For plant engineers, finding new ways to avert costly failures in their facilities is practically second nature. Particularly when it comes to industrial maintenance, thermography testing is one approach that's become much more popular over the last five years or so. Used as a condition-monitoring technique, thermal imaging enables users to identify potential areas of equipment failure and limit downtime.
As part of a comprehensive preventive or predictive maintenance program, it's a good idea to create a regular inspection route that includes scanning systems associated with critical assets — those whose failure would threaten people, property, or product. That way, you'll have baseline images for comparison, which will help you determine whether or not a hot spot is unusual and requires repair as well as verify that repairs are successful.
Consider using key safety, maintenance, and operations personnel to quantify “warning” and “alarm” levels for critical assets. In addition, the InterNational Electrical Testing Association (NETA) provides guidelines to determine when immediate repair is required by comparing the difference in temperature (ΔT) between similar components under similar loading. NETA recommends making immediate repairs when the difference in temperature exceeds 15°C (27°F).
Whenever you use a thermal imager and find a problem, document your findings in a report that includes a digital photograph as well as a thermal image. That's the best way to communicate problems you find and to suggest repairs. With the aid of a handheld thermal imager, you can check all electrical panels for loose and corroded connections, and scan critical electrical systems including motor control centers, motor/drive combinations, pumps, fans, compressors, electrical connections, industrial gearboxes, and transformers. Let's take a look at how you can boost your predictive maintenance strategy in each of these areas.
Motor control centers. Thermal imaging can be used to evaluate the operating condition of the components within motor control centers (MCCs) by comparing their relative temperatures under load. A typical MCC is a standalone arrangement with one or more combination motor control units for controlling an AC motor in a specific application. Each unit has an external disconnect, branch circuit and motor overcurrent protection, and a magnetic motor starter along with pilot devices located on the panel door.
Underwriters Laboratory (UL) allows panelboards and switchboards for branch-circuit protection within an MCC, provided they do not constitute a major portion of the center. That means a complex MCC can contain bus bars, controllers, starters, contactors, relays, fuses, breakers, disconnects, feeders, and transformers.
Use your thermal imager to scan all components and connections within MCCs with the enclosures open and the equipment running. Measure the load at the time of each scan so that you can properly evaluate your measurements against normal operating conditions.
In general, look for components that are hotter or cooler than similar components under similar loads, which can identify broken or undersized wires, defective insulation, faulty (corroded, too loose, or over tightened) connections and electrical unbalance among phases.
Be aware that connection-related hot spots usually (but not always) appear warmest at the spot of high resistance, cooling with distance from that spot. Unbalance, whether normal or out of specification, will appear equally warm throughout the phase or part of the circuit that is overloaded. Harmonic unbalance creates a similar pattern. Note: A cooler-than-normal circuit or leg might signal a failed component.
Since all electrical currents produce some heat, temperature alone is not an indicator of problems. Equally, warm conductors in all three phases represent a “good” pattern. Differentiation between phases should be investigated.
Motors. Thermal images of electric motors reveal their operating conditions as reflected by their surface temperature, capturing infrared temperature measurements of a motor's temperature profile as a two-dimensional image. Unlike an infrared thermometer that only captures temperature at a single point, a thermal imager 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.
Ideally, you should check motors when they are running under normal operating conditions. All motors should list the normal operating temperature on the nameplate. And while the infrared 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 on the outside surface.
Remember that each motor is designed to operate at a specific internal temperature. The other components should not be as hot as the motor housing. Thus, an experienced thermographer who is also 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 a motor (Photo 1).
Pumps, fans, and compressors. Used in combination with other predictive analysis techniques such as vibration monitoring and ultrasound, thermal imaging is especially useful for monitoring motor-driven rotating equipment such as pumps, fans, and compressors. Since many impending failures are accompanied by overheating, use thermogrpahy to capture a two-dimensional image representing the apparent surface temperatures of equipment (apparent temperature can differ significantly from actual temperature, due to the emissivity of a material's surface).
Monitor rotating equipment while it is in operation and under load. Look for hot spots, and pay special attention to differences in temperature between similar units operating under similar conditions.
On pumps and fans, get thermal profiles of the housings — scans that are likely to reveal any problems with bearings or seals — as well as scans of shaft couplings or drive belts and sheaves. For a compressor, use several images, if necessary, to get a thermal profile of the entire unit.
On a pump, a difference in temperature along a seal or gasket is the “signature” of a failure. A spot on the housing adjacent to a bearing may signal an impending bearing failure, although you probably won't be able to ascertain the root cause from a thermal image alone. Perhaps there is a lubrication problem or maybe misalignment in the drive.
An overheating bearing on a fan also signals a problem, but a thermal image alone is not definitive. The root cause could be a lack of lubrication, the wrong lubrication, drive misalignment, or unbalance in the fan itself. Further investigation may be required.
Many industrial and most building-system fans are belt-driven, as are some pumps. According to Snell Infrared, Montpelier, Vt., a belt-and-sheave drive that is designed and installed correctly generates very little heat, and the belt moving through the air tends to cool it near ambient temperature. Overheating, detected by thermography, reflects a problem with the drive's design or installation, perhaps mismatched belt and sheaves, or misalignment. Vibration analysis and/or an alignment check will confirm the latter condition.
Compression produces heat while expansion cools, so a thermal imager can see a “heat machine” like a compressor as it works. To check the efficiency of compressors, look for belt slippage on cooler fans, shaft misalignment, bearing problems, and blocked or leaking valves.
Loose or corroded electrical connections. You can use thermal imaging to troubleshoot loose, over-tight, or corroded connections in electrical systems by comparing the temperatures of connections within panels. Check panels with the covers off and power at ideally at least 40% of the maximum load. Measure the load, so that you can properly evaluate your measurements against normal operating conditions.
