November 2020

O ften, when a compressed air system struggles to meet the current demands on the system, spare compressors are rented and used as back-ups or time and effort is placed in installing an additional compressor to the existing system. Both strategies are expensive, and depending on the size of the compressors needed, could mean hundreds of thousands of dollars. Since 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. Such a program is designed to identify and repair compressed air leaks before they become a large problem and it can save time, money, and energy. For airborne ultrasound, compressed air & compressed gas leak detection remains the most widely used application. Locating compressed air and gas leaks with ultrasound, and then making the necessary repairs, can have tremendous payback in the dollars lost due to these leaks. Recent advancements in compressed air leak detection and reporting allow for the quantification of the money lost from these compressed air leaks. An effective ultrasonic compressed air leak survey will focus on seven key factors: Evaluation Detection Identification Tracking Repair Verification Re-Evaluation By implementing these steps, a typical manufacturing plant could reduce its energy waste by roughly 10% to 20%. How to Use ultrasound for a compressed air leak survey Select an ultrasound instrument For ultrasonic leak detection, an ultrasound instrument that has frequency tuning capability is recommended, and the suggested frequency setting is 40kHz. For ultrasound instruments that are on a fixed frequency, or where frequency tuning is not a feature, 38kHz is usually the frequency setting that the instrument is fixed at. 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. Turbulence is created when a compressed gas inside of a pipe or vessel exists to low pressure or atmosphere through a tiny crack or orifice. Turbulence is also created when there is air in-leakage, or vacuum leaks. With vacuum leaks, since most of the turbulence is on the inside of the leak, there is not as much ultrasound present; therefore, vacuum leaks are more difficult to find with ultrasound, but is still possible if enough turbulent ultrasonic noise is present. The “gross to fine” method Once an ultrasound instrument 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 bounce and reflect off of solid surfaces. That’s why it is important to scan in all directions with the ultrasound instrument, while adjusting the sensitivity. This will help to 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 on the instrument to more finely narrow the field of view to more precisely identify the location of the leak. This method of compressed air leak detection using ultrasound is commonly referred to as the “gross to fine” method. Creating an inspection route The logistics of the leak detection route should now be considered. It is recommended to perform a walk through prior to the inspection. The inspector should use this as an opportunity to determine the specific zones or areas where the compressed air is being used. Blueprints of the compressed air piping are also a handy resource when conducting the initial walk through. Make note of any safety hazards and any 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. Making note of any areas of potential leakage or misuse of compressed air (such as using air to move parts/product, air knives, etc.) will help to take away any confusion of what the inspector is finding and becoming 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. Process, Controls & Plant Focus on: Compressed Air 22 | Plant & Works Engineering www.pwemag.co.uk November 2020 Best Practices for ultrasonic compressed air Contrary to what some might think, compressed air is not free. In fact, for what it takes to produce it, to what is generated it is often considered the most expensive utility in a typical manufacturing facility. Air Compressor experts have also estimated that as much as 30% of the compressed air generated is lost via leaks in the compressed air system.

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