Energy-Tech Magazine

January 2011 spare rotor install
In January 2011 the spare MAC rotor was installed and new, original design BAC pinion bearings were installed. The BAC stage 1 vibration fell to about 0.6 mils overall and the BAC stage 2 vibration also fell to about 0.6 mils. More importantly, the BAC stage 1 sub-synchronous vibration was less than 0.1 mils and the BAC stage 2 sub-synchronous vibration was approximately 0.15 mils. The vibration levels have remained stable since this outage.

Conclusions
The original B compressor BAC rotor was returned to the compressor supplier for inspection.  The inspection revealed excessive gear tooth pitch line runnout and excessive thrust collar runnout, even though the gear manufacturer quality records showed these dimensions being within tolerance. The excessive runnouts were sufficient to cause the MAC rotor vibration to be transmitted to the BAC rotor through the bullgear, which caused subsynchronous vibrations that led to bearing wear with increasing vibration levels.  

During operation

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there were no indications that there was a problem with the MAC rotor. The MAC rotor vibration levels were stable and there also was no evidence of excessive bullgear vibration.

This made it difficult to identify the MAC gearing as the source of the BAC vibration.  As described in this article, there were many unsuccessful attempts made at diagnosing and correcting the problem. The causes of rotating machinery vibration problems are not always obvious, and this is a good example of a sub-synchronous instability problem that was difficult to diagnose and correct.

Patrick J. Smith is lead machinery engineer at Air Products & Chemicals in Allentown, Pa., where he provides technical machinery support to the company’s operating air separation, hydrogen processing and cogeneration plants. You may contact him by e-mailing editorial@woodwardbizmedia.com.

 

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