Air compressor relief valve: what it does and how to test it
What it does
Normally the pressure switch stops the motor at cut-out pressure. If the switch sticks, the pump keeps running and pressure keeps climbing. The relief valve is set above cut-out and below the tank's maximum rating; when pressure reaches its set point, it pops open and vents.
If your relief valve is releasing air
- It opens while the compressor is running at or above cut-out: the pressure switch likely isn't stopping the motor. Switch off immediately and have the switch checked.
- It leaks below cut-out pressure: the valve is worn or dirty. Replace it with one of the identical pressure rating.
How to test it safely
Many manufacturers recommend testing the relief valve periodically. With the tank pressurized and eye protection on, pull the ring briefly. Air should vent and the valve should reseat when released. If it doesn't vent, or won't stop leaking, replace it. The how-to section below lists the steps.
Never plug a relief valve, replace it with a higher-rated one, or tamper with it to get more pressure. If a compressor needs more pressure, it's the wrong compressor. A tank with rust, dents or leaking welds must be taken out of service. Have a qualified technician inspect it.
Choosing a replacement
A replacement safety valve must match the original's set pressure, thread size and flow capacity, and should be ASME-rated where the original was. The set pressure must be no higher than the tank's maximum working pressure stamped on its data plate. If you're unsure, use the part number from the manual or ask the manufacturer.
Relief valve vs pressure switch vs regulator
- Pressure switch: turns the motor on and off at normal cut-in and cut-out pressures.
- Regulator: lowers the pressure sent to your tool.
- Relief (safety) valve: a last-resort vent if pressure rises above the safe limit. It should never open in normal use.
Tank inspection
The relief valve protects against over-pressure, not corrosion. Drain the tank regularly and look for rust, dents and weeping welds. A damaged or heavily rusted tank should be taken out of service. Never weld or patch a pressure vessel yourself.
How to test an air compressor safety relief valve
Confirm the safety valve opens and reseats correctly.
You'll need: Safety glasses, Hearing protection.
- Pressurize the tank. Run the compressor until it reaches cut-out pressure and stops.
- Protect yourself. Put on safety glasses and stand to the side of the valve outlet.
- Pull the ring. Pull the relief valve ring briefly. Air should vent forcefully.
- Release and check. Let go. The valve should snap closed and stop leaking. If it doesn't vent or keeps leaking, unplug the compressor, drain the tank and replace the valve with one of the same pressure rating.
Frequently asked questions
Why is my compressor's relief valve popping off?
Usually the pressure switch isn't shutting the motor off at cut-out, so pressure climbs to the relief valve's set point. Stop using the compressor and have the switch checked. A valve that leaks below cut-out is worn and needs replacing.
Can I adjust the relief valve to get more pressure?
No. The relief valve protects the tank from rupturing. Never adjust, plug or replace it with a higher rating.
How often should I test my compressor's relief valve?
Follow your manual. Many manufacturers recommend a quick ring-pull test regularly, for example monthly or before heavy use, with eye protection and the tank at normal pressure.
Is a leaking relief valve dangerous?
A valve that leaks below cut-out is worn and should be replaced. One that opens at or above cut-out is doing its job and points to a pressure switch problem; stop using the compressor until it's fixed.