Bearings That Can Handle the Heat
The inlet-air section of a utility boiler is a harsh environment for the dampers that control the flow of preheated air into the combustors. Over time, the hot air dries out the lubricant in the metal bearings, or roller elements, that control the movement of the dampers, causing the dampers to seize.
This was a problem at the Sioux Power Plant of Ameren UE (Union Electric), near St. Louis. The plant has two 535-MW, coal-fired units, each of which has 10 cyclone burners. There are two dampers on each burner -- a control damper and a shut-off -- for a total of 40 secondary air dampers. “On average, we had a bearing problem about every two months,” says James Riegerix, general supervisor at the plant.
He adds that when the changeover was done, the installers found a single bearing from Graphite Metallizing that had been put in a few years earlier. It was still in such good condition it was not changed (figure 1).
The Problem DefinedThe dampers control the flow of secondary air, which accounts for approximately 85 percent of the total combustion airflow to the cyclone burners. The rest consists of primary and tertiary airflows fed separately to the burners, which can be adjusted to control flame quality. Secondary air is preheated to 650 to 750oF (343 to 399oC) by flue gas through a large heat exchanger. It then flows through the windbox (duct) to the cyclone burners, where it is mixed with powdered coal.
Airflow to each burner is determined by a control damper that typically measures 8' by 6" by 2'. A shut-off damper, located just upstream from the control damper, is used to isolate the windbox from the cyclone. Shut-off dampers are used only for scheduled and forced outages.
The problem of lubricant loss in the hot environment was compounded by the fact that the dampers are not moved very often, says Riegerix. “We are a baseload plant, so we run near full load during the day and only need to move the dampers if we go down to 60 to 75 percent load at night. For example, in the spring we may move the dampers twice a day -- in the morning and at night -- or on weekends, but in the summer, we may move them only a few times a week.”
“Lack of regular movement is a killer for lubricated bearings,” adds Andy McGraw, a sales engineer with Graphite Metallizing. “The grease carbonizes and the bearings lock up on you.”
“If an outer bearing failed, we would attempt to replace it,” he says, “but if it was an internal bearing, we would just have to make do. Sometimes, bearing problems would force us to alter our firing methods; other times, we would have to fabricate a device to forcefully move the dampers manually when the operators would not drive them. We would not bring the unit offline just for a bearing problem on a damper. We just lived with the headaches.” Riegerix notes that the bearing problems also caused the cyclone burners to work inefficiently due to inability to control the combustion air.
The proprietary graphite/metal alloy used for the bearings consists of graphite, which is porous by nature, impregnated with metal. Graphite Metallizing produces more than 100 such alloys, and each contains a different metal or combination of metals to exhibit specific physical properties. “We mix and match combinations for the application at hand, which may call heat or chemical resistance, or mechanical strength,” says McGraw.
Of the 40 bearings supplied to Ameren, 30 are pillow-block bearings (three sizes) and 10 are four-bolt-flange block bearings. Two of the three sizes of pillow-block bearings have bolted split housings. The split bearings are supplied as two halves and are bolted together around the damper's steel shaft).
“They gave us the temperature and loading criteria, and we made sure everything worked,” says McGraw. At Ameren, the mechanical loading on dampers is “usually pretty minimal,” he says, and the most important considerations were ability to handle the high temperature and improving the movement of the dampers. “Because the material is self-lubricating and can handle high temperatures, you can leave a damper in one position for six months, then operate it, and the bearing will work just fine.” PH