Similar beginnings, but different outcomes, and completely different roles in the greater scheme of things...We see these facets play out in many areas of everyday life. It happens in predictive maintenance as well, and it may surprise many of you to discover one scenario. Two technologies springing from similar beginnings, but moving into very different everyday uses in plants worldwide.
In predictive maintenance, we are blessed with technologies that help us transcend our senses' limitations. They allow us to see, hear, and feel events that give us the information about our machinery which we need in order to make informed decisions.
Infrared Technology allows us to see the spectrum of light that is invisible to our eyes. This capability becomes very useful if we want to know if there is a temperature variance between different objects or of the same object at different times.
Airborne Ultrasound extends our ability to hear sound waves that are above our hearing capability. There are many sources of high frequency sound waves: turbulent flow of material passing through a restricted opening is one, friction energy generated from the rolling elements of a bearing as it rotates inside the housing during operation is another, ionization energy also produces high frequency sound waves, and impact energy generates high frequency sound waves. Do the descriptions above give you some insights as to which areas in your plant may possibly have high frequency sound waves?
Vibration Analysis extends our sense of touch by a few leaps. It detects and measures vibration movements in machineries that are too small for us to feel, then allows us to identify the sources of these vibrations, which are then categorized according to severity. Informed decisions can then be made from there. We have made a few leaps from putting a coin on top of a motor to see how badly it is vibrating. Of course, with the coin, you'll see the coin vibrate, but you can't measure its vibration.
Oil Analysis gives us the ability to look at the chemical composition of used oil in the machines to see if it is normal or not. Any unacceptable deviation from the norm can give indications of problems.
Airborne Ultrasound has several applications in your plant. Based on the explanation above, this technology can be useful in locating air leaks (Turbulent Flow), checking for passing valves or steam traps (Turbulent Flow), managing bearing lubrication better (Friction), detecting flow or no flow situations (Turbulent Flow), detecting and locating sources of electrical discharge (Ionization), and in what I Call ultrasound PdM (Friction). For purposes of this article, I'll narrow the ultrasound discussion to the condition monitoring, or ultrasound PdM, application since that is also the area where machine vibration analysis is used. We'll talk about when to use each one by itself, and when to combine these two powerful technologies.
Similar Beginnings
Interesting enough, vibration analysis and ultrasound have similar beginnings. The accelerometers that vibration analysis uses rely on piezoelectric crystal to detect the changes in acceleration as a body oscillates or moves in a repetitive motion. The contact rod that airborne ultrasound uses in bearing inspection also has piezoelectric crystal that moves in response to the amount of ultrasonic signal generated from the friction energy as the bearing rotates in its housing. What differs is the specification of the crystal, the way the signal from the crystal is processed and the resulting information that comes out.
Two (or more) is Better Than One
For Ultrasound PdM, or the bearing inspection application of ultrasound, the inspector gets a reading from his ultrasonic detector that relates to the amount of ultrasonic energy generated from the bearing. Care must be taken, and proper procedure should be followed, to ensure the validity of data. It is quite easy to get an erroneous reading. The inspector should always try to contact the spot closest to the bearing, because then it is likely that the reading is from the bearings and not from something else. What energy did he pick up? Friction? Impacting? Or Both?
For example, I recently conducted both a vibration and ultrasonic test on the outboard bearing of a pump for a customer. The ultrasonic reading was higher than normal. Sound quality was rough, similar to the sound produced when you have sand between your wet hands and you rub your hands together in a rotational motion instead of a back and forth movement. What could I deduce from this information? Namely, that there was more ultrasonic energy being produced than before, and that the sound was rougher than it used to be. Could I say that there was an outboard bearing problem based on the higher reading and the sound quality that I heard through the headphones? No, at least not yet.
