Plant Services - June 2008 - (Page 36)

ndustrial plants throughout the world rely on critical fans for operation, but what happens when a fan rotor, rotating at a peripheral speed of 450 miles per hour, fractures? e scene looks something like this: Pieces of the wheel rip away from the rotor, tear through the fan housing and ﬂy oﬀ. e steel shaft bends and twists like a pretzel. e cast iron I Before we address what you can do to prevent fan failure, consider the real-world examples below when making a case for preventive action. Coal dryer induced-draft fan Here’s an example of what goes right when plants use vibration monitoring. bearing housings break apart. e motor separates from its pedestal and lands on the foundation. e plant is shut down, and it takes weeks to clean up the mess, rebuild the fan housing and install a spare rotor (assuming the plant has one on hand). is scenario has been played out in heavy industrial facilities worldwide. What are the causes of this situation? What steps can maintenance personnel and engineers take to minimize the chances of such a catastrophic failure? 36 A 96 in.-diameter centrifugal fan had been in service for about three years on a coal dryer application in West Virginia. e fan operated at 1,180 rpm with a peripheral speed of 337 mph. Although there was an upstream cyclone separator to provide gas cleaning, some coal dust continually passed through the rotor. Erosion-resistant liners protected the fan wheel, but small particles of coal dust entrained in the high-velocity gas stream slowly eroded the unprotected welds at the edges of those liners. ere was no planned inspection of the rotor and no routine monitoring of the bearing vibration. One morning, minutes after fan startup, the rotor ripped apart, completely destroying the fan and damaging nearby equipment. Fortunately, no operators were in the area. e plant was shut down for nearly three weeks while a new fan was fabricated and installed on an emergency basis (Figures 1 and 2). J .PLANTSERVICES. http://www.PLANTSERVICES.com

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Plant Services - June 2008

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