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Fixing a Case of Burn-out


Its hard to imagine a harsher environment than the inlet-air section of a utility boiler, especially 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 recurring problem at the Sioux Power Plant of Ameren UE (Union Electric), northeast of St. Louis, Mo. On average, we had a bearing problem about every two months, says James Riegerix, general supervisor at the plant. Built originally in the late 1960s, Sioux Power was one of the first to install cyclone furnaces – essentially 10-foot-diameter barrels into which fuel and air are introduced, and where combustion occurs in a swirling, fiery vortex.

The plant has two 535-megawatt, coal-fired units, and each of them has 10 of these ferocious cyclone burners. They consume everything from coal to petroleum coke to tire chips. There are two dampers on each burner (a control damper and a shutoff damper), for a total of 40 secondary air dampers.

The problem of lubricant loss in this 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.

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 are fed to the burners separately and can be adjusted to control flame quality. Secondary air is preheated to 650 to 750 degrees F (343 to 399 C) by flue gas through a large heat exchanger. It then flows through the windbox (duct) to the cyclone burners, where it is mixed with crushed, powdered coal.

Airflow to each burner is determined by a control damper that typically measures 8.5 ft. by 2 ft. (2590 x 610 mm). A shutoff damper is located just upstream from the control damper and is used to isolate the windbox from the cyclone. Shutoff dampers are used only for scheduled and forced outages.

Into the Furnace

Lack of regular movement is a killer for lubricated bearings, explains Andy McGraw, a sales engineer with Graphite Metallizing who helped to address Amerens concerns. The grease carbonizes and the bearings lock up on you.

To end the Sioux plants seizing problem, Ameren replaced the dampers lubricated steel bearings with self-lubricating bearings made from Graphalloy, the trademarked graphite/metal alloy by Graphite Metallizing. 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 done during planned plant outages, which are scheduled every two years, alternating between the units.

Because scheduled outages take place only every two years, damper problems usually have to be 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 degrees F (93 C). The bearings still seized up, although not as often as those inside the duct, he recalls.

If an outer bearing failed, we would attempt to replace it, he continues, 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.

Engineering the Answer

The proprietary graphite/metal alloy used for the replacement bearings consists of graphite – which is porous by nature – impregnated with metal. Graphite Metallizing produces more than 100 such alloys, under the Graphalloy name. 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 for heat or chemical resistance, or mechanical strength, 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 bearings used at the Ameren plant, Graphite Metallizing impregnated the graphite with copper, producing an alloy that can tolerate operating temperatures up to 750 F (399 C).

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 comments, and Amerens most important considerations were ability to handle the high temperature and improving the movement of the dampers (i.e., the avoidance of seize-up).

Because the Graphalloy 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, McGraw adds.

Of the 40 bearings now in place at Sioux Power Plant, 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 a two-bolt and the other a four-bolt style. (Split bearings come in two halves and are bolted together around the dampers steel shaft). The split bearings – each of which weighs 80 to 90 pounds – are the largest of the bearings. The shafts on the dampers are of two sizes, 2.5 and 4 inches in diameter.

Riegerix points out that when the changeover to the first batch of new bearings was done in 2004, 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. So far, he says, there has been no problem with any of the new bearings.

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