For more than 40 years airborne/structure-borne (AB/SB) ultrasound has evolved from working in single applications, such as leak detection, to electrical surveys and inspecting mechanical or rotating equipment in the maintenance and reliability world across all industries. Ultrasound is a versatile, non-destructive technology with multiple applications that is not limited to any one particular scan, survey or trend.
What is AB/SB Ultrasound?
Essentially, AB/SB ultrasound can be described as non-destructive technology that is listening to the frequency range above human hearing. Normal human hearing is between 20 hertz (Hz) and 20 kilohertz (kHz). Any frequencies above 20 kHz would be classified in the ultrasonic range. AS/SB ultrasound is only listening for sounds created by turbulent flow, ionization or friction. It is different from pulse echo or power ultrasound, which send out a signal and receive it or only send a signal from the end of a transducer. The equipment then reads out in decibels (dB) to indicate how loud the ultrasound is at a particular frequency. Table 1 shows examples of frequencies that have been proven over many years to hold true while monitoring different applications with both analog and digital AB/SB ultrasound.
Table 1. Examples of typical monitoring frequencies from ISO 29821-2 “Condition monitoring and diagnostics of machines – Ultrasound – Part 2: Procedures and validation” (Graphics courtesy of UE Systems)Advantages of AB/SB ultrasound include:
- Relatively inexpensive entry point with quick opportunities for return on investment (ROI)
- Ease of learning curve for all skill levels, from top to bottom with software reporting to gathering data
- Proven track record of trending and reporting to make confident analysis in a condition-based maintenance (CBM)/predictive, preventive, prescriptive maintenance program
- State-of-the-art digital equipment allows for user friendly operation
- Ever-growing remote monitoring applications, perfect for assets that are guarded or shielded for safety reasons, a plus for route-based or continuous-monitoring systems
Monitoring Bearings the Ultrasound Way
Vibration, infrared, motor circuit testing and oil analysis have all been technologies used to monitor bearings. Ultrasound has become a heavy hitter due to its ability to detect faults or subtle changes much sooner than other technologies (see Figure 1).
Figure 1. I-P-F Curve illustrates I as the installation point. Point P is the point where early failure could be detected. Point F is where full functional failure has occurred. Ultrasound is an early indicator of potential problems with bearings and rotating equipment.Particularly in pumps, pillow block bearings and ball bearings—regardless of brand—are excellent candidates for using SB ultrasound with a direct contact module or remote monitoring solution. Not only can ultrasound monitor these faults that occur in the harsh environments of an industrial facility, it can also indicate when another action should be taken or not taken, such as lubrication. This area can be dedicated to trending bearings to prevent over-lubrication or under-lubrication by trending the dB. Lubrication related failures are, as many know, a high percentage of bearing, up to 80 percent in some instances. The I–P sections of the curve are where SB ultrasound can prevent over-lubrication and under-lubrication. These are just two issues that can be easily corrected with a little training and monitoring. With the ability to trend the dB, there are known increases in dB value above a baseline value that indicate when to lubricate a bearing or take other actions.
These actions might include making a decision to order replacement parts that give the facility the ability to have inventory on hand for the next scheduled shutdown. When an inspector reaches the point of inflection, the moment grease hits the bearing, he or she will be able to notice the drop in dB and sound change.
Figure 2. Known rises in dB above baseline that indicated the corresponding alarm for high-speed bearings (above 100 RPM) The area of the curve between P and F illustrates when failure happens at Point P, and the early warning signals (P1, P2, P3 …) and when catastrophic failure happens at Point F. Once Point P is reached, using ultrasound to detect the changing conditions will help give users the most time to plan and schedule a replacement or change out of a bearing. It is highly recommended to monitor assets like pumps, bearings and motors with multiple technologies. For example, implementing ultrasound technology could be complemented with a second or third technology, such as vibration and infrared, to confirm the suspected issues or detect failure. This will ensure failure modes will not be missed, thus enhancing condition monitoring efforts.
Figure 3. Trending and listening to the bearing when grease is needed (right). Time Series view of a .wav file demonstrating the inflection point happens when lubricating a bearing, also illustrating when to stop greasing (left).Think Safety, Monitor Remotely
More and more facilities are continuing to engineer safer environments to help prevent near misses—or worse, reportable incidences. With the increase of guarding to prevent workplace injuries, remote monitoring is on the rise. Traditionally, SB ultrasound has been a go-to option due to the mobility of hand-held devices. Remote access sensors (RAS) are becoming more and more affordable to install. These sensors can collect the data via a switch box or continuous monitoring system that can forecast bearing needs or failures. Pump bearings can be inaccessible because they are either out of reach or obstructed by a cage. Possible solutions are a 100-foot cable with an RAS on the end or Wi-Fi sensors. Depending on remote monitoring needs, ultrasound can provide a solution to safely gather data that helps monitor that highly critical asset.
Figure 4. Wired online condition monitoring (left) vs. wireless online condition monitoring (right)Conclusion
With the ever-changing industry, the need to find lubrication issues or faults in mechanical equipment is on the rise. We can all agree downtime does not pay the bills. The prices for commodities and products produced by industrial facilities are set, for the most part. What is a way to increase uptime? Using proven technologies such as AB/SB ultrasound to help prevent lubrication-related failures or indicate there is an essential action that should be taken to help prolong the life of a facilities mechanical rotating equipment. Remote monitoring is a safe way to collect critical data. Adding AB/SB ultrasound technology to condition-based monitoring undoubtedly will increase opportunities to find failures early enough to make necessary repairs before a catastrophic event takes place.