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Air compressor check valve: what it does, why it's there and how it works. | Air Compressor Guide
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Air compressor check valve: what it does, why it's there and how it works.

Have a better understanding of air compressor check valves with this informative article.

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.

image placeholder 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! ;) ) Blow-down simply means that any air pressure inside this pipe is blown-down to 0 bar pressure (this to help the compressor on start-up). The blow-down valve is often integrated in the pressure-switch.

The check valves which is installed between this pipe and the air tank makes sure that only the small pipe is blown-down and not the whole air tank.

Actually, this is an often seen problem on many compressors. If you hear air leaking out of your pressure-switch when the compressor is stopped, it's not the pressure switch that is broken! It is the check valve that is leaking air!

On rotary screw compressors

The air compressor check valve is also used in rotary screw compressors. It depends on the make and model of your compressor how many check valves are used, and what they look like.

Inlet check valve

There a check valve integrated in the inlet (unloading) valve of screw compressors.

This check valve is there for when the compressor stops. The compressed air that is still inside the compressor is trying to find a way out when the compressor stops.

The inlet check valve is there to make sure that the compressed air (and oil!) cannot flow back out through the inlet filter.

Of course, also this check valve can break down or get stuck and I saw on several occasions the result of this: lots of oil coming out of the air inlet filter!

It's like the compressor is 'vomiting'.. It's not a pretty sight, and gives a lot of mess to clean up!

Element outlet check valve

There's often another check valve installed just below the compressor element. This one makes sure that once the compressed air/oil mixture cannot flow back into the compressor element when the compressor stops.

And, there is the check valve on the outlet of the air compressor. Any air that is produced by the compressor is not allowed to flow back into the compressor. It should go only one way: to the place where you need compressed air.

The air compressed check valves make sure that this does not happen.

How it works

Air compressor check valves are actually pretty simple parts. Although they can take many shapes and forms, the principle is mostly the same.

Inside the air compressor check valve, you will find a disc or ball (ball mostly on smaller check-valves) and spring that pushes down on the disc or ball.

When air flow in the 'allowed direction', it pushes against the disc or ball. Since the spring is not very strong, the compressed air will move the disc or ball open. Result: the air can flow through the check valve.

Now, when the air tries to flow from the opposite direction, it pushes against the disc or ball from the same side as the spring does. The spring and the compressed air work together! They push the disc/ball into its seating and no air can pass by it.

In fact, the higher the pressure, the harder it pushes the disc or ball into its seating, and the less likely it is that there will be any air leakage.

Sometimes air compressor check valves leak air at lower pressure and stop leaking air at higher pressure. While this may seem strange, it is easily understood when you know the inner workings of the check valve!