Compressor unloader valves provide an important function for air compressors.
Even if you are not sure what an unloader valve is, you- like most of us, have probably heard an unloader valve at work, one time or another.
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, when the air in the compressor air tank reaches the set point and the pressure switch trips the power supply to the motor off, and at the same time, trips the unloader valve to open.
In the photo above you can see that the unloader attached to the side of the pressure switch in a typical DIY air compressor installation.
It is the pressure switch that, when reacting to system pressure, turns the compressor motor on and off, and in so doing, and in many compressors also opens and closes the unloader valve.
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 is increasing the load against which your muscles have to work to effect the sit up.
Compressed air, captured in the cylinder after the compressor shuts off, would increase the load against which the electric motor would have to work. It may increase the load too much, and the motor may fail to start, or it may pull too many amps and fry a fuse or pop a breaker in the panel.
When the compressor shuts off, the unloader valve operates, it unloads the trapped air to atmosphere, and that problem is solved.
Since it is installed in the air line between the compressor and the compressor tank, when the unloader valve opens, the potential is there for all of the air already compressed into the tank to escape out the unloader valve to.
To prevent that from happening there is a check valve installed, typically where the line from the compressor pump head enters the tank, and that check valve is designed to keep the compressed air in the tank in the tank when the unloader valve opens to void air over the piston(s).
If you have air bleeding from the unloader valve continuously, it is a good bet that the 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 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.