Condition monitoring helps plant operations

Opportunities for process optimisation and cost saving by utilising condition monitoring (CM) techniques have never been greater. Conversely, the pressure for cost and staff reductions means that a single plant is unlikely to be able to employ an engineer with up-to-date knowledge of all the specialist technology, covering vibration monitoring, acoustic emission, infra-red thermography, oil analysis, etc.

There is, therefore, a need to bring in this specialist knowledge from outside, either in the form of training updates, or from a sub-contract service provider. Fortunately, many of the engineers who championed CM techniques - and helped to enable the current developments - are still around, are still enthusiastic, and have established new roles within CM training or consultancy services.

Among this breed, Austin Dunne spent around 27 years within one of the major UK chemicals producers, applying all CM techniques and developing their thermography programme strategy across various plants. He is now one of a worldwide network of consultants that operates within the Institute of Infrared Thermography, providing training on how to use an infrared camera effectively to obtain and interpret the right information from thermographic images.

Dunne has developed his role to concentrate on certification and operator training for process plant thermography. He is on a committee producing the new UK standards for thermography for the British Institute of Non-Destructive Testing, a member of the UK Thermography Association, and helped establish a similar organisation in Ireland.

Infrared thermography typically starts with tracking processes as they move around a plant, exposing thin walls, scaling, leaking valves or blocked heat exchanger plates. It is normal to see hot spots where external pipe insulation has been damaged, which can easily be replaced, but when a rubber lining to a pipe has worn away, the integrity of the pipe itself needs considering.

More importantly, where refractory linings on boilers or chimneys show evidence of thinning and some resulting hot spots, there is a larger concern. Heat exchanger thermography can show up blockages, which would lead directly to poor process efficiency, and an unexpected sludge accumulation in the bottom of a tank can often be seen as a cooler area in the images.

Typically, process liquids are mixed to create a chemical reaction: thermography can show whether the reaction is indeed taking place within the vessel, or whether the main reaction is actually within the feedline. A faulty discharge or feed valve might allow visible discharges of hot liquids back into the supply tank line, and heat traces can show up a partially open safety valve.

Process plants also rely on mechanical equipment like motors and pumps and here thermography can help monitor their condition. However, while camera manufacturers and distributors can explain the camera operation, and specifications, they often have no ’on-site’ experience, according to Dunne.

“To interpret IR images in an industrial environment, he says, “an operator then needs to have a practical understanding of the emissivity of the different plant components, a knowledge of heat transfer concepts, and understand the influence of practical environmental effects on the object of interest.

“Then he has to use this knowledge to interpret the images correctly … to relate the physics to the practical situation, and to suggest some different approaches that can be used to evaluate and overcome field problems.”

As an example, the image above right shows some typical problems that might be experienced in monitoring site pipework. The whole pipe is at a uniform high temperature, but the flanges, which may be carbon steel, show as hottest; the vertical pipe material has a lower emissivity and indicates a colder temperature because it is stainless steel.

The pipework is then reflecting significant amounts of radiation from the local (cooler) environment: all of these factors affecting the final thermographic image. Also, the welded sections appear to show variations in emissivity along the pipe wall.

“By understanding these physical and thermographic effects, related to plant load, operational data and material knowledge, trained operators can begin to access the benefits of infrared cameras in an industrial environment,” concluded Dunne.





Staying in the picture



One approach to applying thermography is to monitor critical assets most often, followed by equipment in harsh environments, suggests equipment supplier Fluke. Priority, for example, could be given to monitoring extra stress on motors, bearings, windings and insulation caused by variations in processes involving sludge, solvents and particulates.

“Every time a piece of equipment is inspected, a thermal image of it should be saved on the computer so that its condition can be tracked over time,” Fluke also recommends. “The baseline data will enable comparisons that will help to determine whether a hotspot (or cool spot) is unusual or increasing over time, and will also help to verify when repairs are successful.”

The company also highlights how thermal imagers can be used to look at product uniformity in specific processes. One example is in paper processing when the paper may be cured by passing it through an oven.

Coatings applied to the paper often require a combination of time and temperature to achieve the right cure point and final moisture level, notes Fluke. Handheld thermal imagers, it says, can be used to examine the thermal uniformity of the product as it comes out of the oven. Thermal variations are often attributable to other process variables such as non-uniformity in moisture or cure.