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Use of ASME VIII Div 1 weld geometry in API 530 creep designed equipment

Use of ASME VIII Div 1 weld geometry in API 530 creep designed equipment

(OP)
A component subject to creep is designed in accordance with API 530. The code provides simple methods based on primary membrane and bending for calculating stresses and thicknesses in a cylinder and elbows and says that butt welded ASME B16.9 fittings are acceptable for use with API 530.

There is a desire to have a corner joint welded from one side in the vessel. Is it possible to take the corner joint design from ASME VIII Div 1 figure UW-13.2(a-d) and use it for API 530 including using the higher API 530 allowable design stress?

API 530 and ASME Section II Mandatory Appendix 1 use a similar method of calculating the allowable creep design stress. The critical difference is ASME VIII adds additional constraints and design margins resulting in a lower allowable design stress to API 530.

My opinion is that designing a vessel to API 530 which appears to be intended for primary membrane and primary bending design and cherry picking a corner weld detail from ASME VIII without using the ASME VIII material design margins is very suspect. You are effectively reducing the design margin by an unknown amount. I don't believe the Engineer is able to assess if the joint remains safe.

The ASME VIII corner joints in question do require that the cylinder be 1.25x thicker at the welded joint. This also provides design margin. Does the safe operation of the joint only rely on this 1.25x extra thickness? If so, this would suggest that using the higher allowable design stress of API 530 would simply reduce the level of safety down to API 530 levels of safety which would be acceptable. API 530 equipment does operate a safe distance from people and within a containment enclosure so perhaps that explains the lower design margins.

My opinion is that the safe operation of this welded joint depends on the design margin provided by both the 1.25x material thickness and the additional ASME Section II design margin and that the weld geometry "will" be less safe and "may" be dangerous if it used by other design codes with a lower design stress. The only way to assess if it remains safe is with experience and testing.

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