Compressed air is one of the most costly forms of energy you can use. It is also one of the most versatile, fast and strong types of energy to do work in the factory or workshop.
When it's "quiet time" in the plant or workshop, wander around the machinery and listen.
Air will possibly be leaking from many air fittings, unions, from broken or split polyethylene air lines, and you can often hear the gentle (or perhaps not so gentle) hissing of air escaping from the exhaust port of your air valves.
If your plant doesn't have a quiet or down-time which would enable you to actually hear the leaks as you wander the shop floor, there are commercially available ultra-sonic compressed air leak detectors on the market. Investing in an ultrasonic leak detector can bring substantial payback in energy savings.
These are remarkable devices that will pick up the sound of air leaking from a valve, a fitting, a union, anywhere, even over the sound of equipment running full tilt in the plant.
While an air valve or an air cylinder are doing work, of course there will be air being exhausted continuously from the exhaust ports of the valve.
It's when the machine is shut down, with the supply air to the machine still turned on, that air should not be escaping through the valve exhaust ports. That loss of compressed air is the sound of your money being wasted.
Usually you'll have one air valve connected to each air cylinder, and often that cylinder will be a double acting type - which means that it will have two air lines running to it from the air valve. As the air valve shifts back and forth in normal operation, air will alternately flow to one cylinder port and then the other. When air is flowing through one line to the cylinder, the other line is allowing the air at the other end of the cylinder to flow back through the valve to exhaust.
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 compressed air will start bypassing into the other side. This means that this escaping compressed air now has an open path from the supply side of the piston, and down the other air line to the valve 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 compressed air supply side into the exhaust side of the valve. And that air, if leaking past the valve seals, is pouring your compressed air dollars into the air.
Have a look at your cylinder. If the cylinder rod is out (extended) air will be entering the air port at the rear of the cylinder. If the cylinder rod is in - retracted, the air will be coming into the cylinder port at the rod end.
Locate the air line to the cylinder that is charged, that is, the air line that is supplying air to the cylinder at that time, 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 valves exhaust port, then it has to be the seal(s) in the air cylinder that's letting air by-pass to get to the exhaust port of the valve.
If, after ensuring that the air to the cylinder is completely stopped by crimping or squeezing the air line tightly, compressed air continues to exhaust from either or both of the exhaust ports of the valve, then it is the seal(s) inside the air valve that are letting air get by.
Regardless of which is the culprit, the air valve seals or the cylinder seals, get it fixed....fast! Compressed air costs a bundle. You sure don't want to waste it.
Try to get your plant to be one of those that don't rack up the industry average of 10+% of compressed air lost to wastage. How many dollars would drop to your "bottom line" if you reduced your compressed air electricity cost by 10%?
It's worthwhile chasing down and eliminating those leaks.