B31.3, question regarding L in paragraph 304.1.3

[nusselt]

I have a question regarding the value of "L" to use in external pressure design per ASME B31.3, paragraph 304.1.3.
For piping under 31.3, paragraph 304.1.3 defines "L" as the centerline distance between any two sections stiffened per Sect 8 div 1 paragraph UG-29. It does not indicate other items that are considered as lines of reinforcement. (In contrast ASME Sect 8, div 1, paragraph UG-28 defines a line of support as a body flange, cone, vacuum ring, closure)
So as an example, i have a run of piping consisting of 3 straight sections, each 10 foot long with 90 degree elbows between them, all 5 (3 straight pipes and 2 elbows) components welded together, 1st section in the x axis, 2nd section in the y axis, and the 3rd section in the z axis. The piping run has no stiffening rings. Is the design length "L" to be used in the external pressure calculation 30 foot?

[zarrafakt]

Dear nusselt,

If a cone to cylinder junction is defined as a line of support then you would be required to prove that there was sufficient moment of inertia local to the junction. Whether an elbow can be used as a line of support would depend on whether it can be considered as an adequate stiffener. If you cannot determine the moment of inertia for the elbow then it cannot be a line of support and your design length 'L' would have to be 30 ft. Also, ASME B31.3 304.2.4 says that a curved segment under external pressure should be designed as per straight pipe.

[ThaMaestro]

Dear nusselt, please refer to eng-tips.com, which is a forum way better than egpet for answering these kind of specific questions.
I doubt if zarrafakt's if answer is correct (B31.3 304.2.4 says a curved pipe under external pressure may be designed as for straight pipe - not should); Im sure eng-tips.com will provide you a correct answer.

[zarrafakt]

Thank you to ThaMaestro for checking ASME B31.3. You are indeed correct that 'may' is the actual term. However, I stand by my original answer. Perhaps eng-tips.com may be able to corroborate.

[link Point to another website Only the registered members can access]

[zarrafakt]

Thank you to ThaMaestro for checking ASME B31.3. You are indeed correct that the proper term is 'may'. However, I stand by my original answer. The developed centerline length should be used for L (30ft) unless additional support can be justified.