Wastewater Conveyance - Wastewater Operators - page 170

Weld metal
Backing
ring
Butt weld
Socket weld
figure 4.8 Two kinds of welding pipe joints.
4.8.5.3.4 Welded joints
For applications involving high pressures and temperatures, welded
joints are preferred. Welding of joints is the process whereby metal sec-
tions to be joined are heated to such a high temperature that they melt
and blend together. The advantage of welded joints is obvious: The pieces
joined become one continuous piece. When a joint is properly welded,
the joint is as strong as the piping itself.
The two basic welded joints are (see Figure 4.8):
1. Butt-welded joints , in which the sections to be welded are placed
end to end; this is the most common method of joining pipe used in
large industrial piping systems.
2. Socket-welded joints , in which one pipe fits inside the other, the
weld being made on the outside of the lap; this type of joint is used
in applications where leakage integrity and structural strength are
important.
4.8.5.3.5 Soldered and Brazed joints
Soldered and brazed joints are most often used to join copper and
copper-alloy (nonferrous metal) piping systems, although brazing of
steel and aluminum pipe and tubing is possible. The main difference
between brazing and welding is the temperatures used with each pro-
cess. Brazing is accomplished at far lower temperatures than those
required for welding, but brazing, in turn, requires higher temperatures
than soldering .
In both brazing and soldering, the joint is cleaned (using emery
cloth) and then coated with flux that prevents oxides from forming. The
clean, hot joint draws the solder or brazing rod (via capillary action)
into the joint to form the connection. The parent metal does not melt in
brazed or soldered construction.