| Background | | | | waxes, will typically require more persistence, skill and |
| Instrumentation for locating levels in tanks and silos is | | | | a greater rate of transitional heat transfer, but the |
| often unreliable. The need for precise information about | | | | results can be startling. |
| levels remains necessary, or even critical, in many | | | | Whether or not liquid/liquid interfaces, such as a mix of |
| instances. For example, in one situation a | | | | oil and water, can be seen depends entirely on their |
| thermographer was employed to verify a liquid level in | | | | differing thermal capacities and, to a lesser extent, their |
| a large storage tank along the Gulf Coast prior to the | | | | viscosity. Simple experiments suggest it is fairly easy |
| arrival of a tanker ship. In continuous processes the | | | | to locate the interface of oil and water, but further |
| operator must know how much capacity is available in | | | | work needs to be done in the field to validate this |
| each tank. Without that knowledge production may be | | | | technique. Some solids, such as coal ash, plastic pellets, |
| impeded or, if an overflow occurs, a potentially | | | | powdered lime and wood chips, behave as fluids and |
| dangerous situation created. Sometimes traditional level | | | | are designated as "fluidized solids." While |
| indicating instruments simply cannot determine levels. | | | | heat transfer in such materials is still primarily |
| Foams and waxes, for instance, are difficult to detect | | | | conductive, mass transfer of heat by the material's |
| and measure accurately. | | | | movement can be significant. For instance, hot ash or |
| | | | lime blown into a silo carries its process heat to the silo. |
| A paper mill experienced a situation in which a tank | | | | Fluidized solids tend to behave similarly to liquids in the |
| was believed to be sized improperly, when in fact it | | | | way they respond to gravity, except for the fact that |
| was simply full of foam rather than liquid. De-foaming | | | | they can "bridge" across areas where |
| the tank proved more cost effective than | | | | liquids typically would not. In fact, locating bridging of |
| unnecessarily replacing it with a larger one! A | | | | fluidized materials is a valuable use for thermography. |
| petrochemical plant hired a contractor to clean out a | | | | Issues to be Considered |
| large tank. When the manway door was opened, | | | | Some tanks are covered in cladding, often unpainted |
| sludge, which had settled to a depth high above the | | | | aluminum or stainless steel. Detecting the kind of fine |
| door, oozed forth creating a dangerous and | | | | temperature differences necessary to reveal levels on |
| environmentally damaging situation. For industries | | | | surfaces such as these-ones having low emissivity |
| needing to comply with the safety and process | | | | and high reflectivity-is nearly impossible. The radiant |
| requirements of OSHA 1910, thermography may prove | | | | difference is simply not detectable. The problem, |
| to be a particularly cost-effective tool to use. Each of | | | | however, is most often easily rectified by applying a |
| these situations represents a real instance where | | | | high emissivity target vertically. A painted stripe or a |
| infrared could have been used to provide or verify | | | | piece of tape on the tank, for instance, can work very |
| information about the condition inside the tank or silo. | | | | well. For outdoors work, use light colors and/or the |
| Level location as well as verification of other level | | | | shady side of the equipment to avoid solar loading. |
| indicating instruments continues to be an important | | | | Occasionally tanks are heated or cooled with a jacket. |
| need in industry. | | | | These often cause thermal imaging cameras to be |
| Thermal Imaging as a Method for Determining Levels | | | | ineffective for level determination . In some instances it |
| Most of the time, the materials in a tank or silo, | | | | may be possible to see the structural "stand |
| whether solids, liquids, or gases, behave differently | | | | offs" between the tank wall and the jacket. |
| when subjected to a thermal transition. The materials | | | | Tanks that are insulated can also prove challenging. |
| often have differing thermal capacitance | | | | Thankfully, insulation levels are typically not great |
| characteristics. Gases typically change temperature | | | | enough that they preclude seeing levels; rather the |
| much more easily than liquids. Water, for instance, has | | | | insulation changes the thermal dynamics to the point |
| a thermal capacity that is 3500 times greater than air. | | | | where a detectable level may not be obvious as often. |
| One Btu of energy added to a cubic foot of water will | | | | Simple techniques, explained below, can help enhance |
| raise its temperature 0.016°F while the same | | | | thermal differences so that they can be detected. In |
| energy added to the same volume of air results in a | | | | some instances it may be possible to cut small |
| 55°F increase! | | | | "plugs" out of the insulation at various levels |
| While the thermal capacity of solids may be similar to | | | | that would more clearly reveal the tank temperatures. |
| liquids, the different way in which heat is transferred | | | | Although solar loading can enhance a pattern, more |
| permits them to be distinguished with an infrared | | | | often it can cause subtle thermal patterns in a tank or |
| camera. Solids, such as sludge, are influenced primarily | | | | silo to be obliterated. It may be possible to view the |
| by conductive heat transfer. Fluids (non-solids), on the | | | | container on the shady side, but sometimes it may be |
| other hand, are strongly influenced by convective heat | | | | necessary to return when the sun's affect is lessened. |
