Unloadervalve

Even if you are not sure what it is, you have, as have all of us, 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, for just a couple of seconds.

That's the unloader valve, hard at work!

Plumbed into the lines between the compressor and the receiver , as part of the pressure switch assembly in most cases, the unloader valve will open when the air in the receiver reaches the set point.

Compressed air that's contained within the compressor assembly will be evacuated to atmosphere to prevent that pressure from affecting the start up of the compressor motor when the pressure switch again calls for air.

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.

The unloader valve operates, "dumps" that air to atmosphere, and that problem is solved.

When The Hissing Continues

Since it's in the air line between the compressor and the compressor receiver, there's a check valve between the unloader valve and the reservoir to stop the already compressed air from flowing back out when the unloader valve operates. If you have air bleeding from the unloader valve continuously, it's a good bet that the check valve had either failed or hasn't seated properly, and air is bleeding back up the line to atmosphere.

Here's more info on a leaking pressure switch which is disguising a check valve problem.