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By R. Keith Mobley, Contributing Editor
Thermography is a predictive maintenance (PdM) technique for monitoring the condition of plant machinery, structures and systems — not just electrical equipment. It uses instrumentation to read infrared energy emissions (surface temperature) to determine operating conditions. By detecting thermal anomalies (areas hotter or colder than they should be), an experienced technician can locate and define a multitude of incipient problems within the plant. Infrared technology works on the principle that objects having a temperature above absolute zero emit energy or radiation.
Infrared radiation is one form of emitted energy. Infrared emissions are invisible without special instrumentation. The intensity of infrared radiation from an object is a function of its surface temperature. However, measuring temperature with infrared methods is complicated, because three sources of thermal energy can be detected from any object: energy emitted from the object itself; energy reflected from the object; and energy transmitted by the object. Only emitted energy is important in a PdM program. Reflected and transmitted energies distort raw infrared data. Therefore, they must be filtered out of acquired data before meaningful analysis can be performed.
Variations in surface condition, such as paint or other protective coatings, can affect the actual emissivity factor for plant equipment. They may change, sometimes radically, both the surface temperatures and heat distribution recorded by the infrared scanner. If the technician fails to compensate this, it will be difficult, if not impossible to accurately diagnose the incipient problems. In too many cases, they will be missed and serious damage or catastrophic failure will occur.
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In addition to reflected and transmitted energy, the user of thermographic techniques must consider the atmosphere between the object and the measurement instrument. Water vapor and other gases absorb infrared radiation. Airborne dust, some lighting and other variables can distort infrared radiation measurements. Because the atmospheric environment is constantly changing, using thermographic techniques requires extreme care each time data is acquired.
Most infrared monitoring systems or instruments use filters to eliminate the negative effects of atmospheric attenuation. However, the user must recognize the specific factors that will affect infrared data accuracy and apply the correct filters or other signal conditioning methods.
Collecting optics and radiation detectors are basic elements of an industrial infrared instrument. Optical systems collect radiant energy and focuses it upon a detector, which converts it into an electrical signal. The instrument’s electronics amplifies the output signal and process it into a form that can be displayed. Three general types of instruments are used for PdM: infrared thermometers or spot radiometers, line scanners and imaging systems.
Infrared thermometers or spot radiometers provide the actual surface temperature at a single, relatively small point on a machine or surface. Point-of-use infrared thermometers are commercially available and relatively inexpensive. Their typical cost is less than $1,000.
Within a PdM program, the point-of-use infrared thermometer can be used in conjunction with many microprocessor-based vibration instruments to monitor the temperature at critical points on plant machinery or equipment. This technique is typically used to monitor bearing cap temperatures, motor winding temperatures, spot checks of process piping temperatures and similar applications. It is limited in that the temperature represents a single point on the machine or structure. However, when used in conjunction with vibration data, point-of-use infrared data can be a valuable tool.
Line scanners provide a single dimensional scan or line of comparative radiation. While this type of instrument provides a somewhat larger field of view (the area of machine surface), its use in PdM applications is limited.
Unlike other infrared techniques, thermal or infrared imaging provides the means to scan the infrared emissions of complete machines, processes or equipment in a very short time. Most imaging systems function much like a video camera. The user can view the thermal emission profile of a wide area simply by looking through the instrument’s optics.
Infrared imaging systems cost between $8,000 for a black and white scanner without storage capability to more than $60,000 for a microprocessor-based, color-imaging system. However, the lower-price units, which only operate in a scanner mode, are not very useful for a long-term PdM program.
Training is critical with the use an imaging system. The variables that can destroy thermal data accuracy and repeatability must be compensated for each time data is acquired. In addition, infrared data interpretation requires extensive training and experience.
Inclusion of thermography into a PdM program will enable you to monitor the thermal efficiency of critical process systems that rely on heat transfer or retention; electrical equipment; and other parameters that will improve both the reliability and efficiency of plant systems. It can also be used to detect problems in a variety of plant systems and equipment, including electrical switchgear, gearboxes, electrical substations, transmissions, circuit breaker panels, motors, building envelopes, bearings, steam lines and process systems that rely on heat retention or transfer.
Equipment used in infrared thermography inspection is usually energized. For this reason, attention must be given to safety. These safety rules should be followed when performing infrared inspections.
When used correctly, thermography is a valuable predictive maintenance and reliability tool. However, benefits derived are directly proportional to how widely it’s used. If it’s limited to annual surveys of roofs or quarterly inspections of electrical systems, the resultant benefits will be limited. When used to monitor critical processes or production systems regularly where surface temperature or temperature distribution indicates reliability or operating conditions, thermography can yield substantial benefits. To gain the maximum benefits from your investment in infrared systems, use it on critical systems that generate capacity in the plant.
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