Abstract
Compressed air is a major cost component in manufacturing. As such, it offers one of the largest savings opportunities. The investment in compressing air to energize it and then letting it escape from the system through leaks, without doing any useful work, is a complete waste. This waste can be minimized by implementing a program of leak detection and repairs. This entry covers the subject of how to use a handheld ultrasonic leak detector to locate leaks and the procedures required to implement repairs. The documentation and informational database required to ensure that leak waste is controlled and that new leaks are prevented is summarized. Different application technologies for controlling leaks are presented, and time and cost controls reviewed.
INTRODUCTION
This entry covers the basic methods of finding and repairing leakages.
The section on Leak Detection presents the most commonly used procedures to identify leakages. The sound signature of air leaks and the use of a handheld ultrasonic acoustic detector to locate leaks are explained. Suggested procedures for tagging and documenting air leaks are presented. The benefit of entering information into a database for historical trending is reviewed.
The section on leak repairs covers the most commonly found air leaks and the procedures to repair them. The logistical problems that create obstacles to expeditious repairs are discussed. The need for after-control, rechecks, and establishing standards are reviewed.
The section on leak control and prevention presents various approaches for managing leaks to minimize waste and ensure that the savings realized are ongoing. The application of flow monitoring and pressure regulation is presented.
Leak costs are discussed and a summary of the entry is presented.
LEAK DETECTION
In a compressed air system, the pressurized air confined by the pipes and vessels escapes from the system through openings as it expands back to atmospheric pressure. Ideally, all of these openings are created intentionally to extract energy from the compressed air in performing a desired task. In reality, however, many of the openings are unintentional, wasteful leaks.
The leak volume is directly proportionate to the area of the opening, the resistance to flow, and the applied pressure differential. The larger the area of the unrestricted opening and the higher the supply air pressure, the greater the leak flow. A chart showing the discharge of air through an orifice is included as a topic. Note that the values listed are based on a 100% coefficient of flow and should be adjusted for other orifice configurations as suggested (Table 1).
When the air expands back to atmospheric pressure, it transitions from a high-pressure laminar flow to a low-pressure turbulent flow. The escape velocities become extreme as the air volume expands. This results in a full sound spectrum of noise, ranging from audible to high frequency inaudible.
One common method of detecting leakages is to use a soap-like liquid that forms bubbles. Products specifically formulated for high viscosity and film strength exaggerate the bubble effect to enhance the detection capabilities. The liquid is poured, sprayed, squirted, or brushed on a suspect area, and the formation of the bubbles is visually observed. This method allows the detection of leaks that cannot otherwise be heard or felt in the normal operating production environment, but bubble detection is time consuming and messy. It requires the inspection of every connection to the air system, and the foaming agent may require material approval before it can be used in a particular facility. It also is not practical for checking overhead ceiling pipes or under, behind, or inside operating machinery.
The more commonly accepted method for detecting leaks is to use a handheld, ultrasonic acoustic detector that can register the high frequency sound signature associated with gas leaks and translate it into an audible signal. Air leaks have a definitive ultrasonic sound component in their noise signature that is beyond the hearing threshold of the human ear. The ultrasonic leak detector translates the ultrasonic noise of the leak signature into an audible sound heard in the earphones worn by the leak surveyor. Some instruments are also equipped with display meters and indicator lights that visually register the magnitude of the air leak. A distinctive, loud rushing sound is produced in the earphones when the leak detector sensor probe is aligned with a leak. With the production background noise suppressed and filtered out by the headphone set, the leakage hissing is heard.
