7/27/2019 Tripping Cooling Water Pump in Overload at 50Hz

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Problem: Tripping Cooling Water pumps on over load during

operation at the frequency of 50 Hz.

Project: North Chennai TPS

HISTORY :

The cooling water pumps were operating near full load right from thecommissioning time while the frequency was in the range of 40 49Hz. In the recent past, the condition further got deteriorated andthe pumps were experiencing more nuisance trippings. The problemwas analysed and the observations are

a. System resistance seems to have increased which is gettingindicated by high differential pressure across the debris filterand deterioration of condenser cleanliness. The filter back washgear drive is not operational in Unit-1 and 2 due to defects ingear box. Understand from TNEB that spares are being ordered

and will be made good in the near future. For improving thecleanliness of the condenser tubes, TNEB was requested toregularly operate ball cleaning system.

b. To prime the system - the operation staff are requested tovent the condenser on regular basis and maintain the vacuum of300 mm of Hg at condenser outlet water box.

c. The cooling water fore bay level is going low due to shortage of

back water from sea. The quality of water is also found tohave deteriorated affecting the density of water. Theconfiguration of the cooling water, particularly in the vicinity ofthe pump inlet by way of Head and density affects the pumpperformance and cavitation effect was noticed. The extensivepitting marks noticed in the bell mouth and impeller surfacesconfirms the above phenomenon of cavitation. The pumps are alsoexperiencing failure of spider ribs and deflector plate assembly

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near tap flange joint. The above two failures were suspected tobe due to fatigue by the disturbance on cavitations.

The customer has agreed to carry out certain modifications in coolingwater intake canals to improve pump inlet head conditions in the nearfuture (in six months) to prevent the above problems.

BHEL/Hyd., also agreed to increase the capacity of the motor withrespect to pump in future to take care of operating conditions toimprove the reliability of cooling water pumps.

Damages observed on Cooling water pumps at North Chennai TPS

Cooling water pumps were operating at rated load since commissioningof the units. The cooling water pump system is experiencing thefollowing failures recently

? Extensive Pitting damage on Bell mouth and Impellors

? Cracks in bottom most support spider ribs

? Flange cracks in stainless steel sleeve

The first two damages are because of cavitations due to low suctionpressure. ;The support spider houses the rubber guide bearingswhich are water lubricated. The failure of support brackets arebecause of fatigue resulting from excessive vibrations due to pumpcavitation. The cooling water fore bay level is not being maintaineddue to shortage of back water from sea. The quality of water is alsogetting deteriorated due to sea water contamination. Certain systemmodifications are being implemented by TNEB and the water intakelevel would improve in six months time.

Failures are observed at weld joints near the deflector plate topflange. The customer has been advised to provide additional sleeve tostrengthen the weld area. However, the failures have been referredto plumps engineering, Hyderabad for further advice.(P1 . refer the sketch in the next page)

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