1、 STP-PT-034 ALIGNMENT OF SUSTAINED LOAD STRESS INDICES IN THE ASME 831 CODE Prepared by: George Antaki Becht Engineering Company Inc. r:MM“E STANDARDS TECl-INOLOGY, LLC Date oflssuance: December 22, 2011 This report was prepared as an account of work sponsored by ASME Pressure Teclmologies Codes and
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9、 20 II by ASME Standards Technology, LLC All Rights Reserved Alignment of Sustained Load Stress Indices in the ASME B31 Code STP-PT-034 TABLE OF CONTENTS Fo:reword . . v Abstract . vi Background I 2 Objectives 2 3 Approach 3 4 Existing Sustained Stress Equations and Indices . 4 4.1 Existing Sustaine
10、d Stress Equations 4 4.2 Summary of Stress Equations for Sustained Loads . 4 4.2.1 ASMEB31.1 4 4.2.2 ASME B31.3 5 4.2.3 ASME B31.4 5 4.2.4 ASMEB31.8 6 4.2.5 ASME B31.7 7 4.2.6 ASME Ill Division 1 for Class 2 and 3 Piping . 7 4.3 Comparison . 7 5 Chronological Bibliography . 10 6 Sustained Stress Ind
11、ices Data 17 6.1 The Meaning of the Stress Intensification Factor i 17 6 .1.1 Experimental Fatigue Tests 17 6.1.2 Theoretical Peak Stress 17 6.1.3 Experimental and Theoretical SIF 17 6.2 The Meaning of the ASME III Stress Index B 18 7 Options and Recommendations . . . 21 7.1 Option 1 Continue as is
12、. 21 7 .1.1 Description . 21 7 .1.2 Advantages . 22 7 .1.3 Disadvantages . 22 7.2 Option 2 Consistent Equations with B Indices 22 7 .2.1 Description . 22 7.2.2 Advantages . 22 7 .2.3 Disadvantages . 22 7.3 Option 3 Adopt B31.3 Case 178 23 7.3 .1 Description . 23 7.3.2 Advantages . 23 7.3.3 Disadvant
13、ages . 24 7.4 Recommendation for Sustained Loads 24 Appendix A: ASME B31.1 , 2007 Sustained Stress Equations . 25 Appendix B: ASME B31.3, 2004 Sustained Stress Equations . 31 Appendix C: ASME B31.4, 200 Sustained Stress Equations . 37 Appendix D: ASME B31.8, Sustained Stress Equations 41 111 STP-PT-
14、034 Alignment of Sustained Load Stress Indices in the ASME B31 Code Appendix E: ASME J NC/ND-3600 Sustained Stress Equations . .49 Acknowledgments . 57 Nomenclature 58 LIST OF TABLES Table I - AJ towable Stress Factors ASME III 1977 a.S11 and I3S11 7 Table 2 -Theoretical and Experimental SIFs for an
15、 Unpressurized Pipe Bend . 17 LIST OF FIGURES Figure 1 -Example Comparison of Margins Sustained Stress I Allowable 8 Figure 2- SIF i (Lower Curve) and Stress Index B2 for 1.50 Bl6.9 Elbow 19 Figure 3 - SIF i (Lower Curve) and Stress Index B2 for Equal Leg Tee . 20 IV Alignment of Sustained Load Stre
16、ss Indices in the ASME B31 Code STP-PT-034 FOREWORD Loads on piping systems and pipelines are categorized in ASME B3 I as sustained, occasional or thermal loads. None of the ASME B3l codes explicitly define “sustained loads.“ But because they are often called out as “sustained loads such as pressure
17、 and weight,“ sustained loads are understood to mean pressure and weight. In the case of buried pipe, the soil weight on the pipe would also be a sustained load. Occasional loads are loads “such as wind or earthquake“ to which we may add pressure transients (waterhammer). Finally, thermal expansion
18、and contraction loads and loads due to thermal gradients constitute the third category of loads on piping systems. Unlike ASME VIII or ASME lll, ASME B3l does not refer to “primary“ or “secondary“ loads or stresses. Established in 1880, the American Society of Mechanical Engineers (ASME) is a profes
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22、lish and maintain the technical relevance of codes and standards. Visit www.stllc.asme.org for more information. v STP-PT-034 Alignment of Sustained Load Stress Indices in the ASME B31 Code ABSTRACT Loads on piping systems and pipelines are categorized in ASME B31 as sustained, occasional or thermal
23、 loads. None of the ASME B3l codes explicitly define “sustained loads.“ But because they are often called out as “sustained loads such as pressure and weight,“ sustained loads are tmderstood to mean pressure and weight. In the case of buried pipe, the soil weight on the pipe would also be a sustaine