Table 1 Discharge of air through an orifice
| Area sq. in. press | 1/64 ” .00019 | 1/32″ .00077 | 3/64 ” .00173 | 1/16″ .00307 | 5/64 ” .00479 | 3/32″ .00690 | 7/64 ” .0094 | 1/8″ .01227 | 9/6400 .01553 | 5/3200 .01973 | 3/1600 .02761 | 7/3200 .03758 | 1/400 .04909 | 9/3200 .06213 | 5/1600 .07670 | 3/800 .11045 | 7/1600 .15033 | 1/200 .19635 | 9/1600 .24850 | 5/800 .30680 | 3/400 .44179 | 7/800 .60132 | 100 .78540 |
| 1 | 0.028 | 0.112 | 0.253 | 0.450 | 0.700 | 1.06 | 1.48 | 1.80 | 2.27 | 2.80 | 4.0 | 5.5 | 7.2 | 9.1 | 11.2 | 16.2 | 22.0 | 28.7 | 36.3 | 44.8 | 64.7 | 88 | 115 |
| 2 | 0.040 | 0.158 | 0.356 | 0.633 | 0.989 | 1.42 | 1.94 | 2.53 | 3.20 | 3.95 | 5.7 | 7.7 | 10.1 | 12.8 | 15.8 | 22.8 | 31.0 | 40.5 | 51.0 | 63.4 | 91.2 | 124 | 162 |
| 3 | 0.048 | 0.194 | 0.436 | 0.775 | 1.25 | 1.74 | 2.37 | 3.10 | 3.92 | 4.82 | 6.9 | 9.5 | 12.4 | 15.7 | 19.2 | 27.8 | 37.8 | 49.5 | 62.5 | 77.0 | 111.0 | 152 | 198 |
| 4 | 0.056 | 0.223 | 0.502 | 0.892 | 1.39 | 2.00 | 2.73 | 3.56 | 4.50 | 5.55 | 8.0 | 10.9 | 14.3 | 18.1 | 22.2 | 32.1 | 43.5 | 57.0 | 72.0 | 88.9 | 128.0 | 175 | 228 |
| 5 | 0.062 | 0.248 | 0.560 | 0.993 | 1.55 | 2.23 | 3.04 | 3.97 | 5.02 | 6.19 | 8.9 | 12.2 | 15.9 | 20.1 | 24.7 | 35.7 | 48.5 | 63.5 | 80.1 | 99.3 | 143.0 | 195 | 254 |
| 6 | 0.068 | 0.272 | 0.612 | 1.09 | 1.70 | 2.45 | 3.32 | 4.34 | 5.49 | 6.75 | 9.8 | 13.3 | 17.4 | 22.0 | 27.1 | 39.1 | 53.0 | 69.5 | 87.9 | 108.0 | 156.0 | 213 | 278 |
| 7 | 0.073 | 0.293 | 0.695 | 1.17 | 1.82 | 2.63 | 3.58 | 4.68 | 5.90 | 7.29 | 10.5 | 14.3 | 18.7 | 23.6 | 29.2 | 42.2 | 57.3 | 75.0 | 94.7 | 116.0 | 168.0 | 230 | 300 |
| 8 | 0.083 | 0.331 | 0.741 | 1.32 | 2.06 | 2.96 | 4.05 | 5.30 | 6.70 | 8.24 | 11.9 | 16.2 | 21.2 | 26.9 | 33.0 | 47.7 | 64.7 | 84.7 | 106.0 | 132.0 | 191.0 | 260 | 339 |
| 12 | 0.095 | 0.379 | 0.856 | 1.52 | 2.37 | 3.41 | 4.65 | 6.07 | 7.66 | 9.42 | 13.6 | 18.6 | 24.3 | 30.7 | 37.8 | 54.6 | 74.1 | 97.0 | 122.0 | 151.0 | 218.0 | 297 | 388 |
| 15 | 0.105 | 0.420 | 0.945 | 1.68 | 2.62 | 3.78 | 5.15 | 6.72 | 8.50 | 10.48 | 15.1 | 20.5 | 26.9 | 34.0 | 41.9 | 60.5 | 82.5 | 108.0 | 136.0 | 168.0 | 242.0 | 329 | 430 |
| 20 | 0.123 | 0.491 | 1.100 | 1.96 | 3.05 | 4.40 | 6.00 | 7.86 | 9.92 | 12.12 | 17.6 | 24.0 | 31.4 | 39.8 | 48.8 | 70.7 | 96.0 | 126.0 | 159.0 | 196.0 | 283.0 | 385 | 503 |
| 25 | 0.140 | 0.562 | 1.26 | 2.25 | 3.50 | 5.05 | 6.88 | 8.98 | 11.38 | 13.99 | 20.2 | 27.4 | 35.9 | 44.5 | 56.0 | 80.9 | 110.0 | 144.0 | 182.0 | 224.0 | 323.0 | 440 | 575 |
| 30 | 0.158 | 0.633 | 1.42 | 2.53 | 3.94 | 5.68 | 7.7 | 10.1 | 12.77 | 15.70 | 22.7 | 31.0 | 40.5 | 51.3 | 63.0 | 91.1 | 124.0 | 162.0 | 205.0 | 253.0 | 365.0 | 496 | 618 |
| 35 | 0.176 | 0.703 | 1.58 | 2.81 | 4.38 | 6.31 | 8.6 | 11.3 | 14.26 | 17.60 | 25.3 | 34.5 | 45.0 | 57.0 | 70.0 | 101.0 | 137.0 | 180.0 | 227.0 | 281.0 | 405.0 | 551 | 720 |
