Leaks can be a significant source of wasted energy in an industrial compressed air system.
If compressed air were hydraulic fluid, leaks would be so visible that we would ensure their reduction. As it is, we accept a low level hiss in our work places as simply a ‘part of the job.’ At a price that is roughly comparable to that of domestic gas, this attitude costs industry dearly.
It is estimated by the U.S. Department of Energy that industrial energy usage is $1.5 billion per year; 20% to 30% of this amount is lost in air leaks. In addition to being a source of wasted energy, leaks can also contribute to other operating losses. Leaks cause pressure loss in systems, which can mean pressure is too low to the application leading to more reject product. Frequently the generation capacity is increased to compensate, rather than simply fixing the leaks.
Where to find leaks
Pipework—Aging pipework is a prime source of leaks. Replace any corroded pipework sections—for safety as well as energy saving.
Fittings, flanges and manifolds—Large leaks are often found at connection points, both in the main distribution system and in off takes. Sometimes when several snap connectors are used together to form manifolds they can be a source of leakage due to worn connectors and poorly jointed pipe work.
Flexible hoses and couplings—Leaks can be caused by damage to hose due to abrasion by surrounding objects, deterioration of the hose material and strain on the joint because the hose is too long or too short.
Old components not maintained; seals start to leak—Check all pneumatic components, e.g. old cylinders and regulators, for worn internal air seals which can cause large leaks.
Condensate drainage valves—Large amounts of air can be lost when drain valves are stuck open or even left open intentionally. These can often be found in remote parts of the system where condensate collects.
Systems left pressured when not in use—Where subsystems have a large amount of leakage, which cannot be avoided, e.g. presses and drop hammers, isolate them from the air supply when not in use. Simple shut-off valves or electrically operated soft start dump valves offer cost effective ways to isolate leaky systems, or areas of a plant when not in use.
Measuring the leakage
You can measure the base leakage easily using one of several methods. Install a flowmeter and pressure transducer in the compressed air feeding main (after any receivers). Connect the output of the flowmeter and the pressure transducer to a chart recorder and take readings over a representative period of time. Measure the flow from the compressor when the system is not working, e.g. at a weekend.
Use a compressor of known capacity to pump the system up to normal operating pressure during non-production hours. The compressor will unload at the operating pressure. As the system pressure drops due to leakage the compressor will load at its minimum running pressure. You can then estimate the leakage rate from the average loaded and unloaded times over a representative period.
Pump the system up to pressure and measure the time taken for the pressure to decay to the lower limit. If you know the total volume of the piping network and the receivers, you can calculate the leakage rate. Use a small flowmeter in branch lines to identify real problem areas.
