Something in the air: Ultrasound for compressed-air leak detection
Contrary to what some might think, compressed air is not free. In fact, for the energy it takes to produce it to what is generated as a result, it is often considered the most expensive utility in a typical manufacturing facility. To add to the problem, the U.S. DOE notes that more than 50% of all compressed air systems have energy-efficiency problems. Air compressor experts have also estimated that as much as 30% of compressed air generated is lost via leaks in the compressed air system.
Often, when a compressed air system struggles to meet current demands on the system, spare compressors are rented and used as backups or an additional compressor is installed. Both strategies are expensive, and depending on the size of the compressors needed, they could equate to hundreds of thousands of dollars.
Because compressed air systems inherently have leaks, regardless of piping, use, and design, implementing a compressed-air leak-management program can be an economical and effective way to improve the efficiency of any compressed air system. Having a compressed-air leak-management program in place that is designed to identify and repair compressed air leaks before they become a large problem can save time, money, and energy. Proper planning and creating a sense of awareness by educating employees on how costly compressed air leaks can be is integral to achieving success with any compressed-air leak-management program.
Compressed air and compressed gas leak detection remains the most widely used application for airborne ultrasound technology. Employing ultrasound to locate compressed air and gas leaks and then making the necessary repairs can have tremendous payback. Recent advancements in compressed-air leak detection and reporting allow organizations to quantify dollars lost and the CFM loss associated with compressed air leaks. An effective ultrasonic compressed-air leak survey will focus on seven key factors: evaluation, detection, identification, tracking, repair, verification, and re-evaluation. By implementing these steps, a typical manufacturing plant could reduce its energy waste by roughly 10% to 20%. As an example, a 1/8” leak at 100 psi of compressed air at 22 cents per kilowatt hour has an annual cost of $2,981.
Airborne ultrasound: How does it work?
There are three generic forms of ultrasound technology: pulse/echo, power, and airborne/structure-borne. Pulse/echo is the most recognized form of ultrasound, as this is the medical form of ultrasound. With power ultrasound, as in an ultrasonic cleaner, high-frequency sound waves are emitted. These high-frequency sound waves have energy, and they clean parts and various materials. The form of ultrasound technology that is used for compressed-air leak detection is airborne ultrasound. Airborne ultrasound relies on high-frequency sound waves that are above the range of normal human hearing. Humans are able to receive sound within a frequency range of 20 Hertz (Hz) to 20 kilohertz (kHz), with the upper threshold of normal human hearing between 16 kHz and 17 kHz. The ultrasonic range begins at 20 kHz. Most ultrasound instruments are capable of receiving or sensing these high-frequency ultrasound sound waves within a frequency range of 20 kHz to 100 kHz. For ultrasonic leak detection, an ultrasound instrument that has frequency tuning capability is recommended, and the suggested frequency setting is 40 kHz. For ultrasound instruments that are on a fixed frequency or where frequency tuning is not a feature, 38 kHz is usually the frequency setting at which the instrument is fixed.
There are different sources of high-frequency sound that these ultrasound instruments detect. For compressed air and compressed gas leak detection, the source of the ultrasound is turbulence.
Airborne ultrasound and compressed air leak detection
Once an ultrasound instrument that will be used for compressed air leak detection has been selected, the planning of the compressed-air survey can begin. One thing to keep in mind while scanning for compressed-air leaks out in the facility is the fact that high-frequency sound is very low-energy. Because it is low-energy, the sound will not travel through solid surfaces but rather will bounce and reflect off of solid surfaces. That’s why it is important to scan in all directions with the ultrasound instrument and adjust the instrument’s sensitivity. Adjusting the sensitivity and scanning in all directions will help pinpoint the location of the compressed air leak. Once the general area of the compressed air leak has been located, most ultrasound instruments will come with a focusing probe that can be slipped over the end of the airborne scanning module to narrow the field of view and more precisely identify the leak’s location. This method of compressed-air leak detection using ultrasound is commonly referred to as the “gross to fine” method.
