November/December 2012
Special Section: Temperature

Thermal imaging detects equipment issues

Portable thermography in automated process environments

Fast Forward

  • Portable thermography is a powerful tool for maintenance and problem diagnosis.
  • Portable thermography allows users to "see" heat patterns.
  • Portable thermography helps find problems quickly.
 
By Michael Stuart
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Automation often provides significant advantages in today's competitive environment; in some industries, it is practically a requirement to stay in business. Unfortunately, human beings have yet to create a machine or system that operates perfectly at all times. All automated systems will need maintenance at one time or another. The saying "the weakest link can break even the strongest chain" is nowhere more true than in highly-complex, highly-engineered automation systems. Every system has an Achilles heel, and it could be a part of the operation perceived to be so minor that it has no fixed monitoring sensor.

 

Measurement best practices:

  • Verify that electro-mechanical equipment targets are operating at a minimum of 40% of load. Lighter loads do not produce as much heat, making it harder to detect problems.
  • Within the safe zone, get as close to the target as you can. (While thermal imaging is non-contact, if you measure live electricity with enclosure doors removed, NFAP 70E safety standards still apply. Wear appropriate PPE, try to stay four feet away from the target, and minimize time spent in the arc-flash zone.)
  • Do not try to shoot through doors; thermal gradients within an electrical cabinet make it impossible to understand the thermal impact inside the cabinet. Infrared does not penetrate glass or plastic safety shields, so you will need to work around those.
  • If inspecting outside, take wind and air currents into consideration-as they could cool any abnormal hot spots-account for ambient temperatures, and watch out for thermal loading (heat from the sun).
  • Remember that not all problems are indicated by excessive heat. Restricted flow in a cooling system could be indicated by a cooler-than-normal reading.
  • When working with low-emissivity assets, consider sources of reflective infrared radiation.
  • When trending assets, it is important to have consistent loads for capturing accurate temperature data over time.
 
 
 

Infrared theory

Thermal imagers create pictures by measuring infrared energy or heat. The thermal imager "reads" the surface temperature of objects and then assigns different colors to the different temperatures, resulting in a picture of sorts. But not all materials emit thermal energy equally. Emissivity is the property that describes how efficiently an object radiates or emits heat. It is expressed as a value from 0 to 1.0. Shiny materials have low emissivity; painted or heavily oxidized metals, and non-metals, have higher emissivity.

 

 

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Murphy's Law dictates that "anything that can go wrong will go wrong." With all of the components that go into an automated system, this can certainly seem to be the case. And once you enter into an automated process environment, the stakes around possible failure become even higher. Fortunately, there is a tool that can help prevent Murphy's Law from actually becoming a reality: the portable thermal imager.

Portable thermography can help you and your team to maintain your automated systems and proactively keep your motors, controls, conveyors, bearings, chain drives, and other electro-mechanical automation equipment in top operating condition. For automated process environments in particular, handheld thermal images can also be used to identify leaks, blockages, and settling in sealed vessels, pipes, steam systems, or heat exchangers, and to capture process temperature readings. And the price point for this technology is just a fraction of what it was even a few years ago. The newest portable thermal imagers are more rugged, more reliable, and easier to use than ever. These tools have enormous potential-maintenance of automated systems and equipment is just the tip of the iceberg.

Motors and thermal imaging

Motors have a few key inspection points: bearings, belts, couplings, electrical connections, movement of heat, overall temperature, and how the equipment functions and fails.

Bearings under equal load should display equal temperatures. A hotter bearing on the sheave side of a motor could indicate over-tightened belts. Sheaves that are hotter around the circumference could indicate slipping belts. Slipping belts can also be indicated when belts do not cool between the motor and blower sheaves. And belts with unequal thermal patterns can indicate misalignment.

Pay attention to overall temperature, especially if there are any indications of poor cooling. Understand the roles that conduction and convection play in moving heat through the equipment. Also learn the manufacturer's operating specifications for precisely how the equipment is supposed to function and what failure might look like.

