Mining and aggregate processing plants throughout the world rely on critical process fans for operation. So what happens when a fan rotor, with a wheel rotating at a peripheral speed of 450 mph, breaks down?

The scene looks something like this: Pieces of the wheel (as large as a coffee table) rip away from the rotor, tear through the ⅜-inch-thick scroll of the fan housing and fly 500 feet into the air. The 12-ton steel shaft — eight inches in diameter at the bearings and 24 inches diameter at the hub of the fan wheel — bends and twists like a pretzel. The cast-iron bearing housings break apart. The 4,000-hp motor separates from its concrete pedestal and lands on the foundation. The entire plant is shut down. It takes weeks to clean up the mess, rebuild the fan housing and install a spare rotor (assuming the plant has a spare rotor on hand).

This scenario has been played out over and over again in mining operations and processing plants worldwide. Supply- and exhaust-ventilation fans are at risk. Process fans may be at even greater risk (due to potential corrosion and erosion of the structural fan rotor materials).

What are the causes of this kind of catastrophic failure? What steps can be taken to minimize the chances of such a failure? Before looking at possible prevention measures, consider the real-world examples presented here.

EXAMPLE NUMBER ONE

A 96-inch-diameter centrifugal fan had been in service for three years at a coal dryer application in West Virginia. The fan operated at 1,180 rpm with a peripheral speed of 337 mph. Although an upstream cyclone separator provided 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. There was no planned inspection of the rotor and no routine monitoring of the bearing vibration levels.

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. The plant was shut down for nearly three weeks while a new fan was fabricated and installed on an emergency basis. (See Figures 1 and 2.)

EXAMPLE NUMBER TWO

An 88-inch diameter, double-inlet centrifugal fan, a variable speed driven unit, was operating at 350° F. The operators noticed some increase in noise and vibration levels at certain speeds but once the fan changed speeds, the vibration seemed to subside, easing their concerns. The fan had no bearing temperature or vibration-monitoring equipment installed and predictive maintenance vibration monitoring was spotty. In this application, the blade generates a pressure pulse when it passes the cutoff of the scroll-type centrifugal fan housing.

The operators didn't realize that, at certain speeds, the fan's blade-passage frequency (and the resulting frequency of pressure pulsations) caused excitations that exactly matched the natural frequencies of some of the fan rotor components. The result was higher than expected deflection and stress levels in those components. This caused high-cycle fatigue cracking of the rotor material near the “toe” of several fillet welds. Left undetected, the cracks grew until they reached a critical crack length. Without warning, the crack growth rate increased dramatically and the fan wheel flew apart. (See Figure 3.)

These two examples demonstrate the significant effects that axial- and centrifugal-fan-rotor failure can have on plant operation and worker safety. In addition to the obvious damage and plant downtime, similar situations have resulted in severe injury and even death to people working near the fan when it failed. In underground mining operations, a shutdown of the supply and/or exhaust fans means an emergency closure and evacuation of the mine. So what can plant managers do to protect their operations and their workers from catastrophic failures?