Most facilities use fixed-mount sensors for permanent temperature monitoring as well as handheld tools for spot-checks, troubleshooting and predictive maintenance. As thermal imagers have come down in price and complexity, they've become an ideal tool for this purpose. Also known as infrared (IR) imagers or IR cameras, thermal imagers capture two-dimensional representations of an object's surface temperature.
How Thermal Imagers WorkThermal imagers provide a quantitative measure of the temperatures at surface points visible in a scanned scene. A thermal imager consists of an optical system capable of passing infrared wavelengths, an infrared detector, and a digital-image recording system.
The optical system includes an objective lens that focuses incoming infrared radiation from the object being scanned. This process creates an image. The infrared detector is a solid-state device that absorbs any infrared energy that falls on it and responds by putting out an electrical signal. Thermal imagers use carefully calibrated signal detectors that ensure accurate measurements over the whole image.
Most imagers come with two days of training, qualifying users to operate the camera and evaluate the brightness of the infrared wavelengths they see, compared to previous images of that object. Over time, users can learn to evaluate the specific temperatures at surface points visible in a scanned scene.
To ensure quick, accurate measurements by a beginning thermographer, the supervisor can upload the inspection route and location-specific instructions to the thermal imager. Each location can be given a descriptive name that lets the user know exactly where to stop (for example, Motor Control Center 6), what data to record, what safety and measurement procedures to follow, and even where to stand.
That is important because, to compare equipment conditions from one thermal reading to the next, thermal images must be consistent. At a given stop, the technician should stand in the same location and capture the same field of view on the imager's display.
The imager also should carry the baseline images taken during the last general inspection, including the associated emissivity and reflective temperature values (RTC), low/high alarm limits and inspection notes. Thermographers at any skill and experience level will need this information, but for beginning thermographers, having the settings and a prior image against which to compare current values is essential.
Emissivity. Emissivity describes the infrared energy emitted from an object that indicates the object's surface temperature. More specifically, emissivity is the efficiency with which the surface material of that object emits that energy. The standard emissivity of most organic materials and painted or oxidized surfaces is 0.95. However, certain materials, such as concrete and shiny metals, are poorer emitters, so using an emissivity setting of 0.95 in calculating surface temperatures of these objects will not yield an accurate thermal image. Be especially alert for bus bars, tubular bus and any large metal electrical connectors.
Emissivity for a particular location can be uploaded to the imager if the correct values are known. Or, with training and time, thermographers can adjust the emissivity on the camera at the time the image is made. For example, for shiny fuse caps, the emissivity might be only 0.6. Users who know that emissivity value can change the image's emissivity setting from 0.95 to 0.6 and see the real temperatures of the equipment.
Of course, reflected temperature isn't always the same as ambient temperature. For example, the background might be a furnace wall at 392oF (200oC). Then, one has to set the camera to take into account an emissivity of, say, 0.6, but also the reflected temperature of 392oF to achieve an accurate temperature measurement.
Level and Gain. Most users, especially beginning thermographers, will work in automatic mode, in which the temperature scale is automatically assigned a temperature range based on the thermal scene in front of the camera optics. For example, if the camera senses a range from 80 to 120 degrees, the camera will automatically display a temperature range between 75 and 125 degrees. A change in the level and gain sensed by the camera will result in an automatic change to the temperature information on the camera's display.
If one looks at a scene in automatic mode with something cool in the foreground and something very hot in the background, the color palette will be spread across a wide range of temperatures and the resolution will be poor. In such cases, the camera user can manually adjust the level and gain to view just the temperatures of the hot or the cool object.
Process ApplicationsIn specific processes, use your thermal imager to look at product uniformity. For example, if you have a paper process, you probably process the paper by running it through an oven to cure it. The coatings applied often require a combination of time and temperature to achieve the right cure point and final moisture level. Use your handheld thermal imager to examine the thermal uniformity of the product as it comes out of the oven. Thermal variations are often attributable to other process variables such as non-uniformity in moisture or cure.
In general, use your handheld thermal imager to look for hot spots, cool spots and other anomalies. Thermal imaging also can be used to monitor critical equipment. Here is a partial of list of equipment to monitor and what it might detect:
- Steam systems (failed traps, obstructed piping).
