ML20236A313
| ML20236A313 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 01/19/1989 |
| From: | TELEDYNE ENGINEERING SERVICES |
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| Shared Package | |
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| References | |
| TR-7049C, NUDOCS 8903170058 | |
| Download: ML20236A313 (45) | |
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{{#Wiki_filter:. . 1 NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT 1717 WAK0NADE DRIVE EAST WELCH, MINNESOTA 55089 Y i TECHNICAL REPORT TR-7049C A DISCUSSION OF TORSION EFFECTS ON LIMIT M0 MENT THEORY AND PIPING CODE CRITERIA JANUARY 19, 1989 ) ) 1 l l ? WTELEDYNE ENGINEERING SERVICES 130 SECOND AVENUE WALTHAM, MASSACHUSETTS 02254 ) g903170058 890308 617-894 3350 PDR ADOCK 0500 ,{ o - - - - - -
Technical Report "RTELEth'NE TR-7049C ENGINEERING SERVICES )
1.0 INTRODUCTION
Concern has been raised regarding the application of the requirements in ANSI B31.1-1967 related to the calculation of stresses due to Occasional ) Loads (Paragraph 102.3.3). The Occasional Load requirements have been used in the design of nuclear power plant piping to evaluate the acceptability of piping design when subjected to seismic events. In summarizing calculated piping stresses due to earthquake loading at the Prairie Island ) Nuclear Station, Fluor Daniel did not include the shear stress resulting from the torsional moment. The torsional stress is part of the calculation performed and is available for each node point; however, as noted above, the stress summary does not include this effect for earthquake loading. i This report addresses the concern by reviewing the ANSI B31.1-1967 rules, a number of classical papers on the collapse phenomena of pipe, and the development of the existing NC-3600 rules of Section III. ) 2.0 ANSI B31.1-1967 Paragraph 102.3.3, " Limits of Calculated Stresses Due to Occasional Loads" states the following: ) The sum of the longitudinal stresses produced by internal pressure, live and dead loads and those produced by occasional loads such as the temporary ) supporting of extra weight may exceed the allowable stress values given in the Allowable Stress Tables by the amounts and durations of time given in Paragraph 102.2.4. } The important word is longitudinal. Paragraph 102.3.3 is quite specific in defining the stress to be calculated as the sum of the longitudinal stresses. This is a continuation of the criteria addressed 3 originally under Paragraph 102.3.2(d) Additive Stress, which states the following: )
Technical Report "RTELEDYNE TR-7049C ENGINEERING SERVICES The sum of the longitudinal stresses due to pressure, weight, and other sustained loads shall not exceed the allowable stress in the hot condition (Sh )- ) Further, the method of calculating the pressure stress is provided as S1p - F/A, the longitudinal stress. Both paragraphs (Additive Stress and Occasional Loads) require the > calculatic,n of longitudinal stresses in order to provide protection against the mode of failure being addressed. Piping subjected to sustained loading such as deadweight, pressure or live loads is susceptible to a collapse type failure. A review of this failure mode and the stress of concern is provided in Section 3.0 of this report. A review of the B31.1 approach to providing protection and the evolutior, of that into nuclear piping criteria follows. ) Since the publication of B31.1 the Additive Stress paragraph has been handled by the designer in a two-step approach which did not directly address requirements. First, the designer would assure that all piping met the minimum thickness and other pressure requirements of Paragraph 104. ) This step assured that the longitudinal stress due to pressure was usually , less than one-half Sh . The second step was to support the piping using the suggested pipe support spacing of Table 121.4. Since this table is based on a maximum longitudinal stress of 1500 PSI in a simply supported beam, ) the combination of longitudinal pressure and sustained load stress was, by inspection, less than S h. It was unusual to see a calculation performed for sustained load stresses and even more unusual to see documented compliance with Paragraph 102.3.2(d). Seismic loading, if any, was } normally handled as a percentage of deadweight (applied in three directions) using a modified pipe support spacing table to maintain longitudinal stresses at an acceptable level. Experience indicates that the B31.1 approach using longitudinal stresses to provide protection ) against collapse has been highly successful. The major reason for this is that piping systems in power plants include elbows, bends, tees and branch connections so that a moment at one location becomes a torque a short )
~
Technical Report "RTELEDYNE TR-7049C ENGINEERING SERVICES s distance away. Therefore, controlling the bending moment controls the torque. For example, on the basis of an elastic stress analysis, limiting the stress intensity at point "R" in the following figure protects point "L". , o M n R. ' y f M-T L ) If y T ) ) Theoretical Limit Load is reached when either: M - 4tr2s y ) T = str2sy Therefore, limit torsional moment is T - xMy/4 < My, 4 - l'
Technical Report TR-7049C ENGINEERING SERVICES s The elastically calculated stresses when T - Ty are: At Point "L" > r - Sy/2 (ShearStress) SL"SY (Stress Intensity) > At Point "R" p-t o -
" - (Max. Longitudinal Stress)
Z wr2t nr tS o - =S rr t ) S "S (Stress Intensity) R Y So ) S ~S ? R L and if the stress is limited such that ) SRIl2Sh 1 1.2 (.625 S )y 1 0.75 Sy then SL s 0.75 Sy ) The advent of commercial nuclear power resulted in more detailed I requirements. However, the issuance of ANSI-831.7 in 1969 was the first time that a piping Code defined that the concern with Additive Stresses and ) Occasional Loads was to assure that an acceptable margin on the theoretical limit load exists. This is defined in detail in the Foreword to B31.7. ) .
Technical Report "WTELEDYNE TR-7049C ENGINEERING SERVICES > It is important to recognize that Paragraph 102.3.2(d) and 102.3.3 of B31.1 as well as 1-705.1 of B31.7 and NB-3652 of Section III are not intended to predict the actual stress in a piping component due to the applied loading but rather to perform an approximate limit load calculation > to assure a margin on the theoretical limit load exists. This leads to a better understanding of why B31.1-1967 discusses only longitudinal stresses when addressing this phenomena. ) 3.0 REVIEW OF LIMIT LOAD THEORY An abundance of technical literature is available on the collapse of piping. In nearly every case the authors deal only with pressure and ) bending moment; that is shear is not considered. A list of References is provided in Section 6.0 and the following is taken directly from them. Reference 4 " Neglecting shear effects is acceptable if the ) shell is sufficiently thin compared to its smallest surface dimension." Reference 7 - At D/t 150 shear has no appreciable effect. At ) D/t 2 75 shear has some effect. Reference 6 - Calculates the collapse mode by determining the strain in the outer fiber of the pipe due to ) ovalization and assumes the shear strain to be zero. Reference 8 - Provides an estimate of the change in yield point due to bending moment as a result of applied torsion. The effect of neglecting torsion in a cylinder is demonstrated in ) Attachment 1, Figure I which relates the ratio of applied torque, (T), over the torque to produce the limit load, (Tu), to the ratio of applied moment, (M), over the moment to produce the limit load, (Mu). In both cases, )
Technical Report "RTELEEt/NE TR-704'9C ENGINEERING SERVICES strain hardening is neglected. The effects of internal pressure and axial load on a cylinder are far more significant than torque in reducing the allowable moment to produce the limit load. (Which is the reason most investigators include pressure and axial load but not torsional effects.) > Figure 2 of Attachment 1 provides the effects of internal pressure on the value of allowable applied moment and Figures 3 provides the effects of axial load. In order to provide a quick comparison, the value of allowable applied moment is provided in the following table for various ratios of > torque, pressure and axial load. Load Ratio M/Mu
.4 .94 T/Tu .6 .85
.8 .68
)
.4 .73 Sm/Sy .6 .55
.8 .32
)
.4 .86 N/Nu .6 .68
.8 .40
) where: subscript refers to the load to cause the limit load under the specified load acting alone, ) M = applied bending moment T applied torque N = applied axial load ) Sm - circumferential membrane stress due to internal pressure Sy = yield stress in simple tension )
. o Technical Report "RTEL.EDYNE TR-7049C ENGINEERING SERVCES The longitudinal stress effects of internal pressure are addressed in all the piping Codes including ANSI B31.1-1967 However, it is noted that none of the piping Codes require consideration of piping axial loads when determining the margin on the theoretical limit load (i.e., when satisfying
