Mitigation of degradation of high energy secondary cycle piping due to FAC and life management in Indian

NPP’s

Thomas Mathew MoolayilNuclear Power Corporation of India Ltd

Second International Symposium on Nuclear Power Plant Life Managementfrom 15-18th October, 2007 at Shanghai China

Introduction to hazards of Flow Assisted Corrosion (FAC)• Wall thinning in carbon steel piping due to FAC had

resulted in rupture of both single phase and two phase high energy systems piping of secondary cycle in Nuclear Power Plants.

• One of the major risks associated with FAC is that it may result in abrupt rupture of the piping causing serious safety concern to plant equipment and personnel.

• Ensuring safe, reliable operation of Secondary Cycle System in Nuclear Power Plant (NPP) is very important not only from power generation point of view but also from the industrial safety concerns.

Few FAC incidents� Surry Power Station had an 18 inch elbow in the feed

water pump suction line rupture in 1986. The cause of the accident was determined to be FAC. There were 4 fatalities, 4 major injuries, and the plant had to be shut down until the pipe could be replaced.

� The most recent major accident related to FAC occurred at Mihama Power station unit-3 Japan On August 9, 2004. The accident occurred in the condensate pipe and rupture led to the injury of six people and the death of five.

� At Kakrapar Atomic Power Station (KAPS-2) in Gujarat, India, a 10% feed line to steam generator ruptured on 9th Feb, 2006. No injuries or fatalities were there.

FAC• FAC in a carbon steel pipe system is characterized by the

simultaneous dissolution of iron from the iron oxide –fluid interface and formation of an iron oxide film at the oxide metal interface

• Flow provides a vital role in providing a sink of dissolution. Factors Influencing FAC• Material susceptibility• Phase of steam• Piping layout and resulting local flow conditions and

turbulence• Velocity• System temperature• Operating conditions• Water chemistry and pH

FAC mechanism

• Hydrodynamic variables Hydrodynamic variables Hydrodynamic variables Hydrodynamic variables –––– Fluid velocity, pipe Fluid velocity, pipe Fluid velocity, pipe Fluid velocity, pipe configuration, flow disruptions (local turbulence configuration, flow disruptions (local turbulence configuration, flow disruptions (local turbulence configuration, flow disruptions (local turbulence enhances FAC) enhances FAC) enhances FAC) enhances FAC) –––– Design considerationsDesign considerationsDesign considerationsDesign considerations• Metallurgical variables Metallurgical variables Metallurgical variables Metallurgical variables –––– Presence of Cr, Mo, Presence of Cr, Mo, Presence of Cr, Mo, Presence of Cr, Mo, Cu enhances FAC resistance (Increase in Cr Cu enhances FAC resistance (Increase in Cr Cu enhances FAC resistance (Increase in Cr Cu enhances FAC resistance (Increase in Cr content reduces FAC to a great extent).content reduces FAC to a great extent).content reduces FAC to a great extent).content reduces FAC to a great extent).• Environmental variables Environmental variables Environmental variables Environmental variables –––– Temperature (FAC Temperature (FAC Temperature (FAC Temperature (FAC maximum at ~150maximum at ~150maximum at ~150maximum at ~150OOOOC), pH, dissolved oxygen C), pH, dissolved oxygen C), pH, dissolved oxygen C), pH, dissolved oxygen (Higher oxygen lower FAC(Higher oxygen lower FAC(Higher oxygen lower FAC(Higher oxygen lower FAC....

FAC is observed when all these variables co-exist in a system

FAC potential high energy systems of secondary cycle

High-energy system --Either maximum operating pressure ≥ 19.3 Kg/cm2 or maximum operating temperature ≥ 93.3°C.

� Boiler Blow Down System� Feed Water System� Condensate System� Aux. Feed Water System� Extraction System� Turbine Drain System

� Steam Drain System� Separator Drain System� Re-heater Drain System� Heater Drain System� Heater & Miscellaneous

Vent System

Rupture of 10 % feed water pipe KAPS-2(220MWe) PHWR (India)On 9th February 2006, in KAPS-2 10 % feed line ruptured at the immediate down stream of flow measuring orifice.

• Process fluid --- Feed water (liquid)• Operating Temp. --- 171 °C • Design Pressure --- 72 kg/cm²• Flow and Velocity --- 35m3 /hr and 2.33 m/sec• Material --- Carbon Steel SA 106 Gr.B• Size and Thickness --- 80 NB and 7.62 mm

Details about ruptured portion of pipe at KAPS-2

• Dissolved Oxygen --- < 5 ppb• pH (feed water) --- 8.8 to 9.5 (maintained

average 9.2)

• Minimum measured thickness observed at the immediate upstream and down stream of the rupture locations were 1.46 mm and 1.63 mm respectively.

