10M and 15M weld neck flanges have designs that allow the operator to
join these flanges to tubular material of the same O.D. and I.D. as the
butt weld end of the flange neck, and of the same yield strength as that
specified for the flange, using a qualified welding procedure, without
further design or engineering consideration.
In practice this proves inconvenient. The specified
API 6BX weld necks don’t correspond to standard pipe sizes.
Should an operator want to weld API 6BX flanges
directly to standard available pipe, he may do so by specifying API 6BX
flanges with special weld necks machined to match the pipe he intends to
join to the flange by welding. Such flanges will not comply with API
specified dimensions and may not bear an API Monogram.
If an operator wishes to purchase such a flange with
an API Monogram, he must specify the flange under the API Spec 6A
This Other End Connector must have a design and
design analysis that demonstrates that the flange, including its neck
dimensions meet the API specified design allowable stress limits for the
stated working pressure. This allows, and may require, derating the
working pressure of the nominal size flange, e.g. 4-1/16: 10M flange
(7,500 # WP) with a weld neck for 5” nominal Sch. 160, X60 line pipe.
The derated working pressure must appear clearly in the flange marking.
For more information
about joining flanges to pipe by welding see:
About Weld Neck
Weld Neck Flange Attachment.
Q & A 16 Weld Necks on 6BX Flanges.
here for a caution about 6BX weld neck flanges.
Q & A 37 API Flange Weld Neck and Matching Pipe I.