Plant Services - June 2008 - (Page 37)

Heating & Cooling Safety Foundry gas-cleaning induced-draft fan Crash and burn This 88 in.-diameter, double-inlet, variable-speed centrifugal fan was operating at about 350°F. The operators noticed some increase in the noise and vibration at certain speeds, but once the fan changed speeds, the vibration seemed to subside a little, easing their concerns. The fan had no bearing temperature or vibration-monitoring equipment installed, and periodic vibration monitoring was spotty. The blades generated a pressure pulse when they passed 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-thanexpected deflection and stress levels. This caused high- Preventive and predictive maintenance techniques alone can’t effectively prevent sudden and catastrophic failures. cycle fatigue cracking of the rotor material near the toe of several fillet welds. Left undetected, the cracks propagated until they reached a critical crack length. Without warning, the crack growth rate increased dramatically, and the fan wheel flew apart. (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 – of workers near the fan when it failed. So what can maintenance personnel do to protect operations and workers from catastrophic failures? Figure 1. This is the impeller from a coal dryer fan following a catastrophic failure. Flying debris Continuous monitoring is your best protection Preventive and predictive maintenance using frequent visual inspections, nondestructive testing of welds and periodic measurement of bearing vibration help, but these techniques alone can’t effectively prevent sudden and catastrophic failures like those described above. The best approach is continuous monitoring of vibration and bearing temperatures with the instrument signals linked to a fast-response automatic fan shutdown control. Here’s why: • If erosion is a possibility, thinning structural components subjected to the high stress resulting from centrifugal forces can suddenly reach a failure point. • Once fatigue cracks have been initiated, they can propagate faster and faster until there is a sudden, unpredicted failure. June 2008 Figure 2. Rotor failure damaged the fan housing and bearings. www.PLANTSERVICES.com 37 http://www.PLANTSERVICES.com

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Plant Services - June 2008

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