Four Decades of Operating Experience on Detection, ConditionMonitoring and Health Assessment of Tarapur BWR NuclearSurface Condensers – Role of NDE, at Tarapur Atomic PowerStation-1&2, NPCIL, India

A. Ramu, A.Jabbar, N.K.Dhanmeher, N.K.Roy, V.S.Daniel, Ravindranath, S.Bhattacharjee andU.RamamurtyTarapur Atomic Power Station -1&2, Nuclear Power Corporation of India Limited, PO: TAPP; Dist: Thane; Maharashtra-401 504, India

E-mail : akkalaramu@gmail.com; aramu@npcil.co.in

AbstractTarapur Atomic Power Station is a twin-unit Boiling Water Reactors (BWRs) is one of the vintage reactors operating at its best efficiencymeeting the needs of both Maharashtra and Gujarat States. The rated capacity of each unit was 210 MWe but it was re-rated to 160MWein the year 1984-85 subsequent to the isolation of Secondary Steam Generators (SSGs). Since then both the units are in operation at its re-rated capacity. The station has completed 40 years of successful, commercial and safe operation. Main condenser is a one of the vitalconventional system components which has direct bearing on cycle efficiency. The tube material is Aluminum Brass with Aluminum-Bronzetube sheets. The tubes are lightly rolled, seal welded without filler material and expanded with the tube sheets. The station has observed variousfailure mechanisms in the condenser tube material & its associated auxiliary equipments and remedial measures were taken from time to timebased on the Operating Experience (OE) feedback received from overseas BWRs. TAPS has established various NDE techniques to effectivelydetect these degradations mechanisms. Ageing effects is one of the concerns of older generation plants and methodologies need to be adoptedto identify various degradation mechanisms.

This paper describes various degradation mechanisms of surface condensers, their detection and condition monitoring & health assessmentusing various NDE techniques. Also mentions about the inspection methodologies followed at Tarapur to effectively detect the degradationof tubes & other auxiliaries by optimizing inspection techniques and detecting capabilities.

Keywords: Surface Condenser, Aluminum Brass, Aluminum Bronze, Boiling Water Reactors, Inlet-end erosion, Fretting failure,De-zincification.

was conducted in association with the expertise from NuclearPower Corporation of India Limited (NPCIL)-HQ and variousagencies of Bhabha Atomic Research Centre (BARC).

1. Introduction

The condenser of Tarapur BWRs is a “Steam Surfacetype of Condenser” and is one of the vital conventionalsystem components which have direct bearing on cycleefficiency. The operating cycle is “Regenerative RankineCycle”, in which part of the steam is extracted from differentstages of turbines (HP&LP) to regenerate the condensatethru. Feed Water Heaters (FWH) for improving cycleefficiency. Initially TAPS-BWRs worked on Dual-Cycle whichis designed to operate with both Primary and as well asSecondary Cycle. 70% power output was from the PrimaryCycle, i.e., from Reactor Pressure Vessel (RPV-Boiler) andbalance 30% was from secondary cycle SSGs. The unit wasdesigned to operate with load following characteristics and

Tarapur Atomic Power Station is a twin-unit BoilingWater Reactors (BWRs) and is one of the vintage reactorsoperating in the world at its best efficiency meeting theneeds of both Maharashtra and Gujarat States. Both theunits were commissioned in the year 1969 with rated capacityof each unit was 210MWe. Similar vintage reactors operatingworld-wide are Oyster Creek in USA and Tsuruga of Japan.These units were re-rated to 160MWe in the year 1984-85subsequent to the isolation of Secondary Steam Generators(SSGs) of secondary cycle due to problems associated withtubes. Since then both the units are in operation at its re-rated capacity. The station has completed 40 years ofsuccessful, commercial and safe operation. Ageing effects isone of the concerns of older generation plants andmethodologies are being adopted to identify variousdegradation mechanisms and are eliminated either by designmodification or by replacements with better resistantmaterials. This process is required to enhance the life ofcomponents for continued service. TAPS had also taken upcorrective steps in this regard and a comprehensive study

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without changing the reactivity mechanism pattern the powercan be varied with SSGs steam out put to HP turbine at 5th

