Tank Top
Locating levels in tanks and silos using infrared thermography
Background
Instrumentation for localization levels in tanks and silos is often unreliable. The need for precise information about levels remains necessary, even critical, in many cases. For example, In one situation, a thermometer was used to a fluid level in a large storage tank along the Gulf Coast before the arrival of a tanker to verify. In continuous processes with administrator must know how much capacity is available in each tank. Without that knowledge production can be prevented or, if an overflow occurs, a potentially dangerous situation created. Sometimes traditional level indicating instruments simply can not determine levels. Foams and waxes, for example, are difficult to detect and accurately.
A paper mill experienced a situation where a tank was mistakenly believed to be chosen, when in fact it was just filled with foam instead of liquid. The foaming cost-effective than the tank, it was unnecessary to replace a larger one! A petrochemical plant hired a contractor to clean a large tank. When the door was manway, sludge, which had drawn a deep high above the door, opened flowed again creating a dangerous and environmentally damaging situation. Need for industry to comply with safety and process requirements of OSHA 1910, thermography may prove to be a highly cost-effective tool to use. Each of these situations is a real case where IR could be used to verify information on the situation in the tank or silo. Level location and checking the other level indicating instruments remains an important need in the industry.
Thermal imaging as a method for determining levels
Most of the time, materials in a tank or silo, or solids, liquids or gases, behave differently when exposed to a thermal transition. The materials often have different thermal capacity characteristics. Gases usually change temperature much easier than liquids. Water, for example, has a thermal capacity is 3500 times greater than air. One Btu of Energy added to a cubic meter of water, the temperature 0016 ° C increase, while the same energy added to the same volume of air results in an increase to 55 ° F!
While the thermal capacity of solids compared with liquids, the different way ensures that heat is transferred distinguish them from an infrared camera. Solids such as sludge, are primarily affected by the conductive heat transfer. Liquids (non-solids), on the other hand, are strongly influenced by convective heat transfer. The result is that the solid layer in close contact with the vessel wall, despite the often high thermal capacity, heat and cold faster than the liquid part, because they do not mix in the same way the liquid does. One question is whether the tank / silo is half full or half empty. This provision requires further research investigator of the materials, container, housing and environmental conditions.
Necessary environmental conditions
Key to determine levels in the tank or silo to observe during a thermal transition. When viewed with an infrared camera, while a thermal steady state with the environment, there will be no differences. In fact, tanks and silos are full or empty often appear identical with no indication of a level. Interestingly, it is hard to find tanks or silos not in transition, but it may not always produce detectable image. Outdoors, the day / night cycle often provides sufficient driving force to create detectable differences. Even indoors, variations in air temperature thermal transitions are often sufficient to clear. Environmental conditions can directly affect the ability to detect the level by thermal imaging. Wind, precipitation, ambient temperature and solar load, all, separately or together, create or negate differences on the surface. Other factors considered are the temperatures of the products are stored in or moved through the tanks and silos and the rates they are moving. Many tanks are insulated, although rarely to the extent that they will always and completely obliterated the thermal patterns caused by level. If insulation is covered with unpainted metal cladding, care must be taken to enhance the appearance, as discussed later.
Patterns of thermal materials in various forms
The most obvious pattern is a result of a liquid / gas interface. In a situation when the product is not heated, the gas usually responds quickly to transient situation, while the liquid reacts slower. During the day, the gas can be hotter than the liquid, the nights are cooler. Liquid / sludge relationships may be more difficult to distinguish. A larger transient may be required to create a detectable image. Thin layers of silt can be differentiated from the tank bottom. Sludge buildup in the middle of the tank (not in contact with the wall) is simply not detectable, although product structure on the sidewalls is often very clear. Foams are often difficult to distinguish from liquids, but may appear on gases. Care must be taken when the pressure of the tank through a rapid thermal transition to the thermal differences to reveal. Locating levels associated with floating material such as washing, will typically require more perseverance, skills and a greater degree of transitional measures heat transfer, but the results can be spectacular.
