Air leaks: Are they worth fixing?

There remain many myths and much confusion about air leaks, costs, techniques and so on. Here, in the first of a twopart article, Chris Dee, executive director of BCAS sets out to clarify the position

At the very first meeting of the British Compressed Air Society in January 1930, the cost of air leaks and associated loss of system performance was discussed and our first air leak awareness programme launched. Reducing air leaks is probably the single most important energy savings measure, applicable to almost all systems. Why then, listening to comments from visitors to the BCAS stand at the recent Maintec exhibition, are compressed air leak programmes often ignored or even discouraged by management? Awareness of the importance of a regular leak detection programme is low, in part because air leaks are invisible and generally do not cause damage or injury. This problem can also be summed up as 'Over Promise' and 'Lack of Delivery'. In the 1990s, the basic compressed air inefficiency energy transfer became a prime target for energy reduction programs promising great results with low investments. Good payback programs - which they are if performed correctly.

The least complicated part of any energy reduction program, easiest to understand and implement was to fix the leaks. 'A single 6mm leak when fixed can give at least £1400/year savings' became the byword.

Management was easily persuaded to support these low cost, high return programs, but, in too many cases, the results were 'underwhelming': Some 'professional leak specialists' were known to find more leaks than the actual air supply.

Promised savings of thousands of pounds worth of air (energy) reductions failed to register even a small blip on the customer's monthly electric bill.

It didn't take long for this situation to often sour management on programs that require financial and manpower investments.

Management might 'talk the talk' but hesitated to 'walk the walk'. That is not to say that it was all bad, there were many successful programs; however, we see the same issues today with poorly implemented programs continuing to disappoint the customer.

Don't overestimate size or magnitude of leaks In the past, most compressed air leaks were sized with an educated estimate by experienced professionals. When a significant number of leaks were identified and repaired, and if the situation allowed, the total leak accuracy was checked with before and after system actions such as flow meter readings in off times, bleed down tests, etc.

The British Compressed Air Society Installation Guide states: When installing a new system the user should agree with the installer the maximum permissible leakage from the piping system when it is subjected to a final leakage test and before connection to the compressors and airline equipment When attempting to reduce leakage with an existing system it should be recognised that there is a point at which the cost of locating and fixing the remaining leaks can no longer be justified by the very small amount of energy that would be saved. For an installation with regular inspection and maintenance, leakage should not exceed 5% The major issue is that inexperienced personnel often tend to overestimate the actual volume (litres/second) which, when the project is completed and the results are disappointing, management support is lost.

Global leak load tests In addition to measuring leak loads from the bottom-up by listing individual leak sizes, it can be useful to measure the overall system leak load. During periods when the plant is not in production, one can measure the total leak volume with a monitored compressor operation or a bleed down test as shown in the BCAS Installation Guide: This method requires a compressor of a known capacity and an accurate pressure gauge, Class 1 and not less than 100mm in diameter, which has been recently calibrated against a test gauge and a stopwatch or timer.

Run the compressor with all equipment isolated until the system is charged to operating pressure and has remained at a constant pressure for 1 minute: note the pressure, P1, and time and stop the compressor.

Due to air leaks the pressure will fall in the system and should be allowed to drop to some suitable point, P2, on the pressure gauge scale not exceeding 1bar below the maximum recorded reading. Note the time in seconds for the pressure to fall to P2 and restart the compressor. Record the time T for the compressor to reach the maximum pressure P1 and immediately it reaches the previously recorded figure stop the compressor.

Repeat this cycle at least four times to obtain average values for T and t. If the compressor capacity is Q L/s then the amount of air delivered is Q x TL. Total leakage time, however, is (T + t) seconds so the average leakage rate is: (QxT)/(T+t) litres per second.

Other Pressure and Leak testing methods are given in the BCAS Installation Guide and in the Pressure and Leak Test Guide.

Leak repair log As leaks are identified and tagged for subsequent repair, they should be tabulated in a log sheet. Ideally this log would include an estimate of the repair time, include columns noting the actual repair date and costs, and document who did the work.

This repair information will be very useful when determining the true value of the leak project.

Valuing the leak After accurately identifying the magnitude of the total leak list and repairing them, the next significant challenge is to value the effect of the leak project on the monthly or annual electric bill. To management this is where 'the rubber meets the road' - are compressed air leaks worth fixing? In the May/June issue of Industrial Plant & Equipment I will show you how to calculate compressed air leaks. You can then decide if yours are worth fixing.