How are you using ultrasonic analysis?

Andy Page
Tags: ultrasonic inspection

My focus this issue will be on airborne and structure-borne ultrasonic analysis. We refer to it as such because we need to distinguish this type of ultrasonic analysis from pulse-echo ultrasound.

Airborne and structure-borne ultrasound is sometimes referred to as passive ultrasound. This is because the source of the ultrasound isn't coming from the ultrasonic meter but from the component being analyzed (e.g. pump bearing, steam trap or high-voltage switchgear). Pulse-echo ultrasound is active ultrasound, named for the fact that the meter/sensor combination is the source of the ultrasound. The signal is generated from the sensor and transmitted into a material. This is done to measure material thickness and/or detect subsurface discontinuities.

For the rest of this article, I will concentrate on passive ultrasound and refer to it simply as "ultrasonics". Ultrasonic analysis has a wide variety of uses. There are numerous examples of applications for detecting failure modes on mechanical, electrical and stationary equipment. The technology is powerful and flexible in that numerous failure modes can be detected on different types of components. It's easy to implement (relative to other technologies) in that a single four-day class is all most people need to start detecting machinery defects. And, it is affordable (relative to other technologies). You can buy a basic ultrasonic device for less than $2,000.

The data collection methods determine the types of detectable failure modes. The two methods are contact (structure-borne) and non-contact (airborne).

Contact methods are typically used for mechanical applications such as bearing faults, lubrication issues, gear damage and pump cavitation. What did you notice about these four faults? What do they have in common? They are all defects that give off high-frequency noise (vibration). We have now discovered one of the limitations of ultrasonic analysis. It's only capable of detecting high-frequency defects. This is a function of how the ultrasonic meter works. By manipulating the ultra-high-frequency sounds, the meter makes it possible for us to hear them. So, the meter was designed specifically for high-frequency sounds. Low-frequency faults like imbalance, misalignment, looseness and coupling issues can't be detected using ultrasonics.

Contact methods are also very useful for detecting electrical faults on motors. Motors with problems like loose/broken rotor bars produce a very high-frequency, rhythmic pattern that is easily detected with the ultrasonic meter. As for stationary applications with contact methods, steam traps are a very common application. Steam traps that have failed often are "blowing by" (steam constantly leaks past the internal seals) and parts of the trap may be rattling. Both of these produce ultrasonic noise that can be detected through contact measurements.

Uses for non-contact measurements include pressure and vacuum leaks on compressed gas (including air) systems and numerous electrical applications. Airborne ultrasonic analysis is the perfect way to find compressed gas leaks. Air leak surveys, as they are often referred to, can produce big money-saving opportunities in the form of reduced production costs for compressed air and extended compressor rebuild schedules. The most well-known electrical application is the use of airborne ultrasonics to detect the presence of arcing, tracking and corona in high-voltage electrical apparatus. This is especially powerful as infrared thermography can't detect arcing and tracking and can only detect corona under very special circumstances. This makes the use of airborne ultrasonics a key part of your electrical asset health program.

Another application that is often overlooked is the use of airborne ultrasonics as an added safety measure when opening electrical cabinets. An example of this might be during a routine IR thermography scan. Pre-screen the cabinet doors with an ultrasonic meter to detect any arcing that may be occurring in the breaker or starter. This arcing will ionize the air inside the cabinet. When the cabinet door is opened, the fresh air becomes a welcomed path to ground, producing an extremely dangerous arc flash.

These are just a few applications for ultrasonics. How many of these does your program have? Does your equipment maintenance plan call for the use of this highly flexible, very capable and quite affordable technology? Are these tools available to operators and crafts personnel for daily inspections and troubleshooting? Or, are they solely the domain of the condition monitoring personnel?

Andy Page is the director of Allied Reliability's training group, which provides education in reliability engineering topics such as root cause analysis, Reliability-Centered Maintenance and integrated condition monitoring. He has spent 15 years in the maintenance and reliability field, holding key positions at Noranda Aluminum (maintenance engineer) and Martin Marietta Aggregates (asset reliability manager). Andy has an engineering degree from Tennessee Tech and is a Certified Maintenance and Reliability Professional (CMRP) through the Society for Maintenance and Reliability Professionals (SMRP). Contact him at pagea@alliedreliability.com.

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