The nominal diameter of the head, also known as the mean outside diameter. Refers to the standard diameter after standardization, expressed in DN, and the nominal diameter of the container in units of mm, such as an inner diameter of 1200 mm, is designated DN1200. It is mainly divided into three aspects:

1. The nominal diameter of the pressure vessel

The cylindrical body made by coil welding has a nominal diameter which refers to the inner diameter. If the diameter of the container is small, the cylinder can be directly made of seamless steel pipe. At this time, the nominal diameter refers to the outer diameter of the steel pipe. When designing, the inner diameter of the equipment initially determined by the elliptical head process shall be adjusted to meet the specified nominal diameter. The nominal diameter of the head is the same as the barrel.
2. Nominal diameter of the pipe The pipe fitting is also indicated by the nominal diameter, which means the same as the pipe. Seamless pipes used in the project, such as seamless steel pipes for conveying fluids (GB 8163-87), seamless steel pipes for petroleum cracking (GB 9948-88), high-pressure seamless steel pipes for fertilizer equipment (GB 6479-86), etc. The elliptical head marking method does not use a nominal diameter, but is expressed by the outer diameter by the thickness. In the standard, the outer diameter and thickness are referred to as the nominal outer diameter and the nominal thickness.

There are two types of seamless tubes for conveying fluids and hot-rolled tubes and cold-drawn tubes for general-purpose seamless steel tubes. The maximum outer diameter of the cold drawn tube is 200 mm; the maximum outer diameter of the hot rolled tube is 630 mm. In pipeline engineering, hot-rolled tubes are often used when the pipe diameter exceeds 57 mm. Cold drawn tubes are often used within a diameter of 57 mm.

3. The nominal diameter of the container parts

The nominal diameter of some components, such as flanges and bearings, refers to the nominal diameter of the cylinder and head that match it. DN2000 flange refers to the flange that matches the DN2000 barrel (container) or head. The DN2000 saddle is a saddle mount that supports a DN2000mm container. There are also some oval head parts whose nominal diameter is the nominal diameter of the tube to which it fits. For pipe flanges, the DN200 pipe flange refers to the pipe flange that connects the DN200mm pipe. There are also some container parts whose nominal diameter refers to an important dimension in the structure, such as the viewing hole of the mirror, the shaft diameter of the stuffing box, and so on. DN80 (Dg80) sight glass with a peephole diameter of 80mm.