Bearing isolator slashes seal failure rates

In this installment of What Works, a cement plant cuts vertical roller mill downtime with bearing isolator.

Headquartered in Scottsdale, Ariz., the Salt River Materials Group (SRMG) produces coal combustion products, aggregates and Portland cements. Though there are both wet and dry processes to manufacture cement, SRMG uses the dry process. In the first stage, limestone, the primary raw material, is quarried and fed through a series of crushers capable of handling pieces as large as a 55 gallon drum. Primary crushers then reduce rock to a maximum size of about 6 inches. Secondary crushers or hammer mills reduce it to about 3 inches or smaller.

After the material is ground and mixed, it’s sent to cylindrical rotary kilns where it’s heated to about 2,700°Fand discharged red-hot from the lower end of the kiln in the form of marble-sized clinkers.

The clinker is cooled, combined with gypsum and ground into gray powder so fine that nearly all of it passes through a 75 micron sieve. One pound contains 150 billion grains. This fine, gray powder is Portland cement.

SMRG uses a vertical roller mill as the finish-grinding method at its Clarkdale, Ariz., plant. Clinker and gypsum are ground on a rotating table under large rollers. Centrifugal force moves the material off the table, and a stream of air carries it to a classifier.

The classifier allows small particles to transfer for collection and the larger particles to fall back to the mill table for additional grinding. The higher the speed of the classifier, the higher the blaine, a measurement of particle size. Product with the correct blaine is sent to shipping.

A rotor within the separator classifies the mixture into the proper blaine. This rotor, more than 21 feet long and operating at temperatures as high as 220°F, rotates on upper and lower bearings. Very fine particles can contaminate bearings, so they were sealed using V-ring shaft-mounted rubber lip seals.

“The lip seals failed miserably,” says Don Hammond, SMRG mechanical maintenance supervisor. “We were experiencing seal failure issues constantly and having to inject grease into the v-ring seals 24-7 just to stay running.” The high heat generated during the manufacturing process caused the lip seals to crystallize, harden and fail. As the lip seals began to fail, they would lose as much as 200 gallons of synthetic lube in a single week.

“At $25 for a gallon of synthetic lube, this was a costly failure,” Hammond continues. “Add to that the fact that replacing these lip seals could take as long as six days working around the clock, and you can see how much this was really costing us. Trying to counter this impending failure by greasing the seals 24/7 wasn’t exactly inexpensive either.”

Joe Johnson, another maintenance supervisor at Salt River, had contacted Inpro/Seal (www.inpro-seal.com) about a problem they were having with gearboxes. When SMRG understood the advantages of bearing isolators, the conversation quickly turned to lip seal failures in the classifier.

“We replaced this inadequate sealing method with one of our Articulated Air Mizer designs,” says George Gillespie, the local Inpro/Seal regional manager. “Two major considerations were addressed. The first was that due to the abrasiveness of the product, stainless steel was used as the material of construction rather than the standard 660 bronze. The second was that because of the difficulty, time and cost of disassembling the equipment to install a solid seal design, a split design was used. These split designs operate just like our standard solid designs.”

The split design did not require removing the shaft and bearings to install the seal. With the air supply system installed ahead of time, seal installation took about four hours. “The Air Mizer operates without contact as the air purge does the actual sealing,” Gillespie says. “Now they have a permanent, non-contact shaft seal that will typically outlast the equipment it is installed on.”

Before meeting with Inpro/Seal, SMRG had tried a number of methods to seal the classifier. “None of them lasted,” Hammond says. “In the past, by now, the lip seals would have failed at least six times, we would have had that many shutdowns, spent lots of man hours replacing the seal and lost a lot of lube. Do the math and you will see how much this product has really saved us."

Though SMRG has not had a problem with the lip seal in the upper bearing, Hammond expects that it also will fail. “And when the upper seals fail, we are ready,” he says, “as we ordered a second bearing isolator, which is ready for immediate installation.”

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