Figure 6: Pneumatic double acting cylinder with two NC valves

### Blow off, pressure release, and vacuum applications

3/2-way valves are suitable for blow off, pressure release and vacuum applications. In most cases externally piloted or direct actuated valves are necessary because they do not require a minimum pressure differential. Take a look at the circuits below for blow-off, pressure release and vacuum valves. In the vacuum circuit the vacuum pump is connected to port (P,1), the atmospheric pressure is connected to port (R,3). The vacuum will be broken when the port (A,2) is connected with port (R,3). The vacuum pad is picking up the object, when the vacuum pump (P (1) port) is switched on.

Figure 7: Externally piloted blow-off 3/2-way valve (left), externally piloted pressure relief valve (right)

Valve symbol of a 3/2-way valve used for a vacuum application: Vacuum filter (A), vacuum pad (B), filter with separator, manual drain (C), vacuum pump

## FAQ

### What is a 3/2 way valve?

3/2-way valve have three connection ports and two positions that can be driven via a solenoid valve. They are used to control a single-action cylinder, for driving pneumatic actuators, used as a blow-off valve, as pressure release valve and in vacuum applications.

### How does a 3 way 2 position valve work?

A 3-way 2 position valve has 3 ports and two position and can be either normally closed, or normally open. A normally closed valve will open when powered and close through spring tension. A normally open valve will close when powered and open again through spring tension. The third port is typically used for venting.

### What is the difference between a 2 way and 3 way valve?

A 2-way valve is any type of valve with two ports: an inlet and an outlet port. A 3-way valve has three ports within the valve body which are used as inlet, outlet, and exhaust.