Infrared thermal imaging is a powerful tool for locating and verifying levels in tanks and silos. Other level indication instruments are often not sufficiently reliable in many situations, or positive verification of the instrumentation readings is required. When properly used, thermal imaging 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 paper discusses the parameters and limitations that must be addressed, shows techniques that can be employed, and illustrates the discussions with numerous thermal images.

Levels can be seen in two tanks (left and center), along with differing solar influences, while the right and appears to be full.

Background

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, in one situation a thermographer was employed to verify a liquid level in a large storage tank along the Gulf Coast prior to the arrival of a tanker ship. 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 created.

Sometimes traditional level indicating instruments simply cannot determine levels. Foams and waxes, for instance, are difficult to detect and measure accurately. A paper mill experienced a situation in which a tank was believed to be sized improperly, when in fact it was simply full of foam rather than liquid. De-foaming the tank proved more cost effective than unnecessarily replacing it with a larger one! A petrochemical plant hired a contractor to clean out a large tank. When the manway door was opened, sludge, which had settled to a depth high above the door, oozed forth 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.

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Each of these situations represents a real instance where infrared could have been used to provide or verify information about the condition inside the tank or silo. Level location as well as verification of other level indicating instruments continues to be an important need in industry.