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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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