Recently I was reviewing equipment drawings for a project, and made a correction to a piping dimension. The dimension was from the centerline of a 4″ tee to the face of a CL 150, raised-face flange (fitting make-up). The dimension was shown as 7 1/4″. I corrected it to 7 1/8″.

When the drawing came back, the equipment designer had added a note saying the dimension was correct because he had included a 1/8″ weld gap. I’ve been designing piping for over 20 years and have never done that and never seen it done. I’ve designed many runs of butt-weld fittings that would not have worked if there were a 1/8″ gap between welds, and I’ve field verified that fabricated runs of butt-weld fittings conform to the fitting charts. I’ve also never had a checker change my dimensions for this reason nor have I ever checked another designer’s work that included an addition for weld gaps.

Does someone have an explanation for what happens to the weld gap between butt-welded fittings? One possibility is that the fittings charts add 1/16″ of an inch to the actual dimension to account for this gap. Another possibility is that the gap is offset when the piping is prepared for welding. The butt ends have to be ground down to clean metal and this probably equals the weld gap, so the installed fitting nets out to the reference dimension.

I’ve gone through all my reference materails but haven’t been able to find a clear answer. Any help would be appreciated.
Wayne