The logistics of the leak detection route should now be considered. Performing a walk-through before the inspection is highly recommended. The inspector should use this as an opportunity to determine the specific zones or areas where compressed air is being used. Blueprints of the compressed air piping are also a handy resource when conducting the initial walk-through. When performing the initial walk-through, note any safety hazards and areas where accessibility to the test area may difficult or may require the use of ladders, extra PPE, or access to locked areas. Also make note of any obvious signs of compressed air misuse, potential areas of leakage, and improper piping installations. Noting any areas of potential leakage or misuse of compressed air (such as the use of air to move parts/product, air knives, etc.) will help eliminate confusion about what the inspector is finding and help everyone become more aware of where competing ultrasonic noise is coming from. Part of the goal of the compressed air leak survey could be to identify areas where compressed air is being misused and look for alternatives that could perform the same function without having to use costly compressed air.
It’s also necessary to determine the type of leaks that ultrasound will be used to detect – for example, pressure leaks in compressed air or compressed gas systems, vacuum leaks, or refrigerant leaks. After the initial walk-through, select one area or zone to test at a time. For consistency, it is a good practice to begin at the compressor (or supply) side and then move to the distribution lines and then to areas where the compressed air is being used. As the compressed air leaks are found with the ultrasound instrument, a tagging system should be in place for tagging the leak at the leak site. The tag should have space for recording the leak number, the pressure, the type of compressed gas, a brief description of the leak location, and the decibel level of the leak that was indicated on the ultrasound instrument once the leak location was confirmed. An estimated cost of the leak may also be helpful in creating awareness of the expense of compressed air or compressed gas leaks.
Documentation and reporting
Beyond repairing the compressed-air leaks that are found during the compressed-air leak survey, the ultimate success of the survey will rely largely on the reporting and documentation of the compressed air leaks. For documentation purposes, you may want to consider using a leak survey app, which can let the inspector easily document the compressed air and compressed gas leaks that are found, along with the associated cost of the leaks.
When reporting the cost and CFM loss of compressed air or compressed gas leaks, it’s important to remember that these are estimated costs. The cost of the compressed air leaks will be based off of the decibel level once the leak has been located, the cost per kilowatt hour of electricity, and the pressure at the leak site. Ideally, the pressure at the leak site is best. For example, the compressed air may start at the compressor at 120 psi, but where the air is actually being used it may be regulated down to 75 psi. Look for the nearest pressure gauge, or if someone from the plant is available when the leak survey is being conducted, have someone who is familiar with the compressed air system. For specialty gases such as helium, nitrogen, or argon, the cost of the compressed gas leak is based off the decibel level reading at the confirmed leak location, the pressure, and the cost of the gas as a dollar amount per thousand cubic feet. When noting the decibel level readings from the ultrasound instrument, and for the ultrasonic leak report to be as accurate as possible, the inspector should note the decibel level readings from the ultrasound instrument approximately 15 inches away from the confirmed leak location. If the decibel level readings are taken too close to the leak location, the report likely will overestimate the cost and CFM loss of the leak. Several independent studies have compared ultrasound leak survey reports to actual energy savings, and they have found that an ultrasound leak survey is within 20% of the actual savings of the compressed air leaks. When done correctly, an ultrasound compressed-air leak survey can have tremendous payback in a short period of time – once the leaks have been repaired, of course.
Compressed air is an expensive utility whose maintenance and cost is generally taken for granted. A successful compressed-air leak survey depends on having the right ultrasound instrument for the survey’s needs, proper training of personnel who will perform the survey, planning for how the survey will be performed by doing an initial walk-through, documentation of the leaks and the associated costs, and initiation of repairs once the leaks have been identified. Through proper documentation and reporting, an ultrasonic compressed-air leak survey can show tremendous payback and energy savings without a significant capital expenditure.
