As part of the design verification process for piping systems, many analysts use the code compliance reports built-in as standard output in CAESAR II.
While this is a necessary step (but not sufficient), it should be mentioned the need for proper qualification of the piping system to the proper code.
Usually, the client (owner) calls the codes generally, but it’s the stress analysts responsibility (among other specialists) to ensure the correct classification of lines and to ensure correct analysis is performed for every system.
I’m surprised to have seen typical B31.3 systems analyzed under the B31.1 code only because '…it’s a far more stringent code…'; even worse cases, in which B31.11 lines were analyzed using B.31.3 code for the same reason.
IMO this is not only a dangerous approach (code stress equations are code specific), but also a reason of great concern: code requirements are not legally met!
Am I missing something here?

Errrggghhh I get @^#%&#^&%&**%^ mad when people say this code is more conservative than that one or another. This is a moronic statement.... some portions of one code may be more stringent than the similar portion of another code but with the code books covering hundreds of pages and everything from design to testing this oversimplification is grossly in error.

The contract should state what is the code of record for the work and that code should be applied in its entirety unless the owner waives a portion of the code for another.

All of this focuses on the owner because in the end the owner is the ultimate responsible party for a piping system.

By the by B31.1 will yield different stresses because its simplification of inplane versus out-of plane SIFS. When analyzed from one to another the locale of highest stress may change. B31.3 takes into account in-plane versus out of plane.

good question, i also faced this kind of situation

I feel soo much better now, I feared my post will be ignored totally!
I wasn't trying to quantify the code differences (by the way: my tutor pointed out that 'B31.3 ignores torsion in the SUS case, and doesn't stress-intensify torsion in the EXP case, while B31.1 does consider both, and the SIF they apply to inplane, out-of-plane and torsional moments are all intensified with the most conservative SIF of the two used by B31.3'). To me, comparing the codes is like comparing apples to oranges (although I'm quite sure someone has written a paper on it already!).
What worries me the most is how easily analysts fall in the trap of following patterns and apply rules of thumb. And how, mistakes made by uneducated clients, are not corrected by people that should be responsible and, in turn, they choose to use contracts as safety umbrellas.
On the other hand, if I may suggest, the gurus here should be more specific in their advice; evasive comments will not benefit anyone. 'get someone with proper expertise to do it for you' is not going to work: you know just as well as I do that there is a substantial shortage of (good) stress people. From other forums my best experience is "go read this" type of answer...
I believe more people need to come here and get educated, not just looking for a specific solution to their challenge of the day.
...
I'm not a specialist with century-long equivalent experience some of you people have and well aware that this is not something to be aquired ovenight. But I discipline myself to apply correct, logic and responsible engineering judgement in performing my duties.
I apologize if at times I will ask the obvious; in the mean time, I always prefer an abrasive response to complete ignorance.
And last but not least, my thanks go to Mr. Luf for his incisive but very helpful reply!