Ultrasound condition monitoring: hearing is believing

Follow this first hand account of ultrasound condition monitoring and learn the basics of this useful practice.

By Roger Earley

The Lubrizol Corp. is a specialty chemical company that produces and supplies technologies to improve the quality and performance of our customers’ products in the global transportation, industrial and consumer markets. Our first operations, in 1928, were in a small garage in Cleveland. Today, the world headquarters in Wickliffe, Ohio, consists of a number of buildings scattered throughout a large campus.

My duties as a maintenance specialist are to perform predictive maintenance surveys on critical and non-critical pieces of equipment using airborne ultrasound, infrared thermography and vibration analysis. My goal is to locate minor problems before they become catastrophic failures that could result in hundreds of thousands of dollars in lost income.

The power of predicting

Companies are in business to be profitable. If a piece of equipment fails unexpectedly, the cost could be enormous. That’s why it’s essential to keep equipment up and running and to avoid breakdowns. If a production line bearing in a paper or steel mill locks up, the costs can run into the millions of dollars.
We have hundreds of pieces of equipment at our Lubrizol facility, each critical to our operation. Together, they have several hundred bearings, essential components in all rotating equipment. Bearings hold and support shafts so that pumps, fans and other machinery can smoothly and freely rotate. As the internal components in moving machinery begin to wear, friction increases, which changes the ultrasonic sound signature. A change in ultrasound becomes apparent before other signs of wear appear, such as vibration.

Figure 1

Like vibration analysis, ultrasound trending can help you make judgment calls. When you ask an equipment operator to take a pump, fan or motor offline because the bearings are bad and need to be replaced, the operator will ask two questions: “Are you sure?” and, “How long will it take?” With trending, you have a history on the bearings that helps you diagnose the problem. Without that history, you are guessing.

Ultrasound allows you to gather more details about how a machine is operating. For example, it might emit a rough grinding sound when it needs to be lubricated. Or, it might give off a rushing water sound when it’s over-lubricated, which can be equally problematic and can cause the bearing to overheat. Using ultrasound to identify bearing wear in its earliest stages is a valuable tool that avoids costly downtime.

How ultrasound works

Airborne ultrasound is an acoustic frequency that a human ear can’t hear. Human-audible sound ranges from 20 Hz to 20 kHz, with the average being approximately 16,500 Hz. Airborne ultrasound allows us to hear sounds in the 20 kHz to 100 kHz range. A transducer mounted in either a scanning module (used for airborne sounds) or a contact module (used to touch an object) captures the sound and transfers it to the instrument. Through a process called heterodyning, the inaudible signal is converted into a frequency that we can hear through headphones. Also, the sound intensity (in decibels) is displayed on the instrument’s meter. The unit has a compact flash card to record sound wave signature.

Figure 2

By either touching the instrument to a test spot or pointing it at a target, you can hear the ultrasonic sounds through headphones and determine the source on the basis of intensity. The advantage of using ultrasound is that the source of the ultrasonic sound is fairly directional and can be easily identified with little interference from competing sounds.

Figure 3

Ultrasound machines are lightweight and portable, and can be used with relatively little training. The applications for ultrasound are numerous, covering most potential mechanical, electrical and leak problems. Also, ultrasonic technology can detect, trend and analyze potential failure conditions earlier than traditional predictive maintenance technologies.

Fundamentals of repeatability

There are five important things to remember when you trend wear in rolling-element bearings:
  • The most important thing is to have repeatability.
  • Select a location on the machine to take your readings, and take them at the same location every time.
  • Approach the machine from the same angle each time.
  • Apply the same amount of pressure when taking the readings. Too little pressure causes the reading to be lower; too much pressure causes the reading to be higher.
  • Know the machine’s speed and loading because differences affect your readings.

What should I monitor?

You'll have to decide. But keep in mind that no piece of operating equipment is too big or too small to be monitored. If it’s important to you, your operation, or your customer, then it’s worth monitoring and trending.

How often should I monitor?

