Thermography

Thermography is a generic term that refers to an imaging method based on temperature readings. The idea behind thermography is that heat energy given off by components can provide information about their operating conditions. It works by detecting the levels of IR emitted by an observed body. Thermal imaging and analysis are some ways that allow nonintrusive inspection of plant equipment and facilities.

Thermography and IR imaging have been standard additions to an organization's arsenal of analysis tools. These methods have proved useful for preventive and predictive maintenance strategies. A few examples of applications of thermography include mechanical analysis, electrical safety, insulation inspection, and leak detection.

Thermography Principles

At its core, thermography is all about measuring thermal energy in the form of infrared radiation. Instruments are used to measure heat work by taking the surface temperature of an observed component. Analysis equipment can generally measure three main types of radiated energy: emitted, reflected, and transmitted.

These types of energy correspond to the following properties of a material:

Emissivity (ε) - the ability of objects to give off radiation

Reflectance (ρ) - the ability of objects to reflect radiation

Transmittance (τ) - the ability of objects to allow radiation to pass through

Different materials will have set values for each of these three properties, where the sum of the values is assumed to be 1. This known behavior of radiation and materials comes from Kirchoff's Law of Thermal Radiation. In simplified terms, these three values come together as the formula ε + ρ + τ = 1.

While all this sounds purely theoretical and fit only for the classroom, it plays a useful and practical role in taking thermal images. By applying the formula, materials with low emissivity would suggest higher portions of reflected radiation. These materials require more care when having their measurements taken, as it would be harder to get accurate readings. Examples of materials with relatively low emissivity are metals with reflective surfaces.

Instrument Selection and Requirements

Thermal imaging cameras have been the go-to technology in thermography due to their high accuracy and level of detail. However, there are a few other types of equipment that can perform the task, depending on your requirements. Some examples include handheld thermometers, heat-sensitive stickers, paints, and the like.

The decision process of selecting the most appropriate instrument boils down to the application and type of component to be measured. Some key considerations in choosing the right measuring equipment include the following:

• Do you require the device's controls to be adjustable depending on the temperature range?
• Is the spatial resolution enough to represent the observed surface area? In other words, are there enough pixels in the display to render a useful image?
• Does the thermal sensitivity allow you to make precise observations?
• Do you require additional protective installations when performing inspections?

Specific Procedures

Expect that the environment and conditions when performing inspections will vary from one event to another. While you cannot guarantee to replicate the exact setting, your procedures should allow adjustments for a broad range of scenarios.

For example, environmental conditions can play a role in the deviation of thermal readings. Wind can cause a cooling effect on a surface that requires inspection. For qualitative observations, noting the characteristics of the wind may be sufficient for the purpose. On the other hand, quantitative inspections might need more data points to allow for correction factors to calculate the final temperature measurement.

Another example is when measuring surfaces of components from different angular positions and distances. Temperature readings can be affected by ambient reflections from nearby sources. In such cases, it would help to obtain a few more readings within the surrounding region to rule out any anomalies.

Also, a practical section to add to your specific procedures would be the steps to calibrate your measuring devices. Ensuring that your measuring devices are calibrated is the first step to obtaining accurate data. Alternatively, you can add this section as an appendix to the SOP to which users can conveniently refer.

Evaluating Results

The data gathered through inspections can only be useful if it makes sense. Your SOP should allow the user to interpret the data collected and evaluate whether further action is required or not.

On an SOP, you can include a matrix that lists inspection values and their corresponding implications. For example, you can start by gathering a list of normal thermal ranges under which your equipment or facilities operate. This will act as your baseline reference. During inspections, you can then calculate the temperature difference from your set baseline. A higher deviation from the baseline could indicate that urgency is required to resolve the issue.

In line with safety and compliance, it is best to align your evaluation with industry standards and best practices. These standards will depend on the type of equipment or facility inspected. For instance, electrical equipment have some generally accepted standards relating to temperature indicators while in operation. Similarly, mechanical components such as bearings, pumps, and compressors have various literature available on applying thermographic inspections.

Thermographic Inspection Standards and Resources

Given the significance of thermography and its limitless applications, it's not surprising how organizations and industry leaders are continually expanding and refining analysis procedures. Here are examples of thermography standards and resources available:

Electrical Equipment Safety

One of the top applications of thermographic inspections is in electrical equipment safety. For instance, the National Fire Protection Association has included guidelines for thermal inspections in the following documents:

- NFPA 79: Electrical Standard for Industrial Machinery

- NFPA 70B: Recommended Practice for Electrical Equipment Maintenance

Rotating Equipment

Rotating equipment, while aiming for reduced frictional forces, still have the potential to generate high temperatures when in operation. Inspection standards in rotating equipment aim to mitigate the risk of faulty operation. Infraspection Institute offers standards on maximum allowable temperature ranges for various rotating equipment such as bearings, seals, and rolling elements.

Standards from the American Society for Testing and Materials (ASTM)

The American Society for Testing and Materials has released several documents that offer standards for using infrared thermography. Applications include mechanical equipment, electrical equipment, and facility-related systems.

- ASTM E1934-99a(2018) Standard Guide for Examining Electrical and Mechanical Equipment with Infrared Thermography

- ASTM C1153-10(2015) Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging