Ultrasound testing and acoustic imaging are predictive maintenance technologies that can be used for condition monitoring as well as detecting leaks and other problems. These detect sound waves at frequencies that are inaudible to humans. Having real-time knowledge of asset health, as well as changes to asset condition, gives maintenance teams the ability to maximize asset lifecycles.
Collecting asset health data over time makes it possible for teams to identify patterns and find potential issues before damage or downtime occur. Once baselines are established, future measurements can be compared to baselines to determine necessary next steps. Ultrasonic and acoustic imaging are highly effective failure detection techniques and can serve as a core part of a condition monitoring program. Condition monitoring improves efficiency and safety, while at the same time minimizing maintenance spending and unplanned downtime.
Ultrasonic testing and acoustic imaging can detect leaks, flow and lubrication problems. The American National Standards Institute defines ultrasound as “frequencies greater than 20 kHz.”
Key terms to know
Ultrasound: Sound waves with an ultrasonic frequency, higher than the limits of human hearing.
Acoustics: The study of mechanical waves, including vibration, sound, and ultrasound.
Decibel: A logarithmic unit for expressing the pressure of a sound.
Applications of ultrasound technologies
Like other predictive maintenance technologies, such as vibration monitoring, there are numerous types of tools that use ultrasound. Ultrasound sensors can be placed directly on assets (structure-borne) and airborne, connected to handheld tools that can be taken on regular routes or inspections. Sensors and handheld ultrasound tools both can pair with software to analyze the data collected.
Ultrasonic tools can be used to monitor a wide variety of equipment and component types.
Ultrasonic monitoring is an effective, non-destructive method of detecting impending faults in bearings, determining whether adequate lubrication is present, detecting cavitation and detecting other common types of machine failures. Ultrasound can also be used to monitor gears and gearboxes, pumps, motors, steam traps and valves. It is a fast and effective method of identifying the condition of mechanical components.
All mechanical equipment produces some sound. Each asset has a sound signature in the same way as it has a signature vibration pattern. And just as with changes to an equipment’s vibration pattern, changes to a sound signature indicate potential faults or failures. These shifts can be analyzed with software.
Advantages of ultrasonic and acoustic imaging
Friction, impacting, turbulence and electrical discharge all produce detectable ultrasound waves. Early warnings of potential faults are detectable by ultrasound earlier than other modalities, such as infrared and vibration.
Because ultrasound picks up signs of problems so early on, maintenance teams have plenty of time to perform checks such as lubricating bearings, fixing leaks and correcting other problems. The testing itself can be performed while equipment is operating.
Ultrasound waves are directional, meaning they can be easily located. This does not just signal an issue — it means that the location can be pinpointed. Because the sound waves are not detectable to human ears anyway, the tools can be used in even the noisiest environments. Ultrasound and acoustic tools are also versatile, able to detect a wide range of issues across many asset types.
Ultrasound and bearing monitoring
Up to 80 percent of premature bearing failures are the result of either over- or under-lubrication. Over-lubrication tends to have the most detrimental effect on bearing life. Other bearing failure possibilities include contamination, using an incorrect grease and material defects. In addition, bearings can experience indirect failures, such as improper machinery mounting.
Ultrasonic technology can be used for bearing monitoring to identify the signs of a defect early on. Using ultrasound as a modality means measuring the decibel, or sound level, of friction that a bearing experiences. And though there will always be some amount of friction, maintaining the right quantity of lubrication will keep friction at an acceptable level.
When it comes to bearings, well-lubricated ones have steady friction. This means that decibel levels are more constant, with peaks and valleys of less than 4 decibels. Increases in friction produce more significant peaks and valleys, which are caused by impacts of rolling elements.
This is an area where trending condition monitoring data over time provides insights into the status and needs of an asset. As a bearing’s condition approaches different thresholds, different reactions and response times are recommended. Peaks and valleys of around 8 decibels indicate that a bearing needs lubrication. It is even possible to add grease remotely using an automatic lubricator. With peaks and valleys of 16 decibels, the bearing has gone beyond needing lubrication alone and indicates damage. 35 decibels above baseline indicates that the bearing is close to failure. All of these ranges can vary slightly, depending on the bearing type, but they are based on the ISO29821-1 standard.
Condition-based maintenance, rather than time-based maintenance, is preferable for bearings. That is because no two bearings are manufactured, installed or lubricated exactly identically. With so many variables, time-based corrective actions rest on a number of assumptions. Monitoring bearings in real time provides a much more accurate view into bearing health.
Why leak detection matters
Acoustic imaging’s leak detection and location capabilities make it an effective tool for minimizing energy waste. Leak detection is also useful for boilers, condensers, chillers, heat exchangers or any other type of system that relies on pressure or vacuum. Steam, water, gas, air or other liquids can all leak and all of these leaks produce sound waves that ultrasound and acoustic tools can pick up.
Acoustic emissions from assets may not be detectable by human hearing, but high frequencies — and their short waves — can be detected and located with the right tools. Ultrasonic waves can be separated out from among other equipment and background sounds.
When a leak occurs, the intensity of the ultrasonic signal is strongest at the source. Compressed air leaks are just one type of leak that can be identified and located this way. Not only is compressed air expensive, but it consumes a great deal of electricity. Up to 30 percent of that electricity goes toward pressurization. Detecting and resolving leaks can save facilities a lot of money.
Condition monitoring and predictive maintenance
Predictive maintenance is a strategy that determines when maintenance should be performed by monitoring and evaluating asset condition. Predictive maintenance ultimately saves time and money because maintenance tasks are only carried out as needed.
Frequent asset health data helps maintenance teams keep all of their assets running better, longer. That is because the data offers insights into which tasks need to be performed when. Teams can prioritize their work not only to prevent failures, but to prevent maintenance tasks from being performed too frequently. This can happen when maintenance is based on usage or a calendar, rather than on an asset’s condition. Maintenance done too frequently is not only wasteful, but it increases the possibilities of errors.
In order to use ultrasound and acoustic imaging as part of a condition monitoring program, first identify which assets to test. The type of assets chosen can help determine which specific tools to use. Establish baselines and thresholds that determine when an asset needs attention.
Ultrasonic and acoustic tools, when paired with condition monitoring analysis software, can be crucial parts of a predictive maintenance program. But most mature maintenance teams use more than one modality in their condition monitoring programs. If your team is struggling to determine which predictive maintenance technologies are right for your specific needs and assets, or needs help getting started and sustaining a program, there are condition monitoring services available with experts who can assist.
As a mechanical application and product specialist with Fluke Reliability, John Bernet works with customers from all industries to successfully implement their reliability programs. He has more than 30 years of experience in the maintenance and operation of commercial machinery and as a nuclear power plant electrician in the U.S. Navy. He holds a Category II Vibration Analyst certification and is a Certified Maintenance Reliability Professional (CMRP).
