Thermography is a powerful tool for locating or verifying levels in tanks and silos. All too often existing level-indication instruments are not reliable or positive verification of instrumentation readings is required.
When properly used, thermography can reveal not only the liquid/gas interface, but also sludge buildup and floating materials such as waxes and foams. Similar techniques can be used to locate levels and bridging problems in silos containing fluidized solids.
This article discusses the parameters and limitations that must be addressed and explains techniques that can be employed.
Instrumentation for locating levels in tanks and silos is often unreliable. The need for precise information about levels remains necessary, or even critical, in many instances.
For example, prior to the arrival of a tanker ship it may be necessary to verify a liquid level in a large storage tank. In continuous processes the operator must know how much capacity is available in each tank. Without that knowledge production may be impeded or, if an overflow occurs, a potentially dangerous situation could be created.
Sometimes existing instrumentation cannot determine levels (Fig. 1). Foams and waxes, for instance, are difficult to detect and measure accurately. A report from a paper mill identified a situation in which a tank was believed to be sized improperly when in fact it was full of foam rather than liquid. Defoaming the tank proved more cost effective than unnecessarily replacing it with a larger one.
A thermographer working in a petrochemical plant relayed a story about a contractor hired to clean out a large tank (Fig. 2). When the manway door was opened, sludge, which had settled to a depth high above the door, oozed out creating a dangerous and environmentally damaging situation. For industries needing to comply with the safety and process requirements of OSHA 1910, thermography may prove to be a particularly cost-effective tool to use.
How does thermography help determine levels?
Most of the time, the materials in a tank or silo behave differently when subjected to a thermal transition (Fig. 3).
The materials often have differing rates of thermal capacitance. Gases typically change temperature much more easily than liquids. Water, for instance, has a thermal capacity that is 3500 times greater than air. One Btu of energy added to a cubic foot of water will raise its temperature 0.016 F while the same energy added to the same volume of air results in a 55 F increase.
While the thermal capacity of solids may be similar to liquids, the different way heat is transferred allows them to be seen. Solids, such as sludge, are influenced primarily by conductive heat transfer vs fluids (nonsolids), which are strongly influenced by convective heat transfer. The result is that the layer of solids in close contact with the tank wall, despite its often high thermal capacitance, heats and cools more rapidly than the liquid portion because it does not mix in the same way the liquid does.
What conditions are necessary?
Key to determining levels is to observe the tank or silo during a thermal transition. If viewed while at steady state with their surroundings, no differences will be seen. In fact, tanks and silos that are full or empty often appear identical, i.e., no indication of a level.
It is difficult to find tanks or silos that are not in transition, although it may not always yield a detectable image. Outdoors, the day/night cycle often provides enough driving force to create detectable differences. Even indoors, variations in air temperature are often more significant than they might seem.
Environmental conditions can influence detectability. Wind, precipitation, ambient air temperature, and solar loading can all, separately or together, create or negate differences on the surface.
Other factors to be considered include the temperatures of the products being stored in or moved through the tanks and silos, as well as the rates at which they are moving. Many tanks are insulated, although rarely to the extent that they obliterate the thermal patterns caused by levels. When insulation is covered with unpainted metal cladding, care must be taken to increase emissivity as discussed below.
What thermal patterns will be seen?
The most obvious pattern is the liquid/gas interface (Fig. 4). In a situation where the product is not heated, the gas typically responds quickly to the transient situation while the liquid responds slowly. During the day it is warmer than the liquid and at night it is cooler.
Liquid/sludge relationships may be more difficult to discern (Fig. 5). A larger transient may be required to create a detectable image. Thin layers of sludge also may be indistinguishable from the tank bottom. Sludge buildup in the center of a tank, i.e., not in contact with the wall, is simply not detectable, although product buildup on the side walls often is quite obvious.
Foams are often not difficult to distinguish from liquids but may appear similarly to gases (Fig. 6). Care should be taken to push the tank through a rapid thermal transition to reveal the differences.
