The bearings on the dampers controlling airflow to two 535-MW coal-fired units at a power plant couldn't handle the heat. High-temperature bearings solved the problem.

FIGURE 1. This secondary air damper bearing was installed in the Sioux plant several years ago and was still in good condition during retrofit, so it was left in place.


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.



This 3' dia. door leads into one of the 10 cyclone burners at Ameren. This burner is approximately 10' dia.

Ameren has 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 of the bearings for one boiler were replaced in the spring of 2004, and those in the other unit were replaced a year later. Both installations were completed during planned plant outages, which normally are scheduled every two years. So far, there have been no problems with the new bearings, says Riegerix.

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).



This closeup view shows a control damper bearing that is located outside the windbox at the Sioux plant. The four-bolt, split-housing pillow-block bearing is the largest of the four types of bearings installed on the cyclone burners.

The Problem Defined

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 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.”

The control mechanisms for both the control and shut-off dampers on the burners are electrically driven. A large four-bolt, split-housing pillow-block bearing is visible just right of center at the end of the control arm for the damper.

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 of 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 200oF (93oC). “The [previous] bearings still seized up, although not as often as those inside the duct.”

“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.

In this side view, a bearing for a shut-off damper shaft, located behind the sprocket to the right, is visible. The sprocket and chain actuate the shaft. On the left, connected to a turnbuckle, is the control arm for the control damper shaft, which has a separate electric drive.

The company creates the alloys 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, the graphite was impregnated with copper, producing an alloy that can tolerate operating temperatures up to 750oF (399oC).

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

For more information about high-temperature bearings from Graphite Metallizing Corp., Yonkers, N.Y., call (914) 968-8400; e-mail sales@graphalloy.com; or visit www.graphalloy.com.

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