) " Additive Stress" and " Occasional Loads" of ANSI B31.1 and " Primary Stress Intensity" of NX-3650). The above table demonstrates the axial load effect is much more significant than torsion. The effect of torsion is minimal. > 4.0 SECTION !!! - NC-3600 RULES The first piping Code developed to address nuclear power piping was ANSI B31.7 published in 1969. This Code provided detailed rules for the I design of Class 1 piping which were based on the criteria document (Reference 9) of Section III. For Class 2 and 3 piping, the rules of ANSI B31.1-1967 were referenced. At the time that Section 111 first included rules for piping, Class 2 and 3 piping rules continued to reference B31.1. ) Therefore, Class 2 and 3 piping continued to be designed and analyzed using the criteria that had been used over the years for fossil power piping. That is, the longitudinal stress was the stress of concern for Class 2 and Class 3 nuclear piping designed to Section III prior to the 1974 issue. ) Section III rules for Class 2 and Class 3 piping (NC/ND-3600) were published in the Winter 1972 Addenda and were based on the rules existing in NB-3600. The NB-3600 rules were taken directly from ANSI B31.7 so an ) understanding of the basis for those requirements is necessary. Paragraph 1-705.1 in B31.7 is entitled " Satisfaction of Primary Stress Intensity." However, stress intensity, based on Reference 9 and the Foreword to B31.7, is defined as "twice the maximum shear stress and is equal to the largest ) algebraic difference between any two of the three principle stresses." for example, for a thin-walled cylinder subject to internal pressure, remote from any discontinuities, the hoop stress is twice the axial stress and the radial stress on the inside surface is equal to the pressure (P) and is ) Setting the hoop stress equal to ch, the three principal compressive. stresses are: ) 1
Technical Report "RTF1 GTWNE TR-7049'C ENGINEERING SERVICES Si = oh S2 " Oh/2 S3 = -P ) Maximum shear stress theory results in a controlling stress intensity of S1-53 equal to oh + P (largest algebraic difference) and maximum stress theory results in a controlling stress of ch. For a thin-walled cylinder there is little difference since oh is much larger than P. However, when > one includes bending stress due to applied moments the results can be different. For a single applied moment on a cylinder, the bending stress is axial and is maximum tensile at one location on the diameter and maximum compressive 180 degrees away. Setting this bending stress, ob, equal to ) (oh) and combining with pressure results in the following: Principal Stress Pressure Moment Total Si oh 0 ch S2 Oh/2 i oh 1.5 oh or - oh/2 S3 -P 0 -P ) the stress differences are: S1-S2 = -Oh/2 or 1.5 oh ) 52-S3 = 1.5 oh + P or - oh/2 + P S1-S3 = oh + P ) and the maximum stress intensity is S2-53 = 1.5 oh + P versus S1-S3" oh + P for the pressure case alone. Maximum stress theory results in a controlling stress of 1.5 ch, again little difference from maximum shear stress theory for a thin-walled cylinder. ) The theory of failure for Class 1 piping rules in B31.7 and Section III is maximum shear stress. However, a close look at 1-705.1, Equation (9), indicates that a modified longitudinal bending stress is being used as )
Tcchnical Report "RTF1 FrWNE TR-7049'C ENGINEERING SERVICES a reasonable approximation to a limit load analysis. In order to comply with established Section III criteria, the margin on the theoretical limit had to be determined using maximum shear stress theory. As discussed in 2.0, this is defined in the Foreword to ANSI-831.7 where the margin on the ) limit load for straight pipe is compared with Equation (9) in Figure 1. Note that the internal pressure used in that formulation is such that the hoop stress is equal to 2/3 of the yield strength of the pipe material. ) Equation (9) is written as follows: PD B1 o+B2 0Hi $ 1.5 Sm 2t 21 ) Setting B1 equal to 1.0, the first term results in an expression for the hoop stress in a thin-walled cylinder, PDo/2t. However, B1 for a straight ) pipe, remote from welos and other discontinuities, is 0.5. Therefore the pressure stress calculated is really longitudinal (PDo/4t) rather than hoop. This stress is added to the bending stress due to the resultant moment and compared with an allowable of 1.5 Sm. Appendix D, Paragraph D- ) 101 points out the general definition of a stress index, B, as B = o/S ) where: o = elastic stress due to load, L S = nominal stress due to load, L ) and for B indices, o represents the stress magnitude corresponding to a limit load. Therefore,1-705.1 is really satisfaction of limit load rather than primary stress intensity. The only difference then between B31.1-1967 and B31.7 or Section III for Class 1 piping is the inclusion of the ) torsional moment in the term Hj. As discussed in Section 3.0 above, this effect is minimal. ?
Technical Report TELEDYNE TR-7049C ENGINEERING SERVICES Having served as Chairman of the Ad Hoc Task Force for Design of B31.7 and of the Working Group piping Design of Section III, it i s my understanding that one of the reasons for inclusion of the torsional moment in Mi in Class 2 and 3 piping was for simplicity. In the formulation of ! Class 1 rules, the committee determined that only one stress index (C2 ) would be specified for calculation of stress due to a thermal expansion. This was different than B31.1 which did not intensify the torsional stress and allowed the use of different intensification factors for in-plane and > out-of-plane bending moments. Once the decision to use a single stress index was arrived at, there was no need to separate bending from torsional moments. When rules for Class 2 and Class 3 piping were developed, the committee followed the formulation established in Class 1 primarily because ) separation of the torsional moment from the bending moments was an unnecessary complication and would require changes to existing computer programs currently in use to perform Class 1 analysis. )
5.0 CONCLUSION
S It is our opinion that excluding torsion from the evaluation of Occasional Loads in B31.1 is complying with the 1967 Edition of that Code. } The Code is specific with respect to requiring the calculation of longitudinal stresses only for both Additive Stresses, Paragraph 102.3.2(d) und Occasional Loads, Paragraph 102.3.3. Experience indicates the effectiveness of this approach. ANSI B31.1 requires inclusion of the ) torsional moment only for thermal expansion stress calculation. A review of a number of papers and reports on piping collapse supports the use of only longitudinal stress when determining the stress magnitude ) corresponding to a limit load. The inclusion of the torsional moment in Equation (9) of 831.7 and NB-3600 was to comply with the theory of failure established by Section III. The inclusion of the torsional moment in the Class 2 and 3 piping rules was a matter of convenience and simplicity rather than a matter of technical Concern. )
Technical Report "RTF1 FrVNE TR-704'9C ENGNEERING SERVICES
6.0 REFERENCES
- 1. ANSI-831.1 - 1967, " Power Piping"
) 2. . ANSI-831.7 - 1969, " Nuclear Power Piping"
- 3. ASME BPVC Section III, Division 1, " Nuclear Power Components,"
1971 Edition, Winter 1972 Addenda
- 4. " Buckling of Shells - Pitfall for Designers," David Bushnell, Volume I of the Proceedings of the AIAA/ASME/ASCE/AHS 21st Structures, Structural Dynamics and Materials Conference,1980.
- 5. "An Experimental Study of the Plastic Buckling of Cylinders in Pure Bending," B. D. Reddy, International Journal of Solid Structures, Vol. 15, 1979.
)
- 6. " Collapse of Cylindrical Elastic Tubes Under Combined Bending, Pressure and Axial Loads," Ole Fabian, International Journal of Solid Structures, Vol. 13, 1977.
)
- 7. " Tests of Circular Steel Tubes in Bending," Donald R. Sherman, Journal of the Structural Division, ASCE, Vol. 102, No. ST11, 1976.
)
- 8. "On the Plastic Distortion of Solid Bars by Combined Bending and Twisting," R. Hill and M. P. L. Siebel, Journal of the Mechanics and Physics of Solids,1953, Vol.1.
- 9. " Criteria of 'je ASME Boiler and Pressure Vessel Code for Design by Analysis in Sections III and VIII, Division 2."
)
- 10. " Limit Loads for Tubes Under Internal Pressure, Bending Moment, Axial Force and Tension," W. F. Stokey, D. B. Peterson and R. A.
Wunder, Nuclear Engineering and Design 4, 1966. ) 1 1 _ _ _ __ _ _ _ _ _ - - _ _ _ _ - _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _
WTri FrVNE Il$!!' "" " ENGNEERING SERVCES ) l ) ATTACHMENT 1 ) ) ) ) ) l 4 ) 1 1
Technical Report Y Ta-7a49c ENGINEERING SERVICES
~
1o t 0.8
\ 3 M/Mu T 0.6
) . 0.4 0.2
\ 1
) 0.2 0.4 0.6 0.8 1.0 T/Tu ) Fiaure 1 Effect of Torsion on Limit Moment ) (Reference 10) 1
x,,. b .A o
"WTELEDYNE IENONalReport ENGINEERING SERVICES 1.0 N !
0.8 A M/Mu 0.6
\
\
\s 0.4 A
0.2 N,
\
\
0.2 0.4 0.6 0.8 1.0 Sm/Sy ) Fiaure 2 Effect of Internal Pressure on Limit Moment (Reference 10) h
1
. Technical Report Y
'TR-704'9c ENGINEERING SERVICES )
1 ) 1.0 l ) 0.8
\ 3 M/Mu 3
) 0.6
\
1 0.4 ) 0.2 I ) i 0.2 0.4 0.6 0.8 1.0 N/Nu ) Fiaure 3 l Effect of Axial Load on Limit Moment ) (Reference 10) _ _ _ _ - _ _ - _ _ _ _ _ _ _ _ _ _ _ _ - _ _ _ _ _ ~ .
- . i
, o ..
Ls FLUOR DANIEL
. Fluor Daniel, Inc.
