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July 13th, 2008 admin Leave a comment Go to comments

Placement levels in tanks and silos IR Thermal Imaging

Background

Instrumentation localising levels in tanks and silos is often unreliable. The need for precise information on the level remains necessary, even critical, in many cases. For example, In one situation, a thermometer was used to assess the level of liquid in a large storage tank along the Gulf Coast to check prior to the arrival of a tanker. In the continuous processes operator must know how much capacity is available in each tank. Without that knowledge production can be hindered or, if an overflow occurs, a potentially dangerous situation arising. Sometimes traditional instruments level indicating simply can not determine levels. Foaming and washing, for example, are difficult to detect and precisely measure.

A paper mill experienced a situation where a tank was believed wrongly sized, when in fact it was just filled with foam instead of liquid. The foaming tank proved unnecessary cost effective than replacing a larger one! A petrochemical plant hired a contractor to clean a large tank. When the door opened manway, sludge, who were located at a depth high above the door, beamed back to create a dangerous and environmentally unfriendly situation. Necessary for industries to meet 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 example where IR had can be used to verify the information on the situation in the tank or silo. Level location as well as verification of other level indicating instruments remains a major need in the industry.

Thermal Imaging as a method for determining levels

Most of the time, the materials in a tank or silo, or solids, liquids and gases behave differently when exposed to a thermal transition. The materials often have different thermal capacity characteristics. Gases usually change temperature more easily than liquids. Water, for example, has a thermal power is 3500 times greater than air. A Btu of energy added to a cubic meter of water will increase the temperature 0.016 ° F, while the same energy added to the same volume of air results in an increase in 55 ° F!

While the thermal capacity of solids is similar to liquids, the different ways in which heat is transferred allows them to distinguish with an infrared camera. Solids such as sludge, are most influenced by conductive heat transfer. Liquids (non-solids), on the other hand, are strongly influenced by convective heat transfer. The result is that the layer of solids in close contact with the vessel wall, despite the often high thermal capacity, heat and cooling faster than the liquid fraction because they do not mix in the same way the liquid. One question is whether the tank / silo is half full or half empty. This provision requires further examination by the examiner of the materials, container, housing and environmental conditions.

Necessary environmental conditions

Test 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, will no differences. In fact, tanks and silos are full or empty often appear identical with no indication of a level. Interestingly, it is difficult to find tanks or silos not in transition, although it may not always yield a detectable image. Outdoors, day / night cycle often provides sufficient driving force behind the observable differences. Even indoors, variations in the temperature of the air often enough to show thermal transitions. Environmental conditions can have a direct impact on the ability to level to detect by thermal imaging. Wind, precipitation, air temperature, and solar energy, all loaded, separately or together, or undo the 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 speed with which they move. Many tanks isolated, although rarely to the extent that they are always full and the thermal patterns caused by levels are obliterated. When insulation is covered with unpainted metal cladding, care must be taken to reduce emissions to increase, 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 where the product is not heated, the gas reacts most quickly to the situation of transient nature, while the liquid reacts slower. During the day, the gas could be warmer than the liquid, and at night it is cooler. Liquid / sludge relationships can be difficult to recognize. A larger transient may be required to create a detectable image. Thin layers of the sludge can not be distinguished from the tank bottom. Sludge buildup in the middle the tank (not in contact with the wall) is just not visible, although product buildup on the walls is often very clear. Foams are often difficult to distinguish from fluids, but may appear similar to the gases. Caution is advised when pushing the tank through a rapid thermal transition to the thermal differences visible make. Locating levels associated with floating material, such as washing, will typically require more stamina, skill and a higher percentage of transitional heat transfer, but the results can be startling.

Whether or not liquid / liquid interfaces, such as a mixture of oil and water can be seen is entirely dependent their different thermal capacity and, to a lesser extent, their viscosity. Simple experiments suggest that the interface is pretty easy to find oil and water, but otherwise work must be done in the field to validate this technique. Some solids such as coal ash, plastic pellets, powdered limestone and wood chips, behave like liquids and are called "fluidized solids." While the heat transfer in such materials is still primarily conductive, mass transfer heat by the movement of the material can be significant. For example, hot ashes or lime in a silo blown bears his trial heat in the silo. Fluidized solids tend to behave similarly to fluids in the way they respond to gravity, except for the fact that they can "bridge" across areas where fluids usually not. In fact, the detection of bridging fluidized bed is a valuable materials for thermography.

I ssues be considered

Some tanks are covered with upholstery, often unpainted aluminum or stainless steel. Identify the type of fine temperature differences that needed to be reflected in the level surfaces such as ones with a low-emission and high-reflectivity is almost impossible. The glorious difference is not detectable. The problem, however, is usually easily remedied by applying a high emissivity target vertically. A painted stripe or a piece of tape on the tank, for example, may well work. Outdoor work, the use of bright colors and / or the shady side of solar equipment to avoid filling. Occasionally tanks are heated or cooled with a jacket. These cause thermal imaging cameras are often ineffective for the mirror to determine. In some cases it may be possible for the structural stand offs between the tank wall and see coat.

Insulated tanks can be no sinecure. Fortunately, thermal insulation and are generally not large enough that they see the road levels, Rather, the insulation changes the dynamics of the thermal point where a detectable level probably is not often obvious. Simple techniques, described below, can help thermal differences, so they can be tracked better. In some cases it may be possible for small "plugs cut from the insulation at various levels, that would clarify the tank temperatures. Although solar energy can load a pattern of improvement, often it can cause subtle thermal patterns in a tank or silo to be obliterated. It is possible to view the container on the shady side, but sometimes you may need to return when the sun affects reduced. Spheroid tanks offers a different kind of challenge in that, seen from a point, their reflectance varies significantly over their curved surface. It's not unusual that the tops of these tanks appear cooler, while the bottom appear warmer, too often both patterns are more akin to reflection or emission. Tanks located inside buildings are not subject to daily heating cycles. Some thermal cycles usually or place, but it can not be bright enough for the differences observed. Again, simple techniques, described below, can be very effectively used to surface increasing differences.

Simple techniques to improve thermal patterns

Often thermal patterns can be improved using simple techniques to transient heat transfer increases. It is possible to add heating or cooling directly in or on the surface of the tank / silo. The gas main in the tank is faster than the liquid. As discussed above, can react in solids a more complex way. An industrial hot air gun can be used to heat the surface of small to medium tanks. Heating even a small area may indicate a dramatic level. Cooling can be easily supplied by the wet surface with water. Such as evaporation, cooling drives transient heat flow and show the different levels or improves. Although these techniques may seem unfeasible for large tanks, this is not the case. Cool in particular can easily be supplied with a stream of cold water sprayed on the tank surface. Add the element of time for cooling force and, in many cases, the image clearly.

Conclusion

Many industries have a critical need for levels in tanks and silos, and to determine the existing level indication instrumentation validation. Infrared thermal imaging provides a simple, cost-effective means of doing both. Conditions for the levels may often be seen at almost any time of day or night and all year. Although the levels are not always immediately obvious, persistence, careful infrared imaging and simple improvement techniques can often produce remarkable results.

Acknowledgments

The authors would like the following people for their help with the work that went into this document: Jeff Backer, Shane Brooker, Matt Clarke, Lee Colgrove, Jeff Cordova, dodder thank Keith, Patrick Lawrence, Greg McIntosh, Rob spring, and Mark Soult.

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