What is a compressor tank check valve? Your compressor likely has one. What does the compressor tank check valve do for your compressor, and why should you care?
I have referred to the compressor tank check valve on a number of pages on this site. Questions about them still come in regularly, so I thought it time to write a page providing details.
In the photo we see a couple of components with numbers. Number 1 in the photo is this compressors tank fitting, which incorporates the air line from the pump (cannot be seen as it is behind the fitting), an air line from the tank fitting over to the unloader valve (#2), and the compressor tank check valve which extends down into the tank from the fitting.
What Is A Compressor Tank Check Valve?
A check valve is a fitting that allows compressed air to flow through it, but only one way. If the air tries to flow back up through the check valve, the valve flapper or similar device shuts, and air cannot escape.
Compressed air flows into the compressor tank from the pump through the tank check valve fitting. As the tank pressure grows air will try to flow back up out of the tank, but cannot, as the tank check valve keeps it in the tank.
This is important, as you will soon see.
There is another device on the compressor called the unloader valve. The unloader valve is shut while the compressor is running, which prevents air from the pump from escaping from the unloader valve when the compressor is pumping air into the tank. However, when the compressor shuts off on high pressure cut out, the unloader valve opens.
The open unloader valve allows air to flow out of the compressor from over the cylinder in the pump – which is what it is designed to do. With that line now open, what stops the air in the compressor tank from also flowing out of the compressor when the unloader valve opens?
It’s the compressor tank check valve.
Troubleshooting the compressor tank check valve.
Compressing air generates water, and a lot of water can accumulate in the compressor tank, in water form and water vapor form. That water can ruse the tank check valve, making it unable to shut tightly. If that happens, air escapes.
How do you know air is escaping from the compressor tank? Well, you may hear it, see the pressure dropping on the tank gauge, or you might feel a leak of air from the unloader valve when the compressor has shut off. That is the key indicator that the compressor tank check valve is not working.
Shut down the compressor, dump all of the air, pull the compressor tank check valve fitting, and clean the compressor tank check valve. With an operating compressor tank check valve, when the unloader valve opens as the compressor stops, no air but a brief puff should escape from the unloader. If air continues to bleed out, that’s a leaking tank check valve.
Compressor tank check valves are not expensive, so if cleaning a fouled up one is a problem, simply replace it. If you Google “compressor tank check valves” you will find a host of suppliers.
Hi Thanks a lot for the information!
I have two compressors which have check valves & unloader valves as you describe. I would like to connect both compressors together. Would this be OK to connect with an airline/T piece only or is an extra 1 or 2 check valves required?
Edward. You are welcome. Since the gauges on a low cost compressor aren’t very accurate, my sense is 3-5 PSI out of true, it may be difficult to accurately measure a 1 PSI pressure drop in the tank. Specifically though, unless the tank check valve or tank drain leak, air pressure in the tank should stay stable indefinitely. Be sure, however, to drain the tank after your test to allow any moisture in the tank to drain out.
Thank you for your very helpful information. I recently got an oil-less compressor with a 30-liter steel tank, and I would like to check for leaks. How long should the filled tank be able to sit unused without losing 1 PSI?
If I understand correctly, your pressure switch is leaking air through the unloader valve when the compressor is off. Is that correct? If so, you likely don’t have a pressure switch problem, but likely a tank check valve leaking. Have a look at this page: https://air-compressor-help.com/what-is-a-compressor-tank-check-valve/ . If you still need some help, add a comment here.
Hi can you tell me
Why releif valve used in this pressure switch
Actually i want to stop the Motor and its stop on for example 7 bar
But due to releif valve pressure release from this port and motor start at 4.5 bar
So its mean Compressor running every time