Graphite alloy solves bearing failure problem - in hot lime conveyor in pulp mill

Switching to a new graphite alloy material solved a bearing failure problem in a hot lime conveyor in a pulp mill. In this challenging application, the drag chain conveyor moves hot lime at 2100?F from the No. 2 lime kiln to a storage tank. The bearings on the original conveyors failed at frequent intervals, requiring considerable maintenance that resulted in downtime at Georgia-Pacific's pulp mill in Cedar Springs, Georgia, USA. The problem was solved by installing a new conveyor from Vulcan Engineering Company, Helena (Alabama) that employs a proprietary alloy drag chain that is supported on the return by chain rollers with pillow blocks incorporating the graphite alloy bushings. The new bushings have eliminated the need for unscheduled maintenance, reducing downtime of the rotary kiln.

Headquartered in Atlanta, Georgia-Pacific is the leading manufacturer and distributor of building products in the United States and one of the world's leading manufacturers and distributors of pulp, paper and building products. It consists of two distinct operating groups - the Georgia-Pacific Group, which includes the pulp, paper and building products business, and The Timber Company, which manages 5.8 million acres of timberland in North America. The company employs 47,000 people at 400 locations in the United States and Canada. At Georgia Pacific's Cedar Springs mill, a rotary kiln discharges lime into the feed inlet of the hot lime drag conveyor. The conveyor transports the lime approximately 100 feet and discharges into the boot inlet of a bucket elevator, which elevates the lime and discharges into a lime storage silo. A lighter drag chain conveyor was installed for the No. 1 lime kiln several years ago. These conveyors used chain carriers supported with outboard roller bearings. Many bearing failures occurred during the one-year operation of the conveyor. Each failure required considerable maintenance that forced the rotary kiln to be shut down.

In an effort to overcome these problems, Georgia-Pacific elected to replace the No. 2 lime kiln conveyor with a heavier conveyor produced by Vulcan Engineering Company, Helena, Alabama. Vulcan Engineering is the largest in?house foundry engineering, manufacturing and installation group in North America. Vulcan produces conveyors using a proprietary alloy drag chain. According to Richard Rucks of Vulcan, pillow blocks featuring Graphalloy bearings were selected to support the return chain rollers in place of the roller bearings that had been used in the past.

Graphite metallising produces Graphalloy, a graphite-metal alloy, and a high temperature bearing material. Graphalloy combines the properties of graphite and metal. The graphite structure can be compared to a deck of cards with individual layers that are able to easily slide off the deck. This phenomenon gives the material a self-lubricating ability that is matched by no other material. External lubricants are simply not necessary. Graphalloy is graphite filled with a metal impregnant to enhance the chemical, mechanical and tribological properties of the material.

The material works at the high temperatures experienced in the lime kiln without the problems caused when oil-based lubricants burn off or plastics fail. Another advantage of Graphalloy is that it maintains its integrity when submerged in hostile liquids such as acids, alkalis, hydrocarbons, black liquor and liquefied gases. The material provides a constant low coefficient of friction, providing a surface layer that helps protect shafting from catastrophic failure. Lubrication is maintained even during linear motion; lubricant is not drawn out and dust is not pulled in. The bushings are available in Graphalloy is that it maintains its over 100 grades of material and can be designed to fit a variety of machinery; standard designs include cylindrical with or without grooves, single or double flanged, split and metal backed. Grades and design are engineered to best serve the application.

As a test, Georgia-Pacific elected to replace only the No. 2 lime kiln conveyor to evaluate the performance of the Vulcan Engineering Conveyor and the Graphalloy pillow blocks before taking further action. The conveyor operated successfully for more than a year without a single breakdown. Georgia-Pacific was so satisfied with the performance of No. 2 conveyor, they replaced the No. 1 conveyor with the same design.

One benefit of using the new bushing material is that engineers no longer have to worry about an unexpected bearing malfunction causing downtime in the plant. Although there is some wear, it is minimal and can be addressed in plenty of time during normal scheduled maintenance. There has also been a significant savings in labor since it isn't necessary to replace the bearings every few months. In addition, having the bearing problem solved frees up maintenance people for other tasks in the mill.

AMT Enviraclean Technologies Pvt Ltd, Mumbai. Tel: 022-61545400, 67987870, 61545402/03/44. Fax: 91-22-66430345. Email: info@ amtgroup.in