The Powerful Synergy of Multi-Modal PdM Analysis
EP Editorial Staff | January 2, 2004
If “one tool fit all,” mechanics’ toolboxes would contain only one wrench, one hammer, and so on. But mechanics must have a wide range of tools—for example, numerous wrench and driver configurations to span a growing range of fasteners.
In a similar way, the PdM professional’s inspection “toolbox” contains a host of options, including infrared (IR) thermography, electric motor circuit analysis, vibration, tribology (oil/lubricant analysis), ultrasound analysis, and the most ancient of all, unaided human visual, tactile, and acoustic inspection.
Field experience has demonstrated that by appropriately combining and relating the results of different inspection options, PdM professionals can create a synergistic solution—one that is far more thorough than if based on only one test or on several nonintegrated inspection testing methods. This article provides a selection of lessons learned in actual cases in which more than one PdM analytical method was used, which led to a robust understanding and optimal resolution of challenging maintenance problems.
Where there’s heat there’s probably vibration
“My infrared camera paid for itself the first time I used it,” said Richard Harrison, maintenance engineer at Consolidated Diesel Co.’s Whitakers, NC, plant. Moments after he turned on his brand-new ThermaCAM PM 290 infrared camera from FLIR Systems, N. Billerica, MA, in the plant for the first time, he detected a surface temperature of about 640 F on the electrical panel of a starter motor that was clearly on the verge of catching fire. The starter was immediately taken off-line and repairs made.
He calculates that if the starter motor had burned, the resulting repairs and the value of lost production would have totaled about $350,000—more than four times the cost of the camera, lenses, and training at FLIR’s Infrared Training Center.
Subsequently, Harrison was performing an infrared scan of an electrical panel located on a mezzanine catwalk high above a parts washer. He happened to scan below in the direction of the parts washer and noticed a tiny, but clearly anomalous, heat signature. “The high resolution of the camera and its sensitivity enabled me to see a small, subtle anomaly in the coupler between the parts washer motor and pump more than 30 ft away,” he said. At ground level he rescanned the anomaly up close (Fig. 1).
The apparent temperature at the coupler was only 110 F, but high enough relative to the surface temperatures of the motor and the pump to make Harrison suspicious. He performed a slow motion study in which he used a strobe light, setting its frequency to the shaft’s rpm, essentially “freezing” the shaft for inspection. The two halves of the coupler, which are joined by a flexible “spider,” appeared to be slapping into one another. Something was definitely wrong.
Vibration analysis detected both mechanical looseness and misalignment (Fig. 2).
During scheduled downtime, the pump was shut down and the entire assembly was disassembled and inspected. Four of the spider’s eight legs were seriously damaged, allowing the coupler halves to make contact, producing vibration and excessive heat (Fig. 3). A new coupler and spider were installed, and the pump was put back in service.
Post-repair vibration testing showed an 80 percent reduction in vibration amplitude and the elimination of misalignment (Fig. 4). A post-repair infrared scan showed the coupler temperature had decreased by 22.2 F. The rule of thumb conclusion was validated: “Where there’s heat, there’s probably vibration.”
Thermography and MCA
A multiple-technology solution to the detection of rotating machinery problems can reduce uncertainty when trying to pinpoint a fault. Howard W. Penrose, PhD, of ALL-TEST PRO, a division of BJM Corp. Old Saybrook, CT, provides the following recent example of combining results of sophisticated motor circuit analysis (MCA), infrared thermography, and vibration analyses.
During a routine infrared predictive maintenance inspection at a major Midwestern automotive transmission manufacturing plant, a thermographer using a FLIR PM 695 infrared camera determined a motor was operating at an excessively high temperature. The 7.5 hp motor powered a coolant pump in a transmission case machining center that is responsible for critical machining on a key component in the assembly plant. Failure of the cooling pump would cause a shutdown of the entire plant.
The PdM program at the plant includes a broad spectrum of predictive/preventive technology options. A work order for additional analysis was generated to determine if the root cause of the fault was electrical or mechanical.
First, MCA confirmed that the motor and cabling tested electrically good. Follow-up vibration analysis identified a bearing fault in the motor. Close monitoring allowed the motor to be run until scheduled downtime, when it was replaced. A post-installation infrared scan showed the new motor was operating within normal parameters. Subsequent cost analysis of this one incident showed a 100 percent return on investment for all instruments used.
Ultrasound complements IR
Mark Goodman of UE Systems, Elmsford, NY, uses a combination of ultrasound, infrared, and vibration analysis to accurately determine the condition of operating equipment as well as to identify the location of a problem.
Applications include leak detection in pressure and vacuum systems, bearing inspection, steam trap inspection, detection of valve blow-by, detection of cavitation in pumps, detection of corona, tracking and arcing in electrical gear, the integrity of seals and gaskets in transformers, and even partial discharge in cable splices, terminations and transformers. He has found the directional nature of ultrasound allows him to detect warning signals—changes in the normal sonic signature of an assembly—long before actual failure.