| 40 | 0.194 | 0.774 | 1.74 | 3.10 | 4.84 | 6.97 | 9.5 | 12.4 | 15.65 | 19.31 | 27.9 | 38.0 | 49.6 | 63.0 | 77.0 | 112.0 | 151.0 | 198.0 | 250.0 | 310.0 | 446.0 | 607 | 793 |
| 45 | 0.211 | 0.845 | 1.90 | 3.38 | 5.27 | 7.60 | 10.3 | 13.5 | 17.05 | 21.00 | 30.4 | 41.4 | 54.1 | 68.0 | 84.0 | 122.0 | 165.0 | 216.0 | 273.0 | 338.0 | 487.0 | 662 | 865 |
| 50 | 0.229 | 0.916 | 2.06 | 3.66 | 5.71 | 8.22 | 11.2 | 14.7 | 18.60 | 22.90 | 32.9 | 44.9 | 58.6 | 74.0 | 91.0 | 132.0 | 180.0 | 235.0 | 296.0 | 365.0 | 528.0 | 718 | 938 |
| 60 | 0.264 | 1.06 | 2.38 | 4.23 | 6.60 | 9.50 | 12.9 | 16.9 | 21.40 | 26.35 | 37.9 | 50.8 | 67.6 | 85.0 | 105.0 | 152.0 | 207.0 | 271.0 | 342.0 | 422.0 | 609.0 | 828 | 1,082 |
| 70 | 0.300 | 1.20 | 2.69 | 4.79 | 7.45 | 10.53 | 14.7 | 19.2 | 24.25 | 29.90 | 43.0 | 58.6 | 76.7 | 97.0 | 120.0 | 173.0 | 235.0 | 307.0 | 388.0 | 479.0 | 690.0 | 939 | 1,227 |
| 80 | 0.335 | 1.34 | 3.01 | 5.36 | 8.33 | 12.04 | 16.4 | 21.4 | 27.10 | 33.33 | 48.1 | 65.5 | 85.7 | 108.0 | 131.0 | 193.0 | 262.0 | 343.0 | 433.0 | 537.0 | 771.0 | 1,050 | 1,371 |
| 90 | 0.370 | 1.48 | 3.33 | 5.92 | 9.25 | 13.34 | 18.2 | 23.7 | 30.00 | 36.90 | 53.0 | 72.3 | 94.8 | 120.0 | 147.0 | 213.0 | 289.0 | 379.0 | 478.0 | 592.0 | 853.0 | 1,161 | 1,516 |
| 100 | 0.406 | 1.62 | 3.65 | 6.49 | 10.50 | 14.58 | 19.9 | 26.0 | 32.80 | 40.50 | 58.0 | 79.0 | 104.0 | 132.0 | 162.0 | 234.0 | 316.0 | 415.0 | 523.0 | 649.0 | 934.0 | 1,272 | 1,661 |
| 110 | 0.441 | 1.76 | 3.96 | 7.05 | 11.00 | 15.82 | 21.5 | 28.2 | 35.60 | 43.90 | 63.0 | 86.0 | 113.0 | 143.0 | 176.0 | 254.0 | 345.0 | 452.0 | 570.0 | 702.0 | 1,016.0 | 1,383 | 1,806 |
| 120 | 0.476 | 1.91 | 4.29 | 7.62 | 11.40 | 17.15 | 23.4 | 30.5 | 38.51 | 47.50 | 68.0 | 93.0 | 122.0 | 154.0 | 190.0 | 274.0 | 373.0 | 488.0 | 616.0 | 712.0 | 1,097.0 | 1,494 | 1,951 |
| 125 | 0.494 | 1.98 | 4.45 | 7.90 | 12.30 | 17.79 | 24.2 | 31.6 | 40.00 | 49.25 | 70.0 | 96.0 | 126.0 | 160.0 | 196.0 | 284.0 | 386.0 | 506.0 | 638.0 | 789.0 | 1,138.0 | 1,549 | 2,023 |
| 150 | 0.582 | 2.37 | 5.31 | 9.45 | 14.75 | 21.20 | 28.7 | 37.5 | 47.45 | 58.25 | 84.0 | 115.0 | 150.0 | 190.0 | 234.0 | 338.0 | 459.0 | 600.0 | 758.0 | 910.0 | 1,315.0 | 1,789 | 2,338 |
| 200 | 0.761 | 3.10 | 6.94 | 12.35 | 19.15 | 27.50 | 37.5 | 49.0 | 62.00 | 76.2 | 110.0 | 150.0 | 196.0 | 248.0 | 305.0 | 441.0 | 600.0 | 784.0 | 990.0 | 1,225.0 | 1,764.0 | 2,401 | 3,136 |
| 250 | 0.935 | 3.80 | 8.51 | 15.18 | 23.55 | 34.00 | 46.2 | 60.3 | 76.15 | 94.0 | 136.0 | 184.0 | 241.0 | 305.0 | 376.0 | 542.0 | 738.0 | 964.0 | 1,218.0 | 1,508.0 | 2,169.0 | 2,952 | 3,856 |
| 300 | 0.995 | 4.88 | 10.95 | 18.08 | 28.25 | 40.55 | 55.0 | 71.8 | 90.6 | 111.7 | 161.0 | 220.0 | 287.0 | 364.0 | 446.0 | 646.0 | 880.0 | 1,148.0 | 1,454.0 | 1,795.0 | 2,583.0 | 3,515 | 4,592 |
| 400 | 1.220 | 5.98 | 13.40 | 23.81 | 37.10 | 53.45 | 72.4 | 94.5 | 119.4 | 147.0 | 213.0 | 289.0 | 378.0 | 479.0 | 590.0 | 851.0 | 1,155.0 | 1,512.0 | 1,915.0 | 2,360.0 | 3,402.0 | 4,630 | 6,048 |
