Page 87 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

PIPING 2.37

pipe after the weld is completed, the insert should be the only method specified for welding
drainage pipe. This insert melts and becomes part of the joint, and so is called a consum-
able insert.
Since metal that is too thick does not make a proper weld, several thin welds on a single pipe
may have to be made. Each weld is called a pass. Multiple passes are usually required.
Welded joints can be made in the shop or in the field. Shop welding is the least expen-
sive and highest quality because there is more control over the entire welding process by the
use of automated welding machines. As many pieces of pipe are assembled that can fit on
a truck (usually about 40 ft long and about 16 ft wide) or that will fit into the building site.
The whole assembled pipe is called a spool. Doing as much work in the shop as possible
will limit the number of field welds.
In general, the following welding processes are recommended for the specific types of
work: TIG welding, with a consumable insert for the first pass of shop-welded joints done
by machine, because it gives the smoothest interior; MIG welding, to weld the rest of the
joint when done in the shop by machine; electric arc welding, the most common method
used in the field to join piping, with consumable inserts used when a smooth interior is
desired.

Welder Qualification
Specifications for and approval of the entire welding process are necessary for both shop
welding and field welding. It is also necessary to qualify welding personnel to ensure that
they have sufficient training and knowledge to produce a weld of the required quality.
Qualifications of welding personnel are difficult to assess. High-temperature, high-pressure
pipe is covered by ASME codes that specify the selection of successive welding type passes,
filler metal composition, joint preparation, movement and handling of the pipe, tack welding
and clamping, welding currents, metal deposit rates, and weld inspection. None of these
code requirements apply to welded nonpressure drainage pipe. If the engineer does not have
the knowledge to specify the minimum requirements for welders and the welding process, it
could be left up to the contractor to determine the correct specifications for the project and
recommend them to the engineer for approval. When this is done, the contractor establishes
minimum criteria that will qualify any individual for welding on this particular project. It
is now up to the contractor to test a welder’s ability to make sound welds under the actual
working conditions and using the same equipment expected to be used on the job and certify
that person as being qualified. These criteria should be reviewed by the engineer for accept-
ability. It is common practice to use an outside, knowledgeable third party for this review
process and to establish welding criteria.

Weld Testing
Defects in welded piping must be found and corrected. Defects occur because the weld does
not actually create a monolithic piece of pipe. The flaws are cracks or voids in the joint.
Nondestructive testing (NDT) methods are:
1. Visual inspection of the weld. This is the least expensive and the least informative. It is
the most widely used, and, coupled with hydrostatic tests of the completed system, will
find most defects.
2. Dye penetration. This requires the use of two liquids. The first, which is a super-wetting
agent, is applied to the weld, allowed to find its way into any defects, and then wiped
off. The second fluid is a developer, which when applied, changes the color of the first
liquid, creating a red mark where any defect occurs. This is used to check flaws that may
not be visible to the eye.

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