Taking a baseline image of all motors gives you the chance to make comparisons over time and to pick out any abnormal hot spots that may crop up, such as under-full load.

Spotting potential problems using thermal imagery

Thermal signatures are often associated with machine health. Normal machine operation has a verifiable signature, and problems often show up as outside of this norm. Identifying these problems requires an understanding of the machine and how it is to fail. A portable thermal imager makes it simple to quickly check the motor's overall temperature at regular intervals-especially for smaller motors, which may sometimes get overlooked when it comes to maintenance.

When determining whether a motor is overheating, use the motor temperature rating on the nameplate as a guide. Exterior motor temperatures are typically about 36°F cooler than interior temperatures.

The ideal routine or preventive maintenance program starts with newly commissioned and freshly lubricated motors and takes snapshots of all key inspection points with the motors running. These images are then used as baselines.

Maintenance with new motors

With new motors, watch the initial start-up through a thermal imager. Any wiring, alignment, or lubrication problems will show up thermally, before any permanent damage occurs. As the motor ages, components become worn and heat-producing friction develops. This will cause the housing to heat up. Taking thermal images at regular intervals will allow for comparison to the baseline to analyze the motor's condition. Generate a maintenance order when any thermal images indicate overheating.

When looking at small bearings, compare thermal patterns of one bearing to similar bearings in the same operation. The inspection can even be done while the equipment is operating. Small bearing failures can result in fire, mechanical stress, belt wear, and increased electrical loads. The inspection of small bearings is an area where infrared shines as a maintenance technology.

When looking at belts and sheaves, be aware that guards may restrict your view. And be sure to re-inspect belts and sheaves after corrective actions are taken. When inspecting pumps and fans, focus on the coupling-look for irregularities. A healthy coupling should have a consistent thermal signature. Component wear will cause abnormal heating. Alignment issues will show up as excess heat before causing bearing problems or any subsequent irreversible damage.

Inspecting tanks

In automated process environments, thermal imaging can also be used to inspect tanks. Scan the outside surface for variances in temperature at different points. Inspect gaskets, seals, and valves at their openings. Monitor tank levels; locate fluid, solid, "floater" levels, and sludge. While large process tanks typically have built-in visual or electronic indicators for tracking product levels, these are not always reliable. Thermal inspections can reveal the interface between the liquid and the gas (usually air) in a vessel, indicating how full it is and whether the contents have settled or separated inappropriately. Knowing what the correct levels are prevents overfilling and ensures reliable inventory figures for raw materials and/or finished products.

When a tank or silo changes temperature, it is often possible to see thermal patterns associated with the levels inside. For instance, it is invaluable to know the sludge level when operating a continuous process or preparing to clean out a tank.

Thermography can also reveal floating materials as well as layers of liquids, gases, and solids-such as the layer of paraffin that sometimes forms between the oil and water layers in separators, which can hinder their operation.

Identifying leaks and other issues

Leaks typically develop in or around gaskets or seals. Less frequently, corrosion will cause a weakness to develop and rupture a vessel. To find leaky gaskets or seals, scan a thermal imager along the seal looking for abnormalities. A large change in temperature along the seal or gasket indicates the loss of either heat or cold, which is the signature of a failure.

Thermal imagers can also monitor process control valves for leakage, stiction (sticking), or excess friction. The excitation coil of a valve may overheat if it works too hard, indicating a problem such as current leakage or a valve size mismatch. Technicians can follow up by calibrating the valve or the valve's positioner.

A damaged refractory or liner will, under certain conditions, show up as hot or cool spots. Most leaks occur when a seal or gasket fails, or when corrosion leads to a leak in a vessel's wall. No matter its origin, a leak will usually manifest itself as a temperature anomaly. When inspecting refractory insulation, hot areas are associated with refractory thinning or failure. Cool areas are associated with internal product build-up.

