First, the bad news: any compressed air system can experience leaks, which amplify the load on compressors and increase costs. Essentially, such leaks are caused by fluid flowing from a high-pressure to a low-pressure environment, creating turbulence, which generates high frequency sounds (so-called ‘ultrasound’). So, what are the most common leakage sources to be aware of? Equipment left running when it is not required; manual condensate drain valves being left open; plus leaking hoses, couplings, pipes and pipe joints.
However, the air leaks themselves are not the real problem, but rather that these are often ignored, since they are not seen as an immediate health and safety hazard. “Yet reducing air leaks is the single most important energy-saving measure operators can take,” says Roy Brooks, technical development officer at the British Compressed Air Society (BCAS). “A high leak rate causes fluctuation in pressure, resulting in hidden costs, such as slower running or the stalling of production lines, and also creates a noisy environment for staff.”
As simple as it sounds, the first step to reducing the impact of compressed air leaks is to identify where the wastage is occurring. “Operators should initially conduct an out-of-hours survey and walk the site, listening for leaks,” Brooks advises. “The location can then be confirmed, using a number of methods.
A soap solution brushed on to pipe fittings or a leak detection spray are tried and tested options; but, for greater speed and accuracy, a handheld, ultrasonic leak detector provides the best means of detection while production is running.”
Superhuman hearing
Over 80% of the acoustic noise produced by a compressed air leak is above the range of human hearing, which is why ultrasound figures so large in the detection of these leaks, being typically used in the 36-40kHz frequency range. The detected sound is then electronically processed to make the signal audible – thereby giving the operator, as Thomas J Murphy, MD, Reliability Team, SDT Ultrasound Solutions, puts it, “the sixth sense of superhuman hearing”.
Ultrasound detectors empower users to do far more than simply find leaks, though. They also deliver the means to predict failures, control energy costs and improve product quality. By listening uniquely in the ultrasonic region described above, the majority of the plant and process ‘audible-to-human’ noises will disappear, “which means that leak detection can be performed any day of the week – and not just on Christmas Day or Boxing Day, as I have been told in the past”, Murphy adds.
Moreover, new-generation ultrasound devices are super-sensitive and can be equipped with sensors that will make possible air leak detection of piping that is over 20 metres distant or way up in the roof. “Reporting has been greatly simplified, too, with the introduction of mobile apps that allow the inspector to take pictures, store leak site information, assign severity, note repair actions needed, compute loss calculations and compile a cloud-based report – all while walking around performing the inspection.”
Ultrasound is no new-fangled ‘saviour’, of course. It has been used for detecting compressed air leaks for more than 40 years. Murphy recalls reading an article written nearly 30 years ago that explained the necessity of leak detection for compressed air. Then he wrote an article himself almost 20 years ago, on the same subject… and 15 years ago, and 10 years ago.
“So, why on earth do we need to revisit the same subject over and over again, with a similar message?” he asks in understandable frustration. “Where is the disconnect between what has been known for decades and the way we run our plants today? Why is this knowledge not being disseminated and acted upon?”
Part of the answer might be to do with the fact that there are tax breaks for investing in new compressors, but no tax breaks for fixing the leaks. Also, 75% of the total life-cycle cost of a compressor is energy, and the typical production budget includes the acceptance of the waste involved in the process. He offers an example of how much a leak might impact the business, in hard cash terms. “A 3mm-diameter leak in a 7-bar system at 5p/kWh, running 24/7, would cost roughly £1,750 per year. How many widgets do you need to sell, produce, package, deliver, invoice and chase the money for, in order to make that amount of profit; which will only pay for that one leak?” Leak savings are cumulative.
Then there is the growing commitment to ‘being green’ and the use of ultrasound to carry out the leak surveys in line with that objective. Yet he notes that carrying out such surveys “does not make you green – fixing the leaks that you find does”. Nor is the occurrence of leaks an historic problem that will gradually disappear as old plant is taken out of service. “There are thousands of wasteful applications of air still designed in to new plant today,” he states. “Without a coherent location and repair strategy, you are probably losing 25-40% of the air you produce; this loss could be reduced to around 5%.”
Under review
Conservative analyses estimate that compressed air systems account for 10% of all energy used in global industry, and that 10-30% of this is wasted on leaks. Even a 3mm leak could lead to energy loss equivalent to 16 tonnes of CO2 emission, cautions Stef Lievens, business line manager for Industrial Air at Atlas Copco Compressors UK.
Nevertheless, compressed air energy audits are not as ubiquitous as they could be. Some users are simply not aware of the extent of any on-site leakage or the associated cost; others are concerned that any audit will be disruptive and time-consuming. “Yet, when done correctly, auditing can be efficient, fast and hassle-free, while highlighting opportunities for energy-saving and process improvements,” he states. “In fact, initial steps can be taken that involve little or no technology. First, a compressor can be run without attached equipment operating to minimise background noise. Some leaks can then be found simply by listening, although larger ones may be inaudible or only occur at certain pressures. Others may be inaccessible.”
But it is ultrasonic devices he also singles out as the best means of detection, with various hand-held, battery-operated probe devices available that can be aimed at the pipework under investigation. One such type of device electronically converts the high-frequency emissions into lower-frequency equivalents audible through a headset, while another is designed as a camera that shows images of the surveyed pipework, with any detected leaks appearing as bright dots.
“These ultrasonic instruments’ money-saving benefits are maximised when they are used within an ongoing compressed-air system monitoring strategy,” says Lievens. “This presents survey results to management in a clearly understood, actionable format; also, the survey can be repeated at regular intervals – typically every six months or so – to catch new leaks as they appear.”
Atlas Copco performs ultrasonic leak surveys as part of its AIRScan energy audit service, which checks flow, power and air quality, and assesses maintenance levels, as well as leaks. A typical AIRScan can lead to 25-30% energy savings, it is claimed. “For example, in one recent Atlas Copco project, a leakage detection programme instigated across a UK vehicle manufacturing plant identified leaks costing £102,000 a year in lost energy. A similar programme, conducted over three days by Atlas Copco’s AIRScan team at the site of a UK-based food manufacturer, pinpointed £49,000 worth of leaks, demonstrating the benefit of conducting this type of audit. If the AIRScan engineers don’t find leaks to cover the cost of the survey, there’s no charge.”
BOX OUT: Sounding out solutions
Below is a taster of ultrasonic leak detection solutions available, although there are many more on the market, so thorough research as to an organisation’s precise requirements is the starting point.
SKF: The SKF TKSU 10 (pictured above) is an ultrasonic leak detector that, states the company, helps users to find leakages quickly in compressed air or vacuum systems. The built-in LED display assists the user in adjusting sensitivity and shows the measured ultrasound noise from leaking air, allowing the quantification of leaks and prioritisation of repairs.
SDT Ultrasound Solutions: SDT’s latest launch, as part of its asset reliability strategy, is the SDT340 data collector and UAS4.0 – a cloud-connected condition monitoring solution that is said to combine the power of ultrasound, vibration, temperature and RPM in one diagnostic package. Data acquisition time is adjustable from a few seconds up to 10 minutes.
Sonotec Ultrasonic Solutions: Whereas ultrasonic testing devices usually record a frequency range around 40 kHz, Sonotec Ultrasonic Solutions says it is breaking new ground with the Sonaphone ultrasonic testing device. “The main difference between this device and the previously available devices is the broadband technology of the sensors in a frequency range from 20 to 100 kHz. This makes applications like automatic leak analysis more reliable.” Using the touchscreen, users can move the frequency they want to hear out of the range of interference.