When it's "quiet time" in the plant, wander around the machinery and listen. You will often hear the gentle (or perhaps not so gentle) hissing of air escaping from the exhaust port of your air valves.
The sound of compressed air "chewing up your dollars" as it wafts to atmosphere can be muted if your air valves have mufflers in the exhaust ports, but nevertheless, it can be heard.
Also, there are commercially available ultra-sonic compressed air leak detectors on the market. If your plant doesn't have a "quiet time", which would enable you to actually hear the leaks yourself, investing in an ultrasonic leak detector can bring substantial payback in energy savings.
Usually you'll have one air valve connected to one air cylinder. Usually that cylinder will be double acting - which means that it will have two air lines running to it, and as the air valve shifts back and forth, air will alternatively flow to the cylinder through one line or the other. When it's flowing into one line to the cylinder, the other line is allowing the air at the other end of the cylinder to flow through the valve to exhaust.
While an air valve and cylinder are doing work of course there will be air being exhausted continuously from the air valve exhaust ports.
It's when the machine is down, when it's doing no useful - and hopefully money generating work for you - that air should not be escaping through the valve exhaust ports. At this point that loss of compressed air is just that; loss - of profits - of money.
Inside, the two ends of the cylinder are separated by a piston. The piston is what drives the rod out and back as the cylinder cycles.
Around that piston will be an air seal that "crunches" between the side of the piston and the inside of the cylinder barrel, effectively stopping air from flowing by (bypassing) the piston.
In time that seal will wear, and air will start bypassing into the other side. This means that this air now has an open path from the supply side down the other air line to the valve, and thence to the exhaust port. And a gentle (or not so gentle) hiss occurs as your compressed air dollars exhaust to atmosphere.
Or....inside your air valve there is, too, a series of seals that normally prevent air from getting from the air supply side into the exhaust side of the valve, and then out the exhaust port. And that air, as it gently (or not so....etc. ) is pouring your compressed air dollars from the plant air supply.
So, which is it that's leaking; the seal around the piston in the cylinder, or the seal inside the valve that stops the incoming air from getting across to the exhaust port without going up to the cylinder?
Have a look at the cylinder. If the rod is out, air will be entering the air port at the rear of the cylinder. If the cylinder is in - retracted, the air will be coming into the cylinder at the rod end.
Take the air line that is charged, that is, the air line that is supplying air to the cylinder, and crimp it. Many air lines are made of polyethylene or polypropylene, and it's quite easy to make a bit of a bend in the air line, effectively shutting off air to the cylinder.
Listen at the valve. If the air has stopped escaping the valve's exhaust port, then it's the seal in the cylinder that's at fault.
If, after ensuring that the air to the cylinder is completely stopped, air continues to exhaust from the exhaust port of the valve, then it's the seal inside the air valve that's at fault.
Regardless of which is the culprit, the air valve or the cylinder, get it fixed....fast! Compressed air costs a bundle. You don't want to waste it.