| 500 | 1.519 | 7.41 | 16.62 | 29.55 | 46.00 | 66.5 | 90.0 | 117.3 | 148.0 | 182.5 | 264.0 | 358.0 | 469.0 | 593.0 | 730.0 | 1,055.0 | 1,430.0 | 1,876.0 | 2,360.0 | 2,930.0 | 4,221.0 | 5,745 | 7,504 |
| 750 | 2.240 | 10.98 | 24.60 | 43.85 | 66.15 | 98.5 | 133.0 | 174.0 | 220.0 | 271.0 | 392.0 | 531.0 | 696.0 | 881.0 | 1,084.0 | 1,566.0 | 2,125.0 | 2,784.0 | 3,510.0 | 4,350.0 | 6,264.0 | 8,525 | 11,136 |
| 1000 | 2.985 | 14.60 | 32.80 | 58.21 | 91.00 | 130.5 | 177.0 | 231.0 | 291.5 | 360.0 | 520.0 | 708.0 | 924.0 | 1,171.0 | 1,440.0 | 2,079.0 | 2,820.0 | 3,696.0 | 4,650.0 | 5,790.0 | 8,316.0 | 11,318 | 14,784 |
Table is based on 100% coefficient of flow. For well-rounded orifice, multiply by 0.97. For a sharp-edged orifice, a multiplier of 0.65 will give approximate results. Values calculated by approximate formula proposed by S.A. Moss. W=0.5303(ACP/Vr); where: W, discharge (lb/s); A, area of orifice (in.2); C, coefficient of flow; P, upstream pressure (PSI, abs.); T, upstream temperature ( °F, abs.); Values used in calculating table: C = 1; T=530°R(70°); P = Gage pressure plus 14.7 psi; weights converted to volumes using density factor of 0.07494 lb/ft3 (correct for dry air at 14.7 psi abs. and 70°F); values from 150 to 1000 psi calculated by Compressed Air Magazine and checked by Test Engineering Dept. of Ingersoll-Rand Co.
The sound wave generated by an air leak is directional in transmission. The intensity of the leak noise is based upon the shape of the orifice opening, the distance to the sensor probe, and the differential expansion pressure. The sound level is loudest at the actual point of the leakage exit. The procedure for detecting leaks ultrasonically uses this characteristic to locate the actual leaks. Initially, the leak detector is set at the maximum practical sensitivity consistent with the specific environment of the area being inspected. A sweep of the general area is performed as the surveyor walks the system. When a leak is detected, the direction of the leak is determined by scanning the area until the loudest noise level registers. With the probe pointing in the direction of the noise source, the surveyor moves towards the leak, adjusting the sensitivity of the leak detector accordingly. The intensity of the sound increases in the proximity of the leak and is loudest at the actual point of air exit. Extension tubes or cones attached to the sensor probe focus the sound and pinpoint the location of smaller leaks. The bigger, more serious leaks can be felt. A further test using a bubble solution can augment the process by visual enhancement of the exact location. One such product is formulated to produce an ultrasound shockwave as the bubbles burst, so the surveyor gains the benefits of both the visual observation and ultrasonic detection.
