Terminology Explained - Control Valves

By Martin Lissenburg, Product Specialist

Wednesday 21 June 2017

Valves are basically simple devices that block or change direction of air in a compressed air system; a direct analogy to electrical switching systems. They can control components or tools directly, or pilot valves can be used to change the position of another valve.


But, the terminology can sometimes be confusing; here, we run through a number of definitions used in specifying and selecting valves.


Valve body and function?

You will see terms such as 2/2, 3/2, etc associated with valves. The first number specifies the number of ports; that is, the number of connections in and out of main part of the body - not including ports in the end caps. The second number identifies the number of positions the valve spool can occupy. So, a 5/3 valve has 5 ports and 3 possible positions for the valve spool.

ISO standards apply

Each type has a unique circuit diagram defined by an ISO Standard. ISO also controls port numbering; as a general guide Port 1 is always supply, even numbered ports are outlets and odd numbered ports are exhausts. The sequence '12' means Port 1 connected to Port 2 and '14' Port 1 to Port 4.

Normally open or closed?

The 'normal' refers to the function of the valve with no external influence acting on it. So, a normally closed (N/C) will block flow in it's rest position; the opposite is true for a normally open (N/O) valve (note, this is the opposite for the electrical switch).

The 'normal' for a 5/3 valve is in the middle position; here, three options are possible. APB means All Ports Blocked - all 5 ports are sealed; COE (Centre Open Exhaust) means the supply is blocked and outlet ports are exhausted. Finally, COP (Centre Open Pressure) means the supply port is connected to both outlets with the exhausts blocked.

The valve operators?

In general, there are three types of operator. Manual, where activation is by a human being, such as push-button or pedal; mechanical, where the action is by machine, such as plunger or roller lever. Finally, there are solenoid and air pilot operators; a solenoid valve is operated by an electric current pulling a magnet into the energised coil, while the air pilot uses air, usually via another valve. Again, the ISO standard covers symbols for all the operators.


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