Connections that are hotter than others signal high resistance possibly due to looseness, tightness, or corrosion. Connection-related hot spots usually (but not always) appear warmest at the spot of high-resistance, cooling with distance from that spot (Photo 2).
Overheating connections can, with additional loosening or corrosion, lead to a failure and should be disassembled, cleaned, repaired, and reassembled. If, after following this procedure, the anomaly persists, the problem may not have been the connection, although a faulty repair remains a possibility. Use a multimeter, clamp meter, or a power quality analyzer to investigate other possible reasons for the overheating, such as overloading or unbalance.
Industrial gearboxes. Many industrial machines use gearboxes to alter and/or vary the standard speeds of electric motors. The lifeblood of any gearbox is the oil within it that lubricates the gears. If the oil level in a gearbox gets too low or loses its ability to lubricate, the gearbox will eventually fail, preceded by overheating. That's where thermal imaging comes in.
Traditionally, preventive maintenance for gearboxes has consisted of regularly checking their oil levels and replenishing lost oil. Some maintenance departments add a predictive element to gearbox maintenance in the form of oil sampling and analysis. These gearbox maintenance measures are time consuming and expensive and require shutting down the equipment. Also, gearboxes often are in inaccessible or unsafe locations that make oil-level checking and oil sampling difficult.
Use your thermal imager to detect when a gearbox is running hotter than similar gearboxes performing similar work in similar environments. Know the load on each piece of equipment, and check each gearbox when it is running at 40% or more of its usual mechanical load so measurements can be properly evaluated compared to normal operating conditions. Also look for leaking seals. Thermal images can reveal hot oil running down gearbox cases.
When you find an overheating gearbox, its thermal image may offer hints as to the cause of its abnormal operating temperature. Be aware that while all excessive heat generated in mechanical drive components is the result of friction, it may have sources other than inadequate lubrication. But whatever the suspected cause of overheating, you can arrange to follow up by checking the oil level, oil quality, and metal-particle content of the oil or perform acoustical testing or vibration analysis.
You can also monitor the temperature of critical gearboxes over time and establish trends that will dictate when maintenance is required to prevent failure.
Transformers. Most transformers are cooled by either oil or air while operating at temperatures much higher than ambient. In fact, operating temperatures of 65°C for oil-filled units and 150°C for air-cooled transformers are common. Nevertheless, problems with transformers often manifest themselves in overheating or hot spots, making thermal imaging a good tool for finding problems.
The following discussion focuses on using thermal imaging to monitor external and internal conditions of oil-filled transformers. Other diagnostic technologies, including built-in temperature and pressure gauges, may be more reliable for assessing the internal conditions in dry transformers.
Keep in mind that like a standard electric motor, a transformer has a minimum operating temperature that represents the maximum allowable rise in temperature above ambient, where the specified ambient is typically 40°C. It is generally accepted that a 10°C rise above its maximum rated operating temperature will reduce a transformer's life by 50%.
In oil-filled transformers, monitor the following external components:
- High- and low-voltage bushing connections
Overheating in a connection indicates high resistance and that the connection is loose or dirty. Also, compare phases, looking for unbalance and overloading.
- Cooling tubes
On oil-cooled transformers, cooling tubes will normally appear warm. If one or more tubes are comparatively cool, oil flow is being restricted, and the root cause of the problem needs to be determined.
- Cooling fans/pumps
Inspect fans and pumps while they are running. A normally operating fan or pump will be warm, one with failing bearings will be hot, and one that is not functioning will be cold.
Monitor the following for internal problems on oil filled transformers:
- Internal bushing connections
Connections will be much hotter than surface temperatures read by an imager indicate.
- Tap changers
An external tap changer compartment should be no warmer than the body of the transformer. Since not all taps will be connected at the time of an inspection, IR inspection results may not be conclusive.
Preemptive planning pays off. So whether you have already established a highly sophisticated preventive maintenance program or you're just getting started on a more proactive approach, thermal imaging can help you prevent major problems before they start in the areas described above. From motor control centers to pumps to gearboxes, thermography is becoming an increasingly popular method to identify potential areas of equipment failure and limit downtime, especially in the field of industrial maintenance.
By remotely gathering predetermined data before disaster strikes, thermography enables end-users to monitor the condition of all components within an electrical system, record readings, establish a baseline, and track trends over time for optimal maintenance and repair.
Pratten is an ANST-compliant, Level II trained thermographer and training manager with Fluke Corp. in Everett, Wash.
Sidebar: Imaging Tips
Tip No. 1 — When you're working with low electrical loads, the indications of a problem may be subtle at best. A minimum of 40% of design load is recommended (as per NFPA 70B), and the higher the load, the better.
Tip No. 2 — Sometimes it's difficult to get a direct view of the component you want to inspect, such as a motor or gearbox mounted high up on the top of a machine. Try using a thermal mirror to see the reflection of the component. An aluminum sheet (⅛-inch thick) works very well.
Tip No. 3 — Winds (or air currents inside) in excess of even a few miles per hour will reduce the surface temperatures you're seeing, causing real problems to seem less significant or even making them invisible. Buy a good quality wind meter and record the wind speed when you record the apparent temperature. Even those with small temperature increases may become critically hot when the airflow is reduced.
Tip No. 4 — Hardware used for electrical connections and contacts is often shiny and will reflect infrared energy from other nearby objects, which can interfere with temperature measurement and image capture. Extremely dirty equipment can also interfere with accuracy. To improve accuracy, wait until the equipment is de-energized and paint dark, less reflective spots on the target measurement areas. Be careful not to use combustible materials such as black paper or plastic tape.