Competing sounds often mask a leak or otherwise distort the directional transmission. If possible, the best way to eliminate a competing sound is to shut the system off. If that is not possible, shielding techniques can be applied. The angle of the probe extension can be changed. The competing sound can be blocked using the body or other solid barrier like a piece of cardboard or clipboard. Cupping the hand over the leak, or using a rag, can often isolate the true source of the sound. Bubble tests can pinpoint the location regardless of the competing sound. It is imperative when working in and around operational machinery that safety be most important. Common sense dictates the extent of effort that should be expended to identify and quantify a specific leak.
The first step in the preparation for performing a leak survey is to establish a pattern for surveying the facility to ensure that all the piping, connected use points, and workstations in an area are inspected. Detected leaks are identified and tagged during the surveillance of the system.
Different color tags can be used to visually indicate the severity of leaks and establish priorities. Typical classifications might include three levels:
Level 1: Not audible in any environment without an ultrasonic detector.
Level 2: Audible in a quiet environment but not in an operating facility.
Level 3: Serious leaks requiring immediate attention.
Level 1 leaks cannot be felt or heard under any conditions and require the use of the previously described procedures to detect. They are less than 1 scfm and are assigned no value, since the cost of the associated logistics and labor do not economically justify the repair, unless it is very simple, such as the ubiquitous push lock fitting on plastic tubing. Level 1 leaks are tagged and documented for future recheck, since air leaks never fix themselves and only grow larger over time. The cumulative effect of the Level 1 leaks on the compressed air system can be better controlled by maintaining a stable delivered air pressure at the lowest optimum level through the applications of pressure/flow control and regulating use points.
Level 2 leaks are in the 2 scfm range and can typically be felt but not heard without the use of an ultrasonic leak detector. Repairs are economically justifiable and should be performed within a 60-day period.
Level 3 leaks in excess of 2 scfm can typically be felt and sometimes heard by the human ear. These require immediate attention, since they not only waste air but impact the operational efficiency of the compressed air system. Leak flow is a real demand that adds to the filter/dryer loading, increases the pressure drop throughout the system, and creates pressure fluctuations that impact production.
While the true flow for any specific leak cannot be measured practically, the surveyor can assign values based upon the chosen leak volume associated with the various leak levels. These can then be totaled at the end of the survey to estimate the cumulative system leak waste. The surveyor will typically overestimate about the same amount of leakages that are underestimated, so the final figure gives a good portrayal of the total leak waste. As long as the survey procedures are replicated during the re-check, the comparative value for trending becomes an accurate measure for evaluating the remedial repair actions taken. A cost figure can be assigned for use in the financial analysis. Take into account power cost and associated compressor maintenance and repair costs, plus the costs to operate and maintain all the auxiliary equipment, when determining the real value of the leak waste.
Efforts have been made to estimate the actual volume of an air leak based upon pressure and the decibel level registered at a specific distance. People have assembled test stands using the most common orifice configurations found in compressed air systems, and then have measured air flow and decibel noise at different pressures and distances. One such Chart, published by UE Systems of Elmsford, NY, is presented in Fig. 1. Note the disclaimer that the values are not stated as “factual CFM” and are provided as a “general guideline.” A leak signature is affected by many factors, and the loudness of the noise generated is by itself not the sole measure of the volume of the leakage. For example, a high-pitched whistle will sound a lot louder than a low-level whoosh sound, but the whistle will consume less air. At best, the leak detection process will provide an estimate for use in planning the priorities of the remedial repair procedures and a value for evaluating trends.
GUESS-TIMATOR CHART FOR UP9000/10,000
dB vs CFM
| DIGITAL READING | 100 PSIG | 75 PSIG | 50 PSIG | 25 PSIG | 10 PSIG |
| 10 dB | 0.5 | 0.3 | 0.2 | 0.1 | 0.05 |
| 20 dB | 0.8 | 0,9 | 0.5 | 0.3 | 0.15 |
| 30 dB | 1.4 | t.l | 0.8 | 0.5 | 0.4 |
| 40 dB | 1.7 | 1.4 | 1.1 | 0.8 | 0.5 |
| 50 dB | 2.0 | 2.8 | 2.2 | 2.0 | 1.9 |
| 60 dB | 3.6 | 3.0 | 2.8 | 2.6 | 2.3 |
| 70 dB | 5,2 | 4,9 | 3.9 | 3.4 | 3.0 |
| 80 dB | 7.7 | 6.8 | 5.6 | 5.1 | 3.6 |
| 90 dB | 8.4 | 7.7 | 7.1 | 6.8 | 5.3 |
| 100 dB | 10.6 | 10.0 | 9.6 | 7.3 | 6.0 |
Fig. 1 Noise vs leak loss at various pressures.
