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SITE: NTPC RAMAGUNDAM UNIT - 3

PROBLEM: VIBRATION OF TURBINE RUB

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 Jan Feb. 07 . HP-IP

rotor was replaced with new rotor from Ansaldo during Overhaul. The

unit was rolled on 20th Feb. early hours after overhaul. The turbine

rotor system is soaked at 1000 rpm for about 1 hour and at 2500 rpm

for 10 minutes during rolling to 3000 rpm.

Vibration monitoring is carried out with permanently mounted shaftrider 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 soaking it was observed that the

eccentricity,bearing 2 (LP front ) and bearing 3 (LP rear ) vibrations

were increasing alarmingly at constant speed of 1000 rpm , the shaftvibrations were exceeding 225 microns peak to peak . The unit was

tripped and was kept on barring gear for 4 hours before rolling back to

1000 rpm .Once again the turbine vibrations rose alarmingly high refer

figure 1 , and the unit was tripped . The noise from pedestal 2 was

distinctively different from a machine that coasts down smoothly.

It was decided to open and inspect shaft areas within pedestal 2 .

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A strong de-colourisation and deep rub grooves were observed at LP

front bearing Oil catcher portion. The fins had rubbed in to the shaft

and were rectified with proper design clearances.

The unit was rolled and all vibrations were normal at 1000 rpm and

2500 rpm soaking speeds. However when rolled to 3000 rpm the

generator front shaft vibrations started increasing to alarm levels.

Refer to figure 2 . The unit was tripped and on inspection of LP rear

pedestal heavy rub marks were observed at LP rear oil catcher. Some

of the fins had come out and had to be re-finned at site.

The unit was rolled back after above corrections . The shaft and

bearing vibration levels were found satisfactory at 3000 rpm and were

steady. The shaft vibrations were less than 100 microns peak to peak

and bearing vibrations were less than 25 microns peak to peak at all

turbine and generator bearings . The unit was synchronized and

vibration levels are well within limits at rated load.

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Figure 1

Speed : 3000

Gen Front

Gen rear

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Figure 2

eccentricity

Brg 2

Br 3

Speed :