This is where you need to set your priorities. How critical is the equipment? If you lose a pump, fan or a motor, will your operation cease? If so, you should put these items on a monthly schedule. Is there a backup? If one fails, can another take over? If so, check them every other month. If you could continue to operate even if a piece of equipment fails, check them quarterly. These monitoring schedules are only a guideline. Every situation is different. Plant size, equipment to be checked and the number of trained personal to take the readings affect your scheduling.

Establishing baseline readings

When trending bearings, it’s essential to establish an initial baseline reading. The first reading is my baseline. The best time to take the initial reading is when a motor, fan or pump is new.
If the equipment has been in service for some time, you can still take your initial readings. If there’s more than one piece of identical machinery, compare the readings from each at the speed and under the same loading. Then, use the lower readings as your baseline. It doesn’t matter if the equipment is new or old, the important thing is to start a trending program. Some ultrasonic units have software that allows you to change the baseline after an overhaul or equipment replacement.

Taking and interpreting readings

Repeatability is an essential part of trending. Select a location on the equipment to take your reading. Mark the spot with a dab of paint so you can take future readings at the same location. Pay close attention to the contact pressure you apply, and approach from the same angle each time. Set the ultrasound instrument range at 30 Hz.

I follow the same route through the plant when collecting vibration data. Whenever I take a vibration reading, I also take an ultrasound reading. I save the decibel reading, and if I notice the bearing is emitting an unusual sound, I record it for later playback and analysis with the software.

Figure 4 

Figure 4

When I return to the office, I download the data into the software, which saves the readings, the date and time they were taken and the sound wave recordings. The software allows me to both hear and see the readings I took. Once I establish a baseline, I can trend the readings and compare each new reading to the previous reading and the baseline. A current reading that exceeds the baseline by eight decibels is a sign that the bearing lacks lubrication. At a difference of 12 decibels, you can expect minor damage and microscopic faults. When you reach a 16-decibel difference, damage is visually apparent. And a difference of 35 decibels or more indicates catastrophic failure.

Better software provides a choice of charts that you can graph: date, time, decibels, low-level alarm, high-level alarm, rpm, temperature, frequency and more. I normally set my low-level alarm at eight decibels over the baseline. My software automatically sets the high-level alarm at twice the low level.
The program allows us to watch what a particular piece of equipment is doing. We can adjust the alarm level to whatever suits our needs. If a reading exceeds the low-level alarm, the date at which it was taken turns yellow, alerting us that this is something to watch more closely. If the high-alarm level is exceeded, the date turns red, which also gets our attention.

When you locate a problem, initiate a maintenance request to repair the equipment. Including a trend chart in a report shows why a machine should be taken offline. And with trending, you can prove you didn’t miss something; you have the records to prove it.

After you make the repairs, take another set of readings to ensure the problem has been corrected and to reset your baseline. However, don’t delete your earlier data. It provides a valuable history. If a problem repeats, you can check the history. If the problem persists, you might decide, for example, that the application, not the bearing, is at fault.

The savings Although I’m a 27-year veteran of Lubrizol, I didn’t use ultrasonic instruments until I assumed maintenance specialist responsibilities in 1998. Back then, we were using only vibration analysis to inspect equipment. However, I discovered a hand-held instrument sitting in a corner, still in the original carton, collecting dust. After I read the instruction manual and learned about its capabilities, I was so enthused that I taught myself how to use it. After I demonstrated what the instrument could accomplish, I took a Level 1 ultrasonic course. Back at the plant, I started a steam trap inspection program. Within the first year, we saved about $85,000 in steam generation cost because of the failed traps we found. Two years later, I took the Level 2 course. I continue to expand the program and incorporate it into all of our surveys.

Although no single instrument does it all, you can use vibration analysis, infrared thermography and airborne ultrasound together to survey equipment. But because of its reasonable cost, the ease of use and its versatility, ultrasound equipment can fit into any reliability maintenance program.

Roger Earley is maintenance specialist at The Lubrizol Corp., Wickliffe, Ohio. Contact him at rle@lubrizol.com and (440) 943-4200.