Welding Neck Flanges:
The welding neck flange is normally referred to as the "high hub" Flange. It is designed
to transfer stresses to the pipe, thereby reducing high stress concentrations
at the base of the Flange. The welding neck flange is the best designed butt-welded
flange of those currently available because of its inherent structural
value. It is expensive because of the design.

Threaded (Screwed) Flanges:
The threaded flange is similar to the slip-on flange, but the bore is threaded. Its
chief merit is that it can be assembled without welding, explaining its use in
low pressure services at ordinary atmospheric temperatures and in highly explosive
areas where welding create a hazard.

Slip-On Flanges:
The slip-on flange has a low hub because the pipe slips into the flange prior to
welding. It is welded both inside and out to provide sufficient strength and prevent
leakage. Slip-on flanges are all bored slightly larger that the O.D. of the
matching pipe. They are preferred over welding neck flanges by many users due to
their lower initial cost, but final installation cost is probably not much less
than that of the welding neck flange because of the additional welding involved.

Lap joint Flanges:
The lap joint flange is practically identical to a slip-on flange except it has a
radius at the intersection of the bore and flange race. This radius is necessary
to have the flange accommodate a lap joint stub end. Normally, a lap joint flange
and a lap joint stub end are mated together in an assembly system.

Blind Flanges:
The blind flange is a flange without a bore. It is used to close off the ends of
a piping system and/or a pressure vessel opening. It also permits easy access to
the interior of a line or vessel once it has been sealed and must be reopened.

Socket Welding Flanges:
The socket welding flange is similar to a slip-on flange except it has a bore and
a counter bore dimension. The counter bore is slightly lager than the O.D. of the
matching pipe, allowing the pipe to be inserted into the flange similar to a
slip-on flange. The diameter of the smaller bore is the same as the I.D. of the
matching pipe. A restriction is built into the bottom of the bore which sets as
a shoulder for the pipe to rest on. This eliminates any restriction in flow when
using a socket welding flange.