MRO Magazine

Ways to use condition-based predictive maintenance technologies

April 9, 2024 | By L. (Tex) Leugner


There are two fundamental methods that can be used to carry out this practice; the first is to establish routes through the plant where technicians can monitor critical equipment by obtaining oil samples, monitoring ultrasound, vibration and temperatures. The second method is to install transducers or other data collection technologies to monitor equipment condition automatically on an ongoing basis. The data collection transducers are mounted near or on critical equipment components such as bearings and data is collected continually where maintenance specialists or operators trained in the interpretation of the conditions can monitor critical machinery or it’s components on a regular basis.

In either case the plant or organization can use the P-F Curve principle to respond to any machine or condition that requires attention. The P-F Curve illustration below highlights condition monitoring technologies and as the curve from point “P” to point “F” illustrates, each technology used might indicate a problem is occurring and corrective action should be taken based on the priority in the curve.

Let’s consider the actions one might use based on the P-F Curve.


Q | How does your maintenance group respond to a problem where ultrasonic energy is detected? 

LOGIC: ultrasonic energy detects liquid or gas leaks moving from high pressure. The intensity of the ultrasound measured in decibels (dB), will be loudest at or near the leak site. Ultrasonic leak detection can be used to detect internal hydraulic leaks, such as oil passing through a closed control valve, engine manifold vacuum leaks, steam traps, air brakes, leaks in seals, heat exchangers, boilers, chillers, vacuum systems, compressor valve operation and warning of bearing failure. An 8 dB increase over the baseline indicates a lack of bearing lubrication or a tiny crack or spall. A 12dB increase indicates potential failure. A 16 dB increase indicates an advanced failure, while a 35–50 dB gain indicates a potentially catastrophic failure. A base line ultrasound level must be established for each bearing, if comparative ultrasonic readings are to be effective. Arcing, tracking and corona discharge of certain electrical systems do not generate heat and ultrasonic testing can be more effective than thermographic imaging. Ultrasonic devices can be connected to a grease gun and used to determine when a sufficient amount of grease has been applied to the bearing.

Q | How does your maintenance group respond when a machine vibration is detected? 

Logic: every piece of equipment contains moving parts, each of which will vibrate at certain frequencies. These frequencies are generated by the vibration sources and will vary across a wide spectrum. Common sources of vibration are unbalance, misalignment, bent shafts, defective bearings or gears, mechanical looseness and electrically induced problems. Technically defined, vibration is the oscillation of an object about its position of rest and the number of these oscillations (or cycles) in a given length of time, is referred to as the frequency of vibration measured in cycles per minute (CPM), or cycles per second hertz (HZ).  The amplitude of vibration is measured in displacement (distance or movement), velocity (speed) and/or acceleration (force), of the source of the frequency. Using an FFT (fast Fourier transform) analyzer, vibrations can be measured and displayed in either a time waveform, indicating vibration severity, or a spectrum analysis, illustrating the frequencies of the various sources of vibration.

Q | Does the maintenance group include a lubrication specialist who understands oil analysis report interpretation? 

Logic: oil analysis should be carried out on every piece of lubricated critical equipment on a regularly scheduled basis when the oil is hot and well-circulated, either by obtaining samples physically or using monitoring devices mounted on lubricated components. Benefits include monitoring component wear rates, viscosity, alkalinity, acidity, additive levels, contaminant types and their sources, including determining optimum drain intervals and remaining lubricant quality. Oil viscosity is reported in CST at 40 and 100 degrees C respectively for multi-grade oils that have a tendency to shear and viscosity changes of 15 per cent or more should be investigated immediately. Water must be monitored carefully depending on the type of equipment.  A “trace” of water is about 0.1-0.2 per cent or 1000-2000 PPM.  It is a mistake to ignore this much water in certain recirculating systems with high oil flow rates and turbulence that may experience foaming problems with as little as 100 PPM of water.  In systems that contain bronze components, too much water can cause severe corrosion. In systems that use biodegradable oils, control of water is critical to the life of the oil. The Karl Fischer water test must be part of any effective oil analysis program.

Oil degradation conditions can also be measured by infrared scan. This test reports soot levels, nitration, oxidation, additive levels and other conditions that affect the oil’s ability to properly lubricate. Oxidation stability can be tested using the RULER “remaining useful life test” to assess the remaining life of turbine and hydraulic oils. Acid number (AN) is a measure of acidity based on ASTM D664 and should be monitored in critical hydraulic, gear drive, turbine, compressor and natural gas engine oils. Accepted recommendation is to replace the oil when the AN “doubles” that of new oil specifications. Base number (BN) is a measure of the reserve alkalinity remaining in diesel engine oils and is related to the detergent/dispersant ability to counteract acids based on ASTM D2896, providing accurate results of BN that decreases as oil nears the end of service life. The oil should be changed when the BN is reduced by ½ that of new oil. (Oxidation rates increase as temperatures increase, while nitration rates increase as temperatures decrease). 

Q | Does the lubrication specialist know the metallurgical makeup of the equipment components and the purpose of the additives in the oils that are in use? 

Logic: In order to avoid confusion and unnecessary reaction it is critical that someone in the maintenance organization is aware of these considerations to ensure proper interpretation of oil analysis reports.

How does the maintenance group monitor dirt and dust contamination to determine filter quality? Effective contamination monitoring requires the use of particle counts as part of ISO 4406 standards.   

Q | Does the maintenance group use preventive maintenance tasking to effectively cover priorities two and three? 


Sound, well planed preventive maintenance is used for machine problems in these categories and equipment not critical to the operation can be run to failure, depending on replacement cost. 

Using condition monitoring ultrasonic and vibration analysis combined with the most effective oil analyses, in combination with temperature monitoring, will effectively provide downtime scheduling, avoidance of unnecessary repairs, shorter repair times, improved planning for operations and maintenance activities, consistently provide accurate equipment condition trending, improved monitoring of maintenance tasks, while offering guidance for continuous equipment reliability improvement.

In summary; to properly establish an effective condition monitoring program it is first necessary to determine the critical machines to which the program is applied. The next step in the process is to apply all of those condition monitoring technologies that will be most effective at providing the best and most complete condition information that applies to specific critical machines. For example, ultrasound and vibration analyses must be applied to rotating equipment, while both oil and vibration analyses and ultrasound should be applied to all critical rotating machines that use lubricants of any quantity. Infrared thermography and oil analyses should always be applied to equipment like transformers that contain oil. 


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