The ASME VIII-1 code’s general approach to bolting is stated succinctly in ASME VIII-1, Appendix S-1:
In the great majority of designs the practice that has been used in the past should be adequate, viz., to follow the design rules in Appendices 2 and Y and tighten the bolts sufficiently to withstand the test pressure without leakage.
In other words, when the design complies with the ASME Code requirements, the bolts can be tightened as required. Thus, when a connection leaks due to relaxation of the joint, “it may suffice merely to retighten the bolts”. (Appendix S-1) This apparently loose approach to the assembly and operation procedures stems from protection against over-tightening that is found within the Appendix 2 & Y calculations.
A Finite Element Analysis (FEA) is performed when the particular configuration doesn’t fall within the standard Code guidelines, such as the flange calculations found in ASME VIII-1 and mentioned above. The component is then approached as a VIII-1 vessel that needs additional rules, which are readily available in VIII-2.
The following study presents two extreme scenarios: a) No preload, & b) Very high preload. The contrast between these two cases is clearly shown by the results:
a) No Preload: unacceptable stress levels.
b) Very High Preload: acceptable stresses with infinite life cycles.
Should bolting preload be included in a Finite Element Analysis?The resounding answer is YES.
When a Finite Element Analysis is performed, the bolting preload must be considered both to realistically model the assembly and also be in compliance with the applicable pressure vessel code.
Questions remaining for future discussions are: How should the preload be applied in the FEA? How can the required preload be calculated? What financial and practical reasons are there for considering bolting preload when the ASME Code doesn’t seem overly concerned with it?
This document has been supplied for information purposes only. For additional clarification please contact Key Design Engineering.
Property of Key Design Engineering Page 3 of 7 7/24/2008
It is necessary to commence by reviewing the deformed displacement plots to make sure that the component’s behaviour is reasonable. In the following plots, please note the following reference points, as illustrated in the first image:
a) Maximum displacement at the inner surface b) Rotation about outer edge
a) The maximum vertical displacement is 0.0028”, at the lower surface.
b) The plate rotates about the outer edge.
c) The plate curves up leaving a significant gap and there is no metal-to- metal contact around the bolt hole.
d) The bolting deformation lines (different shades of colour) cross the bolt at an angle, because the bolt is undergoing significant bending.
a) The maximum vertical displacement is 0.0009”, at the lower face.
b) There is very little movement about the outer edge.
c) The bolting preload is sufficient to keep the lower surface in contact with its mating face, maintaining metal-to- metal contact.
d) The bolting deformation lines run horizontally, indicating that the bolt is primarily in tension and is undergoing very little bending.
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