• Minimum thickness required for continuous operation was 2.89 mm.

• Failure occurred after 10.11 hot years of operation.

• Thickness at fracture reduced to 0.4 mm from original thickness of 7.11 mm

Arrangement of ruptured 10 % feed water line at KAPS-2

Flow scheme of ruptured 10% feed water line to S.G at KAPS-2

HP HTR 6 OUTLET

TO SG 3

SG 4

90% FEED LINE

350 NB250 NB 250 NB TO SG 1 & SG 2200 NB

80 NB

10% FEED LINE

FLOW

FROM RDP DISCHARGE

50 NB

150 NB 100 NB

ABFP DIRECTPATH

FIRE WATER BACKUP65 NB

40 NB

80 NBBreak

FEED WATER LINE TO S. G. (KAPS)

ELEMENTS

Ruptured 10 % feed line Kaps-2

Analysis of KAPS-2 pipeline rupture

0.0030.08 maxVanadium----0.40 maxNickel

0.0010.15 maxMolybdenum

0.0090.40 maxCopper0.0320.40 maxChromium

Unspecified elements0.330.10 minSilicon0.0080.035 maxSulfur

0.0190.035 maxPhosphorus

0.610.29 – 1.06Manganese0.220.30 maxCarbon

Analysed(Wt %)

Specified for A106 Grade B

(Wt %)Element

No difference in the microstructure between the failed region and the unaffected region--Typical Ferrite-Pearlite microstructure

Fracture surface showing clear ductile failure with no evidence Fracture surface showing clear ductile failure with no evidence Fracture surface showing clear ductile failure with no evidence Fracture surface showing clear ductile failure with no evidence of cleavageof cleavageof cleavageof cleavage

Fracture Fracture Fracture Fracture surfacesurfacesurfacesurface

Finer pattern near the Finer pattern near the Finer pattern near the Finer pattern near the fracture surfacefracture surfacefracture surfacefracture surface

Coarser pattern far away from the Coarser pattern far away from the Coarser pattern far away from the Coarser pattern far away from the fracture surfacefracture surfacefracture surfacefracture surface

1 mm 1 mm

ID surface near the fracture having clear horse shoe pitting ID surface near the fracture having clear horse shoe pitting ID surface near the fracture having clear horse shoe pitting ID surface near the fracture having clear horse shoe pitting

Orange peel appearance / Scalloping:Typical features of Typical features of Typical features of Typical features of single phase Flow accelerated corrosionsingle phase Flow accelerated corrosionsingle phase Flow accelerated corrosionsingle phase Flow accelerated corrosion

Schematic arrangement of failed ABFP discharge line (KAPS-2)

ABFP discharge line check valve -KAPS-2

41 mm

41 mm

Remedial measures taken after KAPS-2 10 % feed line rupture

• After KAPS-2 pipeline rupture some more additional components (around 380) pertaining to various high energy systems of secondary cycle of KAPS-1 &2 were inspected.

• Scope of examination enhanced to include all potential high energy system piping component to generate base line data and to assess the overall healthiness.

• Guidelines issued for UT examination, repair / replacement and procedure for examination, weld overlay, grid size criteria for inspection etc.

• Study was conducted to review candidate materials for secondary cycle piping components and select the most suitable material to resist FAC.

UT Examination results of two PHWR (220 MWe) stations

20685No.of components where thickness reduction > 12.5%

662208No.of components where measured min.thk. < N.W.T

341161No.of components where measured min.thk. > N.W.T

1003369Total components examined

%QuantumDescription %QuantumDescription

12401No.of components where thickness reduction > 12.5%

541799No.of components where measured min.thk. < N.W.T

461558No.of components where measured min.thk. > N.W.T

1003357Total components examined

KAPS-2 examination resultsHot operating years 10.1 years

KGS-2 examination resultsHot operating years 5.5 years

Component with life < 2 yrs = 29Component with life 2 -4 yrs = 13

Results of inspection at different stations in India

363886RAPS-4/PHWR/220/6153206RAPS-3/PHWR/220/6293357KGS-2/PHWR/220/5.5233524KGS-1/PHWR/220/5.9333369KAPS-2/PHWR/220/10.1203240KAPS-1/PHWR/220/11.2281500MAPS-2/PHWR/220/1911591MAPS-1/PHWR/220/11.201164RAPS-2/PHWR/200/17.52445TAPS-1/BWR/160/23