- Cooling systems (fouled cooling towers, blocked heat exchangers).
- Furnaces and boilers (damaged refractory, leaking ports).
- Pumps (hot bearings, leaking seals).
- Process piping (ineffective insulation, reduced flow).
- Tanks and vessels (product or sludge levels, leaks).
- Valves (leakage, striction).
- Conveyors (hot bearings and drives).
- Motors (hot bearings and windings).
- Motor control centers and switchgear (imbalance, overloads).
Each time you inspect a piece of equipment, save a thermal image of it on the computer and track its condition over time. That way, you'll have baseline images for comparisons that will help you to determine whether a hot spot (or cool spot) is unusual and also to verify when repairs are successful.
Heat Processes. Among others, paper, glass, steel and food processing and production all require the uniform application of heat. These processes often utilize thermocouples or infrared temperature sensors for thermal control. Frequently, spot measurements are not adequate due to process variations. Line scanners provide continuous thermal profiling in these cases, while portable thermal cameras can troubleshoot problems and determine the optimum spot to install the thermocouple or infrared sensor.
Pipes. In processes, fluids need to be delivered to the right place at the right time and in the right amounts. If a pipe is obstructed, it can cause a chain reaction that throws an entire process loop out of tune. For example, it can create oscillation, which causes motors to cycle on and off too frequently, which in turn causes more frequent current surges that stress the electrical system and add harmonics that lower system efficiency and ultimately lead to equipment failure. Thermography can often pinpoint an obstruction, allowing corrective action before the whole loop goes down, and the loop can be re-calibrated by a multitasking tech using loop calibrators and digital multimeters.
Valves. Process control valves also are critical to delivering fluids to processes at the right time. A thermal imager can monitor for leakage, striction (sticking) or excess friction. Also, a valve's excitation coil may overheat from working too hard, pointing to a problem such as current leakage or valve size mismatch. When thermography indicates a problem, technicians can follow up by calibrating the valve or the valve's positioner.
Power Distribution Systems. Consistent, high quality power is essential for process manufacturing. Thermal imagery can identify bad electrical connections, imbalances, overloads, harmonics and other impending electrical equipment failures and prevent both uneven or inadequate power supply as well as downtime.
Motors, Fans, Pumps and Conveyors. Thermal inspections of the bearings, shafts, casings, belts, gearboxes and other components that emit heat can detect problems before failure occurs, preventing unexpected equipment breakdowns on moving equipment. In particular, the sludge, solvents and particulates found in many processes puts extra stress on motors, affecting bearings, windings and insulation.
Followup ActionsWhenever a thermal image detects a problem, use the associated software to document your findings in a report that includes a digital, visual-light image as well as a thermal image of the equipment. It's the best way to communicate the problems you found and to suggest repairs. In general, if a catastrophic failure appears imminent, the equipment must either be removed from service or, if possible, repaired while operating. PH
SIDEBAR: Thermal Measurement SafetyTo keep your thermography inspections accurate, effective and safe, establish written inspection procedures for measurement collection and interpretation. Following the same steps each time ensures you have consistent thermal images in your database for comparison. When creating inspection procedures, refer to the following standards.
National Fire Protection Association (NFPA) standard 70E requires that all personnel be educated about the risks they face when working near electrical equipment. Personal protective equipment (PPE) must be made available to minimize the risk if an accident should occur. For thermographers, PPE generally includes flash-resistant clothing and a face shield. Visit www.nfpa.org.
Occupational Safety and Health Administration OSHA 29 CFR, 1910 Subpart S Electrical and Subpart I Personal Protective Equipment Safety standards cover electrical systems, safe work practices and maintenance requirements. Visit www.osha.gov.
International Standards Organization (ISO) standard ISO 6781 and, in the U.S., the American National Standards Institute, discuss thermal insulation, qualitative detection of thermal irregularities in building envelopes, and infrared methodology. Visit www.iso.org or www.ansi.org.
ASTM International standards ASTM E 1934, 1213, 1311, 1316, and 1256 provide a guide for examining electrical and mechanical equipment with infrared thermography, and list thermography practices and certifications standards. Also reference ASTM 1060 and 1153. Visit www.astm.org.