More energy, lower cost
IR surveys have proven their worth for Alcoa, so Welz first investigated whether units with IR-transparent windows could be used for closed-door inspections of switchgear. Here's what he found.
IR windows are made of various crystalline materials, all of which reduce to some degree the amount of infrared energy reaching the camera's detector. The windows used on electrical cabinets typically transmit IR energy only in the wavelength of 0.13 to 10 microns. However, the "sweet spot" of an infrared camera is 8 to 14 microns. If the window cuts off at 10 microns, the camera misses more than 70% of the energy emitted by the target. This reduced energy reaching the detector lowers image quality and severely compromises temperature values computed by camera algorithms established for the 8 to 14 micron range.
"The reduction in IR energy is critical when you're looking at switchgear," Welz explained. "Switchgear is usually oversized for the load, so it will run cool. As a result, the IR camera must detect subtle temperature differences in an environment with very low levels of emitted infrared to begin with. This makes it difficult to identify what you're seeing in the camera so you know you're scanning the correct targets. Anything that attenuates the energy reaching the camera's detector is going to compromise results."
IR windows present practical issues as well. The windows' hygroscopic crystal materials absorb moisture over time, reducing image quality. The crystal material also is easily scratched or cracked, so a camera lens barrel cannot be pushed against it. Removing heavy bolt-on switchgear cabinet doors risks breaking the window if the panel flexes or absorbs an impact. And, like any window, an IR window will attract dirt and oils, making cleaning the inside surface difficult.
However, according to Welz, "the greater concern for us was the cost of the windows—$300 to $600 each. Using a camera's standard lens, you'd need more than one window for each cabinet. With a wide-angle lens, you'd probably need a larger, more expensive window because the wide angle usually has a larger-diameter front optic. We swallowed hard and were planning to try some windows, but then we heard about Mikron Infrared's Viewport. At $50 for a nonlocking port and $85 for the locking version, changing direction was a no-brainer. The Viewport is part of a system that requires a special wide-angle lens, but it's one that our thermography service has," he explained.
Simple system
"We currently have about 60 ports installed, and we add more during scheduled outages," Welz said. "With the port and the lens, there's a defined area you can view at a certain distance, and we position the port to adequately cover the cabinet interior. We've been using one port per cabinet. Had we used IR windows, we probably would have needed more than one per cabinet. Standard IR camera lenses don't have a 54-degree field of view, and they focus to only 12 inches. You can't press the lens on the window anyway, so when you back the camera away from the door surface, you never get your full field of view inside the cabinet."
Coverage and image quality (Figure 10) are as good today as when thermographic surveys used to require the removal of switchgear cabinet doors, Welz added. "The time saved is substantial, as well. Using Viewport, we can scan all the switchgear of a generating unit in about 45 minutes. That would take 4 to 5 hours if we removed the doors and either worked the camera remotely or used protective equipment."
10. Inside look. Image of 4-kV breaker taken with a SpyGlass lens through a Viewport. Courtesy: Mikron Infrared Inc.
In fact, the only protective equipment needed when scanning switchgear through the Viewport is what Alcoa classifies as Level 2: hardhat, safety glasses, ear plugs, and no synthetic fabrics. "It's a big step down from the 100-calorie suit and makes the job fast and easy," said Welz. "The ports may turn out to be even more valuable once we begin using them on our 480-V equipment. The close quarters in their cabinets has already proven the worth of close-focusing, wide-angle lenses. Finally, we think the Viewports will facilitate our use of ultrasound detection, which we're currently implementing."
—Contributed by Mikron Infrared. Contact the company at 888-506-3900 or e-mail jon@mikroninfrared.com.
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