2010

BARC/2010/E/005B

AR

C/2010/E

/005

ANALYSIS OF AHWR DOWNCOMER PIPING SUPPORTED ONELASTO-PLASTIC DAMPERS AND SUBJECTED

TO NORMAL AND EARTHQUAKE LOADSby

P.N. Dubey, G.R. Reddy, K.K. Vaze and A.K. GhoshReactor Safety Division

BARC/2010/E/005BA

RC/2

010/

E/00

5

GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION

BHABHA ATOMIC RESEARCH CENTREMUMBAI, INDIA

2010

ANALYSIS OF AHWR DOWNCOMER PIPING SUPPORTED ONELASTO-PLASTIC DAMPERS AND SUBJECTED

TO NORMAL AND EARTHQUAKE LOADSby

P.N. Dubey, G.R. Reddy, K.K. Vaze and A.K. GhoshReactor Safety Division

BARC/2010/E/005

BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT(as per IS : 9400 - 1980)

01 Security classification : Unclassified

02 Distribution : External

03 Report status : New

04 Series : BARC External

05 Report type : Technical Report

06 Report No. : BARC/2010/E/005

07 Part No. or Volume No. :

08 Contract No. :

10 Title and subtitle : Analysis of AHWR downcomer piping supported on elastoplastic dampers and subjected to normal and earthquake loadings

11 Collation : 49 p., 21 tabs., 30 figs.

13 Project No. :

20 Personal author(s) : P.N. Dubey; G.R. Reddy; K.K. Vaze; A.K. Ghosh

21 Affiliation of author(s) : Reactor Safety Division, Bhabha Atomic Research Centre, Mumbai

22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai-400 085

23 Originating unit : Reactor Safety Division,BARC, Mumbai

24 Sponsor(s) Name : Department of Atomic Energy

Type : Government

Contd...

BARC/2010/E/005

30 Date of submission : April 2010

31 Publication/Issue date : May 2010

40 Publisher/Distributor : Head, Scientific Information Resource Division,Bhabha Atomic Research Centre, Mumbai

42 Form of distribution : Hard Copy

50 Language of text : English

51 Language of summary : English, Hindi

52 No. of references : 22 refs.

53 Gives data on :

60

70 Keywords/Descriptors: HWLWR TYPE REACTORS; SEISMIC EFFECTS; IN-SERVICE INSPECTION;THERMAL STRESSES; FINITE ELEMENT METHOD; DESIGN; STAINLESS STEEL- 316L; THERMALEXPANSION; REACTOR CHARGING MACHINES

71 INIS Subject Category: S21

99 Supplementary elements :

Abstract : Three layouts have been considered for AHWR downcomer for codal qualificationin order to ensure its structural integrity under normal and occasional loads. In addition to codalqualification a good piping layout should have less number of bends and weld joints in order toreduce the in-service inspection cost. Less number of bends will reduce the pressure drop innatural circulation and lesser number of weld joints will reduce the total time of in-service inspectionthat finally reduces the radiation dose to the workers. Conventional seismic design approach ofpiping with snubbers leads to high cost, maintenance and possible locking causing undue higherthermal stress during normal operation. New seismic supports in the form of Elasto-Plastic Damper(EPD) are the best suited for nuclear piping because of their simple design, low cost, passivenature and ease in installation. In this report the characteristics of EPD obtained from theory, finiteelement analysis and tests have been presented and comparison has also been made among thethree. Analysis method and code qualification of AHWR downcomer piping considering the loadingsdue to normal operating and occasional loads such as earthquake have been discussed in detail.This report also explains the concept of single support and multi-support response spectrum analysismethods. The results obtained by using both types of supports i.e. conventional and EPD supportshave been compared and use of EPD supports in AHWR downcomer pipe is recommended.

I

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II

Abstract

Three Layouts have been considered for AHWR downcomer for codal qualification in order to ensure its structural integrity under normal and occasional loads. In addition to codal qualification a good piping layout should have less number of bends and weld joints in order to reduce the in-service inspection cost. Less number of bends will reduce the pressure drop in natural circulation and lesser number of weld joints will reduce the total time of in-service inspection that finally reduces the radiation dose to the workers. Conventional seismic design approach of piping with snubbers leads to high cost, maintenance and possible locking causing undue higher thermal stress during normal operation. New seismic supports in the form of Elasto-Plastic Damper (EPD) are the best suited for nuclear piping because of their simple design, low cost, passive nature and ease in installation.

In this report the characteristics of EPD obtained from theory, finite element analysis and tests have been presented and comparison has also been made among the three. Analysis method and code qualification of AHWR downcomer piping considering the loadings due to normal operating and occasional loads such as earthquake have been discussed in detail. This report also explains the concept of single support and multi-support response spectrum analysis methods. The results obtained by using both types of supports i.e. conventional and EPD supports have been compared and use of EPD supports in AHWR downcomer pipe is recommended�

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III

Acknowledgement

Authors acknowledge their sincere gratitude to scientists of Mechanical Metallurgy

Section, Materials Group esp. Dr. J K Chackrvarthy, Head, Mechanical Metallurgy

Section, , Dr. C Gupta and Shri Pyara Singh for their devoted and arduous efforts during

the testing of elasto-plastic dampers, because of which it is possible to present this report

in complete form. We also convey our sincere thanks to the engineers of Centre for Design

and Manufacturing (CDM) esp. Shri Sandip Guha, and CDM staff for their help in making

the fixtures. We also thank RED workshop people for manufacturing the X-Plates and

fixtures.

IV

List of contents

S. No. Title Page no.

Abstract

1. Introduction 1

2. Classification of nuclear components 2

3. Design loading and service levels for NPP components 3

3.1 Piping Qualification as Per ASME Section-III, Subsection-NB 3

3.2 Primary stress intensity check 3

3.3 Primary Plus Secondary Stress Intensity Range Check 4

3.4 Simplified Elastic Plastic Discontinuity Analysis 4

3.5 Peak Stress Intensity Range and Fatigue Analysis 5

3.6 Faulted Condition/ Level-D 5

4. Piping supports 6

4.1 Conventional supports 6 4.2 Energy absorbers 8 4.2.1 Lead Extrusion Damper 8 4.2.2 Visco-elastic Energy Absorbers 9 4.2.3 Friction supports 9 4.2.4 Elasto-plastic energy absorbers 9

5. Multi-support excitation analysis of Layout-2 10

5.1 Methods of response combination 12 5.2 Maximum stresses in Layout-2 13

6. Design and characterization of EPD 13

6.1 Force Displacement Characteristics of X-Plate 14 6.2 Analytical formulations 14

7. Testing of tensile test specimens and 6.0 mm X-plate 15

7.1 Tensile test of SS316L material 15

7.2 Testing of 6 mm EPD plate 15

8. Theoretical cyclic characteristic of 6.0 mm X-plate 18

9. Non linear analysis of 6 mm X- plate EPD 19

10. Non-linear time history analysis of Layout-3 20

V

10.1 Nonlinear time history analysis results of Layout-3 23

10.2 Stress combination and codal qualification of Layout-3 26

10.3 Stresses due to safe shutdown earthquake (SSE) 28

11. Discussion and conclusions 32

Reference 33

Appendix-I 35

VI

List of figures

S. No. Caption Page no.

1. Schematic of Advanced Heavy Water Reactor 1 2. Proposed layouts for AHWR downcomer 2 3. Spring support 7 4. Hanger support 7 5. Pipe guide supports 7 6. Sliding supports 8 7. Lead extrusion damper 8 8. Visco-elastic dampers 9 9. Friction supports 9 10. Elasto-plastic damper 10 11. Multilevel supports 12 12. Characteristics of X-shaped plate 14 13. Rectangular tensile test specimen 15 14. Stress-strain curve for SS316L 16 15. Schematic of test setup 16 16. Single X-plate EPD at 10 mm tip displacement & 0.25Hz frequency 17 17. Single X-plate EPD at 20 mm tip displacement at 0.5Hz frequency 17 18. Single X-plate EPD at 30 mm tip displacement at 0.25Hz frequency 18 19. Force deflection characteristic of 6.0 mm X-plate 21 20. Theoretical hysteretic characteristic of 6.0 mm EPD plate 20 21. (a) 3-D FE model of EPD plate (b) Dimensions of EPD plate 20 22. Hysteresis curve generated from analysis 21 23(a) Input time history in X-direction 21 23(b) Input time history in Y-direction 21 23(c) Input time history in Z-direction 22 24. Downcomer Layout-3 with (a) 4-plate EPD (b), (c), (d) & (e) high

stiffness EPD 22

25. Variation of (a) thermal stress (b) natural frequency with number of EPD plates

22

26. Optimized layout and support positions 24 27(a). EPD tip displacement vs time curve at position-I of 1st downcomer 24 27(b). Force vs time curve at position-I of 1st downcomer 24 27(c). Hysteresis curve at position-I of 1st downcomer 25 28(a). EPD tip displacement vs time curve at position-II of 1st downcomer 25 28(b). Force vs time curve at position-II of 1st downcomer 25

VII

28(c). Hysteresis curve at position-II of 1st downcomer 25 29(a). EPD tip displacement vs time curve at position-III of 1st downcomer 25 29(b). Force vs time curve at position-III of 1st downcomer 25 29(c). Hysteresis curve at position-III of 1st downcomer 25 30(a). EPD Tip displacement vs time curve at position-III of 2nd downcomer 25 30(b). Force vs time curve at position-III of 2nd downcomer 26 30(c). Force vs time curve at position-III of 2nd downcomer 26

VIII

List of Tables

S. No. Title Page No.

1. Load combinations for different service levels 3

2. Comparison of maximum Stresses in Layout-1 & Layout-2. (Sm = 115.20 N/mm2)

13

3. Maximum displacements due to seismic load without and with single plate EPD

23

4. Frequencies of Layouts-1, 2 & 3 23

5. Maximum displacement at position-I in Fig. 26 26

6. Maximum displacement at position-II in Fig. 26 26

7. Maximum displacement at position-III in Fig. 26 27

8. Stress at downcomer-header junction (B1=. 5, B2r=1.67, B2b=1.34, C1=1.50, C2b=2.23, C2r=2.23)

27

9. Stress at first elbow from the header (C1=1.083, C2=1.729, B1= 0.5, B2=1.0) 27

10. Stress at second elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.0) 27

11. Stress at third elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.00) 28

12. Stress at fourth elbow from the header (C1=1.036, C2=1.5, B1= 0.5, B2=1) 28

13. Stress at steam drum and down comer junction (C1=1, C2=1, B1=0.5, B2=1) 28

14. Stress due to SSE at downcomer-header junction (B1=. 5, B2r=1.67, B2b=1.34, C1=1.50, C2b=2.23, C2r=2.23)

28

15. Stress due to SSE at first elbow from the header (C1=1.083, C2=1.729, B1= 0.5, B2=1.0)

29

16. Stress at due to SSE second elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.0)

29

17. Stress due to SSE at third elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.0)

29

18. Stress due to SSE at fourth elbow from the header (C1=1.036, C2=1.5, B1= 0.5, B2=1)

29

19. Stress sue to SSE at steam drum and down comer junction (C1=1, C2=1, B1=0.5, B2=1)

30

20. Comparison of maximum stress for different loading conditions 30

21. Comparison of maximum stress, number of welds and total length of pipe 30

IX

Notations

α Coefficient of thermal expansion at room temperature.

