Detect leaks with acoustic imaging

For years, compressed air leaks have been among the first items to attack when looking for energy savings in a manufacturing plant. Through the years, the method of locating leakage has gone from using the human ear, to applying a soapy solution to pipes and hoses, to using sensitive electronic ultrasonic audio instruments to enable operators to find and record leaks. Recently, a method of detecting leaks has been developed called “acoustic imaging,” which uses audible and visual inputs and has the potential to lower industrial compressed air and process gas costs.

Background: Ultrasonic leak detection

Ultrasonic leak detection has been around for many years. From the start, equipment suppliers realized that using only the human ear has limitations in finding leakage, especially in very noisy industrial settings. Loud background noise and legitimate compressed air uses in typical industrial plants very quickly mask any audible sound emitted by leaks, making the hunt an almost impossible task, even during quiet times during evenings and weekends.

Long ago, it was discovered that a flow of gas moving from one pressure to another emits sound in the ultrasonic frequency spectrum. Using electronic mixing circuits this ultrasonic signature can be brought down to a frequency range that humans can hear, but at the same time, general low frequency industrial noise can be filtered out. This gives ultrasonic detector operators “superhuman” hearing that allows gas leaks to be easily heard in noisy factory environments, even from hundreds of feet away.

The detection method for ultrasonic guns is typically a “point and shoot” method, where a directional audible pickup device is waved around until something is heard in a certain direction. The operator is then led on a search while following the sound, with the signal getting increasingly louder as the detector approaches the source. Sometimes, when near the leak, it is difficult to pinpoint the exact location due to various factors. At times, the leakage may be behind a protective barrier or in an inaccessible location. Other times, the leak might be among a complex network of piping and hoses, requiring the operator to attempt to feel for the leak within the many fittings, sometimes creating a safety risk. Reflections and interference from other ultrasonic emitting devices also can drown out the leak signal, making it difficult to exactly locate.

Once the source of the sound is found, the leak is documented and recorded so the cost can be estimated, and a repair crew can find it again at a later time. This step usually involves measuring and recording a decibel reading at a fixed distance away from the leak, marking the location with a paper tag, then taking several photographs to describe the location for the future repair crew. Once a survey is done, a report is generated that estimates the leakage flow, calculates the theoretical cost of the leaks and displaying the location description and pictures. This process is a tedious exercise where the operator juggles the detector, a camera, a notebook and a bundle of tags each time a leak is detected, often repeating the task hundreds of times per day if working in a large plant.

Enter acoustic imaging

A more efficient way of finding and recording leaks has recently been developed by Fluke Corp. A leak detector instrument, called the ii900 Sonic Industrial Imager, uses a video camera to provide a live image, and an array of 64 different directional microphones to create a “heat map” of ultrasonic emissions on top of the visual display (see Figure 1). This device brings new meaning to the old saying “a picture is worth a thousand words.” Having an image representing both visual and audible signatures emitted by a leak makes the detection exercise a much easier task.

In using the Imager, the operator also “points and shoots,” but in this case the ultrasonic emission shows up as a live feed on the onboard video screen as a colored spot, making the location identification much quicker. As the operator gets closer to the leak, the location becomes easier to identify, even when shooting through protective screens or pointing at locations out of reach without a ladder. When shooting among other ultrasonic emitters, like a bench full of active compressed air powered grinders, the leak shows up as a consistent spot on the screen, with the tools being intermittent, so it is easy to differentiate. Reflections, which can cause time consuming “wild goose chases” with typical audible only detectors, are easy to separate from the leakage, moving the camera from side-to-side moves the location of a reflection, but not the image of an actual leak.

In using the acoustic imager, the first-time operator is impressed at the speed at which leaks can be detected and positively identified. The built-in video camera makes it easy to record a close-up still picture or video of the leak, which is stored on the device memory for later download. The imager software has the capability of automatically measuring the sound level in decibels, and the distance to the leak by triangulation. Using this information, and entered in pressure information for the leak, an estimate of the leakage flow can be calculated for the final report. Once the leakage flow is known, using the estimated efficiency of the compressed air system, and the site power costs, the total cost of all the leaks captured can be calculated in a final automatically generated output, called a “LeakQ report.”

Other acoustic imaging uses

The use of the imager is not limited only to compressed air; other items within a plant generate ultrasonic signals. Ultrasonic detectors are commonly used to detect nitrogen, bulk gas, steam and vacuum leakage. In addition, pump cavitation, noisy bearings and electrical discharge corona also can be detected using ultrasonic detectors. The ii900 has a tunable frequency range that can be used to better differentiate different types of emitters, depending on the sound signal characteristics.

In evaluating an imager for the first time, we deployed the ii900 at a small 10 bay service shop as part of a compressed air efficiency survey. As a value-added service, the operator spent only 10 minutes locating 12 various compressed air leaks totaling about 10 cfm. This flow consumed about 35% of the average facility flow. At the customer’s power rate, the total cost of these easily repairable leaks was estimated at $1,000 per year. If the customer repairs the leaks, it will save this cost and qualify for an extra $1,500 utility grant to help with the purchase a new more efficient air compressor. This represents about $2,500 value in the first year through only 10 minutes of leak detector work, and roughly two hours of repair time.

A second survey was done at a small fiberglass parts manufacturer during full production hours. This time, 51 compressed air leaks totaling about 50 cfm were identified in a one-hour survey. Two vacuum leaks were also identified. Total cost of these leaks is estimated at $6,300 annually. What became particularly striking is that a leak survey was previously done with a standard ultrasonic gun about a month earlier and repairs were made. About half of the leaks found during the acoustic imager survey had developed over a 30-day period or had been missed in the previous work with a standard detector gun.

The survey found many quick connect couplers and rubber hose leaks, which are common problem areas in a plant of this type. When presented with the visual results, the plant managers immediately recognized the problem and started investigating solutions. A wholesale change of connector and hose type is being considered.

Throughout the surveys, the high value of the acoustic imager output became obvious. Figure 2 shows leakage of one pressure regulator in a bank of regulators. A configuration like this makes for difficult work in identifying the source of the leak if done with a standard ultrasonic gun; use of the imager found the leak immediately. Figure 3 shows the identification of a leak in a pressure regulator behind a protective screen, something that would be difficult to do with a standard gun without shutting down the machine.

Experience with the acoustic imagers shows it makes leak detection quicker and more efficient. Once found, leaks can be clearly identified and recorded, and easily differentiated from background noise, reflections and interference. Leaks in overhead piping and behind screened barriers can be identified without incurring any safety risks. And a basic final report can be conveniently generated online at the Fluke website.

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