Better understanding of the air compressor check valve

The check valve is an important part of any air compressor. The air compressor check valve can be found on almost any compressor out there.Check valves are used for example on reciprocating piston compressors, rotary screw compressor and scroll compressors. On many compressors there is even more than one check valve.

What it does

An air compressor check valve has only one function: to let air flow from one side to the other, while blocking air flow in the opposite direction.

If you hold the check valve in your hand and would blow on it on one side, you have no problem blowing air through it. But, if you turn the check valve around, it is impossible to blow any air through it.

The same thing happens inside the compressor or compressed air system. Once the compressed air passes the check valve, there is no way it can go back again.

Why it’s there

There are so many reasons why air compressor check valves are used on air compressors. And, there are sometimes up to 5 check valves on a single compressor. It all depends on the compressor type, size and make/model of course.

As said before, check valves allow the air to flow through in only one direction. With the help of check valves, the compressor can keep certain parts pressurized and other parts de-pressurized.

Check valves also prevent that compressed air that has left the compressor (to the piping / air tank) cannot flow back anymore, back into the compressor.

Where on the air compressor can we find check valves?

As said, it depends on the type of compressor, the size and the make/model of the compressor.

Here’s an overview of some places where a check valves is used, and why!

On reciprocating piston compressors

On a piston compressors (you know, the ones with the piston that moves back- and forth), you will find check valves in several places.

First, there are 2 check valves, or sets of check valves used inside the piston head to control the airflow from and to the cylinder.

The air is sucked in through the inlet (check-) valve when the piston moves down. When the piston moves up again, the air is compressed and flows out of the cylinder through the second check valve: the outlet valves.

Now, what would happen if there wasn’t a check valve on the inlet of the compressor? Air will follow the path of least resistance, which happens to be: back out through the compressor inlet! Not what we want of course. The check valves make sure that once air has been sucked into the cylinder, it is ‘trapped’ and cannot go back anymore.

That’s already two check valves on your piston compressor! But there’s one more!

Where? Between the compressor and the compressed air receiver (the compressed air tank that the compressor is often mounted on).

This air compressor check valves makes sure that once air has been pumped into the air tank, it cannot flow back to the compressor when it shuts down.

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Small check valve on air receiver
(top part). Photo: Atlas Copco

On shut down, the pipe between the reciprocating piston compressor and the air tank (the ‘discharge pipe’) is ‘blown down’ by the blow-down valve (what else!