One of the best ways to improve the efficiency of a compressed air system is to eliminate compressed air waste caused by leakage. How much air are you leaking? To answer this question, you must measure your leakage flow or do some sort of inspection that will allow you to develop a price tag for the cost of the waste. Here are 6 simple ways to do it:
Flow Meter – A simple way of measuring leaks is to use a properly-installed flow meter. There are many choices for these devices at prices so low there is no excuse for not having one. Pick a time where there is no production in the plant and see what flow is being detected. O, odds are that this flow is mostly leaks, and it is over 30% of all the average air flow produced by your compressors.
Compressor Duty Cycle – If you don’t have a flow meter, and your compressors run in load/unload mode, you can estimate the leakage flow by timing both the load and unload cycle during a time when there is no production. The loaded time divided by the total cycle time of the compressor (loaded plus unloaded) gives you a fraction you can use to roughly calculate the leakage flow. If you tTake this fraction and multiply it by the cfm rating of the running compressor, this will tell you how much air is flowing. For eExample, for a 100 cfm compressor with 1 minute loaded time and 4 minutes unloaded, the duty cycle is 1/(1+4) = 0.2, for an . So estimated flow ofis 20 cfm.
Compressor rpm – If your running compressor is a VSD style, and it doesn’t start and stop during non-production times, a rough estimate of the leakage flow can be done by reading the rpm during leakage and dividing it by rpm at full flow. The result is a fraction that can be multiplied by full load cfm of the compressor for the same result as the previous calculation. For eExample, for a 100 cfm compressor running at 2,000 rpm with full load rpm of 4,000, the f. Fractional load would be 2000/4000 = 0.5. Apparent leakage flow then would be 100 cfm x 0.5 = 50 cfm.
Drawdown Test -– If your compressors only run in modulation mode then a special drawdown test needs to be done during non-production. For this test, the total volume of the system storage capacity needs to be calculated by doing an inventory of the connected storage receivers and large piping. Once the volume in cubic feet is known, it is somewhat easy to calculate the flow during non-production by recording the time it takes for the pressure decay to 50 percent pressure when the compressor is turned off, and then plugging it into a special formula. The procedure and formula for this test can be found here: https://www.compressedairchallenge.org/data/sites/1/media/library/factsheets/factsheet07.pdf
Ultrasonic Leakage Tests – Some plants never have a period of non-production. If so, then the only option is to do a thorough leakage survey using an ultrasonic detector. The cost of these devices has come down over the years, with some costing less than $1,000;, therefore, there is really no excuse for not having one in-plant. Most leak detectors come with a smartphone app that allows you to estimate the leakage flow based on the db level of the sound and create a leakage report complete with photographs and descriptions. It is very common for a leak detector effort to pay for itself with only a few hours effort.
About the Author: Ron Marshall