200 West Monroe Street Chicago, lL MM6 j (312) 368-3 % O l l March 3, 1989 l Mr. J. E. Goldsmith, Superintendent FN-8856 Nuclear Technical Services Northern States Power Company File: 000 259.1 Prairie Island Nuclear Generating Plant 1.2 l 1717 Wakonada Drive East J Welch, Minnesota 55089 Ref: None
Dear Mr. Goldsmith:
Northern States Power Company Prairie Island Nuclear Generating Plant ( Project: 21-7450-259 (832642)
Subject:
Use of Torsional Moments In The PIPESTRESS Program l The following References and this cover letter are intended for your use to respond to the NRC regarding the Fluor Daniel interpretation of the USAS B31.1, Power Piping Code. i Reference 1: " Documentation of Methodology Used For The ; Combination Of Moments For Piping Constructed To I The USAS B31.1 - 1967, Power Piping Code", R. F. Petrokas and E. O. Swain l Reference 2: " Sampling Study To Evaluate The Effect Of l Torsional Moments On Pipe Stress Qualification", Report No. R834486-1, Dated 3/3/89. Reference 1 addresses the acceptability of Fluor Daniel's ! interpretation of not including the torsional moment to calculate longitudinal stress for occasional loads. This report supports ) this interpretation of the requirements in the USAS B31.1 - 1967, Power Piping Code. Additionally, Fluor Daniel performed a sampling study, Reference 2, that included torsional moments in the combined stress calculations. Analyses of the randomly selected analytical parts showed that they all meet the USAR criteria. The sample size was selected to provide a 95% confidence level. i
8 t
, , a
- t : .
FLUOR DANIEL f Mr. J. E. Goldsmith FN-8856 March 3, 1989 A previously (non randomly) selected set of analyses indicated that one analysis exceeded USAR criteria after including torsional moments. This analysis is unique because of a combination of large eccentric load, high initial stress, and low allowable stress due to a high design temperature. Also this line was already scheduled for modification resulting from the IE Bulletin 79-14 reconciliation program and, therefore, should not have been selected. This line meets the operability requirements of the IE Bulletin 79-14 program. 1 If you have any other concerns regarding this issue, we will be available to provide assistance. Very truly yours, cE&d % B. . Dickerson Project Piping Engineer l A. Casillo A. V. Setlur Senior Mechanical Engineer Project Director BLD/AC/AVS/lc Attachments To All ; cc: NSP - J Donatell, GA Rolfson, FDI - MP Lane, JK Khanna, D Madan, Ray Patel, AV Setlur, MT Case , 1
o c1, e, 4 SLI-8904 4 DOCUMENTATION OF METHODOLOGY USED FOR COMBINATION OF MOMENTS FOR PIPING CONSTRUCTED.T0 THE USAS B31.1-1967 POWER PIPING CODE 1 I Prepared for Fluor Daniel 4 Prepared by R. F. Petrokas E. O. Swain S. Levy Incorporated 1 3425 S. Bascom Avenue Campbell, CA 95008 February 1989 1 1
L j.: ,' ,,.
~
i . ' ~ ACKNOWLEDGEMENT ?- The authors wish to acknowledge the contribution of Mr. S. E. Moore of Oakridge National Laboratory, who in the course of several phone calls, provided us his thoughtful opinions, as sumarized in Subsection 4.1-1 i l l 1 4 l i I l i
.d 7, ,,
l l TABLE OF CONTENTS j { 1 Section Eggg l-
1.0 INTRODUCTION
1 2.0 EVALUATION OF CODE ANALYSIS 1 REQUIREMENTS (MOMENT LOADINGS) 2 3.0 TECHNICAL DISCUSSION 3 3.1 Use of Bending Moment to Calculate Longitudinal ! Stress 3 3.2 Code Changes 4 3.3 Significance of Torsional Moments 8 4.0 HISTORY OF CODE CHANGES 9 4.1 Individual Contact - S. E. Moore 10 4.2 B31.1 Code Changes 11 4.2.1 Reason for 831.1 Changes 11 ; 4.2.2 Reason for Use of Resultant of Three Moments 12 1 4.2.3 Some Recent Code Actions Regarding Primary Stress Equations 13 4.3 Archival Information 14
5.0 CONCLUSION
S 15 i
6.0 REFERENCES
17 APPENDIX A A-1 I u 1 11 _ _ _ - - _ _ - _ - _ - - _ _ _ _ - _ _ _ _ _ _ _ - _ _ - _ _ _ _ _ _ _ - - - _ - _ - _ _ _ _ _ _ . _ _ - _ - _ _ _ _ - _ _ _ _ _ _ _ _ - _ - _ _ _ _ _ _ - _ - - _ - _ - _ _ =
., .. ~
i .. . P ,. o 4
1.0 INTRODUCTION
L This report has been prepared at the request of Fluor Daniel. ' A study was made of the methodology which Fluor Daniel used for calculating the stresses due to occasional loads acting on piping systems. .The methodology l was reviewed from the standpoint of (a) compliance with and proper l interpretation'of USAS B31.1.0-1967 and (b) the technical adequacy of USAS B31.1.0-1967. It is the purpose of this report to provide the results of this study. 2 { The U' SNRC has questioned whether Fluor Daniel correctly interpreted the intent of USAS B31.1.0-1967 when the original piping design and analysis. { was performed for the Prairie Island and Kewaunee Nuclear Power Plants. The issue to be resolved is whether torsional moments were correctly left out of calculations to determine piping stresses due to sustained and ( occasional loads, including earthquake. It is our understanding that .; compliance with Paragraph 102.3.3 of USAS B31.1.0 was accomplished by meeting the requirements of the following equation: i / gj2 + Mo2 S Pd2
< l.2 Sh -
t = 002-d2+ Z
- where, i
Mi and Mo - in-plane and out-of-plane moments due to weight and operational basis earthquake (OBE). In the summer 1973 Addendum to 831.1, an equation similar to the one above was provided for occasional loads except it included torsional moments under the radical and used 0.751 instead of 1.01. This change to 831.1 was the basis for the NRC to pose these two questions: j q
- 1) First, for a project designed to the 1967 Edition of 831.1, was it a correct interpretation of the Code to use just the two bending j moments in calculating pipe stresses due to weight and earthquake? l 1
. . ~ :
- 1. . .
i l
- 2) Was the Code in error, even though the interpretation was correct?
The report contained herein will show that the exclusion of' torsional moments was an adequate interpretation of the 1967 Code and that piping analyzed to the 1967 Edition of B31.1 is acceptable. i 2.0 EVALUATION OF CODE ANALYSIS REQUIREMENTS (MOMENT LOADINGS) i The basic' Code rules for piping. stress evaluation were derived from the work of. A.R.C Marki and others beginning in the 1940's (see References 1, 2, and 3). The concept of stress intensification (i)- factors came from a series of fatigue tests by Mark 1 in which the fatigue life (under cyclic I moment loadirg) of various piping products were compared to that for a .i reference item. Mark 1's work shows that the use of these. "1" factors was intended for application to bending moments and were to be used for calculation of expansion stresses. The concern for longitudinal (primary) stresses was primarily from pressure and deadweight. The designer was told to calculate longitudinal stresses due to these loads, but no specific j guidance was given on how to do this. The lack of specific guidance in 'I primary stresses continued from the 1955 to the 1973 Editions of the , Code. I Finally, in the summer 1973 Addenda to B31.1, explicit equations to be used l for calculating primary stresses were provided. With this change, the Code ) allowed the use of all three moments to calculate longitudinal stress. This change is probably what initiated questions on the correctness of the ! use of two moments in primary stress calculations performed to earlier Codes. The complete discussion on the evolution of Code primary stress : calculations is presented in Appendix A. This Appendix clearly shows that ' use of the two bending moments to calculate longitudinal primary stresses is entirely consistent with the intent of the 1967 Edition of the Code and earlier work to support the Code rules. The technical justification and 2
)
1 . historical bases for use of two bending moments is further discussed in Sections 3 and 4. 3.0 TECHNICAL DISCUSSION The use of two bending moments to calculate longitudinal stresses is consistent with the historical information on the basis of Code rules and the requirements delineated in the 1967 Code, as was discussed in Section 2 of this paper. However, technical information available in other literature and the changes made to the Code and Section III itself also shed light on the matter. As a preface to this discussion, one should bear in mind the Foreword to l the 1967 Code (Reference 5) which states:
"There are many instances where the Code serves to warn a designer, fabricator, or erector against possible pitfalls; but the Code is not a handbook, and cannot substitute for education, experience, and sound engineering judgment."