Steam traps, lines, radiators, and convectors

Thermal imagers can quickly spot trap and line temperatures into and out of steam traps. Check each transmission line and follow pipe temperatures to the source of any problems. If the temperature is low in steam pipe, low in trap, and low in condensate return, then the trap may be stuck closed. If the temperature is too high in steam pipe, high in trap, and high in condensate return, then the trap may be stuck open. If temperature is high in steam pipe, high in trap, and slightly lower in condensate return, the trap is likely operating properly. Correctly operating steam traps should show a temperature differential from one side to the other.

A trap that is failed open can go undetected for long stretches of time-weeks or even months-and be very costly once it is detected. A thermal imager will display these traps as warm on both sides. If you find a trap that looks like that, make sure it has not just cycled. If it has, and remains hot on both sides after more than a few minutes, it likely is not working properly.

Substances within tanks

Thermal imagers can detect the level of substance inside a tank, provided that there is a temperature difference between the substance inside, the air inside, and the air outside, and provided that the tank is not composed of a shiny material, such as stainless steel.
Gases have higher heat capacity than liquids, meaning that liquid temperatures change more slowly than gas in the headspace. Because tanks are most often located outside, their contents change temperature throughout the day because of solar loading. The temperature difference between the product and the headspace can typically be observed through the tank walls. At times, though, the air and substance within a tank will be the same temperature, effectively rendering the level invisible. The level will once again be visible when the air gains or loses thermal energy.

A thermal image of a tank that is either completely empty or completely full, or that has a shiny reflective surface, will appear uniform. No product level will be visible. Otherwise, the product level will be seen as a thermal separation between the headspace and the product.

Heat exchangers and steam radiators

Thermal inspection of heat exchangers can quickly and safely identify areas of corrosion, mineral deposits, and sludge build-up, as well as a lack of heat transfer caused by external damage such as hail, abuse, or lack of maintenance. But it is important to remember that mechanical heat transfer is one area where sharp lines of temperature difference rarely exist. Heat exchangers do not offer up the clear "hot spots" that you can see in other overheating or malfunctioning equipment. Instead, heat exchangers are constructed to facilitate a temperature exchange. Higher resolution thermal imagers and on-camera adjustments can help capture lower thermal differences (called Delta T), which are  often exhibited by blocked passages or clogged strainers with plate-type exchangers.

Shell-type heat exchangers, on the other hand, often show definite areas of blockage caused by solid build-up of materials. Steam radiators are another type of heat exchanging device. They are commonly found in schools, commercial buildings, and homes. Infrared inspection of these will reveal blocked passages, cracks, and internal damage caused by corrosion. In all cases, infrared inspection allows for specific trouble areas to be diagnosed.

Conclusion

By its very nature, troubleshooting is scenario-specific. Personal experience goes a long way in determining just how useful thermal imagers can be. The more time you spend using a thermal imager, the better you will become at identifying anomalies. As your thermal knowledge and skill build up, they can be combined with existing knowledge of line and equipment functionality, adding up to formidable troubleshooting power and, thus, improved long-term maintenance.

ABOUT THE AUTHOR

Michael Stuart (michael.stuart@fluke.com) is a practicing T/IRT Level III thermographer (certified in compliance with ASNT standards) and has significant experience in the use of thermography for electrical, mechanical, and building inspection applications and analysis. He is the senior product manager for thermal imaging products with Fluke Corporation. Michael conducts training for customers and Fluke personnel in the fundamentals of thermal imaging for troubleshooting, preventive maintenance, predictive maintenance, building inspection, weatherization, and energy auditing. He has actively taught various subjects related to electrical test & measurement and predictive maintenance at NTI and has presented hands-on courses for various regional training events sponsored by the NJATC, IUOE, and others, and has co-authored a book, Introduction to Thermography Principles, with well-respected fellow thermographer John Snell of the Snell Group.

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