Stage.The purpose of Condenser is to condense the exhaust

steam from the turbine for re-use in the closed cycle tomaximize turbine efficiency by maintaining proper vacuum.As the pressure of condenser is lowered (vacuum isincreased), the enthalpy in the turbine also will increase.This results in (a) increased turbine output (b) ReducedSteam flow and (c) Increased Plant efficiency. Therefore it isnecessary to maintain the condition of surface condenservis-à-vis its vacuum as high as possible. The surfacecondenser is once-through condenser at Tarapur and iscooled by sea water drawn from Arabian Sea through intakecanal. In case of TAPS-BWRs low impurity concentrations,especially chlorides & sulfates during operations is primeimportance. Ingress of chloride content in the primary systemaffects the performance of Austenitic Stainless Steelcomponents, Stress Corrosion Cracking (SCC) and hence itis essential to keep the condenser leak tight from ingress ofsea water.

Non-availability of surface condenser results in energyloss thus economic penalty; therefore it is important to detectvarious degradation mechanisms that affects condition ofsurface condensers using different NDE techniques andshould be mitigated appropriately. In view of this, utmostimportance has been given to assess the condition with on-line performance monitoring techniques. TAPS has alsoexperienced various degradation mechanisms and developedvarious inspection methodologies with operating experiencefeed back (OE) from overseas reactors and in-houseexperience as well. The following paragraphs indicate variousdegradation mechanisms detected and mitigated withdeveloped/enhanced inspection methodologies from time totime at TAPS.

2. Description-Surface Condensers

The Surface Condenser is a rectangular-shaped, weldedsteel plate unit of the single-pass, vertically divided type.The general technical specification of Tarapur condensers isgiven in Table 1. The tube material is Aluminum Brass withAluminum-Bronze tube sheets. The tubes are welded in bothtube sheets. The inlet end of tube is flared for smoothentrance flow characteristics. The tube bundle is sloped fromoutlet end to inlet end of the condenser to facilitate drainageof cooling water when the units are not in operation. Thetubes are lightly rolled, seal welded without filler material andexpanded with the tube sheets.

Table 1: General Specifications of TAPS-1&2 SurfaceCondensers

Description Specifications

Surface Area 171,050 Square feet

No. of tubes 16,334

Number of 1"OD 18 BWG tubes 16034

Number of 1"OD 16 BWG tubes(Impingement Areas of Tube Bundle) 300

Tube Material: Main Tube Bundle Aluminum Brass

Tube Material: Impingement Areas Aluminum Brass

Tube sheet Material Aluminum Bronze

Effective tube Length 40’-0"

Actual tube lengths requiredfor tube replacements 40’-2-1/32"

Number of Tube Support Plates 11

Fig. 1 : Dual Cycle of TAPS-1&2 BWRs

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The degradations mechanisms depend upon variousfactors including material of construction, operatingconditions and other external factors. Table 2 gives variouscomponents of surface condenser with Material OfConstruction (MOC). The degradation mechanism of eachcomponent is discussed in detail in the subsequentparagraphs and operating experience of past four decadeshas also been included.

Table 2: Material Specifications of TAPS-1&2 SurfaceCondensers

Component Description Material Specifications

Condenser Shell Carbon Steel

Tube Sheet Aluminum Bronze (P No. 35)ASTM-B-171 (UNS # 61400)

Tubes Aluminum Brass (P No.32)ASTM-B 111 (UNS # 68700)

Tube Support Plates Carbon Steel

Water Boxes Carbon Steel

Condenser shell-to- Carbon SteelWater box XJ

Turbine Exhaust Belt type Dog-Casing-to-Shell XJ Bone shaped Rubber (Neoprene)

Tube Support plates Structuralto shell supports Steel

Impingement Baffles Carbon Steel

Hot well Tray Alloy Steel

Penetration Nozzles Carbon Steel

3. Degradation Mechanisms- an Overview

The units are in operation since 1969 and haveexperienced various degradation mechanisms associated withtubes, water boxes & its associated piping on cooling waterside and structures/internals on steam side. Initial tube failurerate was very high which might be attributed to fabricationrelated defects but subsequently the failure rate has comedone and stabilized for both the units. However, the totalnumber of tubes failure rate is more in unit no.1 as comparedto unit no.2. Therefore, comprehensive study was carried outand condition assessment of all the associated componentshas been done. In this process, some of the tubes wereremoved from each condenser (healthy tube & leaky tube)and sent for failure analysis and condition assessment. Theshell side of surface condenser being radioactive, the analysiswas carried out by Post Irradiation Examination Division(PIED) at BARC Mumbai.

