Piping Load and Machine vibrations

The question below may not be directly related to the Pure Piping design nevertheless of great importance in an operating plant.

In an operating plant a conflict between Rotary and Piping often arises whenever the rotary machine developes malfunction, attributing the cause to each other.

On Line Machine Monitoring System for a high speed Rotary machine say a Centrifugal compressor, gives data like Shaft locus plots, Online vibrations characteristics seen at the DE (Drive end) and NDE (Non drive end) bearings of the machine, including the similar data for the driver shaft /Gear Box (if required) etc. This data when interpreted properly can give information about any malfunction serious enough to cause any unplanned Plant outage.

However, in a particular case, where a sudden increase of vibration level after a satisfactory operation run for several months were recorded at the DE of a centrifugal machine, it was inferred from the locus plot of the shaft , that external pre-load exists on the machine. The bearing oil temperature / pressure recorded normal.

Here my question to the forum is that ,in what manner does the external preload ( which is static Force, due to piping on nozzles) can cause high vibrations seen by the shaft bearings. Can there be any correlation established between the Shaft locus or any other parameter of the machine to the external pre-load seen by the machine. Request your views far and wide.

Regds Received on Thu Jul 11 05:11:00 2002