ε Strain

εy Yield strain

σy Yield strength of EPD material

τ Total shear stress

ξ Damping ratio

a Height of triangular EPD plate

b Width of triangular EPD plate

h Bend parameter

m Mass

rm Mean pipe radius (D0-t)/2

t Thickness of triangular plate EPD

tn Nominal wall thickness, of product, (NB –3683)

B1 Primary stress index (0 � B1 � 0.5)

B2 Primary stress index (B2 � 1.0)

C1 Secondary stress indices for the specific product under investigation.

C2 Secondary stress indices for the specific product under investigation

C 3 Secondary stress indices for the specific product under investigation

d Deformation

dy Yield deflection

D0 Outside diameter of pipe (NB-3683) E Young’s modulus

Eso Strain energy

F Force in the triangular plate

H Rate of strain hardening

I Moment of inertia

K Stiffness

n Ramberg Osgood parameter

M Bending moment

M i Resultant moment due to a combination of design mechanical loads,

P Internal design pressure (gauge)

Py Force at yield point

R Nominal bend radius of curved pipe or elbow

X

Rm Mean run pipe radius

Sm Stress intensity

Tr Nominal wall thickness of designated run pipe

Zb Section modulus of branch pipe

Zr Section modulus of run pipe

1

ANALYSIS OF AHWR DOWNCOMER PIPING SUPPORTED ON ELASTOPLASTIC DAMPERS AND SUBJECTED TO NORMAL AND

EARTHQUAKE LOADINGS

1. INTRODUCTION

Advanced Heavy Water Reactor (AHWR) being designed in India is 920 MW(Th) pressure tube type boiling water reactor, using heavy water as moderator and light water as coolant. Natural convection cooling of the reactor core is one of the major passive safety features of the reactor. Natural circulation loop called main heat transport system (MHTS), consists of four steam drums, 452 feeders, 452 tail pipes, 16 down comers and reactor inlet header (RIH) as shown in Fig.1. The down comers take water from the steam drum to RIH. From RIH through the feeders water enters the reactor and gets heated and travels to the steam drum through tail pipes. From there steam is separated and sent to the turbine. The RIH is a circular pipe of mean radius 8.75m consisting of 24” NB, schedule 120 SS 304 LN pipe. Four downcomers running from each steam drum join to RIH in one quadrant. Three Layouts as shown in Fig. 2, have been considered for codal qualification in order to insure its structural integrity under normal and occasional loads. In addition to codal qualification a good piping layout should have less number of bends and weld joints in order to reduce the in-service inspection cost. Less number of bends will reduce the pressure drop in natural circulation and lesser number of weld joints will reduce the total time of in-service inspection that finally reduces the radiation dose to the worker.

In the beginning a simple layout, similar to layout-3 as shown in Fig. 1(b) was designed as per ASME code using snubbers as support. Snubbers allow the gradual thermal expansion and arrest the sudden motion due to earthquake. From the past experiences snubbers have proved to be very costly, need frequent and expensive maintenance, leakage problem in hydraulic snubbers and they also congest the space because of more space requirement for

1. Secondary containment

2. Primary containment

3 Gravity driven waterpool

4. Isolation condenser

5. Passive containment

isolation Duct

6. Vent pipe

7. Tail pipe Tower

8. Steam drum (123 m)

9. 100m floor

10 Fueling machine

11. Deck plate

12. Calandria with endshield

13. Downcomer (D/C)

14. Pile support

15 Advanced accumulator

16 Pre-stressing gallery

(b)

El.100 m

El. 123 m

El.95 m

(a)

RIH

Fig. 1 Schematic of Advanced Heavy Water Reactor

2

installation. Sometimes it is also observed that the mechanical snubbers lock and cause undue thermal stresses in the piping and nozzles during normal operation. Due to inherent drawbacks and high initial and maintenance cost involved with snubbers are not encouraged. Analysis and design of layout -1, as shown in Fig. (2), was performed by RSD and it is meeting the code requirements but more number of elbows, which increases the pressure drop in natural circulation and also because of crossing and away from tail pipe tower supporting is difficult. Subsequently Layout-2 was designed and analyzed and it was noticed that Layout-2 has lesser thermal stresses than Layout-1 but more number of elbows. To eliminate the drawbacks of Layouts-1& 2, Layout-3 was designed, which is a very simple with less number of elbows and less number of weld joints. In the present analysis it is aimed to have a layout with most of the required qualities numerated earlier. In this report, analysis and qualification of Layouts-2 & 3 have been performed and merits and demerits of layouts 1, 2 & 3 have been compared. Since 16 downcomers are symmetrically attached to the reactor header constituting four groups, four downcomers and one quadrant have been modeled as 3-D pipe element and analysis has been performed for pressure, dead weight, thermal and earthquake. Response spectrum analysis has been carried out for OBE with 2% damping ratio and SSE with 3% damping ratio though a 3% and 4% damping values are recommended for OBE and SSE respectively for all piping sizes by new edition ASME code. Downcomers are supported at various levels; therefore multi-support response spectrum analysis has been performed. Non linear time history analysis of Layout-3 has been done with passive energy absorbers (EPD) with steam drum level time history as base excitation. Finally the stresses obtained for dead weight, pressure, thermal and seismic loads have been combined as stated by ASME code [1] for different service levels. Under following paragraphs the ASME qualification philosophy, e.g. different loading combinations at different service levels has been explained.

2. CLASSIFICATION OF NUCLEAR COMPONENTS

AHWR down comers are the part of natural circulation loop called main heat transport system (MHTS). As MHTS is safety class- I and seismic category-1. It should suffice to AERB safety guide SG-D-1 [2]. For safety class-I systems applicable design code is ASME Sec-III, Subsection- NB [1] and system has to be designed for SSE.

Layout-1

Steam Drum

Downcomer

Header

Layout-2 Layout-3

Fig. 2: Proposed layouts for AHWR Down comer

3

3. DESIGN LOADING AND SERVICE LEVELS FOR NPP COMPONENTS

In NPPs, the safety systems are subjected to a combination of loadings, which introduce different levels of stresses arising out of different service levels. For each service levels system should be qualified to meet the stress requirement imposed by ASME code. According to the probability of occurrences there are four types of service levels described in ASME Sec-III, Div-II, subsection NCA viz. Level–A (normal), Level–B (upset), Level–C (emergency) and Level–D (faulted). Required load combinations are tabulated in Table-1, and the detailed qualification program has been explained under following subsections.

Table-1 Load combinations for different service levels

Load Case Loads Allowable Equation

Design Condition Design Pressure + Dead Weight 1.5 Sm (1)

Normal operation/ Level -A Pressure + Thermal 3.0 Sm (2)

Upset Condition /Level – B Design Pressure +Dead Weight + OBE Pressure + Thermal + OBE

1.8 Sm

3.0 Sm (1) (2)

Emergency Condition/ Level-C Design Pressure + Dead Weight +SSE 2.25 Sm (1)

Faulted Condition/ Level - D Design Pressure + Dead weight + SSE 3.0 Sm (1.b)

3.1 Piping Qualification as Per ASME Section-III, Subsection-NB

Subsection NB of the ASME boiler and pressure Vessel Code, Section- III, details the requirements pertaining to those sections of nuclear piping designated as class-I. According to above categorization advanced heavy water reactor down comer comes under the safety class-I and seismic category-1. Hence these piping have been qualified as per ASME- Sec-III [1] requirements.

The loading requiring consideration in the design of piping under subsection NB are pressure, weights (live and dead loads), impact, earthquake, vibration, and loading induce by thermal expansion and contraction. The stress requirements, which must be satisfied to meet the safety class-I code criterion, are as follows:

3.2 Primary stress intensity check

The primary stress intensity limit is satisfied when requirement of Eq. (1) [1] is met.

B 1 & B 2 = Primary Stress Indices for the specific product under investigation [1]. P = Internal Design Pressure (gauge), N/m2. D0 = Outside diameter of pipe, m (NB-3683). t = Nominal wall thickness, of product, m (NB –3683). Mi = Resultant moment due to a combination of Design Mechanical Loads, N-m. I = moment of inertia, m4.

For branch connection or Tee, the moment term of Eq. (1) will be replaced by B2b (Mb/Zb) + B2r (Mr/Zr).

K = 1.5 for level A, 1.8 for level B, and 2.25 for level C. Sm = Allowable design stress intensity value (N/m2).

0 01 2 (1)

2 i m

PD DB B M kS

t I+ ≤

4

3.3 Primary Plus Secondary Stress Intensity Range Check

This is to account a stress range as the system goes from one load set (pressure, temperature, moment, and force loading) to any other set of which follows it in time. It is the range of pressure, temperature, and moments between two load sets, which is to be used in the calculations. For each specified pair of load sets, Sn is calculated by Eq. (2) [1]:

Mi = Resultant range of moment which occurs when system goes from one service load set to another.

