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Comments on Butt Welds
Questions below came from one of the seminar attendees; Ron Haupt replies to each.
In reviewing the section on SIF's, I realize that in many cases there would be an SIF for a simple butt weld. This is because the mill tolerance for wall thickness alone could result in an internal ID misalignment that would result in a required SIF. Would the SIF for a butt weld be applied at a butt-welding fitting in addition to the SIF of the fitting itself?
The SIF for an as-welded butt weld is 1.0 because the SIFs are referenced against the as-welded butt-weld. The as-welded butt weld SIF developed was developed considering mill tolerance, counter-bore, and mismatch permitted by B31.1. Note, if you ground a butt weld smooth, as they do in the nuclear industry, technically you could have an SIF for a flush-ground butt weld of less than 1.0. Plus, the maximum stress in an elbow is not at either end, where most people determine stresses, but in the middle of the bend. Thus, cross multiplying an as-welded butt weld SIF of 1.0 by the SIF for an elbow would yield a B31.1 stress equivalent to that of the elbow alone and, further, the B31.1 stress at the end of an elbow should be less than at the center of the elbow. The apparent illogic of the above discussion is due to the fact that B31.1 stresses, i.e., iM/Z, are effective stresses and are less than theoretical or strain-gage measured stresses.
Can you just clarify a point or two for me? What do you mean by an "as-welded butt weld"?
An as-welded butt weld may have weld reinforcement, i.e., weld build-up above the adjacent pipe wall surface (see the code for a limit for that) and ripples, undercut, and some acceptable very localized discontinuities (slight grooving, cold lap, weld spatter, pitting, porosity). A smooth-ground flush weld has all those localized discontinuities ground out and may even be hard to distinguish from adjacent base material without metallurgical testing. The former is typical of B31.1 normal service; the latter may be encountered in nuclear and perhaps high temperature B31.1 service.
Is an as-welded butt weld appropriate for welds between two straight spools of pipe as well? If so, when do the SIF's in Table D-1 for welds with t > 0.237" come into play?
An as-welded butt weld is appropriate for B31.1 service. t < 0.237" is thin-wall pipe (and reflects some observations on the performance of thin-wall stainless steel welds) and t > 0.237" is normal pipe (the SIFs for 0.237" < t < 0.237" are somewhat in disfavor and perhaps will change in time with further research).
Author: Mr. Ron Haupt, P. E., of Pressure Piping Engineering (www.ppea.net) is a member of several piping code committees (B31, B31.1, B31.3, BPTCS, and others). He consults with us in the capacity of Nuclear QA Manager.