Technical Report on Capabilities of API Integral Flanges Under Combination of Loading—Phase II

API TECHNICAL REPORT 6AF2 FOURTH EDITION, OCTOBER 2010

Technical Report on Capabilities of API Integral Flanges Under Combination of Loading—Phase II

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API TECHNICAL REPORT 6AF2 FOURTH EDITION, OCTOBER 2010

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Foreword

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Contents

Page

1. Scope 1

2. Introduction 2

3. Three-dimensional Finite Element Analysis 3

   1. Finite Element Modeling 3

   2. Finite Element Results 11

4. Two-dimensional Heat Transfer Analysis 18

   1. Finite Element Modeling 18

   2. Temperature Analysis Results 18

5. Limiting Criteria for Combined Loading 21

   1. Introduction 21

   2. Leakage Criterion 21

   3. Stress Criterion 23

   4. Development of Load Combination Charts 25

   5. Results 26

6. Reanalysis of Failed Flanges 31

7. Effect of Lockdown Screw Holes on Strength 31

8. Conclusion and Recommendations 38

Annex A—Comparison Between Prac 86-21 and Prac 88-21 Results 40

Annex B—Load Combination Charts 45

Tables

1. Nodal Temperatures for Typical Type 6BX Flanges 20

2. Adopted Stress Criteria 23

3. Flanges That Did Not Meet Makeup-only Criterion, Makeup = 52.5 ksi 33

4. Flange Geometry Modifications to Meet ASME Criterion for 52.5 ksi Makeup 33

5. Lockdown Screw Holes 33

6. Effect of Lockdown Screw Holes on Flange Stiffnesses 34

7. Effect of Lockdown Screw Holes on Flange Combined Load Capability 34

8. Modification Coefficients for Bending Capacity at Zero Tension and Zero Pressure 34

1. Comparison for Type 6B Flanges 41

2. Comparison for Type 6BX Flanges 42

B.1 Makeup Values for Flanges with Stress Criterion Governing at Least for Part of the

Range of the Load Combination 45

Figures

1. A Typical Finite Element Model of Flange and Bolt Superelements 4

2. Unit Load Cases 5

3. Superelemental Hierarchy 7

4. Gasket Modeling 8

5. Typical Lotus 1-2-3 Worksheet for Calculating Equivalent Stresses Due to Applied Forces 9

6. Equivalent Stresses Due to Applied Bending Moment 10

7. Deflected Shape Due to Unit Makeup (1.0 ksi) 11

8. Deflected Shape Due to Unit Pressure 12

9. Deflected Shape Due to Unit Tension (1.0 kip) 12

10. Deflected Shape Due to Bending Moment (1.0 K.in.) 13

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    Page

11. Maximum Shear Stress Contours Due to Unit Makeup 13

12. Maximum Shear Stress Contours Due to Unit Pressure 14

13. Maximum Shear Stress Contours Due to Unit Tension 14

14. Maximum Shear Stress Contours Due to Unit Bending Moment 15

15. Von Mise’s Equivalent Stress Contours for 2 1/16 in. 3,000 psi Type 6B Flange Under 52.5 ksi

    Makeup and 6,000 psi Test Pressure 15

16. Bolt Stress (z) Contours for Unit Makeup 16

17. Bolt Stress (z) Contours for Unit Pressure 16

18. Bolt Stress (z) Contours for Unit Tension 17

19. Bolt Stress (z) Contours for Unit Bending Moment 17

20. Typical Heat Conduction 2-D Mesh for Type 6BX Flanges 19

21. Typical Leakage Load Combination Charts for 52.5 ksi and 40 ksi Makeup Loads 22

22. Stress Linearization 24

23. Critical Section Locations 25

24. Comparison Between 2-D and 3-D Results for Leakage Criteria for 2 1/16 in. 3,000 psi 6B Flange 27

25. Comparison Between 2-D and 3-D Results for Leakage Criteria for 16 3/4 in. 3,000 psi 6B Flange 27

26. Stress Limiting Criteria for 2 1/16 in. 3,000 psi 6B Flange 28

27. Stress Limiting Criteria for 16 3/4 in. 3,000 psi 6B Flange 28

28. Leakage Criteria with Temperature Effect for 2 1/16 in. 3,000 psi 6B Flange. 29

29. Leakage Criteria with Temperature Effect for 16 3/4 in. 3,000 psi 6B Flange 29

30. Stress Criterion with Temperature Effect for 2 1/16 in. 3,000 psi 6B Flange 30

31. Stress Criterion with Temperature Effect for 16 3/4 in. 3,000 psi 6B Flange 30

32. Bolt Stress Contours for 5 1/8 in. 10,000 psi Type 6BX Flange 32

33. Bolt Stress Contours for 11 in. 10,000 psi Type 6BX Flange 32

34. Finite Element Model of Flange with Lockdown Screw Holes 35

35. Effect of Lockdown Screw Holes on Leakage Charts 36

36. Effect of Lockdown Screw Holes on Stress Charts 37

A.1 Hopper Chart (from ASME Boiler and Pressure Vessel Code, Appendix 4) 44

Technical Report on Capabilities of API Integral Flanges Under Combination of Loading—Phase II