NOTES:
ALL READINGS ARE COMPENSATED FOR ATMOSPHERIC PRESSSURE. All readings were laken al 40 kHz,
PROCEEDU’RE:
Use the Scanning Module to conduct the broad scanning to pinpoint the air leaks. The Scanning Module with the Rubber Focusing Probe (RFP) is used to determine air losses. The tip of the RFP on the UP9000 should be fifteen (15) inches away from the leak location for determination of the leak rate.
Notice: The values presented in this table ate not stated as factual CFM measurement This table is provided sotety for convenience and should only be used us a general guideline
Factors such at turbulence, leak orifice configuration, pressure, moisture and instrument sensitivity can significantly effect your results.
LEAK REPAIRS
Detected leaks must be visually tagged for future repair. Some tags are configured to enable you to tear off a copy to give to the maintenance supervisor responsible for the leak repairs. Regardless of the configuration of the tag, information sufficient to allow revisiting an individual leak for repair, even if the tag falls off or is missing, should be recorded on a separate worksheet. This typically consists of:
• Recording the unique, sequential tag number assigned to the specific leak.
• Defining its workplace location in a way that is meaningful to the air user.
• Identifying the specific item that is leaking.
• Identifying the actual point of air exit on the leaking item.
• Classifying the degree of leakage so priorities for remedial action are established.
The tag can be used for after-control by providing a place to enter the date and repairperson’s name. The supervisor should check to ensure that the repair has been properly completed before signing off and removing the tag. The repair actions and associated time should be recorded on the original worksheet and entered into a database to establish time and cost control accounting procedures.
The surveyor should record complete information on the worksheet to describe the leak. The probable cause of the leak, such as aging, wear, damage, looseness, mishandling, breakage, or other reasons, should be noted with an explanatory note if required. Determine whether the leak should be repaired or a part replaced, and note it on the worksheet. If replacement is recommended, the surveyor should collect enough information about the item to allow for purchasing the repair part or replacement unit. Someone will have to do this if the leak is going to be fixed, so the surveyor should make the extra effort to record the information at the same time the leak is identified. The air user will also need to know if the leak is repairable without having to shut down the associated machinery. The worksheet should have areas for helpful comments and field notes to facilitate remedial actions or to alert people about other issues and opportunities that come to the attention of the surveyor.
Detected leaks must be repaired in order to realize any savings. Since most leaks occur at the operating machinery in the production area, repair procedures tend to be repetitive. Stresses are applied to all the various hoses and couplings, tubing connections, and pipe joints because of machinery vibration and movement of the connected tools and pneumatically driven devices. Over time, leaks develop at sealing areas. These are easily fixed by reconnecting the hose or reinstalling the pipe fitting after inspection and cleaning. Worn couplings or quick disconnects are replaced. The plastic components of point-of-use devises, such as filters, regulators, and lubrications, tend to age and crack over time. These must be replaced. Gaskets and seals dry out and become brittle, so they no longer seal effectively. Valve stem packing and sealing rings, manifold gaskets, hose reel rotary joints, and cylinder shaft seals wear over time and need to be replaced. Clamps, pipe unions, flanges, and pipe groove seals often require re-tightening. Leaks in the compressor room are found around air treatment equipment, condensate drains, receiver manholes, and control tubing.
Leaks on pipe joints are relatively easy to fix by either tightening or reinstalling a connection. Clean all surfaces before reassembly. Use a non-hardening sealing paste for threaded connections to prevent the possible contamination of the air system from torn or frayed Teflon™ tape.[1] Leaks in main headers and branch lines often require lifts or special rigging equipment to gain access, and may require special plumbing skills to repair. Advance planning and scheduling will be necessary for coordinating the repairs on machinery not accessible during production.
The largest obstacle to repairing leaks is the logistics involved in planning and implementing the repair procedures. These logistical problems often take months to resolve and sometimes impede the process entirely. A typical scenario follows.
