Excessive Vibration From Loose Connection

by Damon E. Woodson & Rodney D. Allen


This brief paper gives an account of how the authors identified and solved what turned out to be a base looseness problem on an air preheater drive that was first suspected to be a problem with the motor. Phase measurements collected along the entire drive and its base were the key to identifying the exact source of the looseness. Once the looseness was corrected, vibration levels dropped and normal operation was restored. This paper is also a great case history where looseness was only indicated in the vibration spectra by high vibration at 1x rpm with no multiples of running speed being seen of any significance. Without the phase measurements and a trained analyst being on-site, the problem could easily have been misdiagnosed.


“Plant Scherer is a large coal-fired electric generating station. Four power plants am under one roof; the problem was in an air heater motor on Unit 1. The 1,780-RPM, 100-HP motor is the primary driver of a large right-angle gear reducer. The gear reducer turns a pin rack used to drive a 33-1/2 VI-96 Ljunstrum air preheater at the very slow speed of 1.16 RPM. The electric motor is the primary driver, but there are two emergency air motor drives in addition to the primary electric motor for the gearbox.
The air preheater is a very large heat exchanger. It is a rotating wheel on a vertical shaft that has a diameter of approximately 50 feet and weighs several tons. All of the weight of the air preheater rests on a very large Kingsbury thrust bearing. The air motors have high torque and are used to start the preheater. After the rotor develops an oil wedge on the thrust bearing, the lower torque 100-HP electric motor is sufficient to turn the machine. The machine heats combustion air going to the boiler of the turbine generator. The preheater uses flue gas that has exited the boiler to heat incoming combustion air; it is required for operation of the 818 MW electric generating station. Two air preheaters are installed for each boiler at Plant Scherer. The machine studied is designated the “A” machine on Unit 1. The gas flow out the machine is approximately 3.5 million cfm at 320°F when the machine is at full load.
Figure 1 shows the stir preheater but does not show detail of the drive unit.
The drive motor failed a polarization index (PI) test doting a predictive maintenance check. The PI test assesses the electrical insulation of the motor. This check is made every few years to anticipate and prevent motor electrical failures. As a preventive measure the motor was removed from service during a planned outage and sent to a reputable motor shop for repair. Before the motor was removed from service, vibration levels were relatively low. The opposite drive end casing reading in the horizontal plane was the highest at 0.12 IPS at operating speed (Figure 2). The overall vibration level was about 0.15 IPS; thus, most of the vibration was at operating speed (1X) and considered relatively low.
When the motor was put back in service, the vibration level at operating speed was almost 0.6 IPS, and the overall level was almost 0.8 IPS. Figure 3 contains data after the repair and reinstallation.”

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