Butt welding reducers are used to decrease or increase the pipeline size at a given location. There are two types of butt weld reducers:

Concentric reducer: In concentric reducers, the open ends of the fittings are aligned and centered one to the other. Generally, the delta in size doesn't exceed the 2 (max 3) sizes to avoid an excessive drop of the pressure in the pipeline (if multiple decreases of the run pipe are necessary, this should be done in gradual steps).

Carbon steel concentric reducer ASME B16.9

Eccentric reducer: these are reducers when the open ends of the fittings are in an "offset" position one to the other. Eccentric reducers are used, generally, for pipelines with an horizontal flowline (whereas concentric reducers are used for vertical pipelines or for the inlets of suction pumps, as top flat eccentric reducers).

Carbon steel eccentric reducer

Eccentric reducers can be top flat ("TF") or bottom flat ("BF"). The BF type are used for pipe-racks (the bottom of the reducer is flat and at the same level of the bigger sized run pipe), while the top flat type finds wider application at the inlet of piping suction pumps as shown in the image below (this setup prevents the formation of vapor traps in the pipeline):

Eccentric reducer for pumps

The two types of reducers differ by type of application.


Butt weld caps are used to close the pipeline. They are manufactured out of steel plates. Steel caps are manufactured in most sizes and either in carbon and stainless steel. Bw caps comply to ASME B16.9 in terms of dimensions and tolerances. The shape of these caps shall be ellipsoidal and shall conform to the shape requirements as given in «ASME Boiler and Pressure Vessel» code.

Carbon steel cap ASME B16.9



Looking for additional info? Contact the pipingonline expert team

Useful links: ANSI | ASME | ASTM | API | MSS | BS | DIN | ISO