Ta, Tb = Range of average temperature on side a or b of gross structural discontinuity or material discontinuity.

αa,b = Coefficient of thermal expansion on side a or b of gross structural discontinuity or material discontinuity at room temperature.

Eab = Average modulus of elasticity of two sides of gross structural discontinuity or material discontinuity at room temperature.

Po = Range of service pressure. If Eq. (2) is not satisfied then for all pairs of load sets, then the component may still be qualified by using the simplified elastic-plastic discontinuity analysis, explained in sec (4.4) below; otherwise, peak stress intensity range should be calculated by Eq. (5).

3.4 Simplified Elastic Plastic Discontinuity Analysis

If Sn exceeds its limit for some pairs of load sets, simplified elastic plastic analysis may be performed if thermal stress ratchet is not present. This analysis is required only for the specific load sets that exceed the primary plus secondary stress intensity range check [1]. The following two sets of equations must be satisfied:

Where: Se = expansion stress, and Mi = Resultant range of moments due to thermal expansion and thermal anchor movements.

Mi = Resultant range of moment which occurs when system goes from one

service load set to another, excluding moments due to thermal expansion and thermal anchor movements.

3C ′ = Stress index for component under investigation. If Sn > 3Sm, the thermal stress ratchet must be evaluated and demonstrated to be satisfactory before a simplified elastic- plastic discontinuity analysis can be performed. This ratchet is function of |∆T1| range only. The following equation must be checked for thermal ratchet.

Where y′ = 3.33, 2.00, 1.20, and 0.80 for x = 0.3, 0.5, 0.7, and 0.8 respectively.

αE

CSyrangeT y

7.0|| 4

1

′≤∆

,1

20

yStPD

x ×=

0 0 01 2 3 | | 3 (2)

2 2n i ab a a b b m

P D DS C C M C E T T S

t Iα α= + + × − ≤

0 0 01 2 3 3 (4)

2 2 i ab a a b b m

P D DC C M C E | � T � T | S

t I′+ + × − ≤

02 3 (3)

2e i m

DS C M S

I= ≤

5

P = Maximum pressure for condition under consideration, C4 = 1.1 for ferritic and 1.3 for austenitic material. E = modulus of elasticity at room temperature.

3.5 Peak Stress Intensity Range and Fatigue Analysis

Ti = Internal surface temperature, To = External surface temperature, Average temperature of pipe wall,

∆T1 = Ti – To, and ∆T2 =+ (To+Ti)/2, Where: t = wall thickness, and T(x) = temperature as function of distance through wall.

For each loading condition specified peak stress value can be calculated by Eq. (4.5),

Where

ν = Poisson’s ratio of the material. |∆T1| = Absolute value of range of temperature difference between temperature of out side surface and inside surface of pipe wall, assuming moment generating equivalent linear temperature distribution. |∆T2| = Absolute value of range that portion of nonlinear thermal gradient through the wall

thickness not included in ∆T1, assuming moment generating equivalent linear temperature distribution

For each Sp, alternating stress intensity can be calculated by Eq. (6) [1]:

Where:

3.6 Faulted condition/Level-D

In year 1995, code permissible limit for service Level-D due to reversible dynamic loading typically experienced during SSE, was revised upward from 3Sm to 4.5Sm (for code equation 9, [3]).

mSZM

Bt

PDB 5.4

22

01 ≤+ (1.a)

Where However, year 2001 [4] version of code has further brought it down to 3Sm limit with revised stress indices B'2 in code equation.

(6)2

palt e

SS K=

0 0 01 1 2 2 3 1

3 3 2

1| |

2 2 2 11

| | (5)1

p i

ab a a b b

P D DS K C K C M K E T

t I ( �)

K C E | � T � T | E T

α

αυ

= + + ∆−

+ × − + ∆−

1.0 3

11.0 1 3 3

( 1)

13

n m

ne m n m

m

n m

for S S

SnK for S S mS

n m S

for S mSn

�� ≤�

� �−�= + × − < <� � �− � ��

≥�

0

1 = ( )

t

aveT T x dxt

� �−� ��

�

6

mSZM

Bt

PDB 3

2'0

1 2 ≤+ (1.b)

Where B'2 = 3/2

9.0h

4 PIPING SUPPORTS

Chemical and nuclear industrial piping systems are generally supported on hangers, rod restraints, snubbers, and friction supports etc. [4]. In normal operating condition, piping system is subjected to dead weight, pressure and thermal loads. On the other hand, during an earthquake, it will also be subjected to abnormal earthquake loads in addition to the normal loads. It is well known that the requirement of piping characteristics for the temperature and the earthquake loads are not the same. In the case of thermal load, piping should be flexible enough to allow free thermal expansion and for the earthquake loads it should be rigid enough such that it attracts lesser load. Therefore, it is a good design practice to have a balance between stiffness and flexibility. Rod restraints will increase the rigidity and reduces the flexibility in vertical direction. These are preferable if thermal expansion of the piping in the direction of the support is low. Hangers are generally called dead weight supports and will have low stiffness. However, these are simple to design and require absolutely no maintenance. Snubbers are good for both the thermal and earthquake loads but they are not preferred due to drawbacks discussed earlier. Energy absorbers such as elasto-plastic, lead extrusion, visco-elastic types etc. are the best substitute for the snubbers and serve the same purpose. In these types of supports, excitation energy is absorbed due to the hysteretic deformation and helps in reducing the response of the piping/equipment due to the earthquake loads. These devices do not require maintenance and are more reliable compared to the snubbers. Another family of the energy absorbing devices is friction supports. It also serves the same purpose like other energy absorbers. The most important requirement of the friction type of support is to have less coefficient of friction to enable free thermal expansion of the piping or equipment system. One such material is Teflon. The hysteretic behavior of the friction support due to the earthquake excitation results in absorbing the energy and hence reduction in the response of the piping or equipment.

4.1 Conventional supports

Spring supports as shown in Fig. 3, are used for piping carrying high temperature fluid/steam in order to enable free thermal expansion. These supports also allow free movement along the axis of the pipe and along the perpendicular direction of the support. Hanger supports as shown in Fig. 4, are used where there is negligible thermal movement of the pipe/equipment and in certain situations it is merely used as a dead weight support.

In these types of supports, limited amount of energy due to oscillatory motion will be dissipated due to the rubbing movement at the hinge joints. The type of the hinge joint will dictate the amount of energy dissipation.

7

Spherical ball and grove type of hinge joint is found to be more effective in dissipating the energy. However, the force displacement characteristics are highly non-linear and the amount of energy dissipation is very small.

Guided supports as shown in Fig. 5 and sliding supports as shown in Fig.6 are originally used for the piping/equipment to allow free thermal movement. Recently it is found that these supports have inherent characteristics to absorb energy due to oscillatory motion. In order to have free thermal expansion, the coefficient of friction between the pipe and the support should be small and should be maintained through out the service of the piping system. To maintain constant friction coefficient, the surfaces of the pipe or the support should not get corroded. To avoid corrosion problem materials such as Teflon, Ferro-asbestos sheets are very often used. For equipment such as pumps, lubricant pads are used. These are made of graphite rings impregnated in steel plates.

Snubbers are generally called seismic supports. There are two types of snubbers and they are hydraulic snubbers and mechanical snubbers. Hydraulic snubbers consist of an orifice, moving piston and a cylindrical casing contained operating oil. Piston is connected to the piping/equipment through connecting rod. In the case of thermal movement, operating oil moves through orifice enabling the piston to move almost with no resistance from hydraulic pressure. On the other hand in case of seismic motion the orifice offers large resistance to rapid motion by hydraulic pressure preventing flow of operating oil.

Mechanical snubbers consist of a ball screw that converts linear motion of piping into rotating motion and a flywheel attached to the end of the ball screw shaft. The flywheel

Fig.5 Pipe guide supports

Pipe Pipe

Fig.6 Sliding supports

Fig.3Spring support

Spring

Pipe

Hinge for connecting to the structure

Fig. 4 Hanger support

Hinge for connecting to the structure

Pipe

Fig. 3 Spring Hanger Fig. 4 Rod Hanger

8

additionally has a brake mechanism, which restrains the movement of the piping/equipment above certain earthquake acceleration level, but allows free movement under normal operation.

Utilities [3] have strong incentives to remove snubbers from operating plants and avoid in new coming up plants. The reasons behind such incentives are several-fold. First of all, it is expensive to maintain snubbers since they require periodic testing to assure meeting stringent functionality requirements. Secondly, snubbers congest the working space and thus impede in-service inspection. Its consequence is the increase in radiation exposure to the operating personnel. Thirdly, inadvertent snubber lock-up can induce higher piping thermal stresses during normal operation, which is undesirable from the viewpoint of piping fatigue.

Due to the above problems, energy absorbers have been developed to replace snubbers as piping supports to mitigate piping dynamic response. These devices are simple, do not require testing and maintenance and can replace snubbers on a one to one basis.

4.2 Energy absorbers

Discussion on various types of energy absorbers such as elasto-plastic, lead extrusion, visco-elastic and friction type is as follows:

4.2.1 Lead Extrusion Damper

There are two types of lead extrusion dampers (LED) [4]. One is rotary type and other one is cylindrical type as shown in Figs.7 (a) & (b) respectively. The LED absorbs vibration energy by plastic deformation of lead and there by mechanical energy is converted in to heat by the extrusion of lead. This process makes suitable for absorbing the excitation energy of piping /equipment during an earthquake. On being extruded the deformed lead re-crystallizes immediately and recovers its original mechanical properties before the next extrusion. Accordingly, work hardening or the fatigue load does not limit the amount of energy absorbed.

The feasibility of application of these dampers was studied [5] in Japan and found rotary type damper is suitable for piping systems and cylindrical type is suitable for heavy components such as steam generators.

