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SUBJECT : LOW VACUUM IN CONDENSER

PROJECT : NORTH CHENNAI/ UNIT-2

PROBLEM :

Vacuum gradually worsened at above site, from 0.892 Ksc at fullload to 0.763 Ksc (150 MW) over 30 days from 17.05.03 to

19.06.03. All efforts, including water tightness test and condenser

tube cleaning was ineffective. A summary of relevant datasubmitted for study is as tabulated below:

Time Date LoadMW

Vacuum

ksc

HotWell

CWInlet

temp

(L)/(R)

CWOutlet

temp

(L)/(R)

CWF Bay

level

(M)

Remarks

1300 17.05.03 210 -0.892 49 32 32 41 40 8.4

1100 22.05.03 210 -0.863 54 34 34 44 43 8.0

1400 27.05.03 210 -0.800 58 35 35 45 45 8.0

1100 02.06.03 200 -0.799 60 35 35 45 44 7.5

0900 07.06.03 100 -0.791 60 36 36 42 42 6.6

0400 12.06.03 150 -0.78 60 37 37 43 43 6.21800 19.06.03 150 -0.763 59 37 37 45 44 6.5

1000 22.06.03 194 -0.759 63 35 35 46 45 6.2

1000 27.06.03 183 -0.763 62 37 37 47 46 6.7

ONEvac.

pump in

service(Unit 2

only)

1600 27.06.03 155 -0.762 64.8 38 38 47 46 6.7 TWO

vac

pumps

Air exhausted from vacuum pumps was exceeding 150 Kg/hr. (Second

vacuum pump was installed for unit 1. The air piping was extended to

unit-2 from unit-1.)

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ANALYSIS

It was decided to check with Helium leak detector. Helium leak

detection instrument was used, From 22.07.03 to 24.07.03, tocheck the following:

? LP Cylinder : Bursting diaphragms, glands of LP seals,parting plane, Neck weld, LP BP downstream piping.

? Extraction piping: LPH 1 and LPH 2 piping and shell.?

Hot well stand pipes & CEP suction and impulse lines? Vacuum pump and main ejector? Flash tanks and drain coolerReadings were obtained at LP parting plane (corner), ejector air inlet

valve bonnet, ejector drain isolation flange, condenser manhole and

two bursting diaphragms. However, ingress was not high.

Condenser vacuum & CW inlet and outlet sea water systems

were thermally mapped (Fig. 1 shows condenser, Fig.2 shows vacuum

pumps) using a laser non-contact thermometer. From this,

we could see that mixing of inlet and outlet seawater was

taking place -- poor condenser performance due to reduced flow

through CW tubes. (Observe how temp. of flow reversal side is

higher than outlet). Also observe that temp. of air to vacuum

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pump is getting sub-cooled only in one half. (In Ennore TPS unit 3,

similar behavior was observed. Inspection had revealed heavy

corrosion of parting plate of inlet/outlet water box. After

correction, in Ennore vacuum went to full value and exhaust temp

came down to 45 Deg C at low loads.)

CONCLUSION

TNEB was advised to make temporary sealing with suitable agent,

(M-Seal) etc at identified air ingress points till unit could be shut

down. To find a long-term solution they were advised:

1.To service the components showing air ingress and makeairtight.

2.Reduce the seal water temp of vacuum pump.3.Inspect and confirm seawater CW mixing at parting plate of

inlet/outlet water box. Make necessary corrections to stop

this in unit 2 and other units of North Chennai.

4.Monitor the thermal and flow behavior (DT, TTD and CWwaterbox temperatures; DP and flow across each waterbox)

of all condensers and water boxes of the running units.

5.Make provision for sacrificial anode in CW system to preventcorrosion due to seawater.

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Note:

To understand condenser performance DT, TTD, DP and flow must

be watched:

DT = CW outlet temp CW inlet temp (difference in

temperature between CW inlet and outlet should be about 8-10 deg

C at full load of 210 MW. More means less CW flow, less could

mean mixing: confirm this by seeing TTD values also.)

TTD = Hot well temp - CW outlet temp

(TTD values of 5-6 deg C are normal. Values above 10 indicate

worsening condenser performance. 20 or more means very poor

heat exchange possibly due to fouling of condenser tubes or mixing

of inlet and outlet CW.)

DP and FLOW: Higher motor currents in axial flow CW pumps could

mean restrictions in discharge flow due to choking in debris filter,

and higher DP across CW tubes may mean choking in the tubes.

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