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1. General aspects of monitoring and diagnostics.

Question 1.4.

Many diagnostic companies offer on-line and portable systems for machine vibration monitoring. They provide also the diagnostics. Does it differ from preventive vibration diagnostics and by what? The question asked by the participants of the seminar "Equipment Diagnostics of Nuclear Power Plants".

The question is answered by Natalia A. Barkova:

The purpose of the vibration monitoring is watching the changes in machine and equipment vibration condition and detection and prediction of dangerous changes. The purpose of diagnostics in such systems is to identify the reasons of these changes.

The purpose of preventive vibration machine and equipment diagnostics is to detect incipient defects and the prediction of their development. Not always the monitoring systems can provide all the functions of preventive diagnostics. There are several reasons of this.

Firstly, the incipient defects excite mostly the high frequency vibration and only in the defect location. It does not require significant forces.

But not always there is a possibility to mount a sensor on each machine unit, especially permanently.

Secondly, the analyses of high frequency vibration requires comparatively complicated methods and instruments, that sometimes are not included in the monitoring systems.

Thirdly, the experts must have a special training in interpretation and analyses of different types of machine units high frequency vibration or the system must have an automated diagnostic module that not all the monitoring system providers include.

Most successfully the monitoring systems can provide the bearing preventive diagnostics because the control points on the bearing shields are usually used by all the monitoring systems. At the same time for example for preventive diagnostics of each turbine wheel it is necessary to measure its case vibration in the zone of each turbine wheel, but these control points are usually not used by the monitoring systems.

So the monitoring system detects most defects by indirect methods, i.e. after development of several related defects. In this case one of these defects is detected by the monitoring system. Naturally that the quality of equipment condition prediction in this case is much poorer than the potentially possible.



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