Ultrasound will give an indication of the change in ultrasonic level and the quality of the sound heard (subjective information), but it cannot tell the specific cause of the change. Some highly experienced ultrasonic inspectors may be able to have a pretty good idea of the bearing condition based on the sound quality, but to transfer that experience to another ultrasonic inspector is difficult. This is when vibration tests come in. It is important to get more detailed information from another source in order to have a better picture as to the condition of this pump. And vibration analysis fits in perfectly in this type of situation.
Vibration Analysis detected that there was serious impacting happening in the bearing housing. Visual inspection showed grease coming out from the bearing seals, a similar situation to that in Figure 4. All three inspection methods - ultrasound, visual, and vibration - gave me much more comprehensive information to make the proper recommendation. This is a simple example of how the different condition monitoring technologies can be used together to increase effectiveness.
Sound Can Be Misleading
We live in a real and complicated world, and sometimes the real world gives us combinations of events that throws us for a loop and changes our perception.
One time I was in another customer's facility doing routine machine inspections. The ultrasonic reading was higher than normal on one pump in particular - high enough that ultrasonic guidelines called for possible incipient bearing failure. Spectra from machine vibration did not indicate any problem. I began scratching my head in thought. Hmmm, why was there so much more ultrasonic signal being generated from this pump?
I went to operations to ask a few questions. They told me that there was an increased flow going through the process. Of course, this meant that the machines were doing more work and, hence, a higher turbulence was being generated by the pumped liquid.
The machines were designed to handle the increased workload. So the ultrasonic energy level changed, but not because of a physical defect on the bearing. I learned the lesson that going back to the basic understanding of where and how ultrasonic energy can be generated is helpful, and important, when doing troubleshooting.
Spectra analysis of machine vibration is useful in identifying the different forcing frequencies that are affecting the machine. These forcing frequencies can be trended over time to establish the rate of machine deterioration due to the specific forcing frequency.
It is very important that a routine machine vibration analysis is performed to catch these changes as well as identify any new forcing frequencies that are attributed to machine problems.
Trend
Airborne ultrasound is useful in two areas of machine condition monitoring. One is in the lubrication of bearings, and the second is in Ultrasound PdM, or condition monitoring. Integrating ultrasound in bearing lubrication is straightforward, as long as one follows the proper lubrication procedure to know when to stop adding grease. In Ultrasound PdM, one can do the comparison, or trending, method. This procedure is also straightforward, but the analysis of dB readings may not be. Inspector knowledge in ultrasound, as an inspection technique, process operation and machine operation comes into play in order to make sense of what the machine is actually telling you. Generally, relying only on Airborne Ultrasound for machine diagnostics will not be enough. Often, you will need to use another inspection tool, like a vibration test, to zero in on machine health condition.
Do Vibration Analysis Diagnosis and Ultrasonic Readings Go Hand in Hand?
If vibration analysis identifies a fault, would you necessarily see a corresponding increase in ultrasonic readings of the machine (provided you have been doing a trending method)? And, conversely, would vibration analysis necessarily confirm a fault if you see an increased decibel reading through ultrasonic testing?
Not always, and that's just life in the real world. One of the reasons is the variety of ultrasonic sources that can exist in an operating machine. A pump is a good example. There is turbulent flow of the fluid being pumped, there is the possibility of cavitations, there is the metal to grease surface contact, there is the metal to metal surface contact in a bearing, there is the distinct possibility of transient ultrasonic signals - all of which carry ultrasonic energy that can be detected by the ultrasonic detector. Have you ever listened to a pump bearing with your ultrasonic detector and heard a sound like a bearing rotating, then moved your sensor away from the bearing and put it right on the pump and heard exactly the same sound quality? It can get confusing. That's where field experience, for which there really is no substitute, as well as vibration analysis, come into play.
In summary, using Airborne Ultrasound and Vibration Analysis together has its own strength. Many times, one technology can validate the other technology's findings. But, perhaps more importantly, the combination of technologies gives the inspector multiple sources of information, and a more comprehensive data set, in order to make the proper diagnosis and recommendations.
All photos used courtesy of ECS2, Group, Inc.