9

4.2.2 Visco-elastic Energy Absorbers

These energy absorbers [5] use deformation characteristics of high viscous liquids or semi solid materials. Energy absorption arises from pure viscous shearing resistance of the material. Typical visco-elastic dampers are shown in Fig.8. In this type of damper visco-elastic material is bonded to each side of a center plate. Due to dynamic motion, energy is dissipated by relative displacement in the visco-elastic material.

4.2.3 Friction supports

Friction supports as shown in Fig. 9 serves the same purpose as other energy absorbers. However, most important requirement of this support is to have less coefficient of friction to enable free thermal expansion of piping system. One of such material is Teflon. The hysteretic behavior of the friction support due to earthquake excitation results in absorbing energy and hence reduction in the response of the piping system. 4.2.4 Elasto-plastic energy absorbers

Elasto-Plastic energy absorber [6] is composed of uniform strength X-shaped beam of layered laminated plates as shown in Fig.10. Some designers recommend single layered plates as shown in Fig. 10(a). In the layered X-beam as shown in Fig. 10(b), cladding metal is bonded on both the sides of the core metal whose yield strength is higher than the each cladding metal. Damping effect is caused by the energy dissipation through heat loss due to the hysteretic behavior in cladding metal only. The core metal is always kept with in the elastic condition, which is essential to bring the support to original position after releasing the load. This is because the generated strain in the core metal is relatively smaller (i.e. within the elastic limit) and its yield stress is higher than that of the cladding metal. The physical dimensions of the laminated plate govern its stiffness and damping characteristics of the energy absorber. Generally, triangular and X-shaped plates are used in this type of

Fig.8 Visco-Elastic Dampers

Visco-Elastic material

Piping connecting member

Pipe

Ferro-asbestos sheet

Pipe

Teflon laminated on steel

Fig.9 Friction supports

10

energy absorbers. These shapes will result in uniform yielding of substantial portion of the cladding material. Depending up on the actual load, the number of plates required can be determined and can be assembled to get final support configuration.

5 MULTI-SUPPORT EXCITATION ANALYSIS OF LAYOUT-2

AHWR downcomers in Layout-2, shown in Fig. 2, run from steam drum to reactor inlet header (elevation 95.0 m) to steam drum (elevation 126.0 m). Downcomers are supported by conventional supports at different elevation. Response spectrum analysis has been performed using multi level supports excitation method. The mathematical formulation of multi support excitation has been explained in following paragraphs.

For a model dynamic equilibrium equation can be written as. [7-14]

}{rx[M]{x}[K]}x{[C]}x{[M] g����� −=++ (7)

Where nnM ×][ = Mass Matrix of the finite element model which is obtained by assembling all the element mass matrices ][M e.

nnK ×][ = Stiffness Matrix of the model, obtained by assembling element stiffness matrices [ ]eK and

][C = Stiffness Matrix of the model, obtained by assembling the element stiffness matrices [ ]eC .

gx�� = The earthquake ground motion, and }{r = The influence vector having the value unity along the direction of excitation and zero for the direction of no excitation.

Computations for Multi-degree of freedom (MDOF) systems can be reduced by computing the natural frequencies and mode shapes by using modal decomposition technique. For free vibration of un-damped structure /equipment /piping the Eq. (7) can be written as:

2[ ]{ } [ ]{ } 0 (8)i i iK Mφ ω φ− =

�i are the natural frequencies of the structure, and {φ}i the corresponding mode shape. The number of natural frequencies and mode shapes in a structure are equal to the number of degree of freedom. Defining a set of generalized co-ordinates we can carry out modal transformation as:

Substituting Eq. (9) in Eq. (7) and pre- multiplying throughout by {φ}iT, the equation of

motion becomes:

{ } [ ][ ]{ } { } [ ][ ]{ } { } [ ][ ]{ } { } [ ]{ } (10)T T T Ti i i i gM y C y K y M r xφ φ φ φ φ φ φ+ + = −�� � ��

{ } { } (9)i i ix y φ=

Connecting lug X shaped plates

Fig.10 Elasto-plastic damper

clad

core

(a) (b)

11

By using the following orthogonality relationship of mode shapes [12]

The Eq. (10) reduces to:

Eq. (12) is uncoupled equation of motion for single degree of freedom (SDOF) system representing vibration in ith

mode. ξi is the damping in the ith mode. The numbers of uncoupled equations are equal to number of modes selected unlike number of degrees of freedom started with. This helps in reducing computations.

=}{][}{ rMTiφ (Γi); the modal participation factor of ith mode. In case of uniform support

motion i.e. all supports are moving in unison. Then }{][}{ rMTiφ takes the form

}1{][}{ MTiφ

Seismic response transferred to the structural systems through its supports contributes considerably to the total loading scenario of nuclear power plant piping system. Piping system of a nuclear power plant may run at single floor or it may cross the different floors and supported at corresponding elevations. When piping or equipments are supported at more than one floor as shown in Fig.11, which has different, ground motions applied at each, the total response of the structure can be obtained by superposition of the response due to each group of support input. For multi support excitation Eq. (12) representing modal response can be written for each support group modal response as:

Where )(kijy = ith normal coordinate due to jth directional excitation of kth support point (or

group).

ξi, ωi = ith modal critical damping and circular frequency of the system.

kgijx = jth directional excitation of kth support group in ith mode.

)(kijΓ = ith modal participation factor of kth support group in the jth direction.

= )(}{][}{ kj

Ti rMφ .

Here {r}j(k); vector of influence functions is obtained for the kth support group in three

global directions (j= 1, 2, 3) for each structural degree of freedom system. For example )2(

1}{r = the vector of structural displacement of structural degree of freedoms obtained by displacing all supports in support group no. (2) by unity in first global direction while keeping all other supports fixed. )(}{ k

jr can be obtained by static analysis of system before the analysis of multi-support excitation problem. Physical significance of )(}{ k

jr can be understood from Fig.11.

This method of multi-support response spectra analysis is also called the independent support motion (ISM) method because in this method only one support group (level) is moved at one time and all other fixed. This means that if influence function vectors are summed up algebraically over all support groups (levels), it will lead to an identity vector i.e.

Ti jTi j i iT 2i j i

{ } [M]{ } 1, for i j; and 0 for i j{ } [C]{ } 2 , for i j; and 0 for i j (11){ } [K]{ } , for i j; and 0 for i j

φ φφ φ ξ ωφ φ ω

�= = = ≠�= = = ≠ �= = = ≠ �

(k) (k) ( ) ( ) ( )2y 2�� y (13)ij i i ij gijk k ky xij ijω+ + = − Γ�� � ��

2i i i iy 2 y { } [ ]{ } ( 1,2,3,..... ) (12)T

i i i gy M r x i Nξ ω ω φ+ + =− =�� � ��

12

Fig. 11 Multilevel Supports

After calculating group participation factors for a mode, modal normal co-ordinate is calculated by combination of group normal co-ordinates as:

Where, N = number of the support groups (levels). are the participation and spectral acceleration in the ith mode for kth group (level) of the support. When yij are calculated for each mode for a particular support group (level) by appropriate combination method explained below, then subsequent modal displacements can be calculated by {x}I = yij{φ}i , after calculating the modal displacements, other quantities of interest like force , stress, can be calculated by using modal displacement and stiffness priorities of the structural elements. Similarly total response can be calculated by superimposition of peak modal responses.

5.1 Methods of response combination

USNRC Regulatory Guide 1.92 [15] provides the method of combining the modal responses for closely spaced and widely spaced modes. This guideline stipulates, widely spaced modes will be combined by SRSS method and closely spaced modes will be combined by any of the following methods.

(i) 10%, Method (ii) SRSS Method (iii)Double Sum Method (iv) Grouping Method

�=

Γ=�=

Γ=N

k i

kijaS

kij

N

k

kijDk

ijyij

1 2

)()(

1

)()(

ω

}1{}{1

)( =�=

N

k

kjr

)()( kija

kij SandΓ

13

Closely spaced modes are those modes whose frequencies do not differ more than 10%. The combined effect of all three orthogonal spatial components (two horizontal and one vertical) of a seismic event is obtained by SRSS method [16].

5.2 Maximum stresses in Layout-2

The stresses in the piping due to thermal expansion are very significant due to high operating temperature. In analysis of Layout-2 it has been aimed to reduce the thermal stresses and make the layout simpler that eventually makes in-service inspection easy. Stresses in the Layout-2, under different loads and combination of loading have been listed in Table-2. It can be observed from Table-2, that maximum thermal stress in Layout-2 is lower as compared to Layout-1. However, the numbers of weld joints remain same. Further another simpler Layout-3 has been proposed for downcomer. This layout is the most preferable for the process point of view because of less bends the friction loses in flow are minimized. Applying the conventional supports to Latout-3 maximum thermal stress is 521.66 N/mm2 (4.53Sm), and the maximum seismic stress is found to be 307.63 N/mm2 (2.67Sm). These values are far beyond the allowable limit of the code. In order to qualify the Layout –3 for seismic event more number of conventional supports will be needed, which will increase the thermal stresses tremendously. Hence Layout-3 cannot be qualified with conventional supports. Therefore to qualify the Layout-3 support needed should be such that it should absorb the seismic energy without much affecting the thermal stresses. In order to qualify the Layout-3 EPD supports have been incorporated.

Table- 2 Comparison of Maximum Stresses in Layout-1 & Layout-2. (Sm = 115.20 N/mm2)

Sr. No.

Loading Condition Layout-1 Layout-2 Allowable Stress

Remarks for Layout-2

1. Design Pr. + Dead Wt. 0.52Sm 0.646 Sm 1.5 Sm Higher but acceptable

2. Operating Pr.+ Thermal Expansion

2.2 Sm 1.73 Sm 3.0 Sm Lesser

3. Upset Pr. + Dead Wt + Operating Basis Earthquake (OBE)

1.47 Sm

1.64 Sm

1.8 Sm

Higher but acceptable

4. Operating Pr. + Thermal Exp. + Operating Basis Earthquake

(OBE)

2.89 Sm

2.84 Sm

3.0 Sm

Lesser

6 DESIGN AND CHARACTERISATION OF EPD

Experiments carried out by RSD on EPDs have reflected very promising results for absorbing the vibration energy during earthquakes [17]. In addition to excellent energy absorbing characteristic, elasto-plastic energy absorbers have other inherent attractive qualities in comparison to other passive energy absorbing devices viz. simple design, easier manufacturing, easy installation, less maintenance, and lower cost. Because of its attractive qualities it is not away when EPDs will become the substitute for costly and unreliable snubbers. The design and test characteristics of EPDs have been explained under following headings.

