Unusual Vibration
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Cracked Shaft Sleeve Causes Unusual Vibration
by J. Howard Maxwell
Abstract
This paper is an interesting case study of a cracked shaft problem for all levels of vibration analysts and engineers. There are numerous plots comparing trend and phase data from a previous cracked shaft event and those from a recent event that initially indicated the same condition, but turned out to be significantly different. It is a very practical example of using historical information to begin an analysis, but recognizing that each event has to be analyzed on its own merits and that early assessments may not be complete or accurate. The paper closes with a good description of the findings after the machine was inspected and the condition was corrected.
PREVIEW
“1996 Rcp Shaft Crack:
In March of 1996, vibration indications on one of the pumps in Unit 1 begin to show evidence of a probable cracked shaft. After 18 days of monitoring the vibration and preparing for a rotor replacement, the Unit was shutdown and the pump rotor replaced. Subsequent inspections showed a guillotine crack approximately 60% of the shaft diameter, at the upper part of the impeller fit area, which was the same location as previously seen on this type of pump. The vibration indications as described in Maxwell [1] (1997), were those of a classic, almost textbook, shaft crack, including amplitude and phase shifting of both the 1 times running speed and 2 times running speed vibration components.
"Initial Shaft Crack Indications In 2001:
On January 4, 2001 during a routine vibration check, the vibration on the Unit 3 18 RCP showed changes that indicated a possible cracked shaft. An intensive monitoring and analysis program was implemented, and after about 4 days, we decided that we probably had a shaft crack. The usual indicators for a cracked shaft are changing amplitudes and phases of both the synchronous vibration and the twice synchronous vibration, and these symptoms were seen on both vibration sensors, see Figure 1.
We were able to apply the 1996 data to a computerized crack propagation model and then compare the results of this model to the actual Unit 3 vibration data (Maxwell and Rosario [2]). Initially the results, as shown in Figure 2, looked promising. By January 20th however, the actual vibration was obviously deviating from the projection, as shown in Figure 3. As the problem progressed, the deviation between the crack shaft model projection data and the actual vibration data continued to grow. As it turns out, there was a completely different kind of phenomena occurring.
"Figure 4 1 x RPM Amplitude and Phase 2/1/00 – 2/17/01:
Figures 4 and 5 shows the 1xRPM and 2xRPM amplitude and phase for a period of about a year prior to the event to the point where the pump was shutdown. The 1xRPM amplitude and phase data shows a step change over the refueling outage in the spring of 2000. This often happens because the pumps are partially disassembled and reassembled during the outage. The event marked “short outage”, could have caused a similar change in vibration, as the pump was shut down and then restarted, but only caused a very small change.”
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