Have you ever seen a mechanic open a toolbox that only had a single wrench or a carpenter with a tool chest containing just one type of saw? For these individuals to be considered true professionals, they must amass giant collections of tools so they can properly complete a task or job.
In a similar way, a predictive maintenance (PdM) professional’s inspection “toolbox” should comprise a host of options, including infrared thermography, electric motor circuit analysis, vibration, oil analysis and ultrasonic/sonic analysis, as well as visual, tactile and acoustic (sensory) inspections.
Field experience has demonstrated that by appropriately combining and relating the results of different inspection options, these professionals can create a synergistic solution. This approach is much more thorough than one based on only one test or on several non-integrated inspection methods. This article will explore how these technologies and tools can work together to achieve far more than when implemented independently.
Heat is often a symptom of eminent machine failure or malfunction. A non-contact infrared imager can be used to quickly obtain a multipoint temperature profile that can easily be assessed. This inspection can be performed with little to no disruption to the facility’s operations. When utilized as a screening tool as part of a daily or weekly inspection, it can frequently be the first method used to witness a pending component failure.
These instruments generally sense sounds in the 20- to 100-kilohertz range and convert them to either auditory or visual signals that can be heard/seen by a technician. These high frequencies are the exact frequencies generated by worn and underlubricated bearings, faulty electrical equipment, leaky valves, etc. This can also be a great way to detect an impending machine failure before it becomes catastrophic.
In the realm of electric motors, the current signature can be measured and recorded. In its infancy, it was primarily employed to detect rotor bar problems, but today with demodulated spectrums, the technology can be used to identify issues with belts and couplings through trending and baselining.
In its simplest form, vibration analysis is a measurement of displacement over time. By measuring displacement, velocity or acceleration, you can get insight into bearing failures, imbalance, misalignment, wear, looseness, etc.
The lubricant is considered the lifeblood of the equipment. Much like a doctor assesses your health through blood analysis, the same can be done for machine health. Oil analysis can be broken down into three main categories: lubricant health, machine health and contamination. Every test performed on an oil sample can be categorized in at least one of these areas.
While some visual or audible observations require interpretation, many are intuitive and only involve a system to manage and act on the information. Most operators and technicians are familiar with the machinery they maintain or operate, and consequently are aware of the “normal” sounds of that machine, making them qualified to identify unusual conditions.
Part of any strong PdM program is the ability to verify a fault with more than one technology. This not only ensures the validity of the fault but also helps make a more accurate and precise repair recommendation. The importance of verification with a second technology is never more evident than on a critical piece of equipment that requires plant outages for repair. For instance, consider the following scenarios:
While making a routine inspection of an electrical panel located on a mezzanine catwalk, a technician noticed a tiny, but clearly anomalous heat signature below in the direction of a smaller component on the ground. Upon further inspection at ground level, the tech discovered an anomaly in the coupler between a small motor and pump. He was able to spot the issue from a quick scan at more than 30 feet away.
The apparent temperature at the coupler was not very high, but it was enough relative to the surface temperatures of the motor and the pump to make the technician suspicious. He performed a slow-motion study using a strobe light, setting its frequency to the shaft’s revolutions per minute. This essentially “froze” the shaft for inspection. The two halves of the coupler, which were joined by a flexible insert, appeared to be contacting one another. At this point, vibration analysis detected both mechanical looseness and misalignment.
During scheduled downtime, the pump was shut down, and the entire assembly was disassembled and inspected. Four of the insert’s eight legs were seriously damaged, allowing the coupler halves to make contact and produce vibration and excessive heat. A new coupler and insert were installed, and the pump was put back into service.
In this case, infrared thermography was used as a screening tool, while a strobe light, sensory inspections and vibration analysis were employed to validate the potential failure, which would have gone unnoticed if only thermography had been used.
During a routine thermography route, the drive-end bearing of a large, oiled electric motor was found to be running 15 degrees hotter than any of the prior samples. Vibration testing showed nothing out of the ordinary based on the previous six months of data. An oil sample was taken, which revealed a viscosity increase of more than 100 percent. The analysis also indicated signs of cross-contamination between lubricant types based on elemental analysis. It was determined that the wrong oil had been added to the motor during the last top-up. To correct the issue, the motor was drained, flushed and refilled with the proper lubricant.
During a routine infrared PdM inspection, a technician determined that a motor was operating at an excessively high temperature. The 7.5-horsepower motor powered a coolant pump in the machining center responsible for critical machining of a key component in the assembly plant. Failure of this seemingly insignificant cooling pump could cause the entire plant to shut down.