| transfer. The result is that the layer of solids in close | | | | Spheroid tanks offer another type of challenge in that, |
| contact with the tank wall, despite its often high | | | | when viewed from one point, their reflectance varies |
| thermal capacitance, heat and cool more rapidly than | | | | widely over their curved surface. It is not unusual to |
| the liquid portion because they do not mix in the same | | | | find the tops of such tanks appearing cooler while the |
| way the liquid does. One issue is whether the tank/silo | | | | bottom appearing warmer; all too often both patterns |
| is half-full or half-empty. This determination requires | | | | are related more to reflectance than emission. Tanks |
| further research by the investigator of the materials, | | | | located inside of buildings are not subjected to diurnal |
| container housing and environmental circumstances. | | | | heating cycles. Some thermal cycling usually does take |
| Necessary Environmental Conditions | | | | place, but it may not be enough to make the radiant |
| Key to determining levels is to observe the tank or silo | | | | differences detectable. Again, simple techniques, |
| during a thermal transition. If viewed with an infrared | | | | explained below, can be used very effectively to |
| camera while at a thermal steady state with the | | | | enhance surface temperature differences. |
| surroundings, no differences will be seen. In fact, tanks | | | | Simple Techniques to Enhance Thermal Patterns |
| and silos that are full or empty often appear identical | | | | Often thermal patterns can be enhanced by using |
| with no indication of a level. Interestingly, it is difficult to | | | | simple techniques to increase transient heat transfer. It |
| find tanks or silos that are not in transition, although it | | | | may be possible to add heating or cooling directly into |
| may not always yield a detectable image. Outdoors, | | | | or to the surface of the tank/silo. The gas head in the |
| the day/night cycle often provides sufficient driving | | | | tank responds more quickly than the liquid. As |
| force to create detectable differences. Even indoors, | | | | discussed above, solids may respond in a more |
| variations in air temperature are often sufficient to | | | | complex manner. An industrial hot air gun can be used |
| make thermal transitions apparent. Environmental | | | | to heat the surface of small to medium sized tanks. |
| conditions can have a direct influence on the ability to | | | | Heating even a narrow area may dramatically reveal a |
| detect levels by thermal imaging. Wind, precipitation, | | | | level. Cooling can be provided simply by wetting the |
| ambient air temperature, and solar loading can all, | | | | surface with water. As evaporation takes place, |
| separately or together, create or negate differences | | | | cooling drives transient heat flow and reveals or |
| on the surface. Other factors to be considered include | | | | enhances the levels. While these techniques may not |
| the temperatures of the products being stored in or | | | | seem feasible for large tanks, such is not the case. |
| moved through the tanks and silos, as well as the | | | | Cooling in particular can easily be supplied with a |
| rates at which they are moving. Many tanks are | | | | stream of cold water hosed onto the tank surface. |
| insulated, although rarely to the extent that they will | | | | Add the element of time for the cooling to take effect |
| always and entirely obliterate the thermal patterns | | | | and, in many cases, the image becomes readily |
| caused by levels. When insulation is covered with | | | | apparent. |
| unpainted metal cladding, care must be taken to | | | | Conclusion |
| increase emissivity, as discussed later. | | | | Many industries have a critical need to determine levels |
| Thermal Patterns of Materials in Different Forms | | | | in tanks and silos and to validate the already existing |
| The most obvious pattern is a result of a liquid/gas | | | | level-indication instrumentation. Infrared thermal imaging |
| interface. In a situation where the product is not heated, | | | | provides a simple, cost-effective means of doing both. |
| the gas typically responds quickly to the transient | | | | Conditions often allow for levels to be seen at almost |
| situation, while the liquid responds more slowly. During | | | | any time of the night or day and throughout the year. |
| the day, the gas may be warmer than the liquid;at night | | | | While levels are not always immediately obvious, |
| it is cooler. Liquid/sludge relationships may be more | | | | persistence, careful infrared imaging and simple |
| difficult to discern. A larger transient may be required | | | | enhancement techniques can often produce |
| to create a detectable image. Thin layers of sludge | | | | remarkable results. |
| may also be indistinguishable from the tank bottom. | | | | Acknowledgments |
| Sludge buildup in the center of the tank (i.e. not in | | | | The authors would like to thank the following individuals |
| contact with the wall) is simply not detectable, although | | | | for their assistance in the work that went into this |
| product buildup on the sidewalls is often quite obvious. | | | | paper: Jeff Backer, Shane Brooker, Matt Clarke, Lee |
| Foams are often not difficult to distinguish from liquids | | | | Colgrove, Jeff Cordova, Keith Dodderer, Patrick |
| but may appear similar to gases. Care should be taken | | | | Lawrence, Greg McIntosh, Rob Spring, and Mark Soult. |
| when pushing the tank through a rapid thermal | | | | Please visit us at |
| transition to reveal the thermal differences. Locating | | | | For more comprehensive White Papers visit our online |
| levels associated with floating materials, such as | | | | Knowledge Center. |