Degradation mechanisms have been identified in Surfacecondenser & its associated components at TAPS are (a)Inlet-end erosion (b) Erosion-Corrosion (c) Mechanicaldamage (d) fretting failure (e) Under-Deposit Corrosion and(e) General Corrosion. These degradations could be detectedand mitigated in a phased manner based on in-houseengineering analysis & Operating Experience (OE) feed backreceived from time-to-time from overseas BWRs.

3.2.1 Inlet-end Erosion [2,3]

This type of degradation occurs as a result of turbulencecreated by cooling water entry from a larger area water boxto much smaller area of tube ID. Due to this eddies forms attube inlet end typically within H”1"-2" distance from tubeentry. The appearance of this degradation starts with pittingand aggravates with operating time hence causes sea water

Fig. 2 : PVC/Nylon Tube inserts at Inlet-end of tube sheet

Fig. 3 : Tube Inserts details in use at TAPS-1&2sheet

in-leakages. This mechanism has been overcome byintroducing Plastic tube inserts, made of PVC material. Thedesign & construction of tube inserts is such that it allowssmooth entry of cooling water into tubes thus the turbulentarea falls within the tube insert. This mechanism is detectedby visual examination using Optical Fibroscopes and wallreduction can be determined by Eddy Current Testing (ECT)method.

One has to be very careful in selection of tube insertmaterial. In appropriate selection/inadequate design of tubeinsert might cause increased turbulences behind the insertend.

3.2.2 Erosion-Corrosion [2,3]

Erosion-Corrosion is a result of disruption of oxide filmtypically formed on the metal surface [1]. This disruption iscaused by turbulences producing local shear forces

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continuously breakdown or prevent the formation ofprotective oxide film on the tube surface. Erosion-Corrosionis often occurs in case of partial blockage of tube, may becaused due to debris and/or foreign material. This mechanismhas been eliminated to some extent by cleaning of the tubeID surfaces at regular intervals by isolating each half ofcondenser water boxes. However, complete mitigation is notfeasible as the cooling water is from Sea-intake canal and thewater silt content depends upon the seasonal variations.

Removal of turbulence causing foreign material from thetubes and preventing re-entry back into the tubes should theoption to eliminate erosion-corrosion degradation mechanism.The condition of protective film would be difficult to estimateby NDE methods, however, sample tube failure analysis wouldindicate the performance of remedial measures adopted, asdetailed above.

Visual Exanimation after thorough tube cleaning withwater jet followed by pneumatic pressure jet is being followedat present. This would ensure the cleanliness of each tubethus absence of foreign material inside the tubes can beverified & documented. Visual Examination is one of theeffective NDE methods with which most of the problems canbe identified during preliminary inspections. VT has beenuseful because, the rubber ball shooting, generally done atthe time of tube cleaning might stuck within the tube alongthe length, which would otherwise block the flow as well ascauses tube erosion-corrosion. Based on the experience thestuck foreign objects may find its way to outlet sidedepending upon the tide level/pump start/stops. Fig.6 & 7shows such phenomenon observed in some of the analyzedtubes of TAPS.

3.2.3 Pitting [2,3]

Pitting is a localized corrosion that occurs preferentiallyunder the deposits at tube ID surface. These deposits occuras a result of break down of protective film. This degradationmechanism has been observed in one of the remove tubeanalyzed for residual life assessment. Pitting has been noticedunderneath of deposits. Tubes fouling in the form of muddeposition, foreign objects are some of the major contributorsto pitting corrosion of Aluminum Brass tube ID surfaces.Cleaning with high–velocity water jets may help to keep thecondenser tubes clean and fouling can be well controlled.This degradation mechanism has been well understood andaddressed at TAPS by scheduling tubes cleaning at regularintervals of unit operation.

