A recent question received by email: I need some help from you regarding air leak calculation. We just purchased a new air leak detector that calculates leakage flow. We have high air leakage in our LPG bottling plant and our leak detector showed a flow of 9130.2 L/S. If we convert this to CFM then it will total of 19,345 CFM flow in our plant. But we cannot understand how our 566 CFM compressor can generate this level of flow!
Answer: That is very interesting, and quite common actually — the leak detector estimate is obviously way off.
If you are calculating the flow using the db reading and a lookup table (or a calculator within the flow meter), the estimates are incorrect. Perhaps you are using an incorrect setting or constant. Realize that using an ultrasonic flow meter for leakage flow calculations is a guess at best! We find in industrial leak detection that the typical average is about 1 cfm per found leak.
It is impossible to have leakage higher than the flow of your air compressor. If the leak flow was actually that high, you would have no air pressure.
If you have a main flow meter, then check the reading of the meter during a time where the plant is shut down for production and only leaks are flowing. If you have no flow meter, then use the test methods in this document to assess the total leakage flow:
compressedairchallenge.org/data/sites/1/media/library/factsheets/factsheet07.pdf
This calculation can tell you what percentage of your total compressor capacity is leakage.
A small portable flow meter mounted on a pipe can be used to test leaks if you want an accurate estimate. You can often isolate the leak and feed it with the air passing through the flow meter. There is also the “bag method’ of estimating, where you use a plastic bag of known volume and measure the time it takes to fill up the bag. There is a calculator for this method within the USDOE MEASUR program, compressed air section located here for download: www.energy.gov/eere/amo/measur
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