Aerial Infrared
– An Asset Management Tool for District
Heating System Operators
By: Greg Stockton
Stockton Infrared Thermographic
Services, Inc.
8472 Adams Farm Road
Randleman, NC 27317-7331
(336) 498-4734
Abstract
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.
Introduction
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.
Qualitative v. Quantitative Evaluations
The imagery
approach described above is qualitative. It identifies and locates problems in
steam systems based on their anomalous heat signatures. This is low-hanging
fruit with regard to return on investment. Now, this method does not quantify the amount of heat loss. In
order to develop quantitative information, if desired, some additional work is
needed in the form of additional field effort in the infrared data acquisition phase
combined with heat transfer analysis of the steam distribution system.
To
understand the quantitative approach, it is necessary to understand how heat
moves and the factors affecting its transfer, as well as the physics involved
in determining the infrared signature. In order to know exactly what the
radiated energy of any object is, the characteristics of the sensor, atmosphere
and the target must be taken into consideration and one must know the
transmission, emissivity and reflectivity of the target. There are big
differences in the emissive qualities of the concrete, asphalt, grass, dirt,
etc. The ability to obtain quantitative measurements is built into a
radiometric imaging system, so one must use a radiometric infrared camera to collect
the imagery and in a form that can be post-processed.
Heat energy
moves by conduction, convection and radiation. In order to make meaningful
quantitative thermal calculations, the pipe's or pipe's content's temperature,
insulation properties and the complete thermal properties of all the materials
in the ground (specifically heat capacity, thermal conductivity, and density)
must all be known and made part of the calculation. As-built drawings and the
thermal properties are not always readily available, if available at all. This
generally means that estimates of the heat loss, implications of temperature
values obtained, and quantitative evaluation of the pipe's performance can only
be developed as estimates.
Even though
some large format thermal imaging systems are fully capable of accurate radiometric
measurements and rapid frame-by-frame digital temperature data acquisition of
every pixel of every IR image, the cost of quantitatively gathering measurements
and using steady-state and transient heat transfer analysis calculations
(typically done with FEA or finite element analysis), make quantitative
measurement a more expensive step, than simply using the image data to make judgments
based on experience of the person analyzing thermal data. Most of the time,
identifying leaks and excessive line heat loss is straightforward, but making
calculations regarding insulation effectiveness and other qualities is an
additional step that adds cost, which may add value, but also could offer a
lower return on investment. In other words, grab the low-hanging fruit first by
identifying leaks and if you have a known issue (or find one) requiring
quantification, then post-process the thermal data. If you expect to need
quantification, it is wise to plan it ahead of time since it will slightly affect
the methods used for IR image acquisition.
Figures 7) Thermal image
of a flat roof (wet areas are lighter).
Ancillary Benefits
Safety is
improved, asset reliability by condition monitoring can be achieved and wasted
energy can be saved by aerial IR imaging, analysis and repair the steam
distribution system. Creating a ‘thermal map’ of a given area has benefits far beyond
that of just steam. A thermal map helps asset managers in the analysis of many other
types of systems, such as HTHW (high temperature hot water) lines, MTHW (medium
temperature hot water) lines, LTHW (low temperature hot water) lines, CHWS
(chilled water supply), CHWR (chilled water return), supply water mains, storm
water drains, sewer lines and any other distribution piping. Electric power
lines and substations can be surveyed to point maintenance personnel at the
facility to electrical problems.
Drawing the
entrained moisture in flat and low-sloped roofs on a CAD drawing with surgical
precision provides a significant predictive maintenance benefit. Roofs are an
expensive and onerous asset to maintain. Entrained moisture (see Figure 7) in
the insulation and other roof substrates is indicative of leaks into the roof
substrates, seam and flashing failures.
However
unfortunate, ‘wholesale’ building heat loss surveys cannot be accomplished with
a NADIR thermal survey, primarily because most building roofs are decoupled
from the heat loss of the building, either with ventilation, with insulation or
by being so reflective that they are immeasurable with IR sensors. Oblique
aerial or on-ground, right-angle infrared surveying of the walls will be necessary
to accomplish building heat loss surveys.
Author Biography
Gregory R.
Stockton is president of Stockton Infrared Thermographic Services, Inc. Based
in
Acknowledgements
Eric
Raymond Stockton of Stockton Infrared Thermographic Services, Inc., Silk Hope,
NC for assistance in preparation of this paper in application of predictive
maintenance applications issues.
Jack M.
Kleinfeld, P.E. of Kleinfeld Technical Services, Inc., Bronx, NY for assistance
in preparation of this paper in application of heat transfer analysis
issues.
Alejandro
Tache Leon of AITscan Division of Stockton Infrared
Thermographic Services, Inc., Orlando, FL for assistance in preparation of this
paper and in application of thermal mapping issues.
Published June,
2007 at the International District Energy Association Conference in Scottsdale,
AZ
Copyright© 2007
Stockton Infrared Thermographic
Services, Inc.