14

6.1 Force Displacement Characteristics of X-Plate [18-20]

The force displacement characteristics were obtained by analytical formulations and also by conducting static tests and cyclic tests on X plates of different thickness.

6.2 Analytical formulations

The force displacement curve for an X- shaped plate was obtained using the beam theory. The expressions are derived for three cases as follows:

Case-1: Considering bending stress in X- plate is elastic The X-shaped plate connects the top of two triangular plates. Thus considering only triangular plate

2

2 (14)x

d yEI F

dx= −

3

3 (15)6

Ebt dF

a=

For X plate,

Where‘d’ is the displacement, ‘b’ is the width, ‘t’ is the thickness, ‘a’ is the height of the triangular plate and ‘E’ is the Modulus of Elasticity.

Case-2: Considering bending stress in X-plate just reaching yield stress,

For triangular plate, 6

btM

2y

y

σ=

(16) 6

2

a

btF y

y

σ=

Substituting Eq. (16) in Eq. (15) we get,

For X plate:

Case-3: Considering the bending stress in which elastic depth of the X-plate reaches 2yo. The stress of the triangular plate before yield for 0<y<y0 is given as follows

(18) 0

1 y

y yσσ =

The stress of the triangular plate after yield, using the strain-hardening rate ‘H’ is given by Eq. (19).

(19) )(

2 E

HEH y −

+=σ

εσ

3

3

122 aEbt

dF

K ==

(17) 22

Et

ad y

y

σ= Et

ad y

y

2σ=

b

a

Fig. 12 Characteristics of X- shaped plate

t

σy σy

y0

t

F

F

15

Using the balance of the moment at the fix point of the triangular plate:

(20))(22/

20

1

0

0

ydyydybFat

y

y

�� += σσ

Substituting values of σ1 and σ2 in Eq. (20) we get,

(21) ))(34(12 0

322

0� �

��

+−−=y

HtEHty

Ea

bF yσ

The force in the X-plate is the same as that in the triangular plate,

(22) 2 0

2

Ey

ad yσ

=

For X- plate, (23) 0

2

Ey

ad yσ

=

7 TESTING OF TENSILE TEST SPECIMENS AND 6.0 mm X-PLATE

7.1 Tensile test of SS316L material

In order to evaluate the mechanical properties of SS316L, the material of X-plates, tensile tests were performed by Universal Testing Machine (UTM) at BARC on rectangular test specimen as shown in Fig. 13. From the load deflection curve thus obtained stress strain characteristics were derived and plotted as shown in Fig. 14.

From the slope of linear portion in Fig. 14, the Young’s modulus (E) was found 1.93 � 105 N/mm2 and yield strength was calculated at 0.2% proof strain and the value of yield strength is 265 N/mm2. Value of strain hardening coefficient (H) was calculated from stress strain curve and found to be 2.58% of E and the yield stress for bilinear curve was 4.98 GPa.

7.2 Testing of 6 mm EPD plate

In order to evaluate the cyclic characteristic of 6.0 mm X- plates made of SS 316L

material, tests were performed on cyclic testing machine at BARC. At a time, two X-plates

were tested because of concentric load requirement of testing machine, a fixture was made

as shown in Fig. 15, and tests were performed by applying tip displacements of 10 mm 15

Fig. 13 Rectangular Tensile Test Specimen

16

mm and 30 mm at either end of EPD plates and measuring the reaction forces by the load

cell at the common end of the plates

Under these tests forces so measured will be on two plates therefore forces have been halved in order to plot the cyclic load-deflection curve of single EPD plate. The hysteresis curve at 10, 15 and 30 mm tip displacements are shown in Figs. 16-18. Under all these tip displacements testing conditions, tests were performed till the failure of at least one EPD plate in order to evaluate the full amplitude cycles which an EPD plate can withstand till failure. From the tests it was found that 6 mm EPD plate can sustain 313 cycles, 52 cycles and 30 cycles of dynamic loading at 10 mm, 20 mm and 30 mm tip displacements respectively. It can be observed that at high tip displacement the number of cycles to failure of 6mm X-plate is lower because of higher stresses.

0.00 0.07 0.14 0.21 0.28 0.35 0.42 0.49 0.56 0.63 0.700.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Stre

ss in

kN

/mm

2

Strain in mm/mm

Young's modulus (E)=193 kN/mm2

Fig.14 Stress-Strain Curve For SS316L

Fig. 15 Schematic of test set-up

17

To achieve large number of fatigue cycles, combination of 3mm thick X-plates can be used which sustain large number of fatigue cycles before failure [17]. During the test it was also observed that the number cycles to fatigue failure mainly depends on tip displacement, it is almost unaffected by the frequency of loading and unloading.

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

-4.5

-3.6

-2.7

-1.8

-0.9

0.0

0.9

1.8

2.7

3.6

4.5

Forc

e in

kN

Deflection in mm

Fig.16 Single X-plate EPD at 10 mm tip displacement & 0.25Hz frequency

-25 -20 -15 -10 -5 0 5 10 15 20 25

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

Forc

e in

kN

Deflection in mm

Fig.17 Single X-plate EPD at 20 mm tip displacement at 0.5Hz frequency

18

8 THEORETICAL CYCLIC CHARACTERISTIC OF 6.0 mm X-PLATE

Using the beam theory Eq. (21) the force displacement curve has been obtained for a 6 mm thick X-plate made of SS 316L with mechanical properties E= 2.1 x 105 N/mm2, H=2.58% of E=5418 N/mm2 and σy= 265 N/mm2. The stress strain characteristic of SS 316L material for 6 mm tensile specimen is shown in Fig. 14. For evaluating the cyclic characteristics of the 6 mm X-plates Ramberg Osgood model has been adopted [21]. Fig. 19 approximates the shape of the force displacement curve obtained by and Ramberg Osgood parameters ‘n’& ‘α’ have been obtained by appropriate curve fitting. Cyclic characteristic curve has been obtained for a 6 mm X-plate EPD, by putting n =7.781 and α=0.0985, in equations (23), (24 a) & (24 b) for different regions of Fig. 20. (i) For the basic branch of loading up to point ‘b’ following Eq. (24) has been used.

Where, dy= yield displacement, Py= Yield force n, α= Ramberg Osgood parameters. P and d are the force and the displacement, respectively at any point on the curve.

(ii) For unloading branch, which starts from point ‘b’ following Eq. 24(a) has been adopted.

1

1 (24)

nd P P

absd P py y y

α

−

= +� �� �� �� �� �� �

� �

11 24( )

2

nd d P P P Pb b babs aPyd Py y

α−− − −= +

� �� �� �� �� �� �� �� � �

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

-7.50

-6.25

-5.00

-3.75

-2.50

-1.25

0.00

1.25

2.50

3.75

5.00

6.25

7.50

Forc

e in

kN

Deflection in mm

Fig.18 Single X-plate EPD at 30 mm tip displacement and 0.25Hz frequency

19

(iii)For reloading branch, which starts from point ‘d’ up to ‘b’ following Eq. 24(b) has been adopted.

The experimental hysteresis curves shown in Figs 16-18 and theoretical the hysteresis curve shown in Fig. 20 for 6mm thick X- plate at different tip displacement are closely matching. The force displacement characteristics can be best represented by R-O model but a bilinear approximation is also a very close to the actual curve.

9 NON LINEAR ANALYSIS OF 6 mm X- PLATE EPD

The 6 mm X-plate was FE modeled using 3-D brick elements and nonlinear analysis was carried out. The 3-D Mesh was generated to get finite elements of Hexagonal shape as shown in the Fig. 21(a). Dimensions of 6mm X-plate EPD has been shown in Fig. 21(b). All degrees of freedom were restrained at the common edge of EPD plates and nonlinear analysis was performed at 10 mm, 15 mm, 20 mm, and 30 mm peak tip displacements. The force and displacement values thus obtained have been plotted at different tip displacements is shown in the Fig. 22. Non-linear analysis specimen was performed using FE software. The stress-strain values after the yield point were input from stress-stain characteristic of the material as shown in Fig. 14, where Young’s modulus and Poisson ratio are taken as 1.93 ×105 N/mm2 and 0.3 respectively. Comparing the hysteresis curves obtained from tests shown in Figs. 16-18, theoretical curves shown in Fig. 20, and analysis hysteresis curves shown in Fig 22, all these three are closely matching and any of these three can be used for modeling the damper.

11 24( )

2

nd d P P P Pd d dabs bPyd Py y

α−− − −= +

� �� �� �� �� �� �� �� � �

0 3 6 9 1 2 1 5 1 8 2 1 2 4 2 7 3 00

5 00

1 0 00

1 5 00

2 0 00

2 5 00

3 0 00

3 5 00

4 0 00

4 5 00

5 0 00

5 5 00 F rom B eam T heory R -O M odel n= 7 .781 , � =0 .0964

F

orc

e (

N)

D isp la ce m e n t (m )

Fig. 22 Force deflection characteristic of 6 mm X- plate 19

20

10 NON-LINEAR TIME HISTORY ANALYSIS OF LAYOUT-3

The AHWR downcomer Layout-3 piping along with EPD supports have been FE modeled using nonlinear 3-D beam element and nonlinear 3-D truss element respectively. Nonlinear time history analysis has been performed using COSMOS finite element package. EPD supports have been employed because the Layout-3 is not meeting the ASME code requirements, when it is supported on conventional supports.

