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Chevron’s vibration-monitoring crew was called to assess the situation. The company has a sophisticated predictive-maintenance program known as Integrated Machinery Inspection (IMI) and a group to administer it. On its way to greatly reducing maintenance cost, the group has amassed considerable experience in diagnosing noise and vibration problems.

According to Chevron’s IMI Supervisor John Lizarraga, many times the solution is found by elimination. “The complex dynamics that result from the interaction of all the moving parts in the machinery make finding the answer difficult,” he says.

Still, Mr. Lizarraga and his crew checked a number of places made obvious by experience. First they wanted to eliminate the possibility of bearing damage that could have occurred during motor shipment or installation. Mr. Lizarraga explains that sometimes bearings can be damaged in transit if the motor isn’t packed properly. The damage is called brinelling, produced by the balls striking the bearing raceway repeatedly in the same spots. Brinelling can also occur during installation if a bearing is hammered onto a shaft.

Mr. Lizarraga gathered time and frequency data and analyzed it with a Scientific Atlanta SD 385 analyzer. Tests were inconclusive, but the frequency of the troubling noise pointed to one of two things: a bearing problem or an antirotation ratchet. (Some pump motors include a mechanism to prevent impeller reversal. It has been known to cause vibrations in some instances.) At this point, Chevron contacted pump manufacturer Gould. Unfortunately, it too, was at a loss to explain the noise. The refinery then turned to the motor manufacturer, GE Motors.

GE Sales Engineer Frank Judy inspected the pump and motor installation and noticed a shroud installed to protect the motor from moisture. His first recommendation: Eliminate the possibility of sympathetic resonance in the shroud. “Sometimes the inherent vibration of the machinery will excite a natural frequency in the shroud,” Mr. Judy says, “so we had our service shop weld a gusset to the shroud. The idea is to change the stiffness of the shroud so its natural frequency is altered.” When this procedure was done, though, the sound persisted.

Mr. Judy decided to call on GE Motors’ sound laboratory, headquartered in Ft. Wayne, Ind. I responded. The central problem in these matters is to determine if the noise is due to a defect or if it is simply the machinery’s response to a harmless force. We had eliminated the latter with the gusset experiment.

Tool price is right

Our team had been solving noise and vibration problems for decades, using an array of test equipment, including an anechoic chamber. The difficulty: The troublesome noise was 2,000 miles from the lab. Previous experience with these situations, though, gave us a quick and inexpensive tool — a $70 Radio Shack tape recorder. The analog data from a little tape recorder can tell everything we needed about the problem — no need to equip our sales engineers with expensive analyzers.

The analog recording captures all the data, including the time domain and frequency domain of the noise or vibration in question. This is an advantage over readings of frequencies alone because the analysis can be done in three dimensions: frequencies and their amplitudes plotted over time. Viewing the noise over time was especially important in this situation because the noise persisted even as the motor coasted down when de-energized. This is a good way to determine if the noise is due to a mechanical source like a bearing, or a sympathetic response like the shroud.

I directed Mr. Judy to tape the pump and motor in several operating modes. First, he recorded the motor running detached from the pump, which allowed evaluation of the motor sound separately. Next Mr. Judy recorded the motor and pump running at full load to get a perspective on how the two interacted. And finally, readings were taken while the motor was slowing when de-energized. He made all three measurements by walking the microphone a full 360 deg around the motor.

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