Thermography Basics - Keeping it Simple

 Thermography Basics - Keeping it Simple

 

Thermography is a predictive maintenance technique that can be used to monitor the condition of plant machinery, structures and systems, not just electrical equipment. It uses instrumentation designed to monitor the emission of infrared energy, i.e. surface temperature, to determine their operating condition. By detecting thermal anomalies, i.e. areas that are hotter or colder than they should be, an experienced technician can locate and define a multitude of incipient problems within the plant.

Infrared technology is predicated on the fact that all objects having a temperature above absolute zero emit energy or radiation. Infrared radiation is one form of this emitted energy. Infrared emissions, or below red, are the shortest wavelengths of all radiated energy and are invisible without special instrumentation. The intensity of infrared radiation from an object is a function of its surface temperature. 

 

However, temperature measurement using infrared methods is complicated because there are three sources of thermal energy that can be detected from any object: energy emitted from the object itself, energy reflected from the object, and energy transmitted by the object. Only the emitted energy is important in a predictive maintenance program. Reflected and transmitted energies will distort raw infrared data. Therefore, the reflected and transmitted energies must be filter out of acquired data before a meaningful analysis can be completed.

 

Variations in surface condition, paint or other protective coatings and many other variables can affect the actual emissivity factor for plant equipment. In addition to reflected and transmitted energy, the user of thermographic techniques must also consider the atmosphere between the object and he measurement instrument. Water vapor and other gases absorb infrared radiation. Airborne dust, some lighting and other variables in the surrounding atmosphere can distort measured infrared radiation. Since the atmospheric environment is constantly changing, using thermographic techniques requires extreme care each time infrared data is acquired.

Most infrared monitoring systems or instruments provide filters that can be used to avoid the negative effects of atmospheric attenuation of infrared data. However, the plant user must recognize the specific factors that will affect the accuracy of the infrared data and apply the correct filters or other signal conditioning required to negate that specific attenuating factor or factors. 

Collecting optics, radiation detectors and some form of indicator are the basic elements of an industrial infrared instrument. The optical system collects 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, which can be displayed. There are three general types of instruments that can be used for predictive maintenance: infrared thermometers or spot radiometers line scanners and imaging systems.

Three types of instruments are generally used as part of an effective predictive maintenance program: infrared thermometers, line scanners, and infrared imaging systems.

1.  Infrared thermometers or spot radiometers are designed to provide the actual surface temperature at a single, relatively small point on a machine or surface. Within a predictive maintenance program, the point-of-use infrared thermometer can be used in conjunction with many of the 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.

2. Line scanners provide a single dimensional scan or line of comparative radiation. While this type of instrument provides a somewhat larger field of view, i.e. area of machine surface, it is limited in predictive maintenance applications.

3. Infrared imaging provides the means to scan the infrared emissions of complete machines, process or equipment in a very short time. Most of the imaging systems function much like a video camera. The user can view the thermal emission profile of a wide area by simply looking through the instrument's optics. 

There are a variety of thermal imaging instruments on the market ranging from relatively inexpensive, black and white scanners to full color, microprocessor-based systems. Many of the less expensive units are designed strictly as scanners and do not provide the capability of store and recall thermal images. This inability of store and recall previous thermal data will limit a long-term predictive maintenance program.

Training is critical with any of the imaging systems. The variables that can destroy the accuracy and repeatability of thermal data must be compensated for each time infrared data is acquired. In addition, interpretation of infrared data requires extensive training and experience.

Inclusion of thermography into a predictive maintenance 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. Infrared techniques can 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 included in an infrared thermography inspection is usually energized. For this reason, a lot of attention must be given to safety. The following are basic rules for safety while performing an infrared inspection:

Plant safety rules must be followed at all time. A safety person must be used at all times. Because proper use of infrared imaging systems requires the technician to use a viewfinder, similar to a video camera, to view the machinery to be scanned, he or she is blind to the surrounding environment. Therefore, a safety person is required to assure safe completion. Notify area personnel before entering the area for scanning. Qualified electricians from the area should be assigned to open and close all electrical panels.

Where safe and possible, all equipment to be scanned will be on line and under normal load with a clear line of sight to the item. Asset with covers that are interlocked without an interlock defeat mechanism should be shutdown when allowable. If safe, their control covers should be opened and equipment restarted.

When used correctly, thermography is a valuable predictive maintenance and/or reliability tool. However, the derived benefits are directly proportional to how it is used. If it is limited to annual surveys of roofs and/or quarterly inspections of electrical systems, the resultant benefits are limited. When used to regularly monitor all critical process or production systems where surface temperature or temperature distribution is indicative of its reliability or operating condition, thermography can yield substantial benefits.

To gain the maximum benefits from your investment in infrared systems, use it full power. Concentrate your program on those critical systems that generate capacity in your plant. 

There are many maintenance and reliability training companies around the world who offer classes on Thermography and other PdM Technolgies. My recommendation is to attend one of these classes, typically the basics are 3-5 days, you will be glad you did.

The Author: Ricky Smith CMRP, CMRT, CRL

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