The philosophy behind the statement is part of all of the Codes referenced l in this report. The 1967 Code allowed the use of two bending moments to calculate longitudinal stress. However, it was still up to the designer to l use " sound engineering judgment." l 1 3.1 Use of Bendino Moment to Calculate Longitudinal Stress ' The 1967 (Reference 5) Code is specific in its requirement that longitudinal stresses due to pressure, weight, other sustained loads, and occasional loads be calculated. One question, regarding the reasonableness of using bending moments to calculate longitudinal stresses has been posed by the NRC. This is a reasonable approach. To begin with, the classic definition of stresses due to bending moments in beams and similar structural items (like 3
. . j e
pipe) results in the moment being resisted by longitudinal stresses across the section of the beam. It is also interesting to note that this i interpretation is also used by the power industry. For example, Reference 10 is a report on NRC funded work to evaluate "End Effects on Elbows Subjected to Moment Loadings." Reference 10 notes that longitudinal stress , is calculated using (M i+ H )l/2 2 where Mi and M2 are the out-of-plane and in-plane bending moments, respectively (Reference 10 also notes that the actual maximum stress found in the elbow is less than that predicted by the SRSS sum of all three moments). Reference 11 is a similar report regarding nozzles in pressure vessels and piping. Reference 11 specifically defines bending moment, Mb as (Mg + 2H )l/ . Reference 12 notes that bending stress as defined by B31.3 is Sb " I(I t Mj)2 + o(j og)2)l/2/Z Again, this is based on two bending moments. Therefore, calculation of longitudinal stress based on bending moments is reasonable and meets the stated requirements of Reference 5. 3.2 Code Chanaes In Section 2 we discussed that, in the summer 1973 Addenda, the Code ! changed to allow the use of all three moments. This strongly suggests that use of three moments is a deviation from past practice. This is discussed further in Paragraph 4.2.2. The moment portion of the primary stress calculation changed from i
/(iNbp) + (i Mbt) 0.751 MA 0.751 MB
+ {
i Z Z Z l 1 _ __ _ _ - _ _ _ _ _ _ _ _ _ _ . _ . . _ _ _ - _ _ _ _ _ - _ _ - -_ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - ' ' - ' ' ~ - - -
., +
, .~
c . a b where:. M bp is in-plane bending moment Mbt is out-of-plane bending moment MA is resultant moment due to sustained loads MB is resultant moment due to occasional loads Several changes are evident: 1.' Resultant' moment rather than bending moment "may" now be used.
- 2. Moments for sustained loads are separated from those for occasional loads.
3.. 0.751 is now used in place of 1. n The change.to use of 0.751 was made for several reasons. It'was recognized that the stress intensification factor "i" was derived from fatigue tests, and may be in error by as much as a factor of 2 in predicting stresses. This can be seen in the data presented in References 1, 2, and 3 and is i
. discussed further in NC-3673.2(b) of Reference 8, which notes that i is approximately C2 K/2(C2 2 and K2 are stress indices defined in Subarticle NB-3680 of-Section III). This is discussed in more detail in Reference 13. i According to NB-3680 (Reference 8), the index 2B , which is for primary .j stress calculation is equal to 0.75 C 2. Therefore, the change to 0.75i was y made to approximate 82 by the use of 0.751. 1 i
The change to the use of all three moments was probably made for several i i reasons. First, there was a desire to ease bookkeeping. Since ASME Class 1 rules had always required the use of resultant moment, it was deemed advisable to require this for Class 2 and 3 piping. Second, there was a desire to maintain consistency between ASME Class 2 and 3 rules and 831.1. Also, ASME Section III is tending to make the Class 2 and 3 piping' design rules more like Class 1 by requiring the use of stress indices, rather than ' the "1" factors. This also required the use of resultant moments. The 1 history of this change is discussed further in Subsection 4.2. Note that i l 5
~,
., 4.,
c . l' currently, the ASME Code (Reference 9) does use the B 2 index, rather than 0.751 for primary stress calculations in piping. Because of. the change of ASME to the B 2 indices for primary stress, B31.1 initiated a study of the conservatism of 831.1 compared to Section III (Reference 19). This study showed that in most cases, the B31.1 rules were significantly more conservative than Section III in the determination of maximum' allowed moment.. The few instances of non-conservatism were found to be small and the authors of Reference 19 concluded that there was no L immediate safety concern with the B31.1 rules. In fact, the authors L recommended a modification to the 0.751 term and a possible increase in allowable stress to 1.5 S h from the current 1.2 S
- h Finally, Table 3-1 shows a compilation of computed stress indices and the 82 index for several piping products. Note that for the welding elbow and welding tee, the 82 index currently required by the ASME Code is approximately 1-1/2 times higher than the i factor required by Reference 5 and twice 0.751 required by Reference 7. This again shows another :
technical issue in which the Codes have evolved to new criteria over a period of time. One should not interpret designs made to these earlier codes to be wrong, and the same interpretation should apply to primary stress calculations performed to Reference 5. In other words, the integrity of the original Design Code should be recognized as intended by ! Section III (NCA-ll40) and Section XI (IWA-7210). ! l
, Therefore, the changes made to the analysis requirements of 831.1 and Section III were based on an evolution of knowledge and data on how to i calculate primary stresses. The codes evolved to their current forms i regarding stress calculations and enumerable other requirements because of 1 this evolution and not because of explicit errors.
l l 1 6 l
,'e'.
I ', TABLE 3-1 1 0.751 Bl ITEM SIZE T Rt r2 h B31.1-67 -B31.lb-73 ASME-8( 2"/80 .218 3 1.078 .5628 1,32 .99 1. 9l WELD 8"/40 .322 12 4.15 .2244 2.44 1.83 3. 5! 8"/80 .5 12 4.06 .3640 1.77 1.32 2.5! ELBOW 12"/40 .375 18 6.17'. 1773 2.85 2.14 4 . 11 12"/80 .687 18 6.03 .3401 1.85 1.39 2.69 16"/80 .5 24 7.58 .2089 2 56 1.92 3 . 65 24"/80 1.218 36 11.39 .3380 1.85 1.39 2. 6! 2"/80 .218 - 1.078 .8898 .97 .73 1. 4 f WELD 8"/40 .312 - 4.15 .3308 1.88 1.41 2.8l 8"/80. .5 - 4.06 .5419 1.35 1.02 2 . 01 TEE 12"/40 .375 -
'6.17 .2674 2.17 1.63 - 3. 2l 12"/80 .687 -
6.03 .5013 1.43 1.07 2 .1l 16"/80 .5 - 7.58 .2902 2.05' 1.54 3. 0C 24"/80 1.218
- 11.39 .4705 1.49 1.12 2 . 21 SOCKET AI T4 - - - -
1.30 1.575 1. 5: WELD Note: These are calculated values of i and 82 . B2 , i, or 0.751 are not to be taken as less than 1.0 in calculations. L l l i 7
~
=
9 3.3 Significance of Torsional Moments Even though we have seen that Reference 5 did not. require the use of torsional moments in primary stress calculations, what is the practical difference between including the torsional moment and leaving it out7 If this comparison is made, it ought to be between the requirements of the 1967 Code using 1.01 and the sumer 1973 addenda, which uses 0.751 (where < 0.751 > 1.0). To check equivalence let i(Mf+Mf)l/2 " 0.751 (M +Mf+Mf)l/2 . Z Z assuming i has not changed (not true for socket welds) (Mo+Mf)l/2 - 0.75(Mf+Mf+Mf)l/2 M o+Mf=.56(Mo+Mf+Mf) 0.44(Mf+Mf) - 2 Mt 0.56-Mt .88(Mf+Mf)1/2 Therefore the torsional moment needs to be 88% of the resultant bending moment to match the stresses calculated using the 1967 Code. Torsional moments smaller than this would result in calculation of stresses lower l than calculated using the resultant bending moment in the 1967 Code. 8 _ _ _ - - - _ - _ . b
- y. ,
.- i If one were to ignore the "0.751" factor as part of the change, one might ask wnat is the effect of torsion when. combined with bending. stresses?