The PdM program at the plant included a broad spectrum of predictive/preventive maintenance technology options. A work order for additional analysis was generated to determine if the root cause of the fault was electrical or mechanical.
Initially, motor current analysis confirmed that the motor and cabling tested electrically sound. Follow-up vibration analysis identified a bearing fault in the motor. Close monitoring allowed the motor to be run until the scheduled downtime, when it was replaced. A post-installation infrared scan confirmed that the new motor was operating within normal parameters. Subsequent cost analysis of this one incident showed a 100-percent return on investment for all the instruments used.
When used in conjunction, these technologies can be employed in a wide variety of applications, including leak detection in pressure and vacuum systems, bearing inspections, detection of valve blow-by, steam trap inspections, detection of corona, tracking and arcing in electrical gear, detection of cavitation in pumps, checking the integrity of seals and gaskets in transformers, etc.
Technicians can easily use infrared thermography and ultrasound analysis to inspect steam valves. First, touch the upstream and downstream sides of the valve with an ultrasonic sensor’s contact probe. Steam passing through a leaking valve producing turbulence can be heard through headphones as a gurgling or rushing sound. A blockage will emit no sound. Since valve blow-by in steam systems will generate a higher temperature reading downstream, infrared thermography can be used to detect the thermal anomaly along the pipe run and confirm the analysis.
Effectively using some condition monitoring tools, like vibration or oil analysis, requires a considerable amount of training. Sensory inspection, on the other hand, can be performed by non-maintenance personnel such as operators. This can be an advantage when the maintenance staff is occupied with reactive maintenance tasks. Something as simple as detecting an oil leak or a gearbox that sounds weird could and often does lead to the prevention of a catastrophic failure, avoiding tens of thousands of dollars in losses. Therefore, the value of utilizing your senses should not be underestimated or overlooked.
Heat can be a good indicator of a leaking hydraulic valve. The frictional forces of fluid moving through a leak can produce heat as a byproduct. This has been useful in aircraft inspections. However, not every leaking hydraulic valve will generate heat, and the proximity of valves in certain configurations can lead to a potentially inaccurate diagnosis due to heat (and in some instances sound) transference.
This inspection process can be aided by incorporating ultrasound with infrared thermography. A leaking valve will emit a louder sound downstream. By comparing infrared results with upstream and downstream ultrasonic readings, you can quickly make a positive diagnosis.
Technicians can use airborne ultrasonic translators to detect corona, tracking and arcing. Ultrasound can detect faults through small openings or door seals on switchgear cabinetry, through the outer shell of oil-filled transformers, and in the switchyard emitting from bushings, busbars and insulators. Using highly sensitive airborne sensors, these ultrasonic detectors can isolate electric faults on high-tension transmission and distribution lines at distances of more than 150 feet.
Note that corona and tracking do not show up with an infrared scan in electrical systems having a potential of less than 240 kilovolts and that ultrasonic detection can find electric faults in systems well below this threshold. This alone demonstrates the need for the inspection and safety industry to marry temperature imaging and ultrasound scanning techniques.
For example, in one case both an infrared camera and airborne ultrasound were used to inspect 15 13.8-kilovolt rectifier panels during a routine inspection with the panels closed. Thermography did not detect noteworthy temperature anomalies through the closed panels. However, significant levels of airborne ultrasound were detected at the lower right corner of one of the panels. Keep in mind that many technicians employ only thermography to quickly scan the panels for issues.
Several qualified electrical technicians were able to safely listen to the signal, identify its signature as that of a breaker and take definitive action. The vacuum breaker was removed, and a direct current was applied to it, revealing a fault. The intervention averted the loss of electrical power, a shutdown of the plant’s compressed-air system and possible injury to personnel from fire or shock. Since then, airborne ultrasound inspection of all switchgear has been added as part of the regularly scheduled infrared preventive/predictive maintenance program.
While all of these technologies can be used by themselves, the advantage of utilizing multiple technologies is that problems can be cross-diagnosed and decisions to make or delay repairs can be made much more confidently. Success will come to those organizations that have a versatile and experienced workforce from diverse engineering backgrounds and with formal training and certification in various PdM technologies.
The return on investment is clearly positive and substantial, providing management and purchasing decision-makers with verifiable data to justify procurement of the diversified toolbox that defines the modern PdM professional.