24、d load. Occasional loads are loads “such as wind or earthquake“ to which we may add pressure transients (waterhammer). Finally, thermal expansion and contraction loads and loads due to thermal gradients constitute the third category of loads on piping systems. Unlike ASME VIII or ASME Ill, ASME B3 i
25、 does not refer to “primary“ or “secondary“ loads or stresses. Each of these load categories (sustained, occasional, thermal) have their own design equations. In the current codes, there are three areas among the piping and pipeline codes design equations for stresses due to sustained loads which de
26、serve attention, clarification and possibly improvement: Consistency of design equations Consistency in the use of stress indices and stress intensification factors Use offatigue-based factors to calculate stresses due to sustained loads The objective of this report is to address these areas and to
27、propose design equations for sustained loads which would be technically sound, practical, and could be applied consistently by all ASME B3 1 Code books and the ASME Section III Code. VI Alignment of Sustained Load Stress Indices in the ASME 831 Code STP-PT-034 1 BACKGROUND Loads on piping systems an
28、d pipelines are categorized in ASME B31 as sustained, occasional or thermal loads (the latter also referred to as displacement strains in ASME B3l). Interestingly, none of the ASME B31 codes explicitly define “sustained loads.“ But because they are often called out as “sustained loads such as pressu
29、re and weight,“ sustained loads are understood to mean pressure (internal or external) and weight. ln the case of buried pipe, the soil weight on the pipe would also be a sustained load. In contrast, occasional loads are loads “such as wind or earthquake“ to which we may add pressure transients (wat
30、erhammer, explosions, etc.). Finally, thermal expansion and contraction loads and loads due to thermal gradients constitute the third category of loads on piping systems. Unlike ASME VIII or ASME III, ASME B3 l does not refer to “primary“ or “secondary“ loads or stresses. There are in the current co
31、des three areas among the piping and pipeline codes design equations for stresses due to sustained loads which deserve attention, clarification and possibly improvement. These are: a. Consistency of design equations. Since the stresses caused by pressure and weight in pipes and pipelines are the sam
32、e irrespective of application (power, process, pipelines), the ASME III and ASME B3l codes should have the same design equations for sustained loads. This is not the case today. b. Consistency in the use of stress indices and stress intensification factors. While ASME III uses stress indices (Sis su
33、ch as B, C and K indices), ASME B31.1 and B31.3 use stress intensification factors (ii and i0 or i). As stated in the request for proposal for this project “since they are defined by principles of engineering mechanics, there should be no differences in stress indices from book to book.“ In fact, th
34、e SIFs and Sis relate to two different failure modes, and are different. c. Use of fatigue-based factors for sustained loads. It is questionable whether stress intensification factors “i“ developed based on cyclic fatigue tests are appropriate for the analysis of sustained loads. As stated in Rodaba
35、ugh and Moore (1984) “There does not appear to be a good reason, however, to use the stress intensification i-factors to evaluate primary loadings.“ 0.75i is only an approximation of a sustained stress index. STP-PT-034 Alignment of Sustained Load Stress Indices in the ASME B31 Code 2 OBJECTIVES The
36、 first objective of this report is to address the discrepancies listed in Section I : Consistency of design equations Consistent use of stress indices and stress intensification factors Use of fatigue-based factors to calculate stresses due to sustained loads. The second objective of this report is