Figure 1.5.3.5 of Reference 14 is an interaction diagram for circular tubes ; with bending and torsional load. For a torsional moment of 50% of the l total allowable torsion moment, the resulting bending. stress can still be 85% of the-allowable stress in pure bending. This shows that low to 3 moderate levels of torsional moments do not significantly lower the bending-capacity of straight pipe. Figure 1 of the Foreword to Reference 15 has a similar interaction diagram for pipe under high internal pressure (membrane stress - S ). This figure also shows that low to moderate levels of torsional moment do not significantly lower bending capacity. This is also shown in Reference 20, which investigated the effects of pressure, moment, axial force and torsion on tubes. Figure 10 of this paper shows that a torsional moment equal to 40% of the ultimate torsional moment only reduces the bending capacity of the pipe (tube) by about 12%. 4.0 HISTORY OF CODE CHANGES l This discussion concerning'the intent or correctness of the early piping Code (Reference 5) would not be complete without an historical perspective from those who were responsible for writing these specific Code rules,'for they are the ones best suited to interpret what their interest was. Of the authors of this paper, one (R. F. Petrokas) was a member of the ASME Code Committee for Nuclear Piping and attended meetings of this group beginning i around 1974. The other (E. O. Swain) was a long-time member of the ASME Code Committee for Nuclear Piping and is a long-term member of 831. The discussions which follow in Subsections 4.1 and 4.2 relate historical information on interpretation of the intent of the early piping codes from two individuals who were present at the time. One of these individuals is l a co-author of this report. The other, Sam E. Moore, has been a long-term participant on the ASME Working Group on Piping. L_____ _ _ _ _ _ _ _ _ _ _
J. ,
- o. ,
]
i 1 4.1 Individual Contact - S. E. Moore Conversations took place with Mr. Sam Moore of Oak Ridge-National' Laboratory. Mr. Moore has a long history in research into the behavior of piping and piping products. He is a co-author of References 10 and 11. Mr. Moore's discussion revealed his opinion that 831.1.0-1967 (Reference 5) was based on the maximum stress theory and its evaluation requirements did not include all stresses. With the publication cf B31.7-1969 (Reference 15), evaluation criteria for Class 1 piping were based on the maximum shear stress criteria which required the evaluation of all three principal stresses (which in turn requires the use of all three moments). Later changes to nuclear piping rules imposed the use of 0.751 rather than the i factor to calculate primary stresses as a way to approximate the 82 stress index, but still allow the use of "1" factors (which were so familiar to designers of 831.1 and ASME Class 2 and 3 piping). The Code was not entirely correct in the way primary stresses were handled, and this was changed like a lot of other information in the Code. He felt there were plenty of other conservatism available to cover any concerns these changes may have caused., Among these are: l l
. Adding (peak) pressure and moment coincidentally in time. This I only briefly occurs during an earthquake. '
1 Maximum stress vectors do not all occur at the same point in a I fitting. Use of minimum properties for material strength. , The Codes have recently begun to discover that dynamic loads I like earthquakes do not behave like static loads anyhow. ; The fact that changes were made to analysis requirements does not invalidate work done to the old Code. In fact, ASME Section III, NCA-1140, recognizes the legitimacy of the original Code of Construction. Finally, Mr. Moore philosophically agreed that there was probably not much practical difference between using two moments with an "i" intensification and three moments with "0.751". 10 _ _ _ _ _ - - - _ _ _ _ _ _ _ _ _ _ _ _ _ _ - l
~
I l i 4.2 831.1 Code Chanaes 1 The following discussion is based on the 831 participation of E. O. Swain. ! 4.2.1 Reason for 831.1 Chances As can be seen from References 17 and 18, E. O. Swain initiated and led the effort to introduce the equations that. define the longitudinal primary stresses due to sustained and occasional loads into B31.1. The principal and perhaps only reason for the addition of these equations to the Code was 1 to answer the many questions and problems that arose on how the Code criteria was to be interpreted and applied. There was never a feeling that changes were needed to correct Code error or unsafe criteria. The motivations for placing these equations in the Code are discussed below. Most of the piping being designed, analyzed, and installed in nuclear power plants in the late 1960's and early 1970's was 831.1 piping. Because of the many Quality Assurance and NRC regulatory requirements it was important to have~ reasonable consistency throughout industry on how the stress criteria of B31.1 was being applied. Experience at the time showed there was a wide difference in how the 831.1 criteria was being applied. The equations were aimed at providing that consistency. Because of the large number of nuclear plants being designed and constructed and the large amount of piping in each of these plants, there was a demand for very large numbers of piping engineers. Many of the new engineers entering the piping field had little piping analysis experience and limited experience in working with piping codes. General criteria were not always enough under these circumstances. Detailed requirements of the type supplied by the stress equations provided a way to get more uniform interpretations of the Code by these many new piping engineers. 11
~
)
\
The large amount of complex stress analysis, particularly earthquake analysis, required that nuclear piping analysis be done on computers. The computers perform the analysis in two basic steps. First, a structural or flexibility analysis is made on the piping system to determine the forces, moments and deflections from each of the external loading conditions. Second, the moments are used to l calculate pipe stresses for comparison with allowable stresses and allowable stress ranges. The Code committees had to provide clear, consistent, and easily understood requirements so computer programs, stress reports, and licensing documents would be reasonably consistent throughout the industry. 4.2.2 Reason for Use of Resultant of Three Moments The equations added to 831.1 allowed, but did not require (1), the use of the resultant of three moments rather than longitudinal'. bending stresses that excluded torsional effects. The Code Committee was careful not to change the Code wording of the criteria itself. The written criteria in Paragraphs 102.3.2(D) " Longitudinal Stresses" and 102.3.3 " Limits of Calculated Stresses Due to Occasional Loads" still use the expression
" longitudinal stresses."
There were two reasons the resultant of three moments was allowed. First, the resultant of three moments was easier for the computer to calculate and easier for the analyst to interpret. Nuclear piping was generally defined by global coordinates and conversion of global moments to moments about a local axis was often confusing and a source of error, particularly for piping running on a skewed axis. The second reason was to provide consistency with B31.7 and with ASME III. (1) Paragraph 104.8.4 A of Reference 7. 12
1 . 3 4 , 4.2.3 Some Recent Code Actions Recardina Primary Stress Ecuations There has recently been a large amount of testing throughout the world that shows earthquake or any other building filtered loads do not create a collapse failure mechanism in piping. EPRI and the NRC have recently sponsored an extensive and comprehensive series of high level tests of piping components and piping systems to examine the failure mode of piping subjected to vibratory loads many times more powerful than earthquake' and other dynamic. design loads. These tests, as well as many tests performed by others, show the failure mode to be one of-fatigue or fatigue ratcheting, not collapse (see Reference 21). Although there,are several reasons why collapse, the primary stress failure mode, did not occur, it is clear that the inertia loads reverse in direction so rapidly there is not time for a collapse of the piping system to occur. The main purpose of Equations 11 and 12 in ANSI B31.1 (Reference 7) and Equation 9 in NX3600 of Section III (Reference 8) is to provide protection against collapse. Because the many test results show earthquake does not produce a collapse failure mode, Section III has passed two Code Cases j (Code Cases N-451 and N-462) covering Class 1, 2 and 3 piping that allows i earthquake and other building filtered loads to be excluded from Equation 9 for Service Level B loads (References 22 and 23). Because of this new understanding of piping response to cyclic loadings i such as earthquake, the method of moment combination in the primary stress equations is becoming of second or third order importance. I 4.3 Archival Information Other documentation that has come to light also reinforces the opinion that combining two bending moments met the intent of B31.1.0-1967. 13
e ,_ > , Reference 16. is an update of the document which was prepared to provide
' guidance on mechanical design to members of the B31 Committee. The stated purpose of this nonmandatory document "is for guidance purposes to help estabitsh a unanimity of engineering logic and mechanical design criteria among the various B31 Committees." In the portion of Reference 16 which discusses the flexibility factor and stress intensification factor (i) for each piping product, the intensification factor for torsion is always noted as being unity (also noted in Reference 13). This is consistent with the application in Equation 2, above, the equation required for expansion stress evaluation. No specific requirements on the methodology to be used for determination of primary stresses are given except as discussed in the following sentence, although the stresses due to weight and other sustained I loads are specifically stated as being longitudinal stresses. It is especially important to note that Reference 16, page 4, notes "in determining the longitudinal stress due to weight, stress intensification factors need not be appiled." This shows that the B31 Committee did not intend for the "1" factors to be applied to primary stresses, leading one to conclude that analyses were conservative when "i" factors were applied to primary bending moments. It would therefore be a correct interpretation of the 1967 Edition of B31.1 to omit the stress intensification factor as well as torsional moments.
References 17 and 18 are' Code Correspondence from around the time of j discussion of the summer 1973 rules. Reference 17 notes that torsional l moments can probably be excluded from primary stress calculations, but that l this is not stated in the Code. Reference 18, in discussing the proposed (summer 1973) new Code rules, notes that there are currently questions on how to combine moments.
5.0 CONCLUSION
S
.From the discussion contained herein, the following conclusions can be drawn:
14 t I
} T - l J
- 1. USAS'B31.1.0-1967 did not require that torsional moments be considered when calculating longitudinal stresses in accordance with Paragraph 102.3.2(D), " Additive Stresses" and Paragraph 102.3.3,
" Limits of Calculated Stresses Due to Occasional Loads."
I (
- 2. USAS B31.1.0-1967 did not require the application of stress intensification factors when calculating longitudinal stresses in accordance with Paragraph 102.3.2(D), " Additive Stresses" and Paragraph 102.3.3, " Limits of Calculated Stresses Due to Occasional Loads."
- 3. The equations for calculating primary stresses for sustained and occasional loads were added (Reference 7) for clarification and consistent Code application throughout a rapidly growing power piping industry. The equations were-not added to correct an error or deficiency in the Code.
- 4. The interpretation of the 1967 Edition of B31.1 as defined in the equation below is acceptable.
I Pd2 i / gj2 + go2 S , L " Do2-d2+ Z < 1.2 Sh (OBE) i j where, l Mi and Mo = in-plane and out-of-plane moments due to weight and ! earthquake, l
- 5. It is the policy of the Code Committees to not require design or analysis performed to one edition of the Code to be updated to later revisions of the Cods.
