Use Thermography to Expose What’s Hidden

Don’t be kept in the dark. Use thermal imaging to reveal the true colors of your process upset.

The need for temperature measurements to aid troubleshooting efforts is considered during the design and construction of most new chemical process industries (CPI) facilities, and adequate temperature measurement instrumentation is typically installed. However, many older production facilities only have instrumentation to provide temperature data required for daily operations, which can make troubleshooting a long and tedious process.

Temperature monitoring in CPI plants usually relies on direct-contact temperature instruments, such as thermocouples and resistance temperature detectors (RTDs) (1). These devices are robust and accurate, but installing them in a process that is already operational can require downtime and considerable capital expense.

Indirect, or noncontact, temperature instruments, such as infrared (IR) thermal imaging cameras, can be used in applications that are too severe, impractical, or unsafe for direct-contact temperature instruments (2). Thermal imaging cameras can record in real time and provide temperature profiles of large areas, rather than capturing single points at specific moments in time. These devices cannot penetrate an object, and only provide surface temperatures. Objects can become indistinguishable at narrow temperature ranges, which can make interpreting data difficult.

This article explains how to use thermal imaging to gather useful data for troubleshooting investigations.

Thermography basics

Thermography is a technique for detecting and measuring variations in heat emitted by objects (e.g., electrical and mechanical equipment, structures, the human body, etc.). Thermal imaging devices transform heat data into visual signals that are displayed and documented as thermal images. Thermography is widely used for medical screenings, building diagnostics, electrical and mechanical inspections, and other applications.

Thermal imaging cameras detect radiation in the far infrared range of the electromagnetic spectrum. Infrared radiation is emitted by all objects with a temperature above absolute zero, according to the black body radiation law, and the amount of radiation emitted by an object increases with temperature (3).

The camera converts the infrared energy emitted by the measured object into an electrical signal in the imaging sensor and displays a range of colors that correspond to temperature. The data are presented as either a still image or a video. Users can identify areas of concern, such as regions of increasing or decreasing temperature, by evaluating the color variations of the image. Portable thermal imaging cameras are extremely easy to use; if you have operated a video camera before, you should have no trouble operating a thermal imaging camera.

The radiation received by infrared cameras comes from the object of interest as well as the surrounding environment. The air between the object and the camera (i.e., measurement path) reduces the combined radiation. Other factors can influence the radiation received by the camera, such as sunlight scattering in the atmosphere or stray radiation from an intense radiation source outside the field of view, but these factors are generally hard to account for, and in most cases, are small enough to disregard (3).

Emissivity must also be considered during thermal imaging. Emissivity is a measure of the amount of radiation emitted from an object compared to the amount of radiation emitted from a perfect blackbody at the same temperature as the observed object (4). Emissivity varies with the surface condition of an object and also with temperature and wavelength (5).

Preparing the camera

Before you can begin troubleshooting, you must enter several parameters into the camera, including:

• the object’s emissivity
• the relative humidity
• the temperature of the atmosphere
• the distance to the object
• the reflected apparent temperature.

Setting these parameters can be challenging, particularly estimating the emissivity and the reflected apparent temperature. There is no easy way to determine accurate values for these parameters, but their influence is generally insignificant if there are no large and intense radiation sources in the surroundings (4).

Several practical methods for determining the emissivity are available, but emissivity tables are the most common method (4). The manual for the thermal camera usually includes emissivity tables, and constants are also available in various books and literature. Keep in mind that the constants were recorded under specific measurement conditions, and if those conditions cannot be replicated, the constants might not be accurate. However, these constants can be used as a reference point to set the emissivity on the camera. The camera’s instruction manual or the vendor may also provide additional guidance for setting the emissivity.

There are several methods for determining the reflected apparent temperature. The camera’s manual should provide a procedure for obtaining this value (4–6).

Radiation that does not come from the object of interest that contributes to the radiation received by the camera is referred to as background noise. To gather the most accurate data, it is important to minimize background noise by shortening the measurement path and/or positioning the camera so that it is not directly in front of an object with a higher temperature than the object of interest. When determining a measurement path length, position the camera at a safe distance from equipment, especially rotating equipment. The thermal camera’s manual will also provide guidance on reducing background noise (4).

Intrinsically safe thermal cameras are available that can be used in areas classified as an explosion hazard. If the model you have is not intrinsically safe, a hot-work permit and explosion limit tests will be required for the duration of the investigation.

Gathering data

Different levels of accuracy are required for various types of thermography investigations. To determine a distinct temperature difference, such as in the case of a warm liquid being added to a tank containing a cool liquid, approximate temperatures are sufficient, because determining the temperature gradient is the goal. In these...