Tube cleaning is done after reducing the power up to60MWe, and isolating each half of condenser water box oneby one. It is also ensured that debris is removed completelyfrom the tube sheets at inlet and tube inserts are in place asper the design intent. Visual inspection is essential forensuring satisfactory removal of debris from the tube sheet.The failure analysis of one of tube indicated this aspectpredominantly and hence cleaning methodology has beenchanged since then and through visual examination of all thetubes is being followed. The methodology has improved thetube leak detection in hydrostatic testing in four ways: (1)Proper tube cleaning with high pressure water jet followedby pneumatic ensured positive removal of mud/debris fromeach tube (2) Cleaning also removes the fine saw dust whichis used for on-lining plugging of very minute seepage leaksfrom tubes (3) it allows the leakage path opened fro leak

Fig. 5 : Tube ID surface showing Erosion and fine pits at thedisrupted oxide film location

Fig. 4 : One of the failed tubes shows disruption of oxide film

Fig. 7 : Tube ID surface showing axially grooved regioncontaining perforation.

Fig. 6 : Tube ID surface showing axial grooves formed due tomovement of foreign objects.

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detection during surface condenser hydrostatic testing and(4) Positive identification & subsequent leaky tubes pluggingwould facilitate trouble-free unit’s operation as well aseliminates chloride intrusion in the primary system, which aprime contributing factor for SCC.

3.2.4 Fretting/Vibration Induced failure mechanism [2,3]

Aluminum Brass tubes are susceptible to vibrationinduced damage. This may occur when tube are excited bythe exhaust steam flow from LP turbine to vibrations. Thisphenomenon can only be seen on Outer surface of the tubebelow the tube Support Plates (TSPs). The removed tubeswhich were analyzed did not show this type of degradationmechanism. At TAPS surface condenser shell side inspectionis possible during refueling outages (once in 18 months) orany other planned outages.

The Surface Condenser in discussion is in service forthe past 4-decades hence ageing effects of material shouldalso be taken into consideration while assessing thecondition of tubes for continued service. In view of this,Eddy Current Testing of the tubes has been taken up in aphased manner for condition assessment. So far nodegradation of the examined tubes was observed due tovibration induced failure mechanism.

3.2.5 Uniform Corrosion [2,3]

Aluminum Brass tubes are susceptible to uniformcorrosion. The tube removed for failure analysis andcondition assessment was thickness measured by ultrasonictesting and no considerable reduction in thickness noticed.Hence, this type of degradation mechanism has not beenexperienced by TAPS, however localized pitting results inperforations initiated from tube ID were observed at manylocations.

Fig. 8 : Deposits at tube ID at 20X magnification

Fig. 9 : Internal shallow Pittings observed under the deposits20X magnification

Fig. 10: Plugged tube is beingremoved for failureanalysis

Fig. 11: External surfaceof tube at TubeSupport Plate-noreduction in Thk.

3.2.6 Mechanical damage due to external factors [2,3]

Surface Condenser tubes are supported by 11 TSPs andthese in turn take support of condenser shell with variousinternal structurals. Shell side of condenser has manycomponents such as baffles, internal piping/partition platesetc., which are provided to meet the functional requirementsof the system. Total 71 penetrations exist in each condensert facilitate various systems requirements. One of such criticalsystems is Primary Feed Pumps (BFPs) minimum recirculation

Fig. 12: Perforations observed within the erosion area of tubeInner Surface-initiated from ID

Fig. 13: Perforation as seen from OD surface

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lines laid below the tube bundle with supports from hot welltray. The failure of supports leads to tube puncture due tomechanical damage. This problem was reviewed & analyzedby in-house expertise and design modifications have beenimplemented to overcome such failure mechanism.

A detailed inspection was essential to ascertain thecause of failure and its effect on adjacent tubes. All thetubes can not be assessed for extent of damage due toexternal impingement attack, Eddy Current Testing wasadopted to identify & confirm the healthiness of affecte5dtubes.

4. Non-Destructive Testing (NDE) Methods forDetection, Monitoring & ConditionAssessment [2]

The performance of surface condenser is monitored whilein operation in terms of heat duty, various performanceparameters like LMTD, ÄT, ÄP, vacuum and also thecondensate water quality in terms of conductivity etc., anysea water in-leakages will rise the conductivity which is aconcern, as most of the primary system piping is made ofAustenitic Stainless Steel which is susceptible to StressCorrosion Cracking (SCC). Performance of the surfaceCondenser depends on may influencing parameters out ofwhich tube fouling affects not only the heat transfer rate butalso contributes to tube failure rate. Therefore, it is necessaryto monitor the above performance parameters and scheduletubes cleaning & inspection. Inspection methodologies areselected such that positive identification of leaky areas

Inspection & testing plays a key role during shut down indetection & identification of leaky tubes.

