VIB ANALYSIS & UNBALANCING-JUN 2008.pdf

7/28/2019 VIB ANALYSIS & UNBALANCING-JUN 2008.pdf

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JUNE 2008

FEED BACK: 2

PROJECT: Unit - 2 , NTPC Ramagundam

PROBLEM: Vibration analysis and balancing

NTPC Ramagundam units 1 to 3 are Ansaldo Units of rating

200 MW . The turbine consists of Combined HP IP and LP

Modules . The first stage of the combined HP-IP turbine is

Curtis stage .The HP-IP rotor is of single journal at

bearing 1 and LP rotor is supported at bearing 2 and 3.

The thrust bearing location is at bearing 1 and pedestals 1

and 2 are sliding type. The generator bearing are end

shield mounted .

The unit had undergone capital overhaul during May

June. 08 first time by BHEL . LP rotor was replaced

during Overhaul. The unit was rolled on 14th June after

overhaul. The turbine rotor system is soaked at 1000 rpm

for about 1 hour and at 2500 rpm for 10 minutes duringrolling to 3000 rpm.

Vibration monitoring is carried out with permanently

mounted shaft rider probes at turbine and generator

bearings . Each bearing is instrumented with one shaft

rider probe mounted at approximately 60 degrees to the

horizontal . In addition an eccentricity probe is provided at

pedestal 1 .

During the initial rolling at 3000 rpm , Bearing 2 (LP

front ) and Bearing 3 (LP rear ) vibrations shaft

vibrations exceeded the alarm levels 160 microns pk-pk .


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The coast up plot of LP Rear shaft vibrations are given in

Fig.1. The vibrations were predominantly of running

speed component. Since the vibration increase had a steep

slope around 1500 rpm which is in the zone of LP 1st

critical speed and were predominantly 1X, it was decided tocarry out balancing at site using LP plane.

A total of 9 numbers of balancing weights each of 60 grams

weight were used for balancing LP rotor using LP front

plane..

The LP shaft vibration levels after balance correction ,

were brought around 90 microns peak to peak and bearing

vibrations around 20 microns peak to peak, which are wellwithin acceptable limits. The coast up plot of LP Rear

shaft vibrations after balance correction is given in Fig.1.

The Unit was loaded to rated load and the vibration levels

were found steady.

Observation

Incidentally this LP rotor underwent Slow speed balancing

at 250 RPM carried out by customer. This has not yielded

desired results and the rotor once again had to be balanced

at 3000 rpm. Slow speed balancing may help for rotors

whose criticals are above operating speed but for flexible

rotors it is always recommended to carry out balancing at

operating speeds.


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Documents

VIB ANALYSIS & UNBALANCING-JUN 2008.pdf