Author: Kirk E. Staller, Graphite Metallizing Corporation
When an American steel mill needed a solution for maintaining its bleeder valves on blast furnaces, a self-lubricating material was used to create custom-designed bushings that would perform without grease at all pivot points.
A blast furnace is a special type of furnace used at steel mills for smelting iron from ore. Blast furnaces are the biggest chemical reactors and are generally very large. They can be up to 200 feet tall and around 50 feet in diameter.
These huge structures each contain three automatic bleeding valves—sometimes called air release valves (ARV)—that are used to automatically release trapped air from a heating system. When used in a steel mill, the gasses (often containing carbon monoxide) rise to the top and burn. The valves are actuated one to five times a day by a large air cylinder on each valve to control the pressure inside the blast furnaces. Electronic position feedback sensors are located on each valve. The assembly has a combination of linkages with bushings and counterweights. If there is an upset, the counterweights allow the valves to pop open to avoid an explosion of the vessel. Major repairs require removal from the structure by a crane.
At a large steel mill in the United States, a blast furnace utilized bronze bushings at each pivot point on the bleeder valves. Despite being on top of a furnace, the bushings do not get extremely hot. Depending on the wind direction and activity in the furnace, temperatures average around 250 to 300°F but can sometimes reach as high as 400°F. The original design had grease-lubricated bushings fed by automatic lubrication lines. Over time, the old grease would build up on the valve and, when there was an upset, embers could ignite the grease. The resulting fire often burned off the lubrication lines and the attached electrical instrumentation. Because of these safety hazards, the steel mill wanted an alternative to the traditional lubricating system. Initially, they tried a hard metal-on-metal material – a manganese-treated steel bushing. The metal is hard and not supposed to require grease. However, after a year of service these bushings began to stick. The mill maintenance team then installed the capability to force grease into the bushings if they got stuck but the complication of grease buildup and a potential explosion from embers still existed.
Graphite Metallizing Company presented the mill with a self-lubricating bushing solution. Bushings made of Graphalloy, a graphite-metal alloy, were installed into all the pivot locations. The valves have four different size bushings installed in the linkage system to allow operation.
The steel mill wanted something that would last at least three years, the time interval at which they reline and rebuild the furnaces. Each blast furnace has three valves. They bought enough Graphalloy bushings to upgrade two of the furnaces. These two furnaces worked effectively during the three-year threshold, with no issues.
The Graphalloy self-lubricating bushings worked so well that a nearby machine shop decided to use the same solution for bleeder valves at another steel mill.
The biggest advantage of using Graphalloy bushings for this application is the self-lubricating feature. Graphalloy can operate at high temperatures (up to 1000°F) and is dimensionally stable. It maintains its size and shape and will not cold flow or deform under pressure. Because no grease or oil is used, these bushings do not attract dust or foreign matter. Because Graphalloy eliminates the need for oiling schedules, it prevents oil or grease damage to products while operating in inaccessible locations where maintenance can be difficult to perform.
GRAPHALLOY®, a graphite/metal alloy, is a unique self-lubricating bearing material that offers superior performance in hundreds of mechanical and electrical applications. GRAPHALLOY standard and custom-designed products provide lifetime cost savings and significant operating advantages over conventional bushings and bearings. Learn more at graphalloy.com.
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