They're two different things, not related to each other. API 1104 is a standard for the qualification of WPS and welders for oil and gas pipelines. ASME B31.3 is a standard for the design and erection of chemical plant and oil refinery piping.
In my opinion, there's no sense in saying that one is more stringent than the other, unless you want to know it just for curiosity.
Giovanni S. Crisi
Sao Paulo - Brazil
By Arctic 510 
Date 08-21-2009 23:41
Edited 08-21-2009 23:50
They are unrelated, as Mr. Crisi points out, and there are major differences in RT acceptance criteria. Both, however, aim to deliver an overall safe, functioning piping system. My company has piping that falls under ASME B31.3, but we also own and inspect some API 1104 stuff as well.
In my opinion, acceptance criteria in API 1104 is much less stringent. Especially if you look at 31.3's "severe cyclic conditions". API 1104 allows cracks, where 31.3 does not allow any, even in benign "Category D Fluid Service". 1104 allows much more incomplete penetration as well. There are many comparisons that can be made, what specific defects are you interested in?
The short answer to your question is that ASME B31.3 is more stringent than API 1104 when it comes to RT acceptance criteria.