LOGISTICAL PROCEDURES AND OVERHEAD ASSOCIATED WITH LEAK REPAIRS
1. Meetings and Planning
2. Maintenance requisitions
3. Purchaser—product and supplier identification
4. Order costs—cost per placed order
5. Transportation
6. Control of receipt—administration
7. Storage—space and logistics costs
8. Labor schedule—days/weeks
9. Leakage cost per week/month
10. Time control—verification and administration
LEAK MANAGEMENT
Air leaks grow bigger over time, and repaired leakages usually reappear within six months to one year after they are fixed. Steps must be taken to control the growth rate of leaks and to prevent reoccurrences after repairs are completed. The key to managed leakage control and prevention is rechecks and documentation. Periodic rechecks at predetermined intervals ensure that the leak rate is stabilized at a low level. Through documentation, the trends become obvious and developing patterns, both good and bad, are identified. Problems are recognized before creating issues that are more serious. Taking appropriate actions drives the leak trend downward until it reaches the target established by management, typically 5%-10% of the total air demand. This historical information is used to institute leak prevention measures and for calculating the most economical interval for rechecks to ensure that the gains realized are maintained in the future. Establishing standards and good practices minimizes future leakage. With the time and costs documented, controls can be put in place to properly administer a leak management program.
An alternative approach to implementing a full leak management program is to simply fix the leaks immediately upon discovery, assuming a system is checked for air leaks on a regular basis. The technician brings along a tool tote with the appropriate equipment needed to fix the most commonly found leaks. Usually, only the more serious leaks are addressed in the simple seek and fix approach. Little, if anything, is documented.
The total leakage for a facility can be estimated using techniques that measure pressure degradation over time when there are no production demands on the system.
One such method is to measure the load/unload cycle time of compressors when production is shut down and the only air demand on the system is leakage. Start the compressor(s) and record the on-load time and off-load time over a sampling period long enough to provide a representative average. Calculate the leakage lost as a total percentage of compressor capacity using the formula:
Leakage (%) = [(T X 100)I(T + t)]
where: T = average on-load time, and t = average off-load time.
In systems configured with compressor controls other than load/unload, leakage can be estimated based upon the total system capacitance. The total estimated volume (V) of all air receivers, the main piping distribution, and other significant air containment vessels must be calculated in cubic feet. Pressure in the main header must be measured at the start and end of the evaluation test period. Production must be shut down so that the only demand on the system is leakage. The compressors are then started in order to pressurize the system to its normal operating pressure (Pj). The compressors are turned off, and the time (T) it takes the system to drop to a pressure equal to half the normal start pressure (P2) is measured. The leakage is estimated using the formula:
Leakage (cfm of free air)
where: V is the volume in cubic feet, Pj and P2 are in psig, and T is the time in minutes.
Because air escapes from the system at a rate proportional to the supply pressure, the leak volume rate at the normal start pressure will be much greater than the leak volume rate at the end of the timed cycle when the pressure is half. A 1.25 correction factor is applied to compensate for the difference in the leak rate and to provide a more accurate estimation of the loss.
Installing a flow meter to measure and record the actual flow improves the accuracy of the air leak estimate over using a calculated capacitance based upon estimated system volume. A properly configured flow meter can also be used to monitor the system consumption in order to
(1) establish a baseline for evaluating the performance improvements realized from any remedial actions taken,
(2) verify trends, and (3) verify that the gains continue to return the investment into the future.
Many of the leaks in an industrial compressed air system are intentional or planned. Condensate drainage and disposal, spot cooling, fume venting and exhausting, material conveying and blowing off, and drying are examples of intentional leaks. Devices are available to eliminate or mitigate the air used to perform these types of assigned tasks.
No air loss condensate drains collect the condensation in a vessel, until it fills with water. A float or sensor detects the high liquid level and opens a drain port, allowing compressed air to displace the water and forcing it to discharge from the vessel. The sensor shuts off the drain port before the vessel is completely drained, so that no air is lost with the water discharge. Some designs are entirely pneumatic, so no electric power is required at the use point. Electrically activated designs require a power source. While this is sometimes inconvenient, electric units have the advantages of (1) indicator lights that show the operational status of the drain and (2) contacts that can be interfaced with a building management system for remote monitoring.
High efficiency blowing devices are available to entrain surrounding ambient air in the primary air stream to increase the impingement force, so that less compressed air is required to perform the equivalent task. Air knives, nozzles, and jets are offered with a variety of different airflow patterns to better suit a specific task. Air amplification ratios as high as 40:1 over open blowing are achievable.[2] Supply pressure at the point of use can often be regulated to a lower level to further reduce compressed air consumption and the associated noise.
