Recent years have seen an increase in the acceptance and use of infrared thermography for preventive and predictive maintenance. While early applications were confined primarily to electrical and structural situations, today's industrial environment has found new and diverse applications for thermal imaging and noncontact temperature measurement.
The introduction of focal plane array (FPA) imagers during the early 1990s revolutionized infrared imaging by providing high-resolution imaging systems while greatly reducing size and weight. Thermal imaging systems have evolved from cumbersome systems often weighing more than 20 kg (44 lbs.) to systems resembling a video camera that fit in the palm of the user's hand.
These high-resolution infrared imaging systems allow thermography to be applied to more applications than ever before, such as with mechanical systems, intricate process equipment, and printed circuit boards. Infrared thermography can detect unseen problems such as loose or deteriorated electrical connections. Timely repair of these incipient failures can provide tremendous cost savings by avoiding unscheduled downtime.
Infrared thermography also can provide substantial savings by helping to detect problems in products or processes. Permanent improvements in such systems often offer the greatest cost benefit because the repairs are permanent and savings are realized every day that the process operates. Even greater savings are realized when the process or product output is increased.Theory
Thermal imaging is both noncontact and nondestructive. Since it is noncontact, it is useful for inspecting energized electrical systems as well as mechanical systems and rotating equipment. Since the infrared energy emitted from a surface is proportional to its temperature, imaging radiometers are capable of providing surface temperatures as well as images.
Early sensor technology typically used a mechanical scanning system to focus infrared energy onto a single element detector. As a result, displayed thermal images often had poor resolution. Visible light photographs were often required in order to help identify the object of interest in a thermogram.
Early infrared sensors also required liquid nitrogen or compressed gas in order to cool the sensor. The introduction of Stirling cycle and thermoelectric coolers in the 1980s eliminated the need for user-installed cryogenic fluids and gases.
Many infrared imagers now use FPA detectors. These multi-element, solid-state detectors are arrayed together to provide a high-resolution image and eliminate the need for a mechanical scanning system within the optical path.
Detector size is often expressed in terms of the number of horizontal and vertical elements. Typically, FPA detectors have more than 70,000 elements or pixels. As a result of the large number of pixels, thermograms taken with an FPA imager often do not require a corresponding visible light control photograph to help identify the object.
There are currently two types of FPA imagers being offered: cooled and uncooled. Cooled FPAs have been commercially available since the early 1990s. These systems operate in the 3-5 micron range and generally provide excellent sensitivity.
The newest FPA imaging systems use uncooled detectors. Unlike previous infrared systems that sensed photons, these systems operate by sensing changes in electrical resistance across the detector. The microbolometers produce high-resolution images but do not require cryogenic cooling systems. Currently all microbolometers operate in the 8-12 micron range. The increased resolution found on FPA and microbolometer systems enables users to discern minute temperature variations and provides highly accurate temperature readings.
Originally designed for military and aerospace applications, early microbolometers did not provide temperature measurement. Since 1998, many manufacturers have begun to offer microbolometers that can measure temperature. Although they represent the newest detector technology, it is expected that microbolometers will gain in popularity within the next few years.
Indoor electrical systems
Outdoor electrical systems
Highly reflective targets
Boiler/heater tubes - gas fired
Boiler/heater tubes - coal fired
Traditional, new applications
Infrared thermography can be applied anywhere the knowledge of heat patterns and associated temperatures will provide meaningful data about a process, system, or structure. Infrared thermography is useful for condition assessment, forensic investigations, and quality assurance inspections.
Using infrared thermography to detect incipient failures within electrical systems is well documented. Over the past 20 years, the inspection and subsequent repairs of electrical distribution systems have saved companies millions of dollars in avoided downtime.
Infrared thermography continues to be used successfully to inspect building envelopes and flat roofs, boilers and steam systems, underground piping systems, refractory systems, and rotating and process equipment. Results and opinions regarding thermography's effectiveness for rotating equipment inspections have been mixed. However, recent research has found that infrared thermography can be used to accurately detect problems in belted and mechanically coupled rotating equipment.
In 1997, a cross-technologies study was conducted at Eli Lilly in Indianapolis, IN. The study results found that infrared thermography detected misalignment, over/under lubrication of bearings, and improper tension in belted systems more readily than vibration analysis. The study also found that temperature readings taken on the drive-end bell housing within 1 in. of the drive shaft closely approximate the internal winding and bearing temperatures.
For optimum results, a baseline inspection must be made upon installation or retrofit of mechanical equipment. Equipment then must be inspected periodically and results trended. Further investigation or corrective action can be undertaken when an alarm limit is reached.
From the results of the cross-technologies study, predictive maintenance procedures at Eli Lilly were modified to increase infrared thermographic inspections of rotating equipment. This change has allowed more equipment to be inspected while reducing the unit cost for each item inspected and increasing the overall effectiveness of the maintenance program.
Thermal imaging systems vary greatly in their performance and capabilities. The spectral response of a system is dependent upon the type of detector and lens materials used in the construction of the system. While it is possible to buy filters and accessories, some imagers may not be suited for certain applications due to their spectral response.
Spectral response for commercial imagers generally falls into two categories: 2-5 microns (near infrared) and 8-14 microns (far infrared). Commercial infrared imagers and radiometers are not manufactured in the 5-8 micron range due to atmospheric absorption of infrared energy at these wavelengths. The accompanying table shows recommended spectral responses for general PM applications.
It is important to note that there is currently no single imager that will perform every type of infrared inspection. The selection of the imaging system is dependent upon the object being inspected. For some applications such as plastics, it may be necessary to consult with the manufacturer to determine if a particular system can achieve the desired results.
Infrared imaging systems have become more sophisticated; however, they are often easier to use than older systems. Because of this, many people mistakenly believe that infrared thermography can be performed with little or no training. While infrared thermography is a science, it is also an art.
Since the greatest limiting factor in an infrared inspection is often the thermographer, proper training is critical to success. This includes knowledge of infrared theory, heat transfer principles, weather influences, and radiometer operation and limitations as well as a thorough understanding of the system being inspected.
Because of the many variables involved in procuring an accurate radiometric reading, the thermographer will have to address all variables that affect the object being inspected. Some of these variables include target emissivity, background radiation, target size, weather and atmospheric influences, spectral response of the imaging system, and specialty filters. While advances in technology continue to improve the performance and capabilities of thermal imaging systems, proper use of infrared imaging equipment requires formal training.
In-house or contract service
Whether starting or expanding an infrared predictive maintenance program, a company must decide whether to use in-house personnel or outside consultants. If frequent infrared inspections are planned and corporate management is committed to investing in proper equipment and training of personnel, using on-site employees may be appropriate.
If infrequent inspections are planned or the company cannot afford the initial investments in equipment and training, an outside consultant may be a better choice. While arguments can be made for either arrangement, properly trained and equipped personnel can help to increase the effectiveness of a PM program and a company's bottom line. MT
Craig K. Kelch and R. James Seffrin are president and staff engineer, respectively, with the Infraspection Institute, 3240 Shelburne Rd., Suite C, Shelburne, VT 05482; (802) 985-2500; Internet www.infraspection.com