A thermocouple was installed in a Stainless steel seamless pipe line for monitoring the temperature of the fluid. Because the piping was all stainless steel, the material surface was too reflective to directly observe the fluid level with the thermal imager. So, to improve emissivity enough to allow camera use, the engineer applied some black electrical tape around the area of the pipe where the thermocouple was installed. The thermal imager revealed that the pipe was less than one-third filled with fluid. The thermocouple was barely making contact with the fluid, resulting in erroneous temperature measurements. A vapor lock had produced the unwanted headspace.
A thermal imager may improve your monitoring and troubleshooting of equipment and products. The infrared (IR) camera can supplement or supplant traditional techniques, and provide insights about material storage, equipment heat loss, product moisture content and more.
For instance, plants normally use level indicators to monitor how much material is inside a tank. Yet, many sites increasingly are turning to IR cameras to do the same thing. They want to avoid false indications from level gauges — and the resulting risks of either running out of product or, worse, overfilling a tank that was supposed to be empty. As former President Reagan was noted for saying: “Trust, but verify.”
Typical thermal images show the contents of the container and give quantifiable verification of the material inside. Users, by applying their knowledge of materials and physics to the thermal differences they see with an imager, can deduce the level of fluid in a tank. Figure 1 clearly shows the liquid level because the tank contains two different materials: liquid and air in the headspace.
Because of that human deductive element, the meaningfulness of the examination depends upon the person’s knowledge and the type of result desired.
Thermal cycling
Tanks located outside undergo thermal cycling. During daylight, they and their contents absorb heat from the sun and the air, as well as from whatever processing might be taking place. During the night, they give up heat to the surrounding air. This thermal cycle and the different thermal capacities of the materials involved affect how accurately a thermal imager can measure product level.
Uninsulated tanks such as the one shown are 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. Typically the thermal difference is at its maximum two times a day — once in the morning and once in the evening.
At other times, it may not be possible to clearly identify the liquid level with the thermal imager because the liquid and the air in the headspace may approach the same temperature. Reflections from the sunlight during daylight also can make it difficult to observe thermal differences.
Of course, tanks hold materials other than liquids. Dry bulk materials tend to pile up against the sides and have very uneven levels. Thermal imagers enable you to see these irregularities (Figure 2). Also, many liquids contain particulates that may settle out inside the tank, forming a layer of sediment. This layer generally has different thermal properties than the liquid and, so, often can be identified by imaging.
Understanding what the tank is constructed of is important. Many tanks have low-emissivity shiny metal surfaces or insulated walls that make it difficult or impossible to observe internal thermal differences. Being aware of such factors is crucial when evaluating what a thermal imager appears to be telling you.
Use caution and apply knowledge!
For instance, look at the reactor image (Figure 3). The color temperature bar indicates that dark blue is approximately 95?F and the red at the top of the scale is more than 200?F. Notice the dark blue, apparently cool, band where the lid sits on the vessel. What we really are seeing is a very low emissivity ring of stainless steel around the top of this otherwise painted vessel. The painted portion has a much higher emissivity. The bare stainless steel actually is at the same temperature as the painted portions it contacts <em dash>—<em dash>more than 160?F, hot enough to seriously burn skin.
It is fairly obvious to use a camera to examine furnaces and ovens for heat loss. However, thermography also can offer insights for cooling equipment. For instance, a new process freezer for removing heat from cooked meat patties exhibited numerous areas of condensation on its exterior surface, indicating voids in the insulation system in the walls. The manufacturer drilled holes in the metal sidewalls of the freezer where the condensation was located, trying without success to find the voids. The exterior freezer walls were polished Stainless steel seamless pipe which is very highly reflective. The thermographer dried the areas of condensation, placed black tape over them, and then used the camera to pinpoint the coldest spot. He was able to drill a 2-in. hole at the exact location of the insulation void.
