Page 72 - Pipeline Rules of Thumb Handbook

Construction 59

Courtesy of Pipeline & Gas Industry.

pipe welders face pipe which may be covered with a variety Internal undercut
of coatings; these include primers, epoxy, tar, paper, varnish,
rust, scale or moisture. See Figures 1 through 5 for x-rays of the various defects
While joint cleanliness is important in all welding, it is espe- that can cause rejects. Of these, one of the most common and
cially so in the root pass of pipe. Even a thin film of contam- troublesome is internal undercut (undercut on the inside of
inants, which may be difficult to see with the naked eye, may the pipe). This is understandable because it occurs on the
be present. Failure to recognize and properly clean joints can “blind side” and the operator cannot see it happening. Con-
result in a hollow bead or other welding defects. sequently, he cannot immediately do anything to correct it.
Follow these instructions to minimize costly defects: To make matters worse, he seldom gets a chance to see the
x-ray negative or the actual weld itself. All of this makes it
1. Remove all moisture and condensations of any type
difficult for him to correct the situation.
prior to welding. The joint must be completely dry.
Undercut may be the direct result of poor joint prepara-
2. Clean BOTH ends of the pipe INSIDE AND OUT to
tion or fit-up. If the joint does not conform to the details spec-
remove traces of paint, rust, scale, oil, epoxy, or organic
ified in the established procedures (see Figure 6), every
materials. The area to be cleaned should extend at
reasonable effort should be made to correct it before starting
least 1 inch (25mm) from the end of the bevel (per
to weld.
ANSI/AWS D10.11-80 Page 1) on both the INSIDE and
Internal undercut will tend to occur if:
OUTSIDE surfaces.
3. A recommended method for cleaning in the field as 1. The root face (Land) is too small.
described above is the use of a heavy duty straight shaft 2. The root opening is too large.
grinder with a rubber expanding wheel and a carbide 3. If a severe high-low condition exists.
coated sleeve. The small shaft and reduced overall 4. The current is too high.
weight allows easy access to the inside and outside sur- When any undesirable variation occurs in the joint prepara-
faces of the pipe. tion, the normal reaction is to compensate for it by juggling