- 6. It is now widely recognized that earthquake and other similar cyclic loadings do not cause primary stress failures (collapse) in piping 15 l
_ _ _ _ _ _ _ _ _ _ _ _ - l
.6 . . . .i , <
'*e .
systems. Showing that stresses due to earthquake comply with present { primary stress limits is unnecessarily conservative and may lead to excessive seismic restraints and undesirable loss of piping flexibility. Therefore, use of the SRSS sum of the two bending moments to calculate ' stresses due to sustained and occasional loads meets the requirements of USAS B31.1.0-1967, i I l 16 i
. . ~
l l
6.0 REFERENCES
- 1. " Fatigue Tests of Welding Elbows and Comparable Double-Mitre Bends,"
by A.R.C. Markl, Trans. ASME, Vol. 69, 1947.
- 2. "c atigue Tests of Piping Components," by A.R.C. Markl, Trans. ASME, Vol . 74, No. 3,1952.
- 3. " Piping Flexibility Analysis," by A.R.C. Markl, Trans. ASME, February 1955.
- 4. ASA B31.1-1955, " Code for Pressure Piping," Published oy ASME.
- 5. USAS B31.1.0-1967, " Power Piping," Published by ASME.
- 6. ANSI B31.1-1973, " Power Piping," Published by ASME.
- 7. ANSI B31.lb-1973, " Summer Addenda, Power Piping, ANSI B31.1-1973,"
Published by ASME.
- 8. ASME Boiler and Pressure Vessel Code, Section III, Subsection NC, 1977 Edition, Published by ASME. .
1
- 9. ASME Boiler and Pressure Vessel Code, Section III, Subsection NC, 1986 Edition, Published by ASME. )
l
- 10. "End Effects on Elbows Subjected to Moment Loading," by E.C.
Rodabaugh, S.K. Iskander, S.E. Moore, ORNL/sub-2913/7, March 1978. !
- 11. " Stress Indices and Flexibility Factors for Nozzles in Pressure Vessels and Piping," by E.C. Rodabaugh, S.E. Moore, NUREG/CR-0778, June 1979.
- 12. " Accuracy of Stress Intensification Factors for Branch Connections,"
by E.C. Rodabaugh, WRC Bulletin 329, Published by Welding Research Council, December 1987.
- 13. " Overview of Structural Design of Piping Systems," by R.W. Schneider, Published by Pressure Fittings Division, G&W Energy Products Group, Allentown PA, 18105, 1978.
- 14. " Military Standardization Handbook, Metallic Materials and Elements for Aerospace Vehicle Structures," MIL-HDBK-5C, Published by U.S.
Department of Defense,1976.
- 15. USAS 831.7-1969, " Nuclear Power Piping," Published by ASME.
- 16. " Revision of October 1958 ASME B31 Mechanical Design Committee Report to B31 Main Committee," ASME Code for Pressure Piping, September 1982.
17
-b ,
<4 l
- 17. 831 Committee Correspondence, E.0. Swain to W.G. Hegener, March 29, 1971.-
- 18. Letter Ballot,' ANSI B31, Section Committee 1 - Power Piping, August 3, 1972. .
- 19. 831.1 Committee Correspondence, E. O. Swain and R. Haupt to Distribution, " Primary Stress Comparison ANSI B31.1 vs. ASME Section III Class 2," Item 21340 Design Subgroup Minutes, January 20, 1982.
- 20. " Limit Loads for Tubes Under Internal Pressure, Bending Moment, Axial Force, and Torsion," W. F. Stokey, D. B. Peterson, R. A. Wunder, Nuclear Enoineerina and Desian, 1 (1966) pp 193-201.
. 21. " Commentary on Code Cases for Alternative Rules for the Analysis of Piping Products Under Seismic Loads," Attached to Minutes of the ASME Working Group on Piping Design, February 9, 1987.
- 22. ASME Boiler and Pressure Vessel Code Case N-451, " Alternative Rules ,
for Analysis of Piping Under Seismic loading, Class 1," dated July 27, ' 1987.
- 23. ASME Boiler and Pressure Vessel Code Case N-462, " Alternative Rules for Analysis of Piping Products Under Seismic Loading, Class 2 and 3 Piping Systems," dated July 27, 1988.
l 1 1 l l 1 l i 18 t
o , s
. 1 APPENDIX A EVOLUTION OF CODE ANALYSIS REQUIREMENTS (MOMENT LOADINGS) l i
I Much of the bases of 831.1 (Code) and ASME Section III (Section III) have. l their roots in the work of A.R.C. Markl in the. late 1940's and early 1950's (References 1, 2, 3). Markl's work and the resulting Code design rules were based on the premise that " Cyclically varying bending moments resulting from thermal expansion and contraction, from pressure pulsation, or from vibrations":(Reference 3) needed to'be evaluated to appraise the safety of piping.. The concept of " occasional loads" (defined in subsequent piping codes) is not mentioned as being a factor or concern in Markl's work j as documented in References 1, 2, and 3. Note that an actual set of proposed Code design rules is attached to Reference 3. These rules were produced by a 831 working group consisting of Messrs. Markl, Spielvogel, Blair, and Wallstrom. Reference 1 is Markl's paper which first discusses the need to predict the fatigue life of piping. He recognized the need to predict the behavior of l piping products (e.g. fittings and flanges) as well as the straight pipe { itself. Marki talks about the use of allowable stress range as the failurt criterion in effect in B31 at the time. Reference 1 also describes initial set of tests on 90* elbows and mitre bends. The specimens were tested to failure by cyclic bending in and out of the plane of curvature t3 the point ! of failure (a crack). The concent of stress intensification associated I with, bending stresses was introduced. Finally, Reference 1 attempts to develop several failure criteria which match the test data. One of these criteria uses torsional (shear) stress as part of the formulation. No ! mention is made regarding concern for occasional loads. Reference 2 greatly extends the information available in Reference 1 and ) i appears to be about five years farther along in time. This paper reports on a number of tests of straight pipe, welded pipe, elbows, mitre bends, tees and intersections. Fatigue test data for these fittings are compared - A-1 i
o- ;
. . , 's l
to a reference. The fatigue life of the fitting is compared to that of the reference, which was chosen to be a piece of straight pipe with a girth butt weld. The fatigue life of each item was determined to be approximately related by (1) i S = 245,000 N-0.2 where i - stress intensification factor N -. number of cycles of stress reversal to failure S= stress amplitude corresponding to N cycles It should be emphasized that the reference (base) case (i = 1) is a butt welded pipe. If an unwelded piece of pipe'had been used as a base case, ' data showed that i for that case should.be approximately 0.5. This should indicate that use of "i" to calculate primary (longitudinal) stress is not entirely accurate. 'In reading Reference 2, note that data regarding the fatigue strength of all fittings was derived from tests using bending moments. Torsional loadings were not used. In summary, Reference 2 itself l states that "the primary use of the data is to provide design constants for the analysis of stresses caused in piping systems by thermal expansion." L Reference 3 follows on Markl's work by applying the test results from the first two references to the flexibility analysis of piping systems. In l Reference 3 we see the development of proposed rules which were incorporated into ASA B31.1-1955 (Reference 4). In fact,: Appendix 1 of i l Reference 3 is the proposed text for what was to become of the design rules of Reference 4 (e.g. equations 13 and 14) produced by a 831.1 working group ' in which Marki participated. Note that Appendix 1 is still talking about expansion stresses with.the formula: (2) SE" !h+4S S A-2 L_ _ - l
. i
[ . where Sb " I Mb/Z St " Mt/2Z
'i = stress intensification factor defined in Eq. (1), above l
Mb is the resultant bending ir.oment and is. distinguished from the torsional momenttM .. Note that the "i" factor is only used with the bending stress in keeping with its derivation from bending test data. It is of further
~
interest to note that Markl, in Appendix 1 of Reference 3, talks about the need to design and space supports to assure that longitudinal stress due to weight and pressure does not exceed the hot. allowable stress, S *' h Apparently this is the beginning of the separate set of Code requirements to protect against collapse due to primary stress. However, as mentioned' l . earlier, the concern that drove Markl's work and the resulting code rules, i including the concept of stress intensification factors, was cyclic i stresses due primarily to expansion. i As discussed above, part of Marki's and the B31 working group's work was suggested Code rules for piping evaluation. These were incorporated into the 1955 Edition of B31.1 (Reference 4). Starting with this Code, we can trace the development of Code rules for the evaluation of the expansion ( stress range and longitudinal (primary) stresses due to external loads
-{
(including pressure). Table A-1 summarizes the evolution of Code and { Section III rules for the evaluation of primary stresses in piping. l Table A-1 shows that the early (1955) Code was primarily interested in expansion stress and provided an explicit equation for calculating it using both bending and torsional moments. The designer is told to calculate longitudinal stresses due to weight, pressure and other sustained loads. No specific guidance on how to do this is given, but the requirement for longitudinal stress leads one to conclude that bending stresses (due to bending moments) should be used. The 1967 Code still does not provide an i A-3
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explicit equation for longitudinal stress, but again the requirement leads one to use bending moments. The 1967 Code is clear in defining bending- 1 stress as one based on the SRSS sum of the in-plane and out-of-plane ! bending moments. This consistency is maintained up to the 1973 edition, which continues to define bending stress as being based on the SRSS sum of two bending moments. Finally, in the summer 1973 addenda t'o the Code, the { requirements for primary stress calculation have been s scifically stated and an equation is given for calculation of pressure and sustained and occasional loads. For the first time, the Code suooests that the resultant moment required (see Table A-1) be the SRSS of all three moments, including torsion. Also, note that instead of using "i" for stress intensification, "0.751" is used. 1 Finally, it is worthwhile to note that changes beyond what occurred in 831.1 have taken place. For example, in the 1986 Edition of Section III l (Reference 9) we see that "i" factors are no longer used in calculating primary stresses in Class 2 piping. They are used strictly for expansion ; stresses (which is why they were originally derived). Now the Bi and 82 stress indices from NB-3680 are used for primary stresses. These indices 'l are more representative of the actual state of stress in the fitting than ; are the use of "i". i
- In summary, the use of the two bending moments for calculation of i longitudinal piping stress as required by USAS B31.1.0-1967 is correct and consistent with changes made to later editions of the Code. The later introduction of all three moments in the summer 1973 addenda to B31.1 was part of the continuing evolution of the Code and similar rules in Section III.