Air volume amplifiers are available to create directional air motion in their surroundings and to efficiently move air and light materials. A small amount of compressed air is used as a power source to amplify the flow of entrained ambient air. Airflow is directional, with an inlet and outlet, to exhaust and/or sweep an area in a shaped pattern. Air volume amplifiers can create output flows up to 25 times the compressed air consumption.[2]
LEAK CONTROL AND PREVENTION
Leaks of all sizes, both intentional and unintentional, can be controlled by supplying them, at minimum, an acceptable, delivered air pressure. An air system that has a cumulative equivalent of a 5/16″ leakage orifice, for example, is illustrated in Fig. 2.
The application of Pressure/Flow Control in the compressed air system primary is a good method for minimizing leak waste. The smaller leaks, determined to be uneconomically repairable, leak less at the lower delivered pressure. The Pressure/Flow Control also prevents the supply pressure from rising because of the lower demand that stems from leak repairs. Without some method of supply side pressure control, the system pressure increases inversely with demand, forcing leaks and other unregulated use points to consume more air. The savings achieved by lowering the leak demand are offset by air that is shunted out elsewhere in the system because of the rising pressure.
Fig. 2 Cumulative system leak demand at different pressures.
The application of point-of-use pressure regulation is another method for minimizing leakages. Setting and securing pressure regulators to supply air at the lowest minimal acceptable pressure maintains the respective leak losses at their lowest possible level.
Shutting off the air to non-productive workstations and assembly lines is another good method of leak control. Lock out valves and isolation valves can be installed to completely shut off the air to machinery that is shut down. The procedure can be manual or automated through the installation of actuated shut off valves that are activated by an external signal. Automation eliminates the dependency on a human action to stop the waste.
LEAK COSTS
The cost of leaks must be determined to allow management to make proper decisions about the compressed air system. The Chart in Fig. 3 illustrates the cost of air consumed by leaks.
In addition to the power cost shown in Fig. 3, consideration should be given to other associated costs, such as labor to log daily operations, scheduled maintenance, repair services, and periodic major overhauls. Leaks are a real demand that require real airflow to satisfy. There is an added cost burden that results from treating the leak air, removing condensation, additional compressor wear, and increased power to compensate for the greater pressure drop because of the higher flow. The final cost figures can more than double the cost based solely upon electrical power.
An effective leak control and prevention program requires continuous monitoring and verification that the gains realized are ongoing into the future. At a minimum, a leak survey should be performed several times a year in a system recheck. Results should be entered into a database and analyzed. Flow monitoring systems are available to measure actual flow. These can interface with management information systems that have remote access. Measuring real flow allows the true cost of the delivered air to be calculated in $/mmcf (Dollars per million cubic feet). Charting the savings in reports for management ensures continued support for the program.
SUMMARY
In summary, a good leak control and prevention program for a compressed air system starts with a leak survey. Ultrasonic leak detectors are the best tool to find air leaks and pinpoint their location. The information about the air leaks is recorded in a worksheet and documented in a database for use in generating reports and identifying trends. Savings are only realized if leaks are repaired. Repair procedures must be established and an investment made in satisfying all of the logistical obstacles before the actual remedial actions can be taken. Consideration should be given to contracting out the repairs, along with the logistical requirements, to expedite the process and realize the savings as soon as possible. Rechecks and monitoring are necessary to drive the leak trend down and keep it at the targeted rate.
Air consumed by leaks at 100 psig
| Diameter in. | SCFM Leakage | Annual volume | Cost per year* | |
| 1/64(.016) | 0.41 | 215,496 | cf/yr | $47.41 |
| 1/32(.032) | 1.62 | 851,472 | cf/yr | $187.32 |
| 1/16(.063) | 6.49 | 3,411,144 | cf/yr | $750.45 |
| 3/32(.094) | 14.6 | 7,673,760 | cf/yr | $1,688.23 |
| 1/8(.125) | 26.0 | 13,665,600 | cf/yr | $3,006.43 |
| 5/32(.156) | 40.5 | 21,286,800 | cf/yr | $4,683.10 |
| 1/4(.250) | 113.0 | 59,392,080 | cf/yr | $13,066.42 |
*Based upon rate used by US Dept. of Energy.
Fig. 3 The typical cost of air leaks.