4.1 ON-Line detection (Vacuum) Method [2]

Tube leaks are detected by off-line as well as on-line, incase of on-line one of the condenser water box will be isolatedafter reducing the power to lower value and tube leak checksare done with either (a) manometer (b) Cellophane paper(18µm/27-28gm/m2) and/or (c) soaping (soap suds) methods.During this check the tube side in under full vacuum and itsdetection sensitivity better. However, due to field constraints(water box area temperature, 100% RH and poor ventilation)it would be difficult to achieve the desired results. Moreoverit would not be possible check all the tubes for positive tubeleak identification, as 8167 tubes are to be checked aftertemporarily plugging the outlet end of each tube under test.It is a time consuming job and hence most vulnerable areassuch as (a) tubes adjacent to the earlier failure zones and (b)tube in front of impingement baffles are covered & checkedspecifically. The high humidity and high radiation levelspresent in the vicinity due to general contamination restricton-load tube plugging operations to the minimum.

Fine saw dust addition at inlet to Circulating Waterpumps can tolerated for tiding over building up ofconductivity due to minor leaks in condenser tubes. This willenable for proper scheduling of tube leak checks “off-line”.Saw dust is added with a CAUTION to prevent entry of largewooden pieces etc., which might lodge in tubes, causeserosion damage to tubes.

4.2 OFF-Line Methods [2]

Surface condenser & its associated components conditionassessment is done using various NDE methods. Theinspection methods used are (a) Visual Testing (VT-1) (Direct& Remote visual techniques) (b) Liquid penetrant Testing ofWeld joints (c) Ultrasonic Testing (UT) for thickness gauging(d) Eddy Current Testing (ECT) and (e) Hydrostatic Testing(VT-2). Specific Codes are applicable to detect & applicationof these NDE methods for surface condensers are not fullyavailable. These inspection methods/techniques have beenestablished by TAPS from four-decades of Operating/Feedback experience (OE). Tube leak checking procedureshave been established and enhanced based on OE. Aftertubes cleaning, shell side is filled with demineralized waterup to 30’-0" static head, ensures sufficient hydrostaticpressure for positive leak identification.

a) Visual Testing (VT-1& VT-3):

This method is very effective in detecting variousdegradations associated with the surface condensercomponents. In Visual Examination, both direct & remotevisual examination techniques are utilized to assess thecomponent’s integrity. The components covered in VT are(a) shell side internals (b) water box internals surfaces (linedwith neoprene rubber) (c) waste plates/sacrificial anodes.The condition of inlet tube ends are assessed by remotevisual examination method, using Fibroscopes. This is tomonitor tube inlet conditions, vulnerable to inlet-end erosion

Fig. 14: Tube punctured due to BFP minimum recirculation lineend plate.

Fig. 15: Tube damage due to external structural member

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as explained above. This degradation mechanism is can alsobe detected by ECT & confirmed by VT. Conditionassessment of waste plates is also required as it protects thetube sheet during unit’s operation.

Integrity assessment of 42 anode plates located in eachcondenser water boxes ensures protection of tube sheets &tubes. Physical verification of sacrificial anode plates iscarried out & documented to preclude possibility of leavinganode plate inside the water box inadvertently, whichdamages the tube sheet & water box internals while unit isin operation. Rubber lining VT & holiday testing ensures theintegrity and protection of water boxes against corrosion.

b) Liquid Penetrant Testing (LPT):

By design all the tubes are flared, light rolled, seal weldedand expanded at both at inlet & outlet end. During tube leakchecking if the seepage noticed at the seal weld in VT; needto be confirmed by LPT also to assess the extent ofdegradation and establishing the leak repair procedure.

c) Ultrasonic Thickness Gauging (UTG)

Surface Condenser shell, water boxes and inlet/outletpiping is made of carbon steel material. Health assessment ofthese components is felt necessary for continued service.Thickness gauging of water boxes was done, assessed,evaluated & documented.

d) Eddy Current Testing (ECT)

Tarapur Condensers have been in operation for the past40years and hence assessment of tubes for continued serviceis essential. Therefore, tubes are subjected to periodic ECTfor estimating the extent of degradation. Results are analyzedby the expertise and compared with the metallurgical studiesconducted on failed tube samples. Tube plugging criteria hasbeen established on these parameters.

e) Hydrostatic Testing (VT-2)-Condenser tubes

Prior to conducting this test, tube sheets dryness isachieved either by air flushing and/or by heating the tubesheets by high energy flood lights. Tubes integrity isassessed by filling the condenser shell by demineralizedwater step-by-step so that sufficient time is allowed toseepage water to come out of the leak path. Qualifiedinspection personnel in VT are employed to perform thisactivity. The leaky tubes identified in VT are plugged as perthe established procedures.

f) System Leakage tests(VT-2)System leakage test is necessary to ensure the integrity

of surface condenser pressure boundary which is opened tofacilitate the above inspection & testing. This also ensuresthat performance of components after the maintenance worksperformed during the shut down.

