By Teresa Hansen, Associate Editor
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 dampers’ movement, causing the dampers to seize.
This was a problem at the Sioux Power Plant of Ameren UE near St. Louis, Mo. 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 shutoff damper – for a total of 40 secondary air dampers. “On average, we had a bearing problem about every two months,” says James Riegerix, the plant’s general supervisor.
Ameren solved the problem by replacing the lubricated steel bearings with self-lubricating bearings made from Graphalloy, a graphite/metal alloy by Graphite Metallizing Corp., Yonkers, N.Y. All the bearings for one boiler were replaced in the spring of 2004 and those in the other unit were replaced in March 2005. Both installations were performed during planned plant outages, which are scheduled every two years. “So far, there has been no problem with the new bearings,” says Riegerix.
The 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 air that is fed to the burners separately and can be adjusted to control flame quality. Secondary air is preheated to 650 F to 750 F 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.
A control damper that measures approximately 8 feet 6 inches by 2 feet determines airflow to each burner. A shutoff damper, located just upstream from the control damper, is used to isolate the windbox from the cyclone. Shutoff dampers are used for only scheduled and forced outages.
The problem of lubricant loss in the hot environment was compounded by the fact that the dampers are seldom moved, 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.”
“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.”
Because scheduled outages take place only every two years, damper problems are usually dealt with while a unit is online. Riegerix notes that half the bearings are located inside the windbox and half are outside. In the latter case, the shaft goes through the duct, so the bearings are exposed to a lower temperature, around 200 F. “The bearings (outside the duct) still seized up, although not as often as those inside the duct.
“If an outer bearing failed, we would attempt to replace it,” Riegerix 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.” He notes that the bearing problems also caused the cyclone burners to work inefficiently, due to inability to control the combustion air.
The Graphite Solution
The proprietary graphite/metal alloy used for the bearings consists of graphite (porous by nature) impregnated with metal. Graphite Metallizing produces more than 100 such alloys under the name Graphalloy. Each contains a different metal, or combination of metals, to exhibit specific physical properties. “We mix and match combinations for the application at hand,” says McGraw.
The company creates Graphalloy by forcing molten metals into the pores of graphite under high heat and pressure. “We make a rough shape of what will be the final part from graphite and impregnate it with the metal,” says McGraw. “After the part has cooled, we machine it to the final size required.” For the Ameren plant bearings, Graphite Metallizing impregnated the graphite with copper, producing an alloy that can tolerate operating temperatures up to 750 F.
Of the 40 bearings, 30 are pillow block bearings of three different sizes and 10 are four-bolt flange block bearings. Two of the three types of pillow block bearings have bolted split housings, one being a two-bolt and the other a four-bolt style. The split bearings, each of which weighs 80 to 90 pounds, are the largest of the bearings.
“They gave us the temperature and loading criteria and we made sure everything worked,” says McGraw. The mechanical loading on dampers is “usually pretty minimal,” he says. The most important considerations were handling the high temperature and improving the movement of the dampers (the avoidance of seize-up). “The Graphalloy material is self-lubricating and can handle high temperatures,” says McGraw.