Everything gives off heat. And interestingly, many things give off more heat before they fail. Whether it's an electrical contact on the verge of meltdown, or a motor bearing ready to seize in its raceway, the amount of heat an item radiates can be a good indication of a problem. Thermal imaging, a way of visualizing the heat emitted by an object, adds another tool to reduce the chance of catastrophic equipment failure.Thermal imaging is done with special infrared cameras that detect the infrared energy or heat given off by an object. The detected energy is used to create an image that uses color to represent the level of thermal emissions from the object. The process is called thermography and the image it creates is a thermogram.
The selection of which color equals what temperature is arbitrary, with the camera operator or thermographer setting the range. Typically, cooler areas are marked by a bluish-black color fading to red, orange, yellow, and then white as temperatures climb.
Early infrared cameras were cumbersome affairs requiring several hours to setup and calibrate. They typically required cryogenic cooling of their detector using liquid nitrogen to eliminate the thermal background noise created from their own infrared emissions.
Today's modern infrared cameras, now called thermal or infrared imagers, are compact, handheld devices that work at room temperature. The thermogram they form appears as an image on a built-in video screen or it can be transferred to a computer for detailed analysis, printing, and archiving.
Thermal imaging is finding its way into multiple fields with many different applications. Some areas where thermal imaging is being used include asphalt inspection, building diagnostics, food transportation, home inspections, law enforcement, and manufacturing. A growing area for thermal imaging is in predictive and preventive maintenance (PPM) programs for factories and machinery.
The addition of thermography to a PPM system requires more than just the purchase of a thermal imager. Budding thermographers should practice reading thermographic images. Personnel involved with thermal programs should use the camera two to three times weekly over a period of six months to gain expertise. Everyone should plan the work, track the findings, and document results from the beginning.
Thermography should never be used as the one and only tool in a PPM program. However, when combined with data obtained through such other predictive technologies as vibration, motor circuit analysis, airborne ultrasound, and lube analysis, it offers effective support to determine the overall condition of a machine. It may also provide indications of a developing problem before it becomes detectable by other means.
A newly hired thermographer starting work in a plant may require training beyond the scope of thermal imager operation. The National Fire Protection Association (NFPA) 70E standard mandates education about the risks all personnel face when working near electrical equipment. Companies must also supply personal protective equipment (PPE) to minimize risks in the event of an accident. For thermographers, PPE generally includes flash-resistant clothing and a face shield.
Thermographers should avoid prioritizing findings based on temperature alone. Temperature measurements identify problems extremely well and may help characterize problems, but they aren't the best way to determine why a component is failing. So inspection procedures should not just address ways to locate problems using thermography. They should also factor in other technologies to troubleshoot further.
When taking a thermogram, ensure the imager is within calibration by viewing a black body reference or conducting a simple "tear duct check." If the imager has a memory function, make sure it's cleared of previously recorded data. One exception to that is if the area being inspected has undergone previous inspection. Then it might be wise to upload past results to the imager for comparison with the latest findings. Assemble all additional equipment required, such as clamp meters for reading loads, a voice recorder for notes, any special protective gear, and make sure all is in good working order.
Take a moment to get oriented whenever entering an inspection area. Determine an emergency exit strategy and note any potential hazards. Many thermographers begin an electrical inspection by looking first at the panel covers while they are still closed. Any appearing abnormally warm may require taking further safety precautions before accessing the equipment inside. Airborne ultrasound-detection equipment provides a very useful supplemental signature for examination and a level of assurance that things are safe. If necessary, post signs or barricades around an area during the inspection.
Unless the inspection team is conducting a first-time baseline inspection, only record thermal images when problems or exceptions are located. Take time to look at the findings from several different angles and collect any other data useful for analysis, including additional visual images of the component. Don't worry about actually measuring temperatures until after a problem is found. Then, if appropriate, use the correct emissivity and reflected temperature correction (RTC) to obtain the true temperature. Additional analysis is often easier to do back in the office at the computer.
For electrical enclosures such as motor-control cabinets, open only as many panels as is safe. Problem hot spots may cool off if enclosure doors are left open too long.
There are a number of items that can make inspections easier, safer, and more effective. For example, high-emissivity targets installed on components such as bus bars, tubular busses, or any large metallic electrical connectors helps improve the reliability of radiometric temperature measurements.
Unfortunately, there are no standards on how to create such targets. Many plants report good success using flat spray paint. The color appears immaterial if inside, but white works best outside. Electrical tape and paper stickers also can make good thermal targets when installed near connection points.
Infrared transparent windows, made from a crystalline material or a special plastic, installed in electrical-panel covers, make it possible to inspect components without opening the enclosure. The safety aspect dealing with high-voltage enclosures is obvious. Install these only in locations that allow for complete inspection.
The clear-plastic "touch-safe" covers becoming more prevalent inside electrical-control cabinets are not transparent to infrared. A possible modification to these adds hinges or routes small holes in them directly over the connectors and fuse clips.
Bearings and couplings on conveyance systems are often hidden under equipment guards and covers making thermal imaging difficult. Consider installing a small hinged door or using a metal mesh instead of solid metal, as long as it doesn't compromise safety.
Thermal mirrors — thick sheets of plate aluminum — can make it easier to see a thermal signature. To view the end bearings of large vertical motors, mount a thermal mirror above and angled down. Place the thermal mirror on the floor to view up under a process or machine.
Some software that comes with thermal imagers supports simple but useful comparisons of asset condition over time. For example, an alarm temperature setpoint alerts the thermographer if an image exceeds the preset value. The alarm setting along with the previous image can determine the extent of any changes between inspections.
Imagers typically have an area measurement tool that shows the maximum, minimum, and average temperatures for the area, rather than a spot measurement.
It's better to use this tool whenever possible as it ensures identification of the true maximum temperature. It's also important to report equipment loading and environmental variables. Note the temperature corrections used for both the emissivity of the object imaged and the reflected background.
Long-term analysis of data is important, so accumulate the information in a form that helps this process. The benefit is twofold. First, trends become visible that may not be obvious in a day-to-day analysis. For instance, analysis of long-term data may uncover that the motor shop is doing a poor job rebuilding motors, or that problems follow a certain brand of fused disconnect.
The second benefit shows what is and is not working about your maintenance program. It highlights where problems continue to occur or may indicate reducing inspection frequency because few problems are being found.
Thermography lends itself to manufacturing process applications. For example, a process had shut down repeatedly because a heat exchanger couldn't adequately cool the operation. Engineers were planning to add a larger exchanger to "solve" the problem. A thermographer found that hot exhaust air from the production process was blowing directly onto the heat exchanger, defeating its purpose.
Another thermographer in an automotive assembly plant happened to look at the incoming tires and noticed how cold they were. When he showed the image to the area manager, the two quickly connected this condition to a seasonal problem they'd had for years in which the tires failed to mount properly on the rims. The solution was as simple as bringing the tires inside long enough to warm up before mounting.
Of course, thermographers looking at processes are not limited to simply measuring temperatures or seeing thermal images. Moisture, thickness, coatings, material types, and parts presence all have their own characteristic thermal signature. Manufacturing processes are not always simple to look at thermally, but doing so often yields a perspective that may uncover solutions to costly problems.
Thermography and industrial standards
Standards developed by national and international standards organizations cover many aspects of infrared thermography.