Infrared basics

Published:  02 June, 2016

Predictive maintenance and RCM (reliability-centred maintenance) programmes at manufacturing facilities regularly use thermal imaging cameras for inspecting a countless number of mechanical components. FLIR tells PWE how to get started.

An infrared image that integrates accurate temperature data provides maintenance experts with crucial information about the condition of all kinds of equipment. As a non-contact measurement tool that also makes invisible heat issues visible, thermal cameras let technicians inspect production equipment safely, even during peak operation. Along with troubleshooting, thermal images can also help optimise the production process itself as well as monitor quality control.

Typical examples of mechanical equipment where thermography is commonly used includes: pumps (looking for issues such as overheated connections, fuse problems and overloaded electrical cables); process valves (open, closed, leakage); storage tanks (sludge levels); pipelines (scale build-up); motors (overheated bearings, misalignment, overheated windings); and conveyor belts (overheated bearings). Further applications include gearboxes, energy delivery systems, air handling units, vacuum systems, duct systems, engines, heat exchangers, boilers, kilns and steam traps, to list but a few.

All mechanical systems generate thermal energy (heat) during normal operation. Here, high temperatures can be produced by factors that include friction, cooling degradation, material loss and blockages. With this in mind, infrared cameras are proven to be adept at checking for the presence of excessive heat.

One of the most common causes of heat, friction, is created through mechanisms such as wear, misalignment, over- or under-lubrication or system misuse. In these cases, heat energy will travel from generation points to the surface to be imaged through a process called indirect heat transfer, and will subsequently show clearly on an infrared thermal image.

Of course, simply identifying a thermal pattern does not mean a problem has been located, which is why equipment familiarity is key to successful thermography. First, a thermal signature for the system in normal operating conditions must be obtained and understood. Only in this way will any future deviation provide evidence of a potential problem. Some bearings and motors, for example, are purposely designed to run at very high temperatures. Unless the thermographer is aware of this, he or she might think it was an issue.

These baseline infrared surveys are very important as variables can include environmental conditions (especially temperature), equipment type and equipment load. A typical baseline will show a thermal gradient pattern when operating normally, which means the best strategy is compare, compare and compare again over a predetermined time period. Regular planned checks will show anomalies, and using and infrared camera gives the advantage that any changes will be visible immediately.

Thermography is a great tool for indicating the cause of overheating in mechanical systems – quickly and easily. Typically, IR precedes complementing condition monitoring technologies such as vibration analysis, oil analysis and ultrasonics, which are deployed to try and establish the specific root cause of the anomaly identified on the thermal image.

There are many advantages to using infrared cameras. For instance, not only is the process fast and versatile, it is also non-invasive, which means there is no need to shut the system or equipment down. Furthermore, the thermal images produced by the process are far easier to understand than many other NDT techniques.

In a specific example, such as a motor, temperatures are generally compared from test period to test period. If there is a 5, 2 or even 1 degree change in temperature, it will be detected by the infrared camera and provide the user with the opportunity to ask why?

Of course, there might be plenty of innocuous reasons for the difference, such as shifts in ambient temperature, the motor’s environmental conditions, application heat transfer from other sources, frame size/mass or casing type.

On the subject of the casing, it’s important to remember that casing temperature is not the true temperature of the motor. As a rule of thumb, if the motor case temperature is at least 22°C higher than the ambient temperature, it is considered overheating, while an increase of at least 44°C above ambient is deemed a serious heat problem where the motor could be at failure point. Another tip is to always wipe away any grease present in the target area being imaged as it can often show hotter than the actual surface temperature.

Another specific example is steam traps, which are notoriously more difficult than other valve or piping systems to diagnose. This is because pressure has an effect on steam temperature in saturated systems: as pressure decreases, so the temperature of the steam also decreases. As a result, improperly operating traps may go unnoticed.

With this in mind, it’s important to inspect the exit of a steam trap. Here, the thermographer should see temperature fluctuations as the trap dumps condensate. If variations in temperature cannot be seen, it could mean that the trap has failed in the open or closed position, or it is undersized for the condensate load, or maybe the thermographer simply hasn’t waited long enough to see the full trap cycle.

Ultimately, infrared thermography is a powerful inspection method to assist in the reduction of maintenance costs on mechanical equipment and is therefore a critical factor in the operational reliability of industrial or commercial facilities. Put simply, no manufacturing or process plant should be without an infrared camera.

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