Justin Sheard, Senior Engineer and Application Specialist at Fluke Corporation, looks at the ways electrical professionals can utilise thermal imaging tools to identify potential issues with industrial equipment.
Monitoring equipment performance with thermal imaging cameras can reduce the likelihood of unplanned downtime (due to equipment failure), reduce reactive maintenance fees and equipment repair costs, and extend the lifespan of machine assets. Thermal imaging is great for spotting a lack of uniformity in equipment by indicating hot and cold spots in surface temperature, through infrared image capture. Heat is often an early symptom of equipment damage or malfunction, making it important to monitor in preventive maintenance programs.
Thermal inspection methods
Because there is no universal solution for all infrared inspections with a thermal camera, you need to match your method to the equipment being inspected and level of detail required. There are three typical methods of thermography that cover most situations:
Baseline: This is a good place to start for just about any application. First, you scan the equipment when it’s first commissioned or later in the lifecycle when it is working the way it should be, and then use that as a reference point for future inspections. Whether you compare the thermal images on your camera in the field, or on your PC using software tools, this baseline approach paves the way to helping you spot anomalies down the road.
Thermal: Once you’ve set your baseline, you can use thermal trending inspections to compare how temperature is distributed in the same components over time. This can help you detect declining performance over time so that you can hopefully schedule downtime maintenance before equipment schedules it for you.
Comparative: This means you scan similar components with your thermal camera under similar conditions and compare the results. This method relies on the idea that you expect similar or identical components, under similar loads, to have similar temperature profiles. Once you have three or more components it’s relatively easy to pick up an anomaly. There’s one more level of complexity to consider: depending on the components being compared, the actual temperature difference that can be considered an anomaly will vary.
ASSET MAINTENANCE AND TROUBLESHOOTING
Motors and drives
Electric motors are the backbone of industry. Thermal cameras are very useful for both troubleshooting problems as well as for condition monitoring, for long-term preventive maintenance. Using a handheld thermal camera, you can capture infrared temperature measurements of a motor’s temperature profile as a two-dimensional image.
What to check: Ideally, you should check motors when they are running under normal operating conditions. Unlike an infrared thermometer that only captures temperature at a single point, thermal cameras can capture temperatures at thousands of points at once, for all of the critical components: the motor, shaft coupling, motor and shaft bearings, and the gearbox. Note that each motor is designed to operate at a specific internal temperature – the components should not be as hot as the motor housing.
What to look for: All motors should list the normal operating temperature on the nameplate. While the thermal camera cannot see the inside of the motor, the exterior surface temperature is an indicator of the internal temperature. As the motor gets hotter inside, it also gets hotter outside. Thus, an experienced thermographer who is knowledgeable about motors can use thermal imaging to identify conditions such as inadequate airflow, impending bearing failure, shaft coupling problems, and insulation degradation in the rotor or stator in the motor. If you suspect overheating is the result of one of the following, consider the action described:
Inadequate airflow: If a brief shutdown is possible without affecting the plant process, shut off the motor long enough to perform minor cleaning on the air intake grills. Schedule a thorough motor cleaning during the next planned plant shutdown.
Unbalanced voltage or an overload: The usual cause, a high-resistance connection in the switchgear, disconnect, or motor connection box, can usually be pinpointed by a thermographic inspection and confirmed using a multimeter, clamp meter, or a power quality analyzer.
Impending bearing failure: When the thermal images indicate an overheating bearing, generate a maintenance order to either replace the bearing or lubricate the bearing. Vibration analysis can often help determine the best course of action.
Insulation failure: If it will not too greatly impact production, de-rate the motor in accordance with NEMA standards. Generate a work order to replace the motor as soon as possible.
Shaft misalignment: In most cases, vibration analysis will confirm a misaligned coupling. If a shutdown is possible, dial indicators of laser-alignment devices can be used and the misalignment can be corrected then and there.
Electrical panels
Thermal images are an easy way to identify apparent temperature differences in industrial three-phase electrical circuits, compared to their normal operating conditions. By inspecting the thermal gradients of all three phases side by side, technicians can quickly spot performance anomalies on individual legs due to unbalance or overloading.
What to check: Capture thermal images of all electrical panels and other high-load connection points such drives, disconnects, controls, and so on. Where you discover higher temperatures, follow that circuit and examine associated branches and loads. Check panels and other connections with the covers off. Ideally, you should check electrical devices when they are fully warmed up and at steady state conditions with at least 40% of the typical load. That way, measurements can be properly evaluated and compared to normal operating conditions.
What to look for: Equal load should equate to equal temperatures. In an unbalanced load situation, the more heavily loaded phases(s) will appear warmer than the others, due to the heat generated by resistance. However, an unbalanced load, and a harmonic imbalance can all create a similar pattern. Measuring the electrical load is required to diagnose the problem. When a thermal image shows an entire conductor is warmer than other components throughout part of a circuit, the conductor could be undersized or overloaded. Check the conductor rating and the actual load to determine which is the case.
Current: Use a multimeter with a clamp, a clamp meter or a power quality analyzer to check current balance and loading on each phase.
Voltage: On the voltage side, check the protection switchgear for voltage drops. In general, voltage drop should be within 10% of the nameplate rating. Neutral to ground voltage tells you how heavily your system is loaded and helps you track harmonic current. Neutral to ground voltage higher than 3% should trigger further investigation.
Load: Loads do change, and a phase can suddenly be 5% lower on one leg, if a significantly large single-phase load comes online. Voltage drops across the fuses and switches can also show up as unbalance at the motor and excess heat at the root trouble spot. Before you assume the cause has been found, double check with both the thermal camera and multimeter or clamp meter current measurements.
Harmonics: Neither feeder nor branch circuits should be loaded to the maximum allowable limit. Circuit load equations should also allow for harmonics. The most common solution to overloading is to redistribute loads among the circuits, or to manage when loads come on during the process.
Steam systems
Thermal images of steam systems reveal the comparative temperatures of system components and thereby indicate how effectively and efficiently steam system components are operating.
What to check: Using a combination of ultrasound and thermal inspections significantly increases the detection rate of problems in steam systems. Check all steam traps and steam transmission lines, including any underground lines. In addition, scan heat exchangers, boilers, and steam-using equipment. In other words, examine every part of your steam system with a thermal imager.
What to look for: Steam traps are valves designed to remove condensate as well as air from the system. During inspections, use both thermal and ultrasonic testing to identify failed steam traps and whether they have failed open or closed. In general, if a thermal image shows a high inlet temperature and a low outlet temperature (<212°F or 100°C), that indicates that the trap is functioning correctly. If the inlet temperature is significantly less than the system temperature, steam is not getting to the trap. Look for an upstream problem – a closed valve, pipe blockage, etc. If both the inlet and the outlet temperatures are the same, the trap probably has failed open and is blowing steam into the condensate line. This keeps the system operating but with significant energy loss. Low inlet and outlet temperatures indicate that the trap has failed closed and condensate is filling the trap and the inlet line.
Roller, chain, and belt conveyors
Thermal imaging is especially useful for monitoring low-speed mechanical equipment like conveyors. Overheating signals the impending failure of many different electrical and mechanical conveyor components, from motors, gearboxes, and drives to bearings, shafts, and belts.
What to check: While they are running, monitor conveyors that are critical to your operations (i.e., those whose failure would threaten people, property, or production). Be sure to scan the conveyors’ drives, electric motors and gearboxes, and follow the guidelines for these units spelled out earlier in this article. Also, with the conveyors running, check the conveyor chain on any critical towline, powered overhead and power-and-free conveyors used in your operations. In addition, scan the bearings in the carrier rolls of powered roller conveyors and in the idler rollers and drive, tail and take-up pulleys on your critical belt conveyors. And, remember to check the belts themselves.
What to look for: In general, look for hotspots and pay special attention to differences in temperature of similar components operating under similar conditions – similar speeds, similar loading, etc. For example, if the end bearings in the same conveyor roller or pulley or the bearings on the same side of adjacent rollers on the same conveyor are running at different temperatures, the hotter one may be trending toward failure. For monitoring some conveyor components (e.g., drives), thermal imaging complements other condition monitoring technologies such as oil analysis, vibration monitoring, and ultrasound. However, tow chains under the floor and elevated chain conveyors, including power-and-free conveyors, are often most easily and effectively monitored from a distance using thermal imaging. Check the chain as well as the roller turns and curves. Overheating chain or rollers may signal lubrication or wear problems. Thermal imaging is an ideal monitoring technique for powered roller conveyors, roller-bed belt conveyors and bulk-handling belt conveyors with idlers, whether these conveyors are elevated or not. On such conveyors, the bearings are usually too small, too numerous or too inaccessible – perhaps all three – to be effectively screened by other strategies.
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