Why use infrared?

Published:  01 April, 2014

Normally, people in industry use level indicators to tell how much product is inside a tank. So why do so many of them also use thermal imagers (infrared cam¬eras) to do the same thing? It’s because of the horror stories - past experiences when the level gauge gave a false indication, resulting in either running out of product, or worse, overfilling a tank that was supposed to be empty. PWE reports.

Typical thermal images seem to exhibit “x-ray vision” - they show the contents of the container and give quantifiable verification of the material inside. Of course, x-rays aren’t actually involved. Instead, the images show appar¬ent thermal differences. When users apply their knowledge of materials and physics to the ther¬mal differences they see with an imager, they can deduce the level of fluid in the tanks.

Importance of timing

The meaningfulness of the examination depends on the person’s knowledge and the type of result they desire. In the case of a tank farm, a thermal imager will clearly show the liquid level inside the tanks. That’s because the tank contains two different materials: liquid and air in the headspace.

Since these tanks are located outside, the tanks and their contents undergo thermal cycling. During the daylight hours the tank and contents absorb heat from the sun and the air, as well as from whatever processing might be taking place. During the night, the tank and the contents are giving up heat to the surrounding air. This thermal cycle, and the varying thermal capacities of the materials involved, all affect how accurately a thermal imager can measure product level. These metal tanks in particular are un-insulated and highly thermally conductive. As night falls, the headspace begins to cool quickly while the liquid volume cools much more slowly. That makes the thermal gradient between the liquid and headspace readily apparent through a thermal imager.

There are typically two times of day when the thermal difference is at its maximum - once during the morning and once during the evening. At other times of day, it may not be possible to clearly identify the liquid level with the thermal imager, because the contents and the air in the headspace may approach the same temperature. Reflections from the sunlight during daylight hours can also make it difficult to observe ther¬mal differences.

Tanks hold materials other than liquids, too. Dry bulk materi¬als tend to pile up against the sides and have very uneven levels. Thermal imagers enable the user to see these irregularities. Also, many liquids contain particulate that may settle out inside the tank, forming a sediment layer. These layers can often be identified as substrates by the thermal differences they produce.

Understanding the construction

Understanding what the tank is constructed of is also important. Many tanks have shiny, low emissivity metal surfaces and/or insulated walls that make it difficult or impossible to observe internal thermal differences. These factors are crucial for evaluating what a thermal imager appears to be indicating.

In a process reactor vessel example a blue (cool) band was indicated where the lid sits on the vessel when what we are actually seeing is a very low emissivity band of stainless steel around the top of this otherwise painted vessel. The painted portion has a much higher emissivity, so in contrast it appears the bare stain¬less steel is cool when it’s actu¬ally the same temperature as the painted portions that it is in contact with: hot enough to seriously burn skin.

Diagnosing process problems

In one process example there was a thermocouple installed in a stainless steel process line for monitoring the temperature of the process fluid. The process was not function¬ing properly and the process engineer was having difficulty determining why the temperature of the process stream was lower than expected. When he sampled and measured the process fluid externally, it had the proper tem¬perature.

Since the piping was all stain¬less steel, the material surface was too reflective to directly observe the fluid level with the thermal imager. The process engineer applied some black electrical tape around the area of the pipe where the thermo¬couple was installed, improving the emissivity. Now the thermal imager revealed that the pipe was less than 1/3 filled with process fluid. The thermocouple was barely making contact with the fluid, resulting in erroneous temperature measurements of the process. A vapour lock had produced the errant headspace.

Finding the solution

As these examples illustrate, tanks and vessels come in a wide assortment of styles and configurations from familiar looking tanks, to reactor vessels, to heat exchangers, and to ovens, furnaces and freezers. Even the piping connecting vessels is a vessel. Thermal imagers can analyse a great many more things than just temperature, once the spectral and thermal dynamics of materials and processes are understood.

The application of thermal imagers is limited only by the knowledge of the person using the equipment. Thermal imag¬ers are now more economical and easy to use, but the camera is only as good as the person using it. Fluke, for example, is running a series of thermography seminars at locations throughout the UK and Ireland.

For further information please visit www.fluke.co.uk

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