5. Mitigation of Degradation Mechanisms-ShortTerm & Long term and Futur e Action Plans toAddr ess Ageing Related Issues

The above indicated degradation mechanisms have beendetected using various inspections & testing methodologiesestablished from time to time. The mitigative measuresestablished and implemented effectively to limit the tubefailure rate considerably. Remedial measures were taken tomitigate condenser tube failures by (a) installation of Plasticinserts at tube inlet end in 1971 (b) tubes cleaning by rubberplugs during each refueling outage (c) debris removal andtube cleaning by water-jet during short shut downs (d) De-silting of intake bays and canal to limit the silt carry overalong with the cooling water flow and (e) regular dosing ofcooling water by “Chlorine”.

As both the condensers are in service for 40-years,hence a comprehensive health assessment has beenperformed with in-house inspection/engineering expertise. Asa long-term measure, replacement of tubes with new one hasbeen considered and a thorough review regarding selectionof material was done. The material selection should eliminatethe above-mentioned degradation mechanisms. Therefore,

Fig. 16: Arrangement of sacrificial anode plates in each waterbox

Fig. 17: Sacrificial anode plates lay out in each water box

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replacement with corrosion resistant material either (a)Austenitic stainless steel or (b) Titanium for sea water coolingapplications has been studied.

Titanium is considered to be the compatible tube materialfor sea water applications considering the extended operatinglife of the units. Titanium material has less heat transfer co-efficient and hence lower wall thickness tubes are to beselected. Reduced wall thickness results in overall reductionof total tube weight and is significant effect on stresses onthe turbine exhaust XJ & excessive strain on condensersupports [4]. This requires less spacing between tubesupport plates to avoid tube vibration related problems. Highyield strength of titanium material requires compatible tubesheet material. Tube sheet holes will require grooving tofacilitate tube rolling. In view of the practical difficulties, re-tubing with the same material has been done as it proved tobe compatible with the system for 4-decades & other designparameters and successfully implemented in unit no.1, in theyear 2008-2009. The performance of condenser after re-tubinghas improved as complete heat transfer area is available totake up the heat load.

6. Conclusion

Tarapur nuclear surface condensers have givensatisfactory service of more than three decades. Due to ageingand influence of various degradation mechanisms,replacement of tubes has been considered to be the onlyoption available to improve the condenser performance. Theoperating experience feedback obtained during this period

has been utilized to improve & enhance various inspectionmethodologies. The procedures established have beenimplemented and followed scrupulously to mitigate thevarious degradation mechanisms effectively.

Acknowledgements

The author is thankful to TAPS-1&2/NPCIL managementin giving this opportunity to publish this technical paper inNDE 2009. The timely support provided by Post IrradiationExamination Division (PIED)/BARC in failure analysis isappreciable Also we are also thankful to Technical Committeeof NDE -2009 in accepting the above technical paper in“National seminar & Exhibition on Non-DestructiveEvaluation-NDE 2009”.

References

1. Ingersoll-Rand, “Instructions for Maintenance and operation ofsurface condenser and auxiliary equipments”, 11 Broadway,New York, Bechtel Corp. Job No.4267; Bechtel Corp. Spec.No.S-4267-M3 (1963)[TAPS/MANUAL/655].

2. K.Nanjundesawaran,”Know Your Condenser”,TAPS/REPORT/615 Rev.01 (15th February 1984).

3. Albert Bursik & Hans-Gunter Seipp, “Condenser tube failures inwater-cooled condensers with copper-based alloys” PPChem,2007 9(9).

4. EPRI, NP-2371, “Condenser Retubing Criteria Manual”, Stone& Webster Engineering Corporation, Boston, Final Report (May1982) [TAPS/ REPORT/ 3165].