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

-6000

-4000

-2000

0

2000

4000

6000

d

ce

b

Forc

e (N

)Displacement (mm)

Fig. 23 Theoretical hysteretic characteristic of 6 mm EPD plate

Displacement in (m)

20

Fig. 24 (a) 3-D FE model of EPD plate (b) Dimensions of EPD plate

(a) (b)

21

21

The analysis has been performed by giving seismic input motion as three independent time histories i.e. N-S & E-W and vertical directions motions. For conservative analysis, time histories at 123.0m elevation have been used. To encounter the uncertainties in modeling, material properties and soil structure interactions along with normal time histories, time histories with ± 15 % expanded frequencies have also been used for analysis. The input time histories have been shown in Figs. 23 (a), (b) & (c). Seismic stress in piping is highest for the case when +15% expanded time history is given as input excitation among three input ground motions. Therefore expanded time history has been used for further analysis. Stresses obtained from the expanded time history have been combined with other loads and compared with code allowable values.

In order to obtain the optimum supporting conditions the locations shown in Fig. 24, with different numbers of EPD plates have been analyzed. By putting single plate, two plates, three plates and four plates EPDs in global X & Z directions at the location shown in the Fig. 24(a), highest thermal stress is within the allowable limit. It can also be observed from Fig. 24(a) that the increase in the thermal stress is very small with increasing the number of EPD plates as compared to conventional support. It can also be observed from Fig. 24 (a) that the increase in the natural frequency is also small with increase in the number of plates in the EPD supports as compared to conventional supports but at the same time it is seen from Table-3, that the reduction in the seismic response is negligible, this is due to inadequate size of the EPD support for a large size, 300 NB schedule 120 downcomer pipe.

0 5 10 15 20 25 30-7.5

-6.0

-4.5

-3.0

-1.5

0.0

1.5

3.0

4.5

6.0

7.5

Time in Sec

Acc

eler

atio

n (m

/sec

2 )

0 5 10 15 20 25 30-4.5

-3.0

-1.5

0.0

1.5

3.0

4.5

Time in Sec

Acc

eler

atio

n (m

/sec

2 )

Fig. 26(a) Input time history in X-direction Fig. 26 (b) Input time histories in Y-direction 23 23

-0.03 -0.02 -0.01 0.00 0.01 0.02 0.03

-6000

-4000

-2000

0

2000

4000

6000

Forc

e (N

)

Displacement (m)

Fig. 25 Hysteresis curve generated by analysis 22

22

Therefore high capacity EPD support needs to be incorporated for downcomer piping. In place of single plate EPD, a supports with 50 EPD plates of 3mm thickness as shown in Fig. 25(b) has been incorporated in each downcomer and the thermal analysis has been performed. The maximum thermal stress observed in the downcomer is 3.41Sm, which is also higher than the allowable value of 2.0Sm. In order to minimize the thermal stresses and control the stresses due to earthquake the support arrangements as shown in Figs. 24(c), (d) & (e) were analysed under thermal and seismic loads. During these analyses from Fig. 24 (c) to Fig. 24(e) sequent reduction in thermal and seismic stresses were observed. In the supporting arrangement of Fig. 24(e) maximum thermal stress is the lowest among all, in addition to less seismic response. The layout shown in Fig.24 (e) comprises of 40 numbers of EPD plates at two locations and 10 numbers of EPD plates at top location.

0 5 10 15 20 25 30-5.5

-4.4

-3.3

-2.2

-1.1

0.0

1.1

2.2

3.3

4.4

5.5

Time in Sec

Acc

eler

atio

n (m

/sec

2 )

Fig. 23(c) Input time history in Z-direction

Fig. 26 Downcomer Layout-3 with (a) 4-plate EPD & (b), (c), (d), (e) high stiffness EPD

(a) (b) (d) (e) (c)

27 24

0 1 2 3 4 5 61.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

(b)

Freq

uenc

y in

Hz

No. of X-Plates

With conv Support

Fig. 27 Variation of (a) Thermal stress, (b) Natural frequency with number of EPD plates

0 1 2 3 4 5 61.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

(a)

Stre

ss R

atio

(Act

ual s

tress

/Sm

)

No. of EPD plates

Conventional support

28

50 50 60 60

25

23

Table- 3 Maximum displacements due to seismic load without and with single plate EPD

10.1 Nonlinear time history analysis results of Layout-3

In order to evaluate the seismic response of the three proposed layouts shown in Fig. 2 were analysed under different supporting conditions. The natural frequencies for significant modes of each layout have been listed in Table-4. To support the Layout-1 conventional supports have been used, because of high stiff conventional supports frequency of Layout-1 is highest, which depicts low flexibility and hence higher thermal stresses. Layout-2 is also supported by conventional supports and it consists of large number of elbows. Elbows increase the flexibility of Layout-2, which eventually leads to lower frequency and ultimately less thermal stress and the same can be observed from Table-3. In spite of less number of elbows in Layout-3, it has enough flexibility, because of flexible EPD supports. The frequency of Layout-3 is comparable to Layout-2 and less than Layout-1.

Table-4 Frequencies of Layout-1, 2 & 3

Mode No.

Layout-1

Layout-2

Layout-3

1. 5.41 3.403 3.91

5. 6.07 3.789 4.44

9 6.55 6.462 5.21 13 6.6 6.973 5.84

Downcomer piping have been analysed with single X-plate EPD support and it was observed that there is no change in the response of the piping, because stiffness of single EPD plate is negligible in comparison to large size downcomer pipes. The changes in the thermal stress and frequency of the downcomer pipe with increasing the number of X-plates and with conventional supports have been shown in Figs. 25 (a) & (b) respectively. It can be observed from Fig. 25 (a) & (b) that the stress and frequency of the pipe is much higher with conventional supports as compared to EPD supports. Further number of X- plates were increased to four and observed that there was very small change in the seismic response. In order to control the seismic response of downcomer piping analyses were performed with various support stiffness and locations. Finally it was found that high capacity dampers at more than one location can only control the seismic response of piping. Downcomer piping was qualified with ten numbers of 6 mm X-plate EPD supports at positions -I & II, in Z-direction and three plates at Position –III in X-direction. The

Displacement at EPD locations (mm)

Without EPD With single plate EPD

Sr. No.

Dx Dy Dz Dx Dy Dz

1. 5.28 10.77 25.3 5.67 10.6 23.3

2. 4.0 6.9 33.12 4.0 6.8 29

3. 4.0 4.1 42.6 3.0 4.4 42.8

4. 7.69 8.74 47.47 7.3 8.4 44.5

24

support positions and directions are shown in Fig. 26.

The variations of reaction force and tip displacements with time at each support positions of 1st downcomer and positions-I of second down comer have been noted and plotted. The hysteresis characteristics have also been plotted for these supports. The plots of variations of force, displacement with time and hysteresis curves have been shown in Figs. 27-30. From the hysteresis curves shown in Figs. 27(c), 28(c), 29(c) & 30(c) it can be observed that EPDs at each support location are deforming in-elastically by virtue of which it absorbing substantial amount of seismic energy. The EPD supports are less stiff as compared to conventional supports and hence low thermal stress. In the following section the maximum tip displacements and stresses at critical points have been listed.

0 3 6 9 12 15 18 21 24 27 30-9.0

-7.5

-6.0

-4.5

-3.0

-1.5

0.0

1.5

3.0

4.5EPD in 1st downcomer at position-I

Tip

disp

lace

men

t in

mm

Time in sec

Fig. 30(a) EPD tip displacement vs time curve

0 3 6 9 12 15 18 21 24 27 30-20000

-15000

-10000

-5000

0

5000

10000

15000

20000EPD in 1st downcomer at position-I

Forc

e in

N

Time (sec)

Fig. 30(b) Force vs time curve27 27

Fig. 29 Optimized Layout and Support Positions

4 3

2 1 Downcomer No.

26

25

0 3 6 9 12 15 18 21 24 27 30-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5EPD in 1st downcomer at position-II

Fig. 31(a) EPD tip displacement vs time curve Time in sec

Tip

Dis

plac

emen

t in

mm

-9 .0 -7 .5 -6 .0 -4 .5 -3 .0 -1 .5 0 .0 1 .5 3 .0 4 .5 6 .0-20000

-15000

-10000

-5000

0

5000

10000

15000

20000E PD in 1 st dow ncom er a t position-I

Rea

ctio

n fo

rce

in N

Tip displacem ent in m m

Fig. 30(c) H ysteresis curve27 28

0 3 6 9 12 15 18 21 24 27 30-20000

-15000

-10000

-5000

0

5000

10000

15000

20000EPD in 1st downcomer at position-II

Forc

e in

N

Time in sec

Fig. 31(b) Force vs time curve

-4 -3 -2 -1 0 1 2 3 4

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000 EPD in 1st downcomer at position-II

Forc

e in

N

EPD Tip Displacement in mm

Fig. 31(c) Hysteresis curve

28 28

-20 -15 -10 -5 0 5 10 15 20

-6000

-4000

-2000

0

2000

4000

6000EPD in 1st downcomer at position-III

Fig.32 (c) Hysteresis curve

Forc

e ( N

)

EPD Tip Displacemnt (mm)

0 3 6 9 12 15 18 21 24 27 30-5

-4

-3

-2

-1

0

1

2

3

4

5EPD in 2nd downcomer at position-I

Fig. 33(a) EPD tip displacement vs time curve

Tip

disp

lace

nmen

t in

mm

Time in sec

29 30

0 3 6 9 12 15 18 21 24 27 30

-15

-12

-9

-6

-3

0

3

6

9

12

15

18 EPD in 1st downcomer at position-III

Fig. 32(a) EPD tip displacement vs time curve

EPD

tip

disp

lace

men

t in

mm

Time in sec0 3 6 9 12 15 18 21 24 27 30

-6000

-5000

-4000

-3000

-2000

-1000

0

1000

2000

3000

4000

5000

6000EPD in 1st downcomer at position-III

Fig. 32(b) Force vs time curve

Forc

e (N

)

Time in sec

29 29

26

10.2 Stress combination and codal qualification of Layout-3

As stated earlier that analysis results have been optimized by incorporating high capacity EPD supports at three locations as shown in Fig. 26. High capacity EPD supports provide the appreciable reduction in the seismic response, which can be observed from hysteresis curves shown in Figs. 27(c), 28(c), 29(c) & 30(c). Maximum thermal and seismic displacement at each support locations of every downcomer have been tabulated in Tables- 5-7. Optimized stress analysis results have also been tabulated at points of expected maxima along with the required load combinations in the Tables-8-13.