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FLOOR DANIEL I l l j I
SUMMARY
REPORT ON BAMPLING STUDY TO EVALUATE - THE EFFEOT OF TORBIONAL MOMENT l ON PIPE STRESS QUALIFICATION. PRAIRIE ISLAND AND KEWAUNEE l NUCLEAR POWER PLANTS i l Project No. 834486 j Report No. R.834486-1 Date: March 3, 1989 , Prepared By: R. N.-Patel Reviewed By: D. L. Dickerson J. K. Kaliyadan Approved By: A. Casillo
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m s FLUOR DANIEL Report No. R.834486-1 Project No. 834486 l 1 TABLE OF CONTENTS ;
1.0 INTRODUCTION
/
SUMMARY
2.0 EVALUATION 2.1 SELECTION OF SAMPLES 2.2 SCREENING OF SAMPLES 3.3 ANALYSIS OF RELEVANT SAMPLES 3.0 RESULT i l
4.0 CONCLUSION
i 5.O REFERENCES ! i L_______-- - _ - - - - - _ _ . - - _ -
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l i c FLUOR DANIEL Report No. R.834486-1 Project No. 834486 Page 1 of 12 i 1.O INTRODUCTION /
SUMMARY
During the recent NRC audit, the question was raised regarding the interpretation of the term " longitudinal stress" in USAS B31.1.0-1967 Power Piping Code, which is the licensing basis for Prairie Island and Kewaunee Nuclear Generating Plants ! (References 2 and 7) . The question was raised: "Does the Fluor Daniel interpretation of using two bending moments in pipe stress calculation for Upset and Faulted loading combinations adequately meet the intent of the codo?" In. response to this question, in addition to the technical justification provided in Reference 6, this study is performed I to determine the effect of the torsional moment and show that the piping systems are within the limits of the USAR criteria. The results of this study shows that it is not necessary to include the torsional monent in the stress calcul,ation and that the interpretation of the term " longitudinal stress" is correct. The inclusion of torsional moment is considered outside the design basis for these plants. This study is based on the statistical method of selecting samples and analyzing them. The evaluation is performed in accordance with the proceduro contained in Reference 4. According to the procedure, the size of the randomly selected sample should be 58 to achieve a 95% confidence level that the total population conforms to the results and conclusions. l
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FLUOR DANIEL Report No. R.834486-1 Project No. 834486 i Page 2 of 12 i i i Section 2.1 describes the random selection of samples. These l samples are screened in regard to the minimum stress levels ! defined in the procedure to determine the relevant samples. There are a total of 13 analytical parts found as relevant as discussed in section 2.2. These relevant analytical parts are analyzed for pipe stress qualification, including the effect ; of torsional moment. The result show that all of the 13 i relevant samples meet the USAR criteria. l L__-________-___ .-. ._ _ _3
m FLUOR DANIEL Report No. R.834486-1 Project No. 834486 Page 3 of 12 2.0 EVALUATION There are a total 1,428 safety-related piping analytical parts existing for' Prairie Island and Kewaunee Projects combined. A sampling study, based on a random selection method, is adopted for this study. The following subsections describe and document the evaluation performed. 2.1 SELECTION OF SAMPLES To randomly select the required samples of analytical parts, a PC program - written in BASIC with the Random Generator function is used. Each of the 1,428 analytical parts is assigned a unique record number from 1 to 1,428. Reference 5 of this report contains the documentation of the random selection process and the list of all analy-tical parts. The 58 randomly selected analytical parts, which are used in this study, are listed in Table 2.2. This sample size achieves a 95% confidence level and'is consistent with the sample size requirement of IE Bulletin 79-02. 2.2 SCREENING OF SAMPLES The objective of screening the samples is to eliminate
,those analytical parts having a stress level too low to cause an overstress when the torsional moment is included in the pipe stress calculation. The screening criteria, as discussed in Reference 4, is as follows: '
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7 m Report No. R.834486-1 Project No. 834486 ag 4 f 2 FLtH3R DANIEL TABLE 2.2
SUMMARY
OF EXISTING STRESS LEVELS FOR 58 SAMPLES RECORD REPORT PART SYSTEM INDEX PIPE MAX STRESS RATIO STRESSES ** NO. NO. NO. ID NO. SIZE + UPSET FAULTED OBE4+ 1/2 Sh 662 PI-233-50 16 CL 126 S 0.3288 0.2598 1124 7500
- 442 PI-216-3 3 RH 010 S 0.4712 0.5404 5309 6500 737 PI-233-27 33B CC 547 L/S 0.3330 0.4370 8253 7500
- 424 PI-213-9 2CA-2 CS 018 S 0.4520- 0.5840 7641 7500 992 PI-233-44 47B CC 653 S 0.1275 0.1000 863 7500 308 PI-211-14 10F WG 037 S 0.0630 0.0052 540 7500 996 PI-223-34 7B CC 657 L/S 0.1440 0.1660 1889 7500
- 792 PI-234-19 3B RC 039 S 0.8135 0.6240 3671 5550 397 PI-211-22 8A-1 WG 134 S 0.1380 0.0967 176 7500 670 PI-233-51 25 CL 134 L/S 0.2457 0.2480 2291 7500
- 18 PI-205-IV 4 8, , SI 011 L 0.7033 0.7567 6748 6500 323 PI-211-15 11NB WG 052 S 0.0687 0.0540 220 7500 929 PI-326-I D FO 034 L 0.1420 0.1060 246 7500 170 PI-206-40 35A VC 557 S 0.3413 0.2771 2367 6683 967 PI-233-38 28A CC 628 S 0.1400 0.1770 2281 7500 788 PI-234-18 2F RC 035 S 0.2600 0.3230 4609 8138 734 PI-233-27 32A(UPPER) CC 544 L/S 0.4660 0.4070 2674 7500 953 PI-233-37 ISB CC 614 S 0.1111 0.1053 1290 7500
- 440 PI-216-3 1 RH 008 S 0.5560 0.6750 7148 7500 247 PI-206-59 39B VC 634 S 0.0217 0.0208 293 9375
- 581 PI-233-XV VI CL 012 L 0.6210 0.7490 9062 7500
- 448 PI-216-V 9A AND B RH 014 L 0.6600 0.7307 8306 7625 650 PI-233-12 1 CL 114 L/S 0.1230 0.0986 690 7500 959 PI-233-38 19A CC 620 S 0.1794 0.2344 2266 750
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, RIport No. R.834486-1 l Project No. 834486 l FLUOR DANIEL Page 5 of 12 I l
TABLE 2.2 (Continued) .