Goodman uses infrared thermography and ultrasound analysis together to inspect steam valves. He touches the upstream and downstream sides of a valve with the contact probe of an ultrasonic sensor. Through the headphones, he can detect the steam passing through a leaking valve producing turbulence that is heard as a gurgling or rushing sound. Blockage will produce no sound. Since valve blow-by in steam systems will produce a higher temperature reading downstream, infrared thermography can be used to detect the thermal anomaly and confirm the analysis.
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 a useful phenomenon in aircraft inspection. However, not every leaking hydraulic valve will produce 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. A leaking valve will produce a louder sound downstream. By comparing infrared results with upstream and downstream ultrasonic readings, an operator can quickly make a positive diagnosis.
Ultrasound and thermography enhance safety
Allan Rienstra and James M. Hall of SDT North America, Cobourg, ON, use airborne ultrasonic translators to detect corona, tracking, and arcing. They point out that 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, buss bars, 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 ft.
They note that corona and tracking do not show up with an infrared scan in electrical systems having a potential below 240 kV, and that ultrasonic detection can find electric faults in systems well below this threshold. “This alone demonstrates the need for the inspection industry to marry temperature imaging and ultrasound scanning techniques,” they said.
Ultrasound scanning can provide safety benefits. Some utilities use ultrasonic detection as a screening device to monitor for severe partial discharge or arcing before entry into manholes and cable vaults. Rienstra and Hall report using ultrasound to detect corona, arcing, or tracking around door seals and air vents of enclosed switchgear in the following example.
Both an infrared camera and airborne ultrasound were used to inspect 15 13.8 kV rectifier panels during a routine inspection of a chemical plant in the southeast—with the panels closed. Thermography did not detect significant temperature anomalies through the closed panels. However, significant levels of airborne ultrasound were detected at the lower right corner of one of the panels.
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 dc 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 regular scheduled infrared preventive/predictive maintenance program.
Will the presses roll until scheduled downtime?
The advantage of using multiple technologies is that a problem can be cross-diagnosed and decisions to repair or to delay repairs can be made much more confidently. James Sullivan Jr., maintenance engineer with The New York Times, noted that the newspaper industry is unique not only in its demanding production schedules but also in its limited availability for planned maintenance. “For over 100 years,” he said, “the newspaper industry has been using run to failure as a maintenance strategy.”
Over the past decade, he reports that The New York Times has incorporated vibration analysis, thermography, tribology, and ultrasound in a robust, integrated PdM program. Two recent situations at the newspaper illustrate the value of this approach.
In one case, a thermographic scan on a press lineshaft identified a temperature of 168 F, 60 percent above normal, on one line clutch. Follow-up vibration data were collected for the clutch, requiring about 2 hours, compared with the 12 hours required to test the entire press lineshaft—a savings of 10 hours. The clutch was repaired during the next available maintenance window with no loss in production.
In a more dramatic case, the integrated PdM approach enabled maintenance engineers to confidently postpone replacing the tucking blade of a critical and failing folding machine until a scheduled downtime enabled the machine to be repaired with no loss of productivity. The folder is the last stage of a printing press and this one marries, cuts, and folds approximately 18 complete newspapers per second. “With such a demanding print schedule,” Sullivan said, “unscheduled folder downtime is forbidden.”
A rise in the vibration spectrum of the folder at 139 Hz and 153 Hz alerted Sullivan to the very beginning of an impending failure. The latest monthly oil samples contained an increase in iron and chromium particles, confirming the problem.
The subsequent month’s vibration and oil data indicated that the problem had dramatically accelerated. Downtime was not scheduled for several more days. The folder was put on “daily sampling” status. After an oil sample showed much higher levels of iron and chromium, the oil was flushed during the next production shift change, avoiding any loss of productivity.
A spare tucking blade assembly was readied for change-out. Could the folder be operated continuously until the scheduled downtime? Vibration monitoring and oil changes were increased to a “per-shift” basis. The scheduled downtime was still 3 days away. Vibration readings were stepped up to twice per shift.
Finally, the scheduled downtime period arrived. The old tucking blade assembly was pulled out of the folder and the new spare assembly was installed without even 1 minute of unscheduled production downtime.
Thanks to integrated PdM analysis, the failing unit was run with confidence until downtime, reaping huge financial benefits. Visual inspection of the pulled assembly determined that a bearing retainer had worn away and an entire shaft was floating in the unit.
Had a critical failure occurred, it would have caused an estimated $275,000 in damage to the printing press and a safety risk to personnel, plus incurring a minimum of 5 days lost production time—worth about $2.4 million. The cost of the change-out was only about $62,000. The damaged tucking blade assembly was repaired at a leisurely pace at a cost of $18,000. “The integration of oil analysis and vibration analysis not only alerted us to a problem,” Sullivan said, “but also gave us more confidence in running the unit for as long as we did.”
By applying and integrating the results of different inspection options, PdM professionals can cross-diagnose a problem and make decisions to repair or to delay repairs more confidently. Success will come to those organizations that have a versatile and experienced work force from diverse engineering backgrounds and with formal training and certification in the various PdM inspection modalities.
The return on investment is clearly positive and substantial, providing management and purchasing decision makers verifiable data to justify the funds for procurement of the multi-modal toolbox that defines the modern PdM professional. MT