Plugs are Fittings that provide flow control by plugging branches and outlets – the name is quite self-explanatory.
Plugs come in several design: bull plugs (either round or hex), round head plugs, and hex head plugs. Most plugs are threaded; however some are provided without threads and are referred to as socket-weld plugs.
Bull plugs have long heads and a smaller lower area where the piece enters the system. This lower area has a thread for threaded plugs, or is smooth (plain) for socket weld plugs. The size of a bull plug corresponds to the thread size and the outer diameter, which is equal to the nominal pipe size.
Bushings are hollow plugs with threads on the outside and the inside. They allow for pipe reductions by threading the bushing into a female thread and then threading the small pipe size into the centre of the bushing. This versatility works well in troubleshooting complex piping systems.
Round head, hex head, and square head plugs are plugs with a round threaded or round smooth end connection and a round, hex, or square head (respectively). The diameter of the round head plug is the same on the top and the bottom, therefore, however the diameters differ between the top and bottom for hex and square head plugs.