Replaced/repaired

Components inspected

Unit / Reactor / MWe/ hot operating years

U.T thickness gauging PSI data for secondary cycle piping of a 220

MWe unit

0500100015002000250030003500

Totalcomponentsinspected

No. ofcomponents

where measuredmin thk>nom.thk

No. ofcomponents

where measuredmin thk<nom.thk

No. ofcomponents

where measuredmin thk<87.5%of nom.thk

Kaiga-3 U.T gauging data analysis

Most commonly affected high energy pipe lines / locations due to FAC

• 10 % feed water line down stream of control valves• 90 % feed water line down stream of control valve• Down Steam of control valve of live steam re-heater drain,

bled steam re-heater drain, separator drain in the normal path and alternate path

• Extraction steam line to MSR• Extraction steam line to top Heater before S.G.• Steam drain system down stream of restriction orifices

(ROs)• Heater / MSR (moisture separator re-heater) vents down

stream of restriction orifices (ROs)• Heater drain system down stream of Control Valves (CV’s)• Boiler blow down system down stream of control valves

near boiler blow down tank

Other findings of examination• Degradation is noticed in many of the secondary cycle

systems and on components such as elbows, reducers, pipe etc.

• Thickness reduction is noticed in boiler blow down system, separator drain system, re-heater drain system etc where the bulk velocity is lower than normal recommended allowed velocities.

• Degradation is noticed in the Secondary Cycle Components in the temperature range of 90°C to 250°C.

• Average wear rate of 150 to 200 microns/year is noticed in some of the commonly vulnerable systems / lines.

Other findings continueIt shows high corrosion at following systems/ locations which may not be due to one factor but a combination of FAC influencing factors.

� Main feed water lines near top Heater before S.G and its downstream line to S.G

� 90% feed water control valve stations� 10% feed water control valve stations� Downstream of boiler feed pump/ auxiliary boiler feed

pump discharge nozzles� Boiler Blow Down lines� Reheater drain line in alternate path to flash tanks

Temp. and velocity of various systems(for one of the PHWR station in India)

--46 °C - 250°CMSR and drain tank vents

--70 °C - 169°CHeater vents

1-2.10 m/sec52 °C - 165°CHeater drain

25-45 m/sec70 °C - 194°CExtraction steam

1-2 m/sec155 °C- 250°CMSR drain

Around 1 m/sec in liquid lines: vent line 14 m/sec

55 °C - 250°CBoiler blow down

25 – 35 m/sec250° CMain steamVelocity rangeTemp. rangeSystem

Temp. and velocity of various systems(for one of the PHWR station)

---45 °C - 250°CSteam drain

4m/sec (max) ABFP discharge nozzle

38 °C - 120°CAuxiliary feed water

2.5 – 4.5 m/sec157 °C - 171°CFeed water

Less than 3.2 m/sec

42 °C - 157°CCondensate

Velocity rangeTemp. rangeSystem

Degraded BFP discharge expander and elbow

Locations showing thinned components in blow down line near S.G

Locations showing thinning in boiler blow down line to Blow down tank-- down

stream of control valves

MSR drain line –location showing thinning

FAC prone locations of secondary cycle in Indian NPP’S

Methodology adopted in Indian NPP’s to mitigate degradation and life

management of secondary cycle systemFAC monitoring program

Initial monitoring programTo collect one timebase line data and asses the conditionOf secondary cycle components

Periodic monitoring programTo examine the mostvulnerable componentsperiodically (every 6 yearsirrespective of life)

Most vulnerable component of high energysystem inspected. Around800 plus components in each unit

All fittings, pipes d/s &u/s up todistance of 1.5 meters of flow disturbing

devices in piping, all main nozzles & branchpiping near equipments of high energysystems are inspected. Around 3000

plus components in eachunit

Basis of identification of most vulnerable components for inspection• Areas where velocity is high• Areas where local flow disturbances are expected• Areas where wetness is high• Areas where two phase flow is expected• Locations based on piping configurations and where

layout is congested• Areas having industrial failure history on other NPP’S

Basis for selection of components for enhanced inspection

Heater drainHeater vents

MSR drain

MSR and drain tank vents

Extraction steam

Auxiliary steamBoiler blow downMain steam

Temp ≥ 93.3° Cor Pr. ≥ 19.3

Kg/cm 2

Temp <93.3° C or Pr. < 19.3

Kg/cm 2System

Basis for selection of components for enhanced inspection

Feed water

Steam drain Auxiliary feed water

Condensate

Temp ≥ 93.3°C or Pr. ≥ 19.3 Kg/cm 2

Temp <93.3°C or Pr. < 19.3 Kg/cm 2

System

All components such as elbows, reducers, tees, bends, pipe and d/s of control valve , FE, RO’s, manual valves are selected for inspectionComponents for inspection identified by sations

Components identified for inspection

Basis for assessment of balance lifeEstablish corrosion/erosion rate as mentioned below

(TREF – TP) *1000Corrosion Rate (CR)= ------------------------- microns/yr

NEstimate the balance life (residual life) using following equation

(TP – TMIN) *1000Balance Life (BL) = ------------------------ years

CRN = Number of hot operating yearsTREF = Nominal wall thickness or previous thicknessTP = Prevailing minimum wall thickness, mm TMIN = Required minimum wall thickness as per

design code, mm

Guidelines and procedures issued to all stations and projects as part of FAC management

program are given below.• To provide step by step method to be followed

regarding initial examination, first examination and successive examination of components selected for inspection.

