Thermography Improves Predictive Maintenance
Thermal imaging, also known as thermography, has always been a powerful predictive maintenance (PdM) tool because it can detect incipient failures in nearly all types of mechanical and electrical equipment. Now, high-performance, infrared (IR) imaging systems are scanning plant equipment and improving PdM programs.
Thermal imaging, also known as thermography, has always been a powerful predictive maintenance (PdM) tool because it can detect incipient failures in nearly all types of mechanical and electrical equipment. Now, high-performance, infrared (IR) imaging systems are scanning plant equipment and improving PdM programs. Getting the new thermal imaging capabilities into the hands of people most familiar with a plant and its equipment has increased the technology's usefulness, meaning that thermography is no longer strictly the domain of outside specialists or consultants.
Wausau Paper Corp.'s Rhinelander Division in Rhinelander, WI, is a case in point. At that plant, Bill Gray, the division's maintenance reliability specialist, oversees an aggressive predictive maintenance program. The program uses a new-generation imager to do thermography at least once every two weeks. Gray assesses the health of mechanical and electrical equipment.
A large percentage of plant equipment failures involve a rise in operating temperature that reveals itself long before catastrophic failure occurs. In the past, the cost of imaging devices and the complexity of extracting useful predictive information from the raw data caused facilities managers to hire third-party thermography specialists to survey plants about once a year. In such operations, production equipment easily could fail between surveys.
Technology that makes the new thermal imaging devices useful is as complex as ever. (See "How a thermal imager works" sidebar.) However, these latest instruments are designed for use by in-house personnel and simplify the data collection process. They offer portability and ergonomic designs while sacrificing none of the performance qualities a PdM program requires. In fact, the new instruments, despite smaller size and ease of use, have significant on-board computer power to support the data-collection process. They also interface with software running on a desktop host computer, making it possible for maintenance managers to extract information needed.
At Wausau Paper, which still employs third-party thermography specialists, Bill Gray learns by double-checking their findings with his thermal imager. "On the mechanical side," he says, "we look at pumps, agitators, drives, and all rotating equipment. On the electrical side, we look at the switching gear, the motor control units, and the electric motors themselves."
On the job
Gray's inspection route provides a good idea of how he uses thermal imaging in his plant. For example, his itinerary recently had him checking a motor control unit that had been replaced earlier. (For more information about creating an inspection route for thermal imaging, see "Mapping a thermography route" sidebar).
"I use a pre-determined route that goes from each motor control circuit to the next one," Gray explains. "To see the actual circuit and measure its operating temperature, I have to open the cabinet while the unit is operating under power. Of course, I must follow safety procedures. Then, I take a [real-time thermal] picture of the circuit board to look for hot spots." For reference, during the same visit, Gray also takes a visible-light digital picture of the unit.
The thermal imager sighting "window" displays the thermal image, as well as readouts of status, mode, and route information. He uses this window to frame the thermal image. Other features manage the temperature-range readout and span, switch the display from color to black and white, and turn on a built-in laser aiming-beam.
"I usually don't use the laser," Gray reveals. "I just look at the picture itself. When I get it to where I want it, I record it, and it freezes the image. If I like it, I can save it. If it isn't acceptable, I can redo it."
To aid in planning his sessions with the imager, Gray keeps what he calls a "thermographic problem list"—a list of suspect equipment that he wants to monitor using thermal imaging technology. When an item on the list appears to require servicing, the next step is to generate a work order directing repairs. After repairs, Gray uses the thermal imager to check the unit again to determine whether the repair has fixed the potential fault.
Gray also keeps a history of each piece of repaired equipment to monitor recurring problems. For each repaired production unit, his database file includes a problem number, date, work order number, and notes. Having this historic data readily available on the desktop computer in his office facilitates the planning of inspection routes.
After planning a route on his desktop, Gray downloads it into the thermal imager's memory. The on-board route keeps each inspection tour on track despite inevitable interruptions that all managers deal with on and off the plant floor.
After collecting thermal images, Gray brings the imager back to his desk and uploads the images into his computer via USB connection for better viewing and data analysis. Results are then organized into reports that include annotation of images, spot measurements at specific locations, and incorporation of comments previously entered into route data. Gray then saves the reports as MicrosoftWord documents, which can then be printed or attached to e-mails for distribution.
"On the electrical side," Gray says, "I give reports to the E&I (electrical and instrument) supervisor of the area, and to the E&I superintendent, who keeps track of them for insurance purposes. On the mechanical side, I give them to the associated mechanical foremen and the mechanical superintendent."
Typically, a report is one page and includes thermal and ordinary visible-light images, date, time, equipment number, problem number, a work order number, descriptive information, and diagnostic comments.
Having thermal imaging available in-house has made Wausau Paper's predictive maintenance program more reliable and self-correcting. For example, the company recently had a problem with a pump control unit. Thermal imaging showed that one of the three-phase power legs was running hot, indicating that the unit was drawing too much current through that leg.
Maintenance personnel executed a work order and attempted a repair to the pump controller. However, subsequent thermal imaging to evaluate the repairs showed that two legs were then running hot.
Maintenance personnel attempted a second repair, and then all three legs ran hot. Gray then made the decision to remove the entire unit and replace it.
As time-consuming as the events may appear, the important result is that all of the tests and attempted repairs occurred before a catastrophic failure happened. There was no emergency shut down. No production time was lost. No work-in-process material was damaged or lost. A sound PdM program takes steps to correct small problems before they become big problems.
Jason Wilbur is thermography segment manager, Raytek Corp;
How a thermal imager works
Users of predictive maintenance devices are often confused about the difference between thermal imagers and infrared (IR) cameras. Infrared cameras provide a qualitative image that allows the viewer to identify objects and features based on "brightness" at infrared wavelengths of light. Thermal imagers, by contrast, provide a quantitative measure of the temperatures at surface points visible in a scanned scene.
A thermal imager consists of an optical system capable of passing infrared wavelengths, an IR detector, and a digital-image recording system.
The optical system includes an objective lens that focuses incoming infrared radiation from the object being scanned. This process creates an image.
The IR detector is a solid-state device that absorbs any infrared energy that falls on it, and responds by putting out an electrical signal. Thermal imagers use calibrated signal detectors that ensure accurate measurements over the whole image.
Mapping a thermography route
Consistency is the key to effective and efficient periodic inspections. Manufacturers of the latest thermal imaging systems provide on-board intelligence and advanced user interfaces to help supervisors ensure that technicians acquiring images visit each piece of equipment along a carefully planned route.
To ensure that a technician visits all equipment on a route in order and creates all images needed at each station, the thermal imager's human-machine interface should incorporate a reminder system showing the planned route, location of each stop, images needed at each stop, and any required reminder notes. The desktop application software helps build and edit a route file, then uploads it into the thermal imager. The imager then uses that file to lead the technician through the route.
Maintenance supervisors must keep three considerations in mind when developing routes:
Travel time —The time required to acquire the images at each stop is typically less than the time required to move from point to point. Therefore, supervisors should plan routes to minimize travel time as well as the time used at each stop.
Logical inspection sequence —Whenever possible, the route should follow a logical sequence. For example, the sequence might follow the plant's production process.
Safety —Written route descriptions should help ensure the safety of technicians as they travel from inspection point to inspection point, as well as when they acquire data.
Today's portable thermal imagers allow supervisors to add safety notes to route information on the PC. Such notes that include, for example, instructions about where to stand when gathering data can have a major impact on safety (while also ensuring data repeatability from trip to trip).