Nominal Diameters For Different Objects

Nominal diameter is also called mean outside diameter. Standard diameter is represented with DN and the unit symbol is mm. For example, there is a vessel with 1200mm inner diameter, thus its nominal diameter can be marked as DN1200. In general situation, there are three kinds of objects that can be described with nominal diameter.

 

For barrels which are coiled and welded from steel plates, the nominal diameter refers to the inner diameter. A barrel can be made directly with a seamless steel tube, if the diameter is small. In this case, nominal diameter is the outside diameter of the steel tube. Heads have the same nominal diameters with barrels. 

 

Pipe fittings can be also described with nominal diameters. For fittings like torispherical heads, ellipsoidal heads and stainless steel heads, nominal diameters are used to express them as well as seamed pipelines. For seamless pipes used in engineering, their ellipsoidal heads are marked with outside diameters multiply by thicknesses instead of nominal diameters only. Seamless steel pipes for transmitting liquids (GB 8163-87), seamless steel pipes for petroleum cracking process (GB 9948-88), high pressure seamless steel pipes for fertilizer facilities (GB 6479-86) all belong to the above mentioned pipelines. Seamless steel pipes for general services are classified into hot-rolled pipes and cold drawn pipes. The maximum outside diameter of cold drawn pipes is 200mm, while that of hot-rolled pipes is 630mm. In pipeline projects, hot-rolled pipes are used when diameters are required to be more than 57mm, otherwise use cold drawn pipes.

 

Nominal diameters of some components, such as flanges, bearings and others, refer to the nominal diameters of their suitable barrels or heads. DN2000 flanges refer to flanges match with DN2000 barrels or DN2000 heads. There are also some ellipsoidal heads which nominal diameters are the same with those of pipelines they match with. For example, DN200 pipe flanges mean pipe flanges that connecting DN200 pipelines.