Shielded with protective visors, operators at Westar Energy’s three 800 MW generating units at St. Marys, Kan., attempted to inspect the plant’s boiler tubes for signs of slag by looking through the boilers’ hot flames.
“That’s typical practice in the utility industry, and we make multiple inspections per day,” says Carl Schultz, Westar’s senior predictive maintenance (PdM) analyst for thermography. “We thought there should be a better way and began exploring the use of an infrared camera to capture what was happening with slag inside the boiler.”
Westar began its search for a better slag monitoring method by researching cameras specifically designed to operate in the 3.9 micrometer (µm) band, a mid-wavelength that allows the camera to look through flames. During the search, operators learned about a broadband (8 to 14 µm) bolometer camera that uses filtering to operate in through-flame mode. The camera’s broadband capability allows it to be used as a general purpose infrared camera for typical thermal inspections when it isn’t being used in the through-flame mode.
“This gave us the potential for double-duty service from our camera investment,” says Schultz. “We could use it for varied thermal inspections as well as for monitoring what was happening with slag inside the boiler.”
Westar bought the camera, a MikroScan 7400, from Mikron Infrared. The uncooled, microbolometer (a specific type of bolometer used as a detector in a thermal camera) camera offers three selectable temperature ranges, including a high-temperature range (400 C to 1,600 C) needed for infrared imaging inside the boilers where combustion temperatures can exceed 1,100 C.
Operators perform thermal inspections by inserting the camera lens into an inspection door in the furnace wall. Photo courtesy of Mikron Infrared.
Infrared filtering on the MikroScan 7400 allows the dual spectral band operation-8.0 to 14 µm long-wave mode or mid-wave with 3.9 µm microfilter for through-flame imaging. According to Jon Chynoweth, Mikron Infrared marketing director, the camera is suited for typical PdM applications because it is unaffected by sunlight or smoke in a plant.
Mikron’s infrared technology enables the bolometer camera to image narrow spectral bands at high temperatures, Chynoweth says. “In this case, we can deliver 3.9 µm through-flame capability along with general purpose 8 to 14 µm utility.”
Mikron calls this technology “spectral tuning.” It allows operators to use the camera for more tasks, such as predictive maintenance monitoring in the long-wave band on motors, bearings and electrical cabinets from ambient temperature to 400 F, says Schultz. “Then we can switch to the 3.9 µm band and high-temperature range to image the boiler tubes for slag condition.”
Along with specialized infrared filtering capabilities, the MikroScan 7400 is also available with a radiation shield and protective window to allow temperature measurement inside a furnace without interference from combustion flames.
It is battery-operated and self-contained in a metal case. It includes a digital voice recording device and can simultaneously record high-definition 14-bit thermal images with digital visual images. It comes with built-in image processing software and stores images and data to PCMCIA (Personal Computer Memory Card International Association) standard cards. Images can also be viewed in real time via video output or optional built-in IEEE 1394 (Firewire) interface-a very fast external bus standard that supports data transfer rates of up to 800 megabits per second (Mbps) and is ideal for video and other devices that need to transfer high levels of data in real time.
Chynoweth says that depending on the resolution of the detector array (commonly 320×240 pixels) microbolometer-based cameras may cost $12,000 to $43,000. “Typical mid-wave cameras that would otherwise be needed for this boiler application are based on a cryogenically cooled (approximately -200 C) detector. Such cameras are larger, heavier and less portable. They commonly cost $50,000 to $80,000, consume batteries at a rapid rate, and require costly servicing if the cooling system fails.”
The MikroScan 7400’s primary role at Westar Energy is imaging the slag buildup on boiler tubes, which reduces heat transfer efficiency and can keep ash from falling to the bottom of the boiler. “In this application, we’re not trying to precisely measure temperatures, but rather view what’s happening inside the boiler furnace,” Schultz says.
Operators perform periodic thermal inspections throughout the day. The camera lens is inserted into a five-inch by 10-inch inspection door in the furnace wall to conduct the thermal imaging. Inspection doors, platforms and decks are at various intervals and heights on the boiler. Operators are exposed to ambient temperatures of 100 F to 120 F on the outside of the furnace.
The MikroScan 7400 camera can help find plant equipment faults or see through furnace flames. Photo courtesy of Mikron Infrared.
The boiler is 85 feet wide and 14 stories tall, divided by a center wall. The Mikron camera allows thermal images to be captured through flame and particulate matter from inspection door to the center wall. “The camera has telephoto capability, letting us image an area of about 225 square feet at the 43-foot distance,” says Schultz.
Westar bought the MikroScan 7400 in mid-2004 and it has operated problem-free since, Schultz says.
When slag buildup is detected, the Westar generating plant uses various methods to deal with it. Blowers and water cannons can be applied to dislodge the slag, says Schultz. During nighttime hours, when electrical demand is lowest, the generating load can be dropped, in effect “cooling” the unit which can also induce slag to slough off. As boiler pressure is reduced and the saturation temperature of the water decreases, the tubes shrink, helping the slag to break loose.