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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.

Reducing air leaks in your plant can save thousands of dollars annually. Compressed air is one of the most costly forms of energy you can use in your plant, of course, it's one of the most versatile, fast and strong too.

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.