Table- 5 Maximum displacement at position-I in Fig. 26

Thermal (mm) OBE (mm) D/C

No.

Node

No. Dx Dy Dz Dx Dy Dz

1 82 -104.56 -92.324 0.329 2.8 5.19 4.87

2 104 -97.276 -93.481 0.281 2.95 5.3 8.14

3 128 -88.265 -94.745 0.246 5.24 6.62 7.57

4 155 -83.175 -95.586 0.223 5.09 5.69 17.94

Table- 6 Maximum displacement at position-II in Fig. 26

Thermal (mm) Seismic OBE (mm) D/C

No.

Node

No. Dx Dy Dz Dx Dy Dz

1 603 -69.672 -63.436 0.284 18.15 5.2 4.39

2 608 62.345 64.571 0.002 18.87 5.3 3.77

3 613 -52.864 -65.812 -0.033 26.09 6.63 4.88

4 618 -46.105 -66.639 0.047 20.32 5.69 5.99

0 3 6 9 12 15 18 21 24 27 30-20000

-15000

-10000

-5000

0

5000

10000

15000

20000EPD in 2nd downcomer at position-I

Fig. 30(b) Force vs time curve

Forc

e in

N

Time in sec

-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6

-20000

-15000

-10000

-5000

0

5000

10000

15000

20000 EPD in 2nd downcomer at position-I

Fig. 30(c) Hysteresis curve

Forc

e in

N

Tip Displacemnt in mm

27

Table- 7 Maximum displacement at position-III in Fig. 26

Thermal (mm) OBE (mm) D/C

No.

Node

No. Dx Dy Dz Dx Dy Dz

1 605 -19.639 -38.444 -0.814 17.87 5.2 5.74

2 610 -16.827 -39.561 -0.611 18.09 4.96 5.58

3 615 -13.426 -40.782 -0.670 23.12 6.63 6.96

4 620 -11.006 -41.596 -0.808 19.21 5.7 5.86

Table- 8 Stress at Down Comer-Header Junction (B1=. 5, B2r=1.67, B2b=1.34, C1=1.50, C2b=2.23, C2r=2.23)

B2×OBE (I) (N/mm2)

B1×Pr.+ B2× (D wt + OBE (I))

Pr.+ C2×(Th + OBE (I))

D/C

No.

C1×Pr.+ C2×Th stress

(C1× Pr stress = 68.47), (N/mm2)

B1×Pr.+ B2× D wt stress (B1× Pr.

Stress=22.82) (N/mm2) Run Branch Allow = 207.73 Allow=345.60

1 117.15 28.10 11.46 30.15 87.64 210.00

2 135.62 28.81 12.15 50.24 116.36 274.13

3 150.35 29.45 10.65 61.23 128.64 310.645

4 165.83 31.50 9.53 62.70 131.43 326.905

Table- 9 Stress at First Elbow from the Header (C1=1.083, C2=1.729, B1= 0.5, B2=1.00

B1×Pr+ B2× (D wt + OBE (I))

Pr.+ C2×(Th + OBE (I))

D/C No.

C1× Pr.+ C2× Th stress (C1× Pr stress = 49.43) (N/mm2)

B1×Pr.+ B2×D wt stress

(B1×Pr stress = 22.82), N/mm2)

B2×OBE (I) (N/mm2)

Allow = 207.73 Allow =345.60

1 203.05 24.03 40.57 64.60 273.195

2 178.78 24.79 39.44 64.23 246.97

3 145.27 25.10 41.53 66.63 217.075

4 132.25 26.87 23.00 49.87 172.07

Table- 10 Stress at second elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.00)

B1×Pr.+ B2×(D wt + OBE (I))

Pr.+C2×( Th + OBE (I))

D/C No.

C1× Pr+ C2×Th stress (C1×Pr stress =47.31),

(N/mm2)

B1×Pr.+ B2× D wt stress (B1×Pr. stress =31.27), (N/mm2)

B2×OBE (I)

(N/mm2) Allow =207.73 Allow=345.60

1 172.83 36.67 43.00 79.67 237.33

2 157.18 37.21 38.56 75.77 215.02

3 132.74 38.75 41.20 79.95 194.54

4 112.42 40.47 34.72 75.19 164.5

28

Table- 11 Stress at third elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.0)

Pr.+ B2× (D wt + OBE (I))

C1×Pr.+C2× (Th + OBE (I))

D/C No.

Pr.+ Th stress (C1×Pr stress = 47.31), N/mm2

B1× Pr.+ B2×D wt stress (B1×Pr stress =22.82),

N/mm2

B2×OBE (I)

(N/mm2) Allow =207.73 Allow =345.60

1 118.12 51.03 11.68 62.71 135.64

2 107.13 54.37 13.83 66.20 127.88

3 94.15 59.92 13.82 73.74 114.88

4 85.39 66.35 12.63 78.98 104.33

Table- 12 Stress at fourth elbow from the header (C1=1.036, C2=1.5, B1=. 5, B2=1.0)

B1×Pr.+ B2×( Dwt + OBE (I))

C1× Pr.+ C2× (Th + OBE (I)),

D/C No.

C1×Pr+ C2×Th stress

(N/mm2)

B1×Pr.+ B2× D wt stress (B1×Pr stress

= 22.82 N/mm2)

B2×OBE (I)

(N/mm2) Allow = 207.73 Allow= 345.60

1 202.78 42.91 38.31 81.22 260.24

2 166.71 44.50 37.1 81.60 222.36

3 129.09 46.54 38.96 85.50 187.53

4 106.26 48.14 37.05 85.19 161.83

Table- 13 Stress at steam drum and down comer junction (C1=1, C2=1, B1=. 5, B2=1.0)

B1×Pr.+ B2×(D wt + OBE (I))

C1×Pr.+ C2×(Th + OBE (I))

D/C No.

C1× Pr+ C2×Th stress (N/mm2)

B1×Pr. + B2×D wt stress (N/mm2)

B2×OBE (I)

(N/mm2) Allow =207.73 Allow=345.60

1 88.22 30.31 58.5 88.81 146.72

2 79.71 31.44 61.89 93.33 141.60

3 70.22 33.48 64.68 98.12 135.08

4 64.07 36.02 61.72 97.74 125.79

10.3 Stresses due to safe shutdown earthquake (SSE)

Nonlinear time history analysis of AHWR downcomer Layout-3 supported on EPD, has been performed. Conservatively the SSE time histories at steam drum elevation have been used as base motion in three orthogonal directions. Since the nominal diameter of AHWR downcomer pipe is 300mm therefore as per ASME code 3% damping has been considered for analysis. The stresses due to earthquake have been combined with pressure and dead weight stresses and compared with the allowable stresses given by the ASME Code. The stresses at six critical locations of the piping have been listed in Tables 14-18 and from the

29

analysis it can be calculated that AHWR down comer piping is safe under SSE loading condition.

Table- 14 Stress due to SSE at downcomer header junction (B1=. 5, B2r=1.67, B2b=1.34, C1=1.50, C2b=2.23, C2r=2.23)

B2× SSE (N/mm2) B1×Pr.+ B2× (D wt + SSE

D/C No.

B1×Pr.+ B2× D Wt stress (B1× Pr. Stress=22.82

N/mm2) Run Branch Allow = 259.66 N/mm2

1 28.10 18.27 43.55 89.92

2 28.81 19.29 59.10 107.20

3 29.45 16.40 67.10 112.95

4 31.50 15.38 80.32 127.20

Table- 15 Stress at first elbow from the header (C1=1.083, C2=1.729, B1= 0.5, B2=1.0)

B1×Pr+ B2× (D wt + SSE D/C No.

B1×Pr.+ B2×D wt stress (B1×Pr. stress = 22.82

N/mm2)

B2×SSE (N/mm2)

Allow = 259.66 N/mm2

1 24.03 38.30 62.33

2 24.79 37.45 62.24

3 25.10 38.47 63.57

4 26.87 22.87 49.74

Table- 16 Stress at second elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.0)

B1×Pr.+ B2×(D Wt + SSE D/C No.

B1×Pr.+ B2×D wt stress (B1×Pr. stress = 22.82

N/mm2)

B2×SSE (N/mm2)

Allow = 259.66 N/mm2

1 24.03 40.90 64.93

2 24.79 37.68 62.47

3 25.10 40.08 65.18

4 26.87 34.44 61.31

Table- 17 Stress at third elbow from the header (C1=1.036, C2=1.5, B1=0.5, B2=1.0)

Pr.+ B2× (D wt + SSE) D/C No.

B1× Pr.+ B2×D Wt stress (B1×Pr stress =22.82), N/mm2

B2×SSE (N/mm2)

Allow = 259.66 N/mm2 1 51.03 21.22 72.25

2 54.37 22.74 77.11

3 59.92 28.63 88.55

4 66.35 24.73 91.08

30

Table- 18 Stress at fourth elbow from the header (C1=1.036, C2=1.5, B1=. 5, B2=1.0)

B1×Pr.+ B2×( Dwt + SSE) D/C No.

B1×Pr.+ B2× D Wt stress (B1×Pr stress = 22.82

N/mm2)

B2×SSE (N/mm2) Allow = 259.66 N/mm2

1 42.91 23.35 66.26

2 44.50 30.62 75.12

3 46.54 30.58 77.12

4 48.14 27.94 76.08

Table- 19 Stress at steam drum and down comer junction (C1=1, C2=1, B1=. 5, B2=1.0)

B1×Pr.+ B2×(D Wt + SSE D/C No.

B1×Pr. + B2×D wt stress (N/mm2)

B2×SSE (N/mm2) Allow =259.66 N/mm2

1 30.31 42.86 73.17

2 31.44 45.50 76.94

3 33.48 48.80 82.28

4 36.02 46.46 82.48

Table-20 Comparison of maximum stresses for different loading conditions

Sr. No.