Locating levels associated with floating materials such as waxes will typically require more persistence, skill, and a greater rate of transitional heat transfer.
Whether or not liquid/liquid interfaces, such as a mix of oil and water, can be seen depends entirely on their differing thermal capacities and, to a lesser extent, their viscosity. Simple experiments suggest it is fairly easy to locate the interface of oil and water, but further work needs to be done in the field to validate this technique.
Some solids, such as coal ash, plastic pellets, powered lime, and wood chips, behave as fluids and are called fluidized solids. While heat transfer in such materials is still primarily conductive, mass transfer of heat by the material’s movement can be significant. For instance, hot ash or lime blown into a silo carries its process heat to the silo (Fig. 7). Fluidized solids tend to behave similarly to liquids in the way they respond to gravity, except for the fact that they can bridge areas where liquids typically would not. In fact, locating bridging of fluidized materials is a valuable use for thermography.
Issues to be considered
Some tanks are covered in cladding, often unpainted aluminum or stainless steel. Detecting the kind of fine temperature differences necessary to reveal levels on surfaces such as these—ones having low emissivity and high reflectivity—is nearly impossible. The radiant difference is not detectable.
The problem, however, is most often rectified by applying a high emissivity target vertically. A painted stripe or a piece of tape on the tank, for instance, can work well. For outdoor work, use light colors and/or the shady side of the equipment to avoid solar loading.
Occasionally tanks are heated or cooled with a jacket. These often prove impossible to work with. In some instances it may be possible to see the structural stand offs between the tank wall and the jacket.
Tanks which are insulated also can prove challenging. Insulation levels are typically not great enough that they preclude seeing levels; rather the insulation changes the thermal dynamics to the point where a detectable level may not be obvious as often. Simple techniques, explained below, can help enhance thermal differences so they can be detected. In some instances it may be possible to cut small plugs out of the insulation at various levels that would more clearly reveal the tank temperatures.
Although solar loading can enhance a pattern, more often it can cause subtle thermal patterns in a tank or silo to be obliterated (Fig. 8). It may be possible to view the device on the shady side, but sometimes it may be necessary to return when the sun’s affect is lessened.
Spheroid tanks offer another type of challenge in that, when viewed from one point, their reflectance varies so widely over their curved surface. It is not unusual to find the top of these tanks appearing cooler while the bottom appears warmer; all too often both patterns are related more to reflectance than emission.
Tanks located inside of buildings are not subjected to diurnal heating cycles. Some thermal cycling usually does take place, but it may not be enough to make the radiant differences detectable. Again, simple techniques, explained below, can be used effectively to enhance surface temperature differences.
Enhancing thermal patterns
Often thermal patterns can be enhanced by using simple techniques to increase transient heat transfer. It may be possible to heat or cool the tank/silo or the surface of the tank/silo. The gas head in the tank responds more quickly than the liquid. As discussed earlier, solids may respond in a more complex manner.
An industrial hot air gun can be used to heat the surface of small- to medium-sized tanks. Heating even a narrow area may dramatically reveal a level. Cooling can be provided simply by wetting the surface with water. As evaporation takes place, cooling drives transient heat flow and reveals or enhances the levels.
These techniques also are feasible for large tanks. Cooling in particular can easily be supplied with a spray of cold water hosed onto the tank surface. Add the element of time for the cooling to take effect and, in many cases, the image becomes readily apparent.
Many industries have a critical need to determine levels in tanks or silos or to validate existing level-indication instrumentation. In many instances, infrared thermography provides a cost-effective means of doing both. Conditions often allow for levels to be seen at almost any time of the night or day and throughout the year.
While levels are not always immediately obvious, persistence, careful imaging, and simple enhancement techniques can often produce remarkable results.
The authors would like to thank the following individuals for their assistance: Jeff Backer, Shane Brooker, Matt Clarke, Lee Colgrove, Jeff Cordova, Keith Dodderer, Patrick Lawrence, Greg McIntosh, Rob Spring, and Mark Soult. MT