Whether or not liquid / liquid interfaces, such as a mixture of oil and water can be seen is completely dependent on their different thermal capacities and to a lesser extent, their viscosity. Simple experiments suggest it is fairly easy to find the boundary between oil and water, but further work needs to be done in the field to this technique validation. Some solids such as coal, plastic pellets, powdered limestone and wood chips, behave as liquids and are designated as "solid fluidized substances. "While heat transfer in such materials is still primarily conductive mass transfer of heat by the movement of the material can be significant. For example hot ashes or lime blown into a silo carries the process heat to the silo. Fluidized solids tend to behave the same way for liquids in the way they react to gravity, except for the fact that they can "bridge" areas where fluid usually not. In fact, locating bridging fluidized materials is a valuable use for thermography.
POINTS to be considered
Some tanks are covered with coating, often unpainted aluminum or stainless steel. Seeking the kind of fine temperature levels required to disclose to surfaces such as ones with low- high-emissivity and reflectivity is nearly impossible. The radiant difference is just not detectable. The problem is usually easily remedied by applying a high emissivity vertical target. A painted line or a piece of tape on the tank, for example, can work very well. For outdoor work, use bright colors and / or the dark side of the equipment sun-filled to avoid. Occasionally tanks are heated or cooled with a jacket. These often cause thermal imaging cameras are not effective for the level determination. In some cases, it possible for the structural stand offs seen between the tank wall and the jacket.
Insulated tanks that may also prove challenging. Fortunately, Insulation levels are generally not large enough to see them off levels, but rather the insulation changes the dynamics of the thermal point at which a detectable level may not be obvious as often. Simple techniques, explained below can help thermal differences, so they can be detected to improve. In some cases it is possible to small "plugs" the insulation at different levels, that would clearly show the tank temperature cut. While solar energy can load a pattern to increase, it can often lead to subtle thermal patterns in a tank or silo to be obliterated. It may be possible for the container on the shady side but sometimes it may be necessary to return when the sun effect is reduced. Spheroid tanks offer a different kind of challenge that, if a point from their reflection on their curved surface varies greatly. It is not uncommon that the tops of such Tanks appear cooler, while the bottom appear warmer, too often both patterns are related to more reflection than emission. Tanks located within the buildings are not subject daily cycles of heating. Some thermal cycling usually occurs, but it can not be bright enough to observable differences. Again, simple techniques, as shown explain it very effectively used to increase the surface temperature differences.
Simple techniques to improve thermal patterns
Thermal patterns can often be improved by using simple techniques to transient heat transfer increase. It may be possible to add heating or cooling directly in or on the surface of the tank / silo. The gas main in the tank reacts faster than the liquid. As discussed above, solids may react in a more complex manner. An industrial hot air gun can be used to heat the surface of small to medium tanks. Heating even a small area can reveal a dramatic level. Cooling can easily be determined by the surface wet with water. The evaporation, cooling drives transient heat flow and shows the levels or improves. Although these techniques can not feasible for large tanks, this is not the case. Cooling in particular can easily be supplied with a stream of cold water sprayed on the tank surface. Add the element time for cooling effect and in many cases, the image is clearly visible.
Conclusion
Many industries have an absolute necessity for the level in tanks and silos, and to determine the existing level meter instrumentation validation. Infrared thermography provides in a simple, cost-effective means of doing both. Conditions often possible for the levels seen at almost any time of day or night and all year. Although the levels are not always immediately clear, persistence, and easy expansion careful infrared imaging techniques can often produce remarkable results.
Acknowledgments
The authors would like to thank the following people for their help in the work that went into this document: Jeff Backer, Shane Brooker, Matt Clarke, Lee Colgrove, Jeff Cordova, Keith dodder, Patrick Lawrence, Greg McIntosh, Rob Spring, and Mark Soult.
Visit us at www.electrophysics.com / Snellsiloab
For more detailed Whitepapers visit our online Knowledge Center.
www.electrophysics.com / Thermal-imaging
Electro Physics – IR Thermography cameras for Professionals
373 Route 46, Fairfield, NJ 07004 Phone: 973-882-0211 Fax: 973-882-0997
About the Author
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