Page 86 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

PIPING

2.36 CHAPTER TWO

Because oxidation causes poor weld quality, an inert gas displacing the air, or a coated
electrode that creates such a gas when heated, is used to keep air away from the molten
metal. In order to properly weld pipe, the pipe ends must be specially prepared according
to pipe thickness, composition, and welding method. The proper weld end preparation is
critical to proper welding and must be diagrammed or described in the specifications.

Types of Welding
Shielded Metal Arc Welding (SMAW). This is the standard electric arc welding. This pro-
cess uses a coated rigid metal stick electrode of the proper composition. The stick electrode
is an anode (+) and the pipe is the cathode (ground). The electrode is used to produce the
arc and is also consumed as filler metal in the process of welding the joint. This electrode
is coated with a substance that, when heated by the electric arc, creates a shielding gas and,
in addition, makes a flux for a protective slag over the cooling weld metal of the joint. The
coated electrode forms a flux on top of the weld that must be removed before the next pass
can be made.
This process is widely used because it is the least expensive, requires the least clearance
between obstructions, and is the easiest to manipulate the actual welding tool in the field.
Gas Metal Arc Welding (GMAW). This process, also known as metallic inert gas (MIG)
welding, uses a metal electrode wire of the proper composition to produce the arc. The
electrode wire also becomes the filler metal in the process of welding the joint. Inert gas,
usually argon, is piped to the weld area from a remote tank and is delivered to the weld site
through the electrode holder. This process is the next least expensive method and produces
a better quality weld faster than SMAW.

Gas Tungsten Arc Welding (GTAW). Also known as tungsten inert gas (TIG) welding, the
electrode is long-lasting tungsten which is not consumed during the welding process. The
inert gas, usually argon, is piped to the weld area from a remote tank through the electrode
holder. Filler metal is added to the joint either automatically through the electrode holder
or separately by hand. This process is the most costly and slowest method, but produces
the finest weld quality of all. This is used most commonly as an automated shop-welding
process.

Oxy-fuel Torch Welding. This process uses oxygen in combination with another fuel gas
(usually acetylene, but propane, MAPP, and natural gas are also used) to produce the heat
needed to melt the metal of the pipe and filler metal. The filler metal is added to the joint
by hand. This process is mostly used for welding pipe in the field.

Orbital Welding. Orbital welding is a welding process used to join stainless steel tubing,
mainly in the pharmaceutical and food-processing industries. It is a TIG process that uses a
welding rig that automatically welds the pipe and produces welds with little or no obstruc-
tion to the pipe interior.
Welded Joint Types. There are two types of joints used for welding, butt joint and socket
joint. Butt welding is the term used to describe two pipes placed end to end and joined with
no overlapping. Socket welding describes one pipe being placed on top of the other when
only one end of the exposed pipe is actually welded to the bottom pipe. This is like a cou-
pling, but with the joint on the outside of the pipe. In general, only pipe 2 in or less (called
small bore pipe) can be socket welded.
In butt welding, the pipe ends must be separated by a fixed distance. This can be done
either with short pieces of wire or a ring called an insert placed between the two ends of
the pipe before the weld is started. Because incorrectly placed wire may extend into the

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