• Criteria for replacement/ repair of degraded components

• Criteria for grid size marking on the components to be inspected

• procedure for U.T thickness measurement • procedure for weld deposit• Steps to identify the suitable material for replacing the

degraded components and generating its base line data

Activity flow chart for replacement/ repair/successive examination

Guidelines for repair/replacement and future inspection

• Once an assessment is carried out basic philosophy is to carry out replacement/ repair/ further assessment before 50 % of the balance life is over.

• Accordingly for the components, having balance life up to 4 years, require replacement/repair/further assessment before 50% of balance life is over.

• For the components, having balance life more than 4 years, further assessment will be carried out at an appropriate time depending on balance life (during a BSD).

Grid marking on pipe fittings for thickness measurement

Guidelines of criteria for grid sizingOf identified components for UT Examination are prepared referring To code case ASME-N-480

Thickness measurement is Thickness measurement is performed at about .4 million performed at about .4 million points in high energy system points in high energy system piping of secondary cycle of each piping of secondary cycle of each stationstation

What literature says about chemistry parameters

QuestionWhether at any reactor oxygen injection practice is followed ?

Indian NPP’S water chemistry aspects- In feed water system pH recommended is 8.8 to 9.5 and dissolved oxygen is less than 5ppb.

FAC inspection plan for secondary cycle equipments/valves

Feed water heaters• Inspection up to a distance of OD of nozzle+150 mm at all

sides on shell from nozzle OD by U.T thickness gauging.• Visual inspection of channel water box side through

manhole.Flash tanks and boiler blow down tanks• Inspection up to a distance of 500 mm above and below

inlet nozzle all around by U.T thickness gauging• Visual examination through manholeDeaerator and other drain tanks/valves• Visual examination through manhole/bonnetNote: After visual examination if any damage found, then

detailed examination by U.T thickness gauging

FAC management action plan for operating stations

� To implement UT thickness gauging monitoring program, including that for base line data, for high-energy system piping of secondary cycle for all operating stations.

� To replace progressively the existing carbon steel pipe and fittings of the lines / portion of piping of high energy systems which are prone for FAC with low alloy steel SA-335 Gr.P22 (for pipes) and SA-234 Gr.WP22/SA-182 Gr.F22 (for fittings) being more FAC-resistant material and with one schedule higher than required.

� To follow water chemistry as per recommendations of advisory committee on steam and water chemistry.

FAC management action plan for projects under construction

• Action has been taken to generate base line data of all installed piping components pertaining to high energy system piping of secondary cycle by UT thickness measurement before start up of plant.

• To implement UT thickness gauging monitoring program for high-energy system piping of secondary cycle.

� To replace progressively the existing carbon steel pipe and fittings of the lines / portion of piping of high energy systems which are prone for FAC with low alloy steel SA-335 Gr.P22 (for pipes) and SA-234 Gr.WP22/SA-182 Gr.F22 (for fittings) being more FAC-resistant material and with one schedule higherthan required.

• To follow water chemistry as per recommendations of advisory committee on steam and water chemistry.

FAC mitigation plan for future plants

• To use better FAC resistant material instead of carbon steel in FAC prone lines / portion of piping of high energy systems.

• To provide higher corrosion allowance for pipes and pipe fitting in FAC prone lines / portion of piping of high energy systems.

• Proper velocity assumptions will be considered wherever felt necessary while sizing the piping system for future projects.

• To develop piping layout considering all required thoughts to minimize flow disturbances.

• To implement UT thickness gauging periodic monitoring program of high energy system piping

ConclusionFAC management program of secondary cycle high energy piping and piping components is being achieved in Indian Nuclear Power Plants through:-

� Continuous examination and monitoring of components in all stations, its residual life analysis, following uniform guide lines for repair / replacement and performing successive examinations.

� Pipes and fittings at FAC prone lines/ portion of piping of high energy systems for operating stations will be replaced progressively with a better FAC resistant material (i.e. low alloy steel) and with one schedule higher.

� For projects under construction base line data of the large number of installed components is also being generated for future reference.

� Water chemistry in various lines of secondary cycle is also maintained as per recommendations of advisory committee on steam and water chemistry.

� For future projects efforts are being initiated to minimize the effect of FAC influencing factors through improved pipe layout, better FAC resistant materials, higher corrosion allowance, and proper velocity assumptions during line sizing etc.

Conclusion