Air compressor unloader valve

Your reciprocating piston compressor will have a smooth start up, thanks to the unloader valve.

The air compressor unloader is valve is a small, but important, part of the reciprocating piston compressor. It blows off the pressure inside the discharge pipe when the compressor stops.

(There is another type of unloader valve, which can be found at the inlet of rotary screw compressors. If you where looking for this valve, clik here.)

How does it work and why is it there?

image placeholder Air compressor unloader valve.

The output of the piston compressor is connected through a pipe to the air receiver. When the compressor stops, the pressure in this pipe is blown off to 0 bar (atmosphere).

This is the ssssssshhh sound you will here when the compressor stops. At the same time, a check-valve makes sure that not the whole air receiver (tank) flows empty, but only this small pipe.

Why? When the compressor starts up again, and this pipe would still be under pressure, the electro motor will not be strong enough to turn the compressor.

Where to find the unloader valve

On smaller units, this valve is often mounted on, or inside, the pressure switch. When the pressure switch switches off the compressor, the unloader valve is also actuated.

unloader valve piston compressor Small modern-type unloader valve, using a
solenoid valve. Photo: Atlas Copco

The check valve (which prevents the whole tank from draning) is often located right at the point where the compressor discharge pipe is connected to the storage tank. From this check valve, a small tube or pipe runs to the unloader valve on the pressure switch.

On newer compressors, a simple solenoid valve is sometimes used to blow down the pressure.

On bigger compressors, there is often a big unloading valve, which is controlled by air pressure from a small pilot valve. In this case, the unloader sits somewhere besides the compressor.

image placeholder Big unloader valve (left) which is controlled by a small pilot unloading valve (right, below the pressure switch). On the left side of the big valve we see the compressor output (top) and the connection to the air tank (bottom)