Somewhere in the garage, the workshop or the plant, the "kathumping" of a reciprocating air compressor echoes throughout. Suddenly the thumping stops, and there's an audible... "psssssssssssschhhhhht", clearly the sound of air escaping, but just for a second or so.
That is the unloader valve, hard at work!
It is often part of the pressure switch assembly. It will, in many cases, be opened by the pressure switch as the switch operates.
When the air in the compressor air tank reaches the cut out set point and the pressure switch trips the power supply to the motor to off at the same time, that movement toggles the unloader valve to open.
In the photo the unloader valve is attached to the side of the pressure switch, in what is a typical DIY air compressor installation.
Different manufacturers have different methods of tripping their unloader valves. The purpose of the typical unloader valve, regardless of its shape or location on the compressor or whether it is mechanically operated or electronically fired, is to unload the air trapped over a compressor piston.
It is the pressure switch then, when reacting to system pressure, turns the compressor motor on and off and and in many compressors opens and closes the unloader valve at the same time.
For example, the last time you did a sit-up, you may have folded your arms over your chest, or if you are particularly masochistic even held a weight to your chest. What you were doing by crossing your arms or holding a weight is increasing the load against which your muscles have to work to accomplish the sit up.
In the same fashion, compressed air, captured in the cylinder over the piston after the compressor shuts off, would increase the load against which the electric motor would have to work. If the load on the motor increases too much the compressor motor may fail to start, or it may pull too many amps and fry a fuse or pop a breaker.
When the compressor shuts off the typical unloader valve opens and unloads the trapped air to atmosphere, and that problem is solved.
Sometimes though, the air evacuating from the unloader valve does not stop coming out. The unloader valve continues leaking air after the compressor is stopped. Why is the unloader valve leaking?
Since the unloader valve is typically installed in an air line running between the compressor pump head and the compressor tank, when the unloader valve opens, all of the air already compressed into the tank could escape out the unloader valve too.
To prevent that from happening there is a tank check valve installed, typically in the fitting where the line from the compressor pump head enters the compressor tank. That check valve is designed to keep the compressed air in the tank when the unloader valve opens to void air over the piston.
If you have air bleeding from the unloader valve continuously, it is a good bet that the tank check valve has either failed or has not seated properly. The compressed air in the tank is bleeding back up out of the tank, down to the unloader valve, and out to atmosphere. The air will continue to bleed out until the tank pressure reaches the cut in pressure setting on the pressure switch, and then the air compressor will start to pump up the pressure in the tank again. This cycle will continue until the check valve has been repaired or replaced.