37、to propose design equations for sustained loads wihich would be technically sound, practical (simple) and could be applied consistently by all ASME B31 Code books and the ASME Section III Code. 2 Alignment of Sustained Load Stress Indices in the ASME 831 Code STP-PT-034 3 APPROACH The report takes t
38、he following approach. 1. Conduct a literature search on the topic including compiling the existing sustained load stress indices of B3 1.1, B31.3, B3 1.4 and B31.8, and Section III, Subsection NC!ND. 2. Gather relevant historical data and references. 3. Obtain and reduce recently published test dat
39、a for sustained stress multipliers. 4. Recommend applicable equations for sustained stress indices and notes for use in B31 books and Section lii, Subsection NC/ND. 3 STP-PT-034 Alignment of Sustained Load Stress Indices in the ASME B31 Code 4 EXISTING SUSTAINED STRESS EQUATIONS AND INDICES 4.1 Exis
40、ting Sustained Stress Equations Existing equations for stresses due to sustained loads are reproduced in Appendices: Appendix A ASME B3 1.1 Power Piping Appendix B ASME B31.3 Process Piping Appendix C ASME B31.4 Pjpeline Transportation Systems for Liquid Hydrocarbons and Other Liquids Appendix D ASM
41、E B31.8 Gas Transmission and Distribution Piping Systems Appendix E ASME Ill Division 1 Rules for Construction of Nuclear Facility Components, Subsection NC Class 2 Components. 4.2 Summary of Stress Equations for Sustained Loads 4.2.1 ASME 831.1 4.2.1.1 P ressure Design 8 31.1 The 1955 edition of AS
42、ME B31.1 design for sustained loads included the pressure design equation in the same form as todays: t = PxD 2x (S+Pxy) The 2005 addendum of ASME B3l.l reduced the design margin against ultimate strength (increased the allowable stress S) from 4 to 3.5. 4.2.1.2 Longitudinal Stresses 8 31.1 The 1955
43、 edition did not include sustained load design equations, but, instead, required piping and equipment to “be supported in a thoroughly substantial and workmanlike manner “ This is. consistent with the practice of the 1950s, as described in the M.W. Kellog Co. manual “Design of Piping Systems,“ which
44、 stated: Other loading which may act on piping systems includes: the weight loads of the piping, including structural members, the weight of the insulation and contents; snow and ice loading; wind loading if exposed; loading due to acceleration imparted by earth tremors; special shock loading, such
45、as gun fire or moving vehicles; an unbalanced static pressure or flow effects. It is possible to include any or all of these loads in a complete solution, following the methods of Chapter 5 Flexibility Analysis by the General Analytical Method. Ordinarily, these effects are not sufficiently critical
46、 to warrant the extra engineering cost of this more precise approach. Instead they are indirectly controlled in a standardized way (e.g. support standards) or individually estimated and controlled so that the sum of all effects will approximately meet the same combined stress criterion. 4 Alignment
47、of Sustained Load Stress Indices in the ASME 8 31 Code STP-PT-034 The 1967 edition of B3 1.1 included the requirement that “The sum of longitudinal stresses due to pressure, weight and other sustained loads shall not exceed the allowable stress in the hot condition S,. Where the sum of these stresse
48、s is less than S, the difference between S, and this sum may be added to the term 0.25S, in Formula (1) SA = f (1 .25 Sc + 0.25 S1JJ for determining the allowable stress range SA. The longitudinal pressure stress s,p shall be determined by dividing the end force due to internal pressure . by the cro
49、ss-sectional area of the pipe S1P = P x cf I (D/ - d2).“ There is no formula for the other longitudinal stresses. There was no equation to calculate the longitudinal stresses. The 1977 edition of B31. 1 contains the stress equation in its current form, PD/(4 tn) + 0.75i MA/Z 1.0 sh The single stress intensification factor i, maximum of in-plane i; and out-of-plane i0 was used, and continues to be used in B31 .1, for sim
