Turning gear slowing down-DEC09.pdf

7/28/2019 Turning gear slowing down-DEC09.pdf

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22 DECEMBER 2009 TS TIDINGS

TECHNICAL SERVICES / PSSR

FEED BACK NO. 2

PROJECT : NALCO, ANGUL, UNIT 9, 120 MW

PROBLEM : TURNING GEAR SLOWING DOWN

Problem details:

This 120 MW unit is provided with a hydromotor to drive theturbine on turning gear. The normal speed of the machine duringturning gear is 110 RPM. On 12/09/09 at 10.15 hrs it was observedthat the turbine turning gear speed was suddenly going down from110 rpm to 96 rpm, and after some fluctuations in speed , itdecreased and finally came to zero ( the turbine rotation stoppedphysically) with rubbing noises in the LP turbine seals. Immediate

checks showed no abnormal conditions i.e. lifting oil pressure, vacuum, seal steam pressures and temperatures were normal. After thirtyminutes, with hand barring, it was made free, and turning gear wasrestored.

Analysis and resolution:The trends of turbine speed with various parameters like seal

steam parameters, vacuum, lifting oil and lube oil pressures werecollected for studying behaviour of the speed drop. It was seen thatthere was no influence of any of these parameters. Further searchand analysis revealed that one hour before speed reduction, LPExhaust hood spray had opened on protection at LP Exhausttemperature of 90 C. This temperature immediately dropped to 33

C and was seen maintaining at this value because of the continuationof LP exhaust hood spray. The restoration of turning gear was doneafter closing this spray.

The next time when unit was tripped and turbine came to turninggear, the same behavior was observed. Turbine speed startedreducing from 110 rpm to 95 rpm and then started to rise afterclosing the exhaust hood spray manually.


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Further study of trends of turning gear speed with seal steamparameters, vacuum, lifting oil , lube oil, and LP Exhaust hood

temperature, it was observed that speed drop was significantlyaffected by LP Exhaust Hood spray only i.e.Turning gear speeddropped when LP Exhaust temperature went below 40 degrees due toopening of LP Exhaust hood spray valve, and then recovered when LPExhaust spray was cut off. This slowing down behavior was seenrepeatedly (observed at least five consecutive times) during trip outs

with vacuum and seal steam in service. The turning gear speed couldbe restored to normal by simply cutting off LP Exhaust Spray

manually. Trends of turning gear showing speed reduction andrestoration with LP exhaust temperature is enclosed for reference.

The original logic given by BHEL, Hyderabad was checked. Theauto close command is given when speed less than 1850 RPM or loadmore than 10% provided LP exhaust temperature is less than 90 degC. The auto closing of LP exhaust spray valve was additionally givenat site with temperature lower than 50 deg C. The auto logic with

opening of LP Exhaust spray valve at 90 C and closing of the valveat 50 C of LP Exhaust temperature was ensured.

CONCLUSION:No such fluctuation in turning gear speed has happened since

this logic was implemented in end September 2009. LP Exhausttemperature was maintaining between 90 to 50 degrees with LPExhaust spray opening and closing in auto mode as per the abovelogic. The problem is thus resolved.


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Turning gear slowing down-DEC09.pdf