The Benefits of Aerial Infrared Thermal Mapping
The imagery (IR) from aerial infrared thermal surveys of facilities, complexes, campuses, military bases and cities can be used for many purposes. Systems like supply steam and condensate return lines, hot water lines, chilled water lines, supply water mains, distribution piping, storm water drains and sewer lines can be monitored by looking at surface temperatures/patterns. In the case of district heating systems, the distribution system can be flown rapidly and inexpensively to provide thermal data for asset management planning and predictive maintenance (PdM). As a result of finding and repairing leaks in the steam system, energy usage can be reduced with all the related benefits.
Leaks and insulation failure in overhead steam lines and underground steam lines (direct-bury lines and in steam tunnels), can result in less than optimal energy efficiency, especially when the steam leaks and line heat losses (see Figure 1), are undetected, inaccessible or difficult to find given the vast acreage at some facilities. The longer that a steam leak, excessive heat loss on a line and/or undetected draining of fluids (see Figure 2) goes undetected; the greater the energy loss, the more make-up chemicals have to be added and the more potential there is for negative environmental impact.
Figure 1) Typical steam system heat losses. (Red is more heat loss than yellow and green is normally operating apparatus.)
Figure 2) Steam line leaking onto the ground surface.
Understanding Aerial Infrared
Aerial infrared can help monitor the steam distribution system so that those charged with the task, can manage the assets better. Checking the boilers, lines and steam traps inside buildings and inside steam tunnels are jobs best accomplished on the ground, but the distribution and condensate return lines are best surveyed from the air. Thermal contrast between active lines and the surrounding ground are usually good depending on the depth of the line, temperature, flow and the materials covering the lines. The entire system can be flown, a mosaic thermal image produced and the areas with suspected problems can be pinpointed and documented. On-ground, hand-held IR surveys on all but very small areas within a system are time-consuming, labor intensive and produce small field of vision imagery and/or paint on the ground. Owing to recent developments in infrared technology and the availability of high thermal sensitivity/high spatial resolution (large format) thermal imaging systems mounted on an aerial platform, the on-ground survey has become outdated.
The methodology for taking aerial infrared thermographs is similar in many ways to taking aerial photographs. To collect the data, the aircraft flies over a given area with a camera mounted to the airframe and oriented looking straight-down to the ground (NADIR). The imagery is stored on a computer hard drive and later post-processed. Where aerial infrared thermography differs from aerial visible photography is the time of day when the survey occurs and the wavelength of the imagery that the detector collects. IR thermography of ground objects is performed at night. Thermography reveals sources of heat and the relative differences in heat from one object to another.
Infrared imagery is a grayscale picture whose scales (or shades of gray) represent the differences in temperature and emissivity of objects in the image. Typically, objects in the image that look lighter are warmer, and those that look darker are cooler…bright white objects being the warmest and black objects, the coolest.
Any object with a temperature above absolute zero (0 Kelvin or –273 degrees Celsius) emits infrared radiation. An infrared picture only shows objects which emit infrared wavelengths in the 3000-5000 nanometer (mid-wave) range or 8000-14000 (long-wave) range. Objects in visible light wavelengths of 400 to 700 nanometers are detected, but only because they also emit heat. An example of this would be a street light that can be seen in the IR imagery because the ballast and bulb are warm.
Infrared imagery is usually recorded on digital media and later copied to DVD-Video, videotape and/or captured as digital image files. The images may then be modified in a number of ways to enhance their value to the end-user, such as creating false-color images and/or adjusting the brightness and contrast of a grayscale image to be used in a report.
Steam Leak Infrared Surveying
Steam and condensate return lines are almost always readily visible with infrared imaging, even when no notable problems exist. This is due to the fact that no matter how good the insulation, there is always heat loss from the lines which makes its way to the surface. Problem areas are generally quite evident, having brighter infrared signatures (see Figures 3) that exceed the norm.
Figures 3a and 3b) IR and visual image of a steam line with leak colorized red.
Steam line faults normally appear as an overheated line or as a large hotspot in the form of a bulge or balloon along the line. Overheated lines often occur when the steam line is located in a conduit or tunnel. If there is a leak in the line, it will heat up the conduit with escaping steam. If a steam line is buried directly in the ground with an insulating jacket, a leak will usually saturate the insulation, rendering it largely ineffective and begin to transfer heat into the ground around the leak, producing the classic bulge or balloon-like hot area straddling the line.
Some leaks show up as an overheated manhole or vault cover. Manholes or vaults that contain steam system control apparatus which are leaking, will often heat the covers to warmer than normal temperatures. Unless these leaks are severe enough to significantly raise the manhole temperature above their normally slightly elevated temperatures, these leaks can be difficult to identify.
In fact, steam line infrared imagery can be a little misleading, unless one understands and interprets the relative brightness and temperature of a given line correctly. For instance, a steam line that is the same temperature from one end to the other that passes under different surfaces and materials can exhibit a variety of real and perceived
temperature variations. Five different apparent temperatures will result from the same temperature line that runs under a grass-covered field, an asphalt roadway, a concrete loading dock, a gravel-covered parking lot and a bare earth pathway.
Figures 4a and 4b) Mosaic visual and infrared imagery of a city steam distribution system.
Thermal Mapping, Ortho-Rectification and Post-Processing
Using a non fixed-mounted high-resolution thermal imager to survey a couple of buildings or a few thousand feet of underground lines can be done by flying over and locating the target(s) in the imagery, saving the data and putting it together into a report. This works for very small areas, but it is not possible to make precise thermal maps of a whole complex, campus, military base or city (see Figures 4) without ortho-rectification of the imagery. In order to produce ortho-rectified thermal maps, much more information must be gathered and tagged to the IR imagery. During the flight, the aircraft flies straight, smooth lines on a pre-planned grid, allowing overlap and sidelap of the imagery. The IR operator manages the sensor data-acquisition following a structured checklist for orderly data file management. The imagery must be collected with a precise direct-digital timing system, a 3-axis ring-laser-gyro and an inertial navigation system (INS), which is tightly-coupled to a real-time differential GPS satellite positioning system that provides x, y, z positioning of the sensor at all times.
After data is collected, the digital infrared imagery is processed into a series of ortho-rectified image tiles, which are then stitched together to create a giant mosaic image. A computer system puts all this information together using a digital elevation model (DEM) of the scene that consists of a uniform grid of point elevation values and the position and orientation of the camera with respect to a three-dimensional coordinate system output. The result is presented as a high-resolution thermal image in the form of a geo-TIFF (see Figure 5), which is compatible with any GIS software such as ESRI ArcView™, AutoCAD® Map 3D, Global Mapper, MapInfo™, etc. Once high quality digital thermal and photographic ortho-rectified maps are created, they can be added as
layers other data sets, to existing or new CAD and GIS systems. Digital data can also be post-processed in other ways, such as creating false color imagery to highlight areas of interest, adding temperature data and/or creating graphic reports (see Figure 6).
Figures 5) Mosaic infrared image (geo-TIFF) of a small college.
Figure 6) Colorized thermal imagery and post-processing example.
Figures 7) Thermal image of a flat roof (wet areas are lighter).
ualitative v. Quantitative Evaluations