Coal power plants today must find ways to operate economically if they are to survive. Every area of cost reduction must be investigated. On the maintenance side, time savings must be sought—to reduce component replacement costs, slash maintenance time, and increase uptime.
Due to the stress of coal plant operations, as well as high temperatures and the heavy presence of particulate, bearing maintenance is of particular importance. Bearings must be durable, long lasting, and able to operate without frequent maintenance. That is easier said than done.
Take the case of a North American plant running three 400-MW steam turbines. Each of its boilers has 24 wind boxes fitted with dampers to control the amount of air for combustion.
These wind boxes operate in a hostile environment, including temperatures of up to 600F, with the dampers subjected to air that is rich in coal dust and fly ash, as the wind boxes are located near the supply of pulverized coal.
The plant manager and staff at the plant found themselves dealing with annoying bearing issues inside their wind boxes. The specially designed roller bearings originally used on the wind box dampers could not be lubricated effectively. Clogging and bearing failures caused the dampers to bind. Personnel spent hours each week in the wind boxes as the combustion mix kept fluctuating, boiler tubes were damaged, emissions spiked, and fuel costs rose. Despite their best efforts, bearings continued to fail regularly.
As the outside diameter of the bearing and the damper shaft were close together, the presence of a little rust, scale, or particulate caused the bearings to seize. Quick fixes included the manual removal of rust and scale, as well as chipping away a little material from the bearing sleeve. This bought a little time but didn’t address the underlying problem. As an additional challenge, the wind box bearings were hard to access.
This wreaked havoc as the flap gates for the dampers would fail to properly open and close. Eventually, they would either be left open or locked shut. The burners were fed either too much air or not enough. Improper combustion led to unburned coal and hot spots damaging boiler tubes. Emissions became a problem. With too much oxygen, nitrogen oxide levels soared. With too little, carbon monoxide levels climbed. Overall availability suffered due to shutdowns and unscheduled maintenance.
There were almost 600 internal and another 600 external roller bearings in use for the wind boxes of the three steam boilers. Eventually, bearing issues ate up too much maintenance time and attention. The scale of the problem finally led the plant to seek out alternatives.
New Bearing Design Tested
The plant manager ran a small pilot project to investigate a new design. He moved from pillow blocks with roller bearings to new blocks with self-aligning bearings (Figure 1) that included a Graphalloy, graphite/metal alloy, self-lubricating bushing in a 2-bolt cast iron flange block assembly designed by Graphite Metallizing Corp. of Yonkers, New York. The manager installed eight of these bearings in the harshest position inside one wind box.
1. This photo shows external four-bolt self-aligning bearings in service. These bearings include a Graphalloy, graphite/metal alloy, self-lubricating bushing in a cast-iron flange block assembly. By switching from pillow blocks with roller bearings to new blocks with graphite-based self-lubricating bushings, the plant greatly reduced maintenance demands while achieving a more predictable combustion mix. Courtesy: Graphite Metallizing Corp.
The new bearings offered more clearance than the previous roller bearings. This meant they stayed clean longer, and technicians found it easier to chip away scale or build-up.
The previous roller bearings could operate without problems for about two months. After that, technicians faced a daily challenge to keep them running. After successfully testing the new bearings, with continuous operation without intervention for more than six months, the plant decided to roll out this technology across the facility.
Almost 600 external four-bolt wind box damper bearings are gradually being replaced with these self-lubricating units, which can be installed without an outage. Another nearly 600 internal two-bolt bearings will be replaced in conjunction with scheduled outages at the facility.
Benefits include an improved combustion ratio, fewer emissions, higher power output, lower fuel consumption, the elimination of hot spots in the tubing, and fewer maintenance headaches. The plant manager expects these bearings will operate untended for as long as five years. ■
—Archie Robb is a writer based in Southern California specializing in business and technology. For more information visit www.graphalloy.com.