SUMMARY
OF EXISTING STRESS LEVELS FOR 58 SAMPLES RECORD REPORT PART SYSTEM INDEX PIPE MAX STRESS RATIO STRESSES ** NO. NO. NO. ID NO. SIZE 4 UPSET FAULTED OBE++ 1/? Sh j 744 PI-233-29 32B CC 554 L/S 0.2069 0.2350 2709 7500
- 837 PI-310-2 4A HC 008 S 0.5480 0.6110 8379 7500 I 205 PI-206-44 40-II VC 592 S 0.0680 0.0520 156 8825 508 PI-221-14 MS-24 MS 061 S 0.4420 0.3962 3849 7500 722 PI-233-24 36 CC 532 L/S 0.1985 0.1531 790 7500 554 PI-224 3 AF 014 S 0.2605 0.2212 1674 7500 972 PI-233-40 27B CC 633 L/S 0.0470 0.0345 310 7500 248 PI-206-59 39C VC 635 S 0.0217 0.0208 293 9375
- 24 PI-205-VIII V SI 017 L 0.5010 0.4610 3496 7156 293 PI-211-12 4D WG 022 S 0.3358 0.3057 2064 7000 604 PI-233-VI CL-SFP CL 035 L 0.1108 0.0845 289 7500 875 PI-221-23 2MS-6R MS 102 S 0.2124 0.1565 460 7500 136 PI-206-26 10Q VC 523 S 0.3270 0.3939 5372 8482
- 802 PI-234-22 RC-25 RC 049 S 0.5068 0.4049 1747 7215 647 PI-233-14 8 CL 111 S 0.4400 0.4150 3317 7500 853 PI-321-1 8 EG 008 L 0.1160 0.0800 85 5400 924 PI-326-8 29A FO 029 S 0.0916 0.0810 951 7500
*1118 CC-31-011 31-011 CC 007 L 0.5050 0.6360 8441 7500 l 1094 KEW-206-16 15 CVC 051 S 0.4190 d.3325 1729 8963 1030 KEW-016-1 4B RBV 004 S 0.1705 0.1440 1525 7500
*1107 KEW-206-15 8(INT.) CVC 064 S 0.6176 0.7448 10419 8688
*1100 KEW-206-14 2(INT.) CVC 057 S 0.6126 0.5862 6483 8688 1289 KEW-MS-5 19 SW 045 S 0.0994 0.0803 658 7500
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, . Report No. R.8344'6-1 8 Project No.. 834486 FLUOR DANIEL Page 6 of 12 1 TABLE 2.2 (Continued)
SUMMARY
OF EXISTING STRESS LEVELS FOR 58 SAMPLES RECORD REPORT PART SYSTEM INDEX PIPE MAX STRESS RATIO STRESSES ** NO. NO. . NO . ID NO. SIZE 4 UPSET FAULTED OBE4+ 1/2 Sh 1195 KEW-MS-8 56 GWP 038 S 0.2685 0.1931 1004 7500 1042 KEW-205-6A 3A SI 011 S 0.2790 0.2432 2706 9155 1291 KEW-MS-6 24 SW 048 S 0.1940 0.1795 1355 7500 1388 SW-02-002 02-002 SW 093 L 0.2199 0.2421 2901 7S00 1098 KEW-206-16 19 CVC 055 S 0.1717 0.1205 818 9375 1126 KEW-210-6 VI CC 018 S 0.2084 0.1769 1692 7500 1279 KEW-233-10 IV.BB SW 034 L 0.2688 0.3136 3683 7500 1258 SW-02-016 02-016 SW 010 L 0.1400 0.1206 1021 7500 1063 KEW-206-7 12 CVC 018 S 0.2320 0.1985 2892 8963 1358 RHR-34-004 34-004 RHR 001 L 0.4522 0.4467 3918 7025 1287 KEW-MS-3 14 ( A) SW 043 S 0.1526 0.1374 1236 7500 i l NOTES:
- These samples are the relevant analytical parts.
** stresses are in psi.
+ Pipe size, S = small bore and L = large bore l
++ Includes torsional stresses.
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4 e i FLUOR DANIEL i Report No. R.834486-1 l'
' Project No. 834486 Page 7 of 12 Any of the analytical parts having either -
l a) maximum Upset or Faulted stress ratio to the allowable 2 0.5, or b) maximum OBE stress, including the effect of torsion, 2 Se h (S h material allowable stress 88 at design temperature) is considered relevant and is evaluated further. For each of the 58 samples, the existing maximum stress ratios for Upset and Faulted conditions and the maximum OBE stress values are obtained from the Analysis of Record (AOR). Table 2.2, which is extracted from Reference 3 and included in this report, summarizes the existing maximum stress ratios and maximum OBE stress, along with the \ S3 values for each of the 58 analytical i parts. After screening the 58 samples, there are 13 relevant i samples which are chosen for further study. These relevant samples are identified in Table 2.2. 2.3 ANALYSIS OF RELEVANT SAMPLES USAS B31.1.0-1967 Tode (Reference 1) does not provide an equation to calculate the combined pipe stress due to pressure, sustained and occasional loads. The method used to calculate the combined stress including the effect of torsional moment is provided in Reference 4. The resultant moment due to deadweight and seismic loadings are calculated using the square root of the sum
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of squares (SRSS) of the three moments with the stress intensification factor applied, as appropriate, to the j l j _____________________m___-_
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, b j FLUOR DANIEL i Report No. R.834486-1 f Project No. 834486 i Page 8 of 12 two bending moments. This approach of including torsional moment is consistent with the calculation of expansion stress presented in the Code (Reference 1).
If USAS B31.1.0-1967 Power Piping Code had required the inclusion of torsional moments, then the method used in this study would have been appropriate. Reference 3 documents the pipe stress analysis for 13 relevant samples, for the inclusion of torsional moment. The revised Upset and Faulted stress ratios to the i allowable are tabulated as shown in Table 3.1. of l i
4.- *' (o , m FLUOR DANIEL
, Report No. R.834486-1 Project No. 834486 Page 9 of 12 3.0- RESULT All of the 13 relevant analytical parts meet the USAR criteria when the torsional moment is included in the pipe stress calculation. Table 3.1 provides the summary of Upset and Faulted stress ratios for "without torsion" and "with torsion" casos for each of the 13 relevant analytical parts. The distrib'41 ion of 58 samples, in regards to the pipe size (small bore and large bore) is shown in Table 3.1 as supplementary information. Increases in combined stresses for Upset and Faulted loadings are nominal when the torsional moment is included in the stress calculation. For the worst case, which is Report Nc, PI-234-19, Part No. 3B, the Upset stress ratio is increased from 0.813 to 0.870 after the torsional moment is included.
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\ a m' u t . w.. FLOOR DANIEL ' Report No. R.834486-1 Project No. 834486 3 ' Page 10 of 12 TABLE 3.1 RESULTS OF SAMPLING STUDY l
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DISTRIBUTION OF 58 SAMPLES: PRAIRIE ISLAND KEWAUNEE ORIGINAL SAMPLES 41 17 STRICTLY LARGE' BORE 7 5 SMALL-LARGE BORE 8 0 STRICTLY SMALL BORE 26 12 RELEVANT ANALYTICAL PARTS MAXIMUM STRESS RATIO WITHOUT TORSION WITH TORSION REPORT NO. PART NO. UPSET FAULTED UPSET
- FAULTED
- PI-216-3 3 0.471 0.540 0.490 0.560 PI-213-9 2CA-2 0.452 0.584 0.520 0.630 PI-234-19 3B 0.813 0.624 0.870 0.G90 PI-205-IV 48 0.703 0.757 0.780 0.810 PI-216-3 1 0.556- 0.675 0.740 0.800 PI-233-YV VI 0.621 0.749 0.650 0.770 PI-216-V 9A&9B 0.660 0.731 0.720 0.790 PI-310-2 4A 0.548 0.611 0.640 0.720
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PI-205-VIII V 0.501 0.461 0.560 0.510 PI-234-22 RC-25 0.507 0.405 0.530 0.430 CC-31-011 31-011 0.505 0.636 0.530 0.670 KEW-206-15 8(INT) 0.618 0.745 0.640 0.760 KEW-206-14 2(INT) 0.613 0.586 0.660 0.650
- NOTE: The stress ratios are conservatively calculated according to Reference 4.
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FLUOR DANIEL
. Report No. R.834486-1 Project No. 834486 Page 11 of 12 4.O CONCLUSION All of the 58 sample analytical parts satisfy the USAR criteria when the torsional moment is included in the combined pipe stress calculation for Upset and Faulted conditions.
From this study, it is concluded that the torsional moment does not contribute significantly to the pipe stresses. I, l
u,,., - 4 l_ '.. FLUOR DANIEL Report No. R.834486-1. Project No. 834486 i Page 12 of 12 l l i
5.0 REFERENCES
(1)' USAS B31.1.0-1967, Nuclear Power Piping Code. ! (2) USAR for Prairie Island Nuclear Generating Plant, Revision dated February 20, 1986. l- (3) " Evaluation of pipe stress, including the effect of torsional moment for 58 sample analytical parts," Calculation No. M.834486.02, Dated 2-24-89, Project No. ] 834486. i (4) " Procedure to perform sampling study to evaluate the j 1 effect of torsional moment on pipe stress qualification," Procedure No. 834486-1, Project No. 834486, Dated February 3, 1989. (5) " Random selection of samples from the total population I of analytical parts contained in Prairie Island and Kewaunee Projects," Calculation No. M.834486.01, Project No. 834486, Dated February 10, 1989. (6) " Documentation of' Methodology used for combination of moments for piping constructed to the USAS B31.1-1967 Power Piping Code," prepared by R.F. Petrokas and E.O. Swain, Dated February, 1989. (7) USAR for Kewaunee Nuclear Power Plant, Revision dated July 1, 1986. __ _______________- _ _ _ -}}