Loading condition

Layout–1 Layout–2 Layout–3 Allowable Stress

Remarks for Layout-3

1.

Design Pressure + Dead Weight

60 MPa (0.52Sm)

74.46 MPa

(0.646Sm)

66.46 MPa

(0.576 Sm)

172.8 MPa (1.5Sm)

Higher than Layout-1 but within code allowable.

2.

Pressure + Thermal Expansion. (Pr Stress = 0.4 Sm)

253.4 MPa

(2.2Sm)

199.3 MPa (1.73Sm)

203.0 MPa (1.76Sm)

230.4 MPa (2.0Sm)

Less than Layout-1

3. Pressure + Dead Weight + OBE

169.3 MPa

(1.47Sm)

188.9 MPa (1.64Sm)

131.43 MPa (1.14 Sm)

207.4 MPa (1.8Sm)

Least among three

4. Pressure + Thermal Exp. + OBE

332.9 MPa

(2.89Sm)

327.2 MPa (2.84Sm)

326.9 MPa (2.837Sm)

345.6 MPa (3.0Sm)

Least among the three layouts

5. Pressure + Dead Weight + SSE

127.20

(1.1 Sm ) 259.66

(2.25Sm)

Much below the allowable

31

Table- 21 Comparison of maximum stress, number of welds and total length of pipe

Description Total length of piping in m No. of welds No. of Supports

Layout-1 609 112 56

Layout-2 661 120 60

Layout-3 520 72 88

It can be observed from the stress values listed in Tables- 8-13, that stresses at all critical points of the downcomer piping due to different loading and combination thereof is within the allowable limit of the code. In order to make comparative study of stresses due to different combination loading in three layouts, they have been also listed in Table-14. From Table-14, it can be observed that stress in Layout-3, due to thermal expansion is less than Layout-1 and higher than Layout-2, but the value of stress is within the allowable limit. From Table-14, it can also be observed that a stress in Layout-3 is the least among the three layouts for combination of loadings due to pressure, dead weight and seismic loads. Other aspects like total length of pipe required, number of weld joints and number of supports have also been compared for three layouts and Listed in Table-15. From Table-15, it can be observed that piping length required to construct the Layout-3 is least among the three, it will leads to a lot of cost saving in terms of material and also the number of weld joints are least in Layout-3, which will finally lead to least in-service inspection cost and inspection time. By reducing the in-service inspection time, radiation dose to the workers can be minimized. Number of elbows/bends is least in Layout-3, which will lead to less pressure drop during natural circulation.

11 DISCUSSION AND CONCLUSIONS

It is observed that, theoretical and analysis results on EPD are matching with the test results. EPDs made of SS316L, 6mm thick X-plates were fabricated and static and fatigue tests were performed to understand the EPD characteristics viz. number of earthquake cycles which a damper can tolerate without failure during seismic event. It can be concluded from the tests and analysis that the characteristics of EPD can be estimated by beam theory also. Fatigue tests on 6 mm EPD plate shown that they can withstand 313 cycles, 52 cycles, 30 cycles at 10mm 20mm and 30mm tip displacements respectively. Dampers are very flexible and they allow almost free thermal expansion and absorb the substantial amount of seismic energy by hysteretic deformation. Shift in the frequency of the piping with EPD support is less than 5%. EPD can be effectively used to reduce the seismic response of piping system as a better, reliable, maintenance free and economic substitute for unreliable and costly snubbers.

From the analysis of three layouts following conclusions can be drawn

1) Layout-3 has less thermal stresses, less number of elbows and almost half the number of weld joints, as compared to other two layouts. Installation of supports is easy because it can be easily attached to tail pipe tower as the pipe runs close to the tail pipe tower. In addition to above Layout-3 is very simple and total length of piping required is also less as compared to other two layouts.

32

2) Supports employed to support the down comer layout-3 are elasto-plastic dampers (EPD), which absorbs a lot of seismic energy without much affecting the thermal stresses in the piping.

3) To support this analysis, static and dynamic tests have been performed on 3mm, 4mm, and 6mm thick EPD plates and found that 3 mm plates can sustain large number of thermal & earthquake cycles.

4) EPDs are passive supports, so its functionality can be guarantied in exigencies unlike active and semi active dampers.

5) Layout-3 supported on EPD supports meets all codal requirements with many other attractive qualities as compared to Layout-1 & 2 hence Layout-3 is suitable layout for AHWR down comer piping.

33

REFERENCES

1. ASME Piping and Pressure Vessel Code Sec –III, Subsection. NB

2. AERB safety guide SG-D-1. 3. Paul R. Smith, P. E. Thomas J. Van Laan, P. E. “Piping and Pipe Support Systems,

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5. Eiji kokubo et al, 1995, “Development of Lead Extrusion Damper for PWR Reactor Coolant Loop System”, ASME, PVP, vol.312, Seismic Engineering.

6. Y.M. Parulekar, Veto Verma, G.R. Reddy and K. K. Vaze, April 2002, “Retrofitting of Existing Structures for Seismic Re-qualification”, Journals of Structural Engineering, vol.29 (1), pp. 77-82.

7. Namita, Y., Masayuki Shigeta et al, 1991 “The application of Elasto-plastic Supports Devices for a piping System (Device Test and Vibration Test of Piping System & Simulation Analysis)”, JSME International Journal, Sec-III, vol-34, pp.34-38.

8. Karl R. Leimbach, Hansjorg Schmid, 1979, “Automated analysis of multi support excitation piping problems”, Nuclear Engineering Design, vol.51, pp. 245-252, North Holland Publishing Company, Rep. Germany.

9. Karl R. Leimbach, Hans P. Sterkel, 1980, “Comparison of multiple support excitation solution techniques for piping systems”, Nuclear Engineering Design, vol.57, pp. 295-307, North Holland Publishing Company.

10. N.C. Singhal, “Earthquake Response Analysis of structural systems subjected to multiple-support excitation”, Lecturer, Dept. of Earthquake Engineering, University of Roorkee, Chapter-XXI.

11. M. P. Singh, “Seismic Response of Multiply Supported Secondary Systems”, Prof., Dept of Engineering Science & Mechanics, Virginia Ploy-technique Inst. & Blacksburg, Virginia, USA.

12. J. K. Biswas, Canada, 1994, “Seismic Response of Equipments Supported at Multiple Levels”, ASME, PVP, vol.65, pp. 133-142, Orindo, Florida.

13. A. Neelwarne, “A report on Development of Software for Seismic Spectral Analysis Under independent Support Motions”.

14. M Subudhi, P Bezler, Y K Wang & M. Hartzman, “Seismic Analysis of Piping Systems Subjected to Independent Support Excitations by Using Response Spectrum and Time History Methods”, BNL, NUREG–31296, Report.

15. R.W. Wu, et al, 1978, “Seismic Response of Structural System Subjected to Multiple Support Excitations”, Nuclear Engineering Design, vol.47, pp. 273-282, North Holland Publishing Company.

16. US Atomic Energy Regulatory Guide, 1.60, 1.61, 1.92.

34

17. ASME Piping and Pressure Vessel Code Sec –III, Subsection. NB, Appendix-N.

18. Y.M. Parulekar, G.R. Reddy and K. K. Vaze, and H S Kushwaha, 2003, “Elasto-Plastic Dampers for Passive Control of Seismic Response of Piping Systems” BARC/2003/E/028.

19. Namita Y. et al, 1995, “Development of Aseismic Design for Piping Systems Supported by Elasto-Plastic Dampers”, ASME, PVP, vol.211, pp. 63-69.

20. Namita Y. et al, 1991, “Development of a Seismic Design for Piping Systems Supported by Elasto-Plastic Dampers”, ASME, PVP, vol-211, pp-63-69.

21. Namita Y. et al, 1991, “Development of the Energy Absorber and Its Application to Piping Systems in Nuclear Power Plants”, ASME, PVP, vol-211, pp-51-56.

22. K. Satishkumar, K. Muthumani, B. Sivarama Sarma, N. GopalaKrishnan, 2000 “Evaluation of X-type metal Passive Energy Absorbers”, Structural Engineering Research Centre (SERC) Report,.

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APPENDIX-1

1. Piping material : SS 304LN

Young’s Modulus = 2.022 × 1011N/m2 Density = 8.01 ×103 Kg/m3

Coefficient of thermal expansion (α) = 17.0 × 10-6 / 0C Primary stress intensity range (Sm) = 115.3 × 106 N/m2

2. DESIGN & OPERATING CONDITIONS

Design temperature = 310 0C Operating temperature = 280 0C Design pressure = 9.81 × 106 N/m2 Operating pressure = 7.1 × 106 N/m2

3. PIPE SIZES

(i) Header Size : 600NB schedule 120 Outer diameter = 609.2 mm Thickness = 46.02mm Moment of area = 8.4148 × 1010 mm4 (ii) Down comer : 300NB schedule 120

Outer diameter = 323.85 mm Thickness = 25.4 mm

4. ELBOW SIZE (i) First elbow from the header Size : 300 NB Schedules 120(LR, 900) Radius of curvature = 1.05 m. Total number of elbows = 16 (four per quarter of header) (ii) Second elbow from the header

Size 300 NB Schedules 120 (LR, 900)

Radius of curvature = 2.20 m

Total number of elbow = 16

(III) THIRD ELBOW FROM THE HEADER Size : 300 NB Schedules 120 (LR, 600)

Radius of curvature = 2.20 m

Total number of elbow = 16

36

(iv) Fourth elbow from the header Size = 300 NB Schedules 120 (LR, 600) Radius of curvature = 2.20 m. Total number of elbows = 16 (v) Valve Details Weight of each valve = 2 T Total Numbers of valves = 16 (one in each horizontal leg of

down comer)

5. PROPERTIES OF EPD PLATES

Material of EPD Plate : Low carbon steel Young’s modulus (E) = 2.0 × 1011 N/m2 Yield strength = 2.20 × 106 N/m2 Width (b) = 56.57 mm Thickness = 6.0 mm Length (2a) = 113.14 mm Stiffness (K)

= =

1.125 ×106 N/m

3

2

12aEbt