A coupling connects two pipes. If their sizes differ, the fitting is known as a reducing coupling, reducer or an adapter. There are two types of couplings: slip and regular
A reducer is the component in a pipeline that reduces the pipe size from a larger to a smaller bore (inner diameter).
The length of the reduction is usually equal to the average of the larger and smaller pipe diameters. There are two main types of reducer: concentric and eccentric reducers.
A reducer can be used either as nozzle or as diffuser depending on the mach number of the flow.
A reducer allows for a change in pipe size to meet hydraulic flow requirements of the system, or to adapt to existing piping of a different size. Reducers are usually concentric but eccentric reducers are used when required to maintain the same top-or bottom-of-pipe level.
These fittings are manufactured in inch and metric size.