Finding & Fixing Leaks and Establishing a Leak Prevention Program
Leaks can be a significant source of wasted energy in an industrial compressed air system, sometimes wasting 20-30% of a compressor's output. A typical plant that has not been well maintained will likely have a leak rate equal to 20% of total compressed air production capacity. On the other hand, proactive leak detection and repair can reduce leaks to less than 10% of compressor output.
In addition to being a source of wasted energy, leaks can also contribute to other operating losses. Leaks cause a drop in system pressure, which can make air tools function less efficiently, adversely affecting production. In addition, by forcing the equipment to cycle more frequently, leaks shorten the life of almost all system equipment (including the compressor package itself). Increased running time can also lead to additional maintenance requirements and increased unscheduled downtime. Finally, leaks can lead to adding unnecessary compressor capacity.
There are two types of air leaks, planned and unplanned. The planned air leaks are the ones that have been designed into the system. These leaks are the blowing, drying, sparging etc. used in the production process. Many times these have been installed as a quick fix for a production problem. Some leaks take the form of "coolers", which are used to cool production staff or equipment.
The unplanned leaks are the ongoing maintenance issues and can appear in any part of the system. These leaks require an ongoing air leak detection and repair program. While leakage can come from any part of the system, the most common problem areas are:
- Couplings, hoses, tubes, and fittings. Tubes and push-to-lock fittings are common problems.
- Disconnects. O-rings required to complete the seal may be missing.
- Filters, regulators and lubricators (FRLs). Low first-cost improperly installed FRLs often leak.
- Open condensate traps. Improperly operating solenoids and dirty seals are often problem areas.
- Pipe joints. Missed welds are a common problem.
- Control and shut-off valves. Worn packing through the stem can cause leaks.
- Point of use devices. Old or poorly maintained tools can have internal leaks.
- Flanges. Missed welds are a common problem.
- Cylinder rod packing. Worn packing materials can cause leaks.
- Thread sealants. Incorrect and/or improperly applied thread sealants cause leaks. Use the highest quality materials and apply them per the instructions.
Estimating Amount of Leakage
For compressors that have start/stop controls, there is an easy way to estimate the amount of leakage in the system. This method involves starting the compressor when there are no demands on the system (when all the air-operated end-use equipment is turned off). A number of measurements are taken to determine the average time it takes to load and unload the compressor. The compressor will load and unload because the air leaks will cause the compressor to cycle on and off as the pressure drops from air escaping through the leaks. Total leakage (percentage) can be calculated as follows:
Leakage (%) = [(T x 100)/(T + t)]
Where: T = on-load time (minutes)
t = off-load time (minutes)
Leakage will be expressed in terms of the percentage of compressor capacity lost. The percentage lost to leakage should be less than 10% in a well-maintained system. Poorly maintained systems can have losses as high as 20-30% of air capacity and power.
Leakage can be estimated in systems with other control strategies if there is a pressure gauge downstream of the receiver. This method requires an estimate of total system volume, including any downstream secondary air receivers, air mains, and piping (V, in cubic feet). The system is started and brought to the normal operating pressure (P1). Measurements should then be taken of the time (T) it takes for the system to drop to a lower pressure (P2), which should be a point equal to about one-half the operating pressure.
Leakage can be calculated as follows:
Leakage (cfm free air) = (V x (P1-P2)/T x 14.7) x 1.25
Where: V is in cubic feet
P1 and P2 are in psig
T is in minutes
The 1.25 multiplier corrects leakage to normal system pressure, allowing for reduced leakage with falling system pressure. Again, leakage of greater than 10% indicates that the system can likely be improved. These tests should be carried out quarterly as part of a regular leak detection and repair program.
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Since air leaks are almost impossible to see, other methods must be used to locate them. The best way to detect leaks is to use an ultrasonic acoustic detector, which can recognize the high frequency hissing sounds associated with air leaks. These portable units consist of directional microphones, amplifiers, and audio filters, and usually have either visual indicators or earphones to detect leaks.
Ultrasonic leak detection is probably the most versatile form of leak detection. Due to its capabilities, it is readily adapted to a variety of leak detection situations. The principle behind ultrasonic leak detection is simple. In a pressure or vacuum leak, the leak flows from a high pressure laminar flow to a low pressure turbulence. The turbulence generates a white noise which contains a broad spectrum of sound ranging from audible to inaudible frequencies. An ultrasonic sensor focuses in on the ultrasonic elements in the noise. Since ultrasound is a short wave signal, the sound level will be loudest at the leak site. Ultrasonic detectors are generally unaffected by background noises in the audible range because these signals are filtered out. This means leaks can be heard in even the noisiest environments.
The advantages of ultrasonic leak detection include versatility, speed, ease of use, the ability to perform tests while equipment is running, and the ability to find a wide variety of leaks. They require a minimum of training, and operators often become competent after 15 minutes of training.
Due to the nature of ultrasound, it is directional in transmission. For this reason, the signal is loudest at its source. By generally scanning around a test area, it is possible to very quickly home in on a leak site and pin point its location. For this reason, ultrasonic leak detection is not only fast, it is also very accurate.
How to Fix Leaks
Leaks occur most often at joints and connections at end-use applications. Stopping leaks can be as simple as tightening a connection or as complex as replacing faulty equipment such as couplings, fittings, pipe sections, hoses, joints, drains, and traps. In many cases leaks are caused by bad or improperly applied thread sealant. Select high quality fittings, disconnects, hose, tubing, and install them properly with appropriate thread sealant.
Non-operating equipment can be an additional source of leaks. Equipment no longer in use should be isolated with a valve in the distribution system.
Another way to reduce leaks is to lower the demand air pressure of the system. The lower the pressure differential across an orifice or leak, the lower the rate of flow, so reduced system pressure will result in reduced leakage rates. Stabilizing the system header pressure at its lowest practical range will minimize the leakage rate for the system.
Once leaks have been repaired, the compressor control system should be re-evaluated and adjusted, if necessary, to realize the total savings potential.
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Establishing a Leak Prevention Program
A good leak prevention program will include the following components: identification (including tagging), tracking, repair, verification, and employee involvement. All facilities with compressed air systems should establish an aggressive leak reduction program. A cross-cutting team involving decision-making representatives from production should be formed.
A leak prevention program should be part of an overall program aimed at improving the performance of compressed air systems. Once the leaks are found and repaired, the system should be re-evaluated.
A good compressed air system leak repair program is very important in maintaining the efficiency, reliability, stability and cost effectiveness of any compressed air system.
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