How to Detect Compressed Air Leaks

Allan Rienstra, SDT North America
Tags: ultrasonic inspection

Compared to water, electricity and gas, pneumatic processes are a necessary utility and an important source of converted energy that has been in use long before the Industrial Revolution.

In today's modern industrial operations, few processes rank higher in terms of importance than compressed air, and no process places a higher demand on energy consumption. As a key utility, its uses include running machinery, conveyance in handling systems, and switching for instrumentation and electrical systems.

However, energy demand is negatively impacted when poor maintenance practices allow inefficiencies to spiral out of control. The single biggest culprit is system leaks. Today's compressed air systems are obviously more complex than those from ancient times and hopefully more efficient. It makes you wonder how the Greeks dealt with leaks.

Below is a simple breakdown of a typical investment a company would need to make for a simple compressed air system. The chart reveals that energy accounts for as much as 75 percent of the total system cost. It's a surprising statistic, as conventional logic would have us believe that upfront capital costs and ongoing maintenance costs should dominate. While capital costs for compressors and delivery systems are significant, they are not ongoing. If a system is specified correctly and maintained well over time, its capital costs can be depreciated. A poorly maintained and leaking system will continually drain resources, have a negative impact on energy and never fulfill demand. An inefficient, energy-wasting system further hurts the environment through additional and unnecessary greenhouse gas emissions.

The fact that most people don’t always think of compressed air in terms of energy consumption explains why so little attention is given to finding and fixing leaks. Leaks are expensive. According to the U.S. Department of Energy, average systems waste between 25 and 35 percent of their air to leaks alone. In a 1,000-standard-cubic-feet-per-minute (SCFM) system, 30 percent leakage translates to 300 SCFM. Eliminating that leak is the equivalent of saving more than $45,000 annually. Depending on your plant's location and your region's energy costs, the amount saved can be three to four times higher.

A better understanding of leak complacency is needed to get to the core of the problem. Why do people pay more attention to energy-efficient lighting and continue to ignore inefficient compressed air systems? One explanation is that compressed air leaks are not seen. I grew up listening to my parents say, "Don't leave the lights on," so efficient lighting was ingrained in me at an early age. They never said much about leaks, yet I can still recall leaking air lines in my father's workshop.

In the factory setting, a steam leak is obvious and an oil leak even more so, but air leaks don't create a visible plume, nor do they make a dangerous and slippery mess on the floor. They don't have an unpleasant odor, and their continual hissing is generally not heard or ignored.

So is it truly a case of "out of sight, out of mind?" Is energy waste and system inefficiency still a low priority for manufacturers? Could it be that compressed air is a background process taken for granted?

Consider your compressed air system and all the areas where pneumatics are employed at your facility. Expand your thinking beyond the factory walls where compressed air makes possible many things in science, technology and everyday living. From jackhammers for road repairs and drills in the dentist's chair to the tires that roll us to work, school and play, compressed air is all around us … and yes, it is taken for granted.

A culture change is occurring in industry where it's needed most. Industry represents the biggest consumer and therefore the largest potential gain. It is a dual challenge and a dual opportunity. The challenge is to invest in more efficient energy and environmentally conscious practices. The opportunity is to improve profitability and slow the effects of global warming.

There is an insatiable thirst for electricity, and the fossil fuels necessary to quench this thirst are being used up at rates we cannot afford. Two concerning side effects are the diminished supply of non-renewable fuel sources and the effect that increased levels of carbon dioxide have on global climate change. Diminished supply means continued higher energy costs, while global climate change represents something much more expensive.

Not all companies are sitting idly by waiting for others to take action. Many have already begun programs that address energy efficiency and specifically target the compressed air system. AFG Glass is the second largest flat glass manufacturer in North America and the largest supplier to the construction and specialty glass market. Founded in 1978, AFG is headquartered in Kingsport, Tenn. With its three divisions, it is a fully integrated supplier. One division is responsible for flat glass manufacturing, another for advanced energy-efficient coatings and a third fabrication division adds value to the finished product through tempering, laminating and insulating. In total, AFG has nine glass production operations, 34 fabrication/distribution centers, four sputter coating lines, five insulating plants and one laminating facility. They have more than 4,800 employees working in their North American operations.

Some of AFG's manufacturing divisions implemented airborne ultrasound programs in 2006. Ultrasound was considered for its reputation as an overall predictive maintenance and troubleshooting tool. When several technicians attended ultrasound certification training, they learned the technology they invested in would be used for much more than troubleshooting.

Ultrasonic leak detectors work like simple microphones that are sensitive to high-frequency sounds ranging beyond the human ear. Early detectors enabled problems with machinery to be heard on the factory floor regardless of background noise. As the technology has grown up, so has its form and function. Today's ultrasound detectors can be simple leak detectors or advanced data collectors capable of trending and diagnosing machine failures and plant inefficiencies.

Ultrasound detectors use a sensitive piezoelectric crystal element as a sensing element. Small high-frequency sound waves excite or "flex" the crystal, creating an electrical pulse, which is amplified and then translated into an audible frequency that an ultrasound inspector can hear through high-quality noise-attenuating headphones.

As a leak passes from a high to a low pressure, it creates turbulence. The turbulence generates a high-frequency sound component, which is detected by the crystal element. Higher frequency sounds are directional by nature. By detecting only the ultrasound component of a turbulent leak, the technician is able to quickly guide the instrument to the loudest point and pinpoint the leak.

A typical compressed air system can be surveyed for leaks in one or two days. Larger plants may take longer, but the benefits to finding and fixing leaks are well worth the investment in time.

Several ultrasonic detectors use parabolic reflectors or elliptical reflectors to enhance and concentrate the leak signal, which can be useful when detecting small leaks or scanning at a great distance. Imagine scanning all the overhead piping without ever climbing a ladder or scissor lift. Parabolic accessories are key to enhanced productivity and operator safety.

Douglas Bowker is the plant maintenance superintendent at AFG Industries' plant in Greenland, Tenn. He has been instrumental in the implementation of ultrasound testing to improve the well-being of his plant's equipment.

"Compressed air is not free," Bowker says. "It costs Greenland approximately $137,000 per year to supply compressed air to the plant. Air leaks therefore cost us money. A small leak that is undetected by the human ear can typically contribute to $3,000 of cost per year. The ultrasonic equipment can now be utilized in a cost-saving manner to detect such leaks and fix them proactively."

Douglas is quick to add that an air leak the size of a pinhole can be detected from a distance of 40 feet. In addition, his technicians can detect natural gas, nitrogen and hydrogen leaks. The ultrasound equipment is also useful in detecting leaky or malfunctioning valves and can help detect flow in pipelines from a distance.


About the Author

Allan Rienstra is the president of SDT Ultrasound Solutions and co-author of "Hear More: A Guide to Using Ultrasound for Leak Detection and Condition Monitoring." He has spent the past 20 years helping manufacturers around the globe establish world-class ultrasound programs. Contact ...