Plant Services - June 2008 - (Page 40)

Heating & Cooling safety Passive support Figure 5. Hydrodynamic sleeve-type bearings such as this one should be monitored by sensing shaft vibration. proximity probe systems. The probes are mounted into the bearing housings, a clean environment that protects the proximity probes against dust and corrosion. These systems monitor the distance between the rotating shaft surface and the probe tip, and the output is the relative motion between the shaft surface and the bearing housing. Of course, the shaft surface quality, diameter and eccentricity must be closely controlled during fabrication so that the output accurately reflects the shaft vibration and not the shaft’s out-of-round condition. Typically, the output is in displacement units (mils peak-to-peak or mm peak-to-peak). ANSI/AMCA Standard 204 includes the recommended maximum displacement of the shaft relative to the available clearance from the bearing liner (Figure 6). On the other hand We’ve seen examples of what can go wrong in the absence of preventive and predictive maintenance. Here’s an example of what goes right when plants use vibration monitoring. 40 www.PLANTSERVICES.com A 7,500-hp induced-draft fan on a cement kiln was subject to erosion because of particulates passing through the fan. After welds at the edges of the liners began to erode, a portion of a blade liner – weighing 50 lbs – loosened, then broke off while the rotor tip speed was about 430 miles per hour. One might expect that the tremendous unbalance forces would cause severe damage. Although the force of the 50-lb piece ruined the fan housing inspection door, there was no other damage to the fan housing, fan shaft, bearings, coupling or motor. The vibration-monitoring system installed on the fan bearings sensed the higher vibration developing as the blade liner began to loosen. The system was interlocked to the fan motor and shut the fan and system down immediately. No plant operator would have been able to react to this situation so quickly. This is an excellent example of the value of continuous vibration monitoring that is interlocked to the main fan motor-driver and is set up for instant response if the vibration exceeds a preset shutdown level. June 2008 http://www.mactekcorp.com http://www.mactekcorp.com http://www.PLANTSERVICES.com

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Plant Services - June 2008

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