1 Scope

The evaluation of the load carrying capacity of API 6A integral flanges is the objective of this work. The applied loading includes the end tension and bending moment in addition to the conventional rated pressure and makeup forces. The effect of a temperature difference corresponding to 250 °F on the inside and 30 °F on the outside was also evaluated.

Three-dimensional finite element meshes were generated for the Type 6B, and Type 6BX flanges. The bending moment load case required a model of one quarter of the flange which was built up from the smaller segments and the half-bolt superelements. The computer program SESAM was used to obtain the stresses at selected critical flange and hub sections and to determine the gasket reaction due to each of the four unit load cases and the temperature difference load case. Leakage criterion was defined as the load combination which reduces the initial makeup compressive forces in the gasket to zero. The stresses in each defined section were linearized in accordance with the ASME Section VIII, Division 2, procedure to determine the membrane and membrane-plus-bending stress intensities. These stress intensities were checked against the allowables specified in API 6A, and the limiting loads were determined. A computer program LCCP was written to carry out this code check and a LOTUS 1-2-3 Release 3 worksheet was used to plot the load combination charts.

The results of the analysis carried out indicate that the leakage criterion governs the capacity of the smaller flanges in the Type 6B flanges. Leakage was governing for up to 9 in. size flanges in both the 52.5 ksi and 40 ksi makeups for the 2000 psi pressure. Leakage was governing the 5 1/8 in. for the higher pressures. Leakage was also found to be governing all Type 6BX flanges for working pressures of up to 5,000 psi. For the 10,000 psi and 15,000 psi flanges, leakage governed only in the larger size range greater than 2 9/16 in. Leakage was governing in all the 20,000 psi API 6BX flanges. The leakage model adopted in this study employs several approximations that have not yet been evaluated. Therefore, the actual leakage forces, i.e. load combinations leading to leakage, may be considerably higher than assumed herein. In reality, the gasket only leaks when its energized capacity is exceeded.

The state of stress at the stress governing hub section under the combined loading of makeup, pressure, tension and bending moment is considered to be “secondary.” However, when pressure, tension, and bending moments are applied together with the necessary makeup to resist these actions without leakage, the state of stress is rendered “primary” and, therefore, the allowable stress intensities are halved. This does not seem to be consistent, and it may by far exceed the intention of the code. However, the oversight subcommittee preferred to adopt the conservative route, which may be overly conservative pending further evaluation. Therefore, it may be concluded that when the hub stresses are treated as primary, most flanges do not possess significant reserve strength beyond the leakage condition. In fact, if the leakage condition was somewhat conservative, the stress condition may become governing for most flanges.

The temperature difference of 250 °F internal and 30 °F external leads to increases in the load-carrying capacity of the flanges. This condition is caused by the compressive forces generated in the gasket due to this temperature difference, and the increase in the allowable stresses when the self-limiting temperature load condition is included. It is recommended that a 3-D finite element, nonlinear material and geometric models of approximately eight flanges be carried out to determine the actual failure mechanism that governs the behavior of these flanges. This includes the prediction of the response of the gasket under increasing load and a more accurate definition of the leakage mechanism. The elimination of the raised face does not significantly reduce the stresses in the hub which caused six Type 6B flanges to fail to meet the ASME criterion for makeup load only (52.5 ksi for 105 ksi bolting). The stress intensities were reduced only by about 5 % when the raised face was eliminated, increasing the thickness of the flange by about 10 %. The hub thickness for these flanges had to be increased by up to about 27 % of their existing thicknesses together with the elimination of the raised face.

The bolt stresses did not govern for any of the flanges analyzed. Bolt stresses are typically within approximately 67 % of their yield strength due to makeup, pressure, tension, and bending moment loads. The bolts are expected to be made up to half their yield. The stresses in the bolts due to temperature differences increase by about 5 ksi to 7 ksi, which is

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