Sunday, 24 November 2024
Continuing with the theme of Non-Destructive Testing (NDT) for proper maintenance of chillers and chilled water plants’ ancillary equipment for the HVAC sector, vis-à-vis predictive maintenance technology, Dan Mizesko discusses infrared thermography in the third part of the series.
Dan Mizesko is the Managing Partner at Al Shirawi Chiller services
In Part I of this column, my focus was on oil analysis via Non-Destructive Testing (NDT) for the HVAC sector. Part II, covered vibration analysis. This time, let’s look at infrared thermography (IR).
What is IR?
IR can be defined as the process of generating visual images that represent variations in IR radiance of surfaces of objects. Similar to the way objects of different materials and colours absorb and reflect electromagnetic radiation in the visible light spectrum (0.4 to 0.7 microns), any object at temperatures greater than absolute zero emits IR energy (radiation) proportional to its existing temperature. The IR radiation spectrum is generally agreed to exist between 2.0 and 15 microns. By using an instrument that contains detectors sensitive to IR electromagnetic radiation, a two-dimensional visual image reflective of the IR radiance from the surface of an object can be generated. Even though the detectors and electronics are different, the process itself is similar to the one a video camera uses to detect a scene reflecting electromagnetic energy in the visible light spectrum, interpreting the information and displaying what it detects on a liquid crystal display (LCD) screen that can then be viewed by the device operator.
Because IR radiation falls outside the realm of visible light (the radiation spectrum to which our eyes are sensitive), it is invisible to the naked eye. An IR camera or similar device allows us to escape the visible light spectrum and view an object based on its temperature and its proportional emittance of IR radiation.
Role of IR in predictive maintenance
We generally forget about the roof until it leaks on our computers, switchgear, MS/CMS or chiller control panels
How and why is this ability to detect and visualise an object’s temperature profile important in maintaining systems or components? Like all predictive maintenance technologies, IR tries to detect the presence of conditions or stressors that act to decrease a component’s useful or design life. Many of these conditions result in changes to a component’s temperature. For example, a loose or corroded electrical connection results in abnormally elevated connection temperatures due to increased electrical resistance. Before the connection is hot enough to result in equipment failure or possible fire, the patterns are easily seen through an IR imaging camera, the condition identified and corrected.
TES tanks, chilled-water piping and chiller evaporator insulation are other areas where IR thermography has proven very useful. I cannot tell you how many times on a District Cooling plant hand-over inspection we have found substandard TES tank insulation/poor chilled-water pipe insulation and evaporators, which had insulation that was damaged and failing. All of these would not have been identified without an IR technology profile. Rotating equipment problems will normally result in some form of frictional change, which will be seen as an increase in the component’s temperature. Loss of a roof’s membrane integrity will result in moisture that can be readily detected as differences in the roof thermal profile. These are just a few general examples of the many possible applications of this technology, and how it might be used to detect problems that would otherwise go unnoticed, until a component fails and results in excessive repair or downtime cost, as well as massive energy loss.
Types of IR detection devices
Many types of IR detection devices exist, varying in capability, design and cost. In addition, simple temperature measurement devices that detect IR emissions but do not produce a visual image or IR profile are also manufactured.
Spot Radiometer (Infrared Thermometer):
Although not generally thought of in the world of thermography, IR thermometers use the same basic principles as higher-end equipment to define an object’s temperature based on IR emissions. These devices do not provide any image. Typical IR spot thermometers are not representative of an object’s thermal profile but rather a value representative of the temperature of the object or area of interest.
Infrared Imager:
As indicated earlier, equipment capabilities, design, cost and functionality vary greatly. Differences exist in IR detector material, operation and design. At the fundamental level, IR detection devices can be broken down into two main groups – imagers and cameras with radiometric capability. A simple IR imager has the ability to detect an object’s IR emissions and translate this information into a visual image. It does not have the capability to analyse and quantify specific temperature values. This type of IR detection device can be of use when temperature values are unimportant, and the object’s temperature profile (represented by the image) is all that is needed to define a problem. An example of such an application would be in detecting missing or inadequate insulation in a structure’s envelope. Such an application merely requires an image representative of the differences in the thermal profile due to absence of adequate insulation. It needs to be noted that exact temperature values are unimportant.
IR cameras with full radiometric capability detect the IR emissions from an object and translate the information into a visible format, as in the case of an imager. In addition, these devices have the capability to analyse the image and provide a temperature value corresponding to the area of interest. This capability is useful in applications where a temperature value is important in defining a problem or condition. For example, if an image indicates a difference between a pulley belt temperature and an ambient temperature, the belt may have worn, be of the wrong size, or it could indicate a misalignment condition. Knowing the approximate temperature differences would be important in determining if a problem exists.
The primary value of thermographic inspections of electrical systems is locating problems so that they can be diagnosed and repaired. “How hot is it?” is usually of far less importance. Once the problem is located, thermography and other test methods, as well as experience and common sense, are used to diagnose the nature of the problem.
A checklist for diagnosis
The following list contains just a few of the possible electrical system-related survey applications:
Rotating equipment applications are only a small subset of the possible areas where thermography can be used in a mechanical predictive maintenance programme. In addition to the ability to detect problems associated with bearing failure, alignment, balance and looseness, thermography can be used to define many temperature profiles indicative of equipment operational faults or failure.
If an image indicates a difference between a pulley belt temperature and an ambient temperature, the belt may have worn, be of the wrong size, or it could indicate a misalignment condition
The old adage, out of sight, out of mind, is particularly true when it applies to flat-roof maintenance. Flat roof profiles are very common in this region. We generally forget about the roof until it leaks on our computers, switchgear, MS/CMS or chiller control panels. Roof replacement can be very expensive, and at a standard industrial District Cooling complex, easily run into hundreds of thousands of dollars. Depending on construction and length of time the roof has leaked and other related factors, actual building structural components can be damaged from leakage and years of neglect, which drive up repair cost further. Utilisation of thermography to detect loss of a flat roof’s membrane integrity is an application that can provide substantial return by minimising the area of repair/replacement. Roof reconditioning cost can be expected to run less than half of new roof cost per square foot. Add to this the savings to be gained from reconditioning a small percentage of the total roof surface, instead of replacement of the total roof, and the savings can easily pay for roof surveys and occasional repair for the life of the building, with some change left over.
Costs and recommendations
As indicated earlier, the cost of thermography equipment varies widely, depending on the capabilities of the equipment. A simple spot radiometer can cost from USD 500 to USD 2,500. An IR imager without radiometric capability can range from USD 7,000 to USD 20,000. A camera with full functionality can cost from USD 18,000 to USD 65,000. Besides the camera hardware, other programme costs are involved. Below is a listing of equipment and programmes recommended:
Payback can vary widely, depending on the type of facility and use of the equipment. A production facility, whose downtime equates to several thousands of dollars per hour can realise savings much faster than a small facility. On an average, a large facility can expect a payback in 12 months or less. A small facility may consider using the services of an IR survey contractor. Again, my recommendation is to outsource all the NDT services to trained and certified professionals. Contracted outsourced services are always the most cost-effective approach for most facilities, large or small.
Training for infrared thermography is available through a variety of system manufacturers and vendors. In addition, the American Society of Non-destructive Testing (ASNT) has established guidelines for NDT (Level I, II and III) certification (NASA 2000). These three levels are designed to take the student from Level I, where the student is competent with equipment function and use, to Level II, where the student is fully capable and experienced and can complete diagnostics and recommendations, to Level III, where the student is fully experienced to supervise and teach Level I and II students.
CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.
Dan Mizesko is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at dan@uschillerservices.com
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