e. Unions. The ground joint and flange unions are the two most common types
of iron pipe unions. Both are used to join two pipes. The ordinary ground joint
union (1, fig 4) has a shoulder piece with female threads, thread piece with female
and male threads, ring or collar with an inside flange which matches the shoulder
of the shoulder piece, and a female thread matching the male thread of the thread
piece. The shoulder and thread pieces have a ground spherical joint. The pipes
are screwed to the thread and shoulder pieces. Both pieces are drawn together by
the collar, making a gastight and watertight joint. Another type of ground union
joint (2, fig 4) is made in two parts, each of which is screwed to the pipes to be
joined and then drawn together by nuts and bolts. The flange union (3, fig 4) has
two parts, each with a female thread, which are screwed to the pipes to be joined.
Nuts and bolts pull the flanges together. A gasket between the flange a makes a
gastight and watertight joint. Plain faced flanges are shown; they may have male
and female faces or tongue and groove faces.
Iron pipe unions.
The ordinary coupling (1, fig 5) is a short fitting with a
female thread used to join two lengths of the same pipe size when making a straight
run. It is not used to join two lengths of pipe already installed and relatively
fixed; a union must be used in such cases.
The reducing coupling (2, fig 5) is
used to join two pipes of different sizes. The eccentric reducing coupling (3, fig
5) has female threads of different sizes and different centers.
It is fitted so
that the bottom of the two pipes joined will form a straight line to promote
drainage. An example is in a steam line where it is undesirable to have a pocket
where water could collect. Another type of coupling is the extension piece (4, fig
5), which has both male and female threads. There are many special couplings such
as the Dresser coupling.