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Pipe Welding
Pipe operating conditions in the handling of oil, gases, water, and other
substances range from high vacuum to pressure of several thousand pounds
per square inch. Mechanical joints are not satisfactory for many of these
services. Electric arc or oxyacetylene welding provide effective joints
in these services and also reduce weight, increase the strength, and
lower the cost of pipe installations.
PREPARATION FOR WELDING
a. Pipe Beveled by Manufacturer. Pipe to be welded is usually supplied
with a single V bevel of 32-1/2 degrees with a 1/16-in. (1.6-mm) root
face for pipe thicknesses up to 3/4 in. (19.1 mm). A single U groove is
used for heavier pipe. If the pipe has not been properly bevelled or has
been cut in the field, it must be bevelled prior to welding.
b. Cutting of Pipe. This operation is necessary when pipe must be cut to
suit a specific length requirement. To ensure a leak proof welded joint,
the pipe must be cut in a true circle in a plane perpendicular to the
center line of the pipe. This may be accomplished by using a strip of
heavy paper, cardboard, leather belting, or sheet gasket material with a
straight edge longer than the circumference of the pipe to be welded. The
material is wrapped around the pipe and overlapped and the pipe marked
along the edge of the material with a soapstone pencil. Pipe with a wall
thickness exceeding 1/8 in. (3.2 mm) should be cut first with a straight
cut, then bevelled with a hand torch to a 30 to 35 degree angle, leaving
a shoulder of approximately 1/8 in. (3.2 mm).
c. Cleaning of Pipe. After bevelling, remove all rust, dirt, scale, or
other foreign matter from the outside of the pipe in the vicinity of the
weld with a file, wire brush, grinding disk, or other type of abrasive.
If the bevels are made by oxyacetylene cutting, the oxide formed must be
entirely removed. The inside of the pipe in the vicinity of the weld may
be cleaned by a boiler tube and flue cleaner, by sandblasting, by tapping
with a hammer with an air blast follow up, or by any other suitable
method, depending on the inside diameter of the pipe. Care must be taken
to clean the scarf faces thoroughly.
Aligning the Joint.
(1) A pipe line up clamp should be used to align and securely hold the
pipe ends before tack welding. A spacing tool to separate the pipe ends
can be made from an old automobile spring leaf. The spacing for
oxyacetylene welding should be approximately 1/8 in. (3.2 mm); for arc
welding, the spacing depends on the size of the electrode used for the
root pass.
(2) If a pipe line up clamp is not available, the pipe section must be
set in a jig so that their center lines coincide and the spacing of
the pipe ends is uniform prior to tack welding. An angle iron
(fig. 12-34) will serve as a jig for small diameter pipe, while a
section of channel or I-beam is satisfactory for larger pipe.
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