1. Nominal diameter DN:
Flange nominal diameter refers to the nominal diameter of the container or pipe with the flange. The nominal diameter of the container refers to the inner diameter of the container (except the container with a tube as a cylinder), the nominal diameter of the pipe refers to its nominal diameter, which is a value between the inner diameter and outer diameter of the pipe, most of which are close to the inner diameter of the pipe. The outer diameter of the steel pipe with the same nominal diameter is the same, and the inner diameter is also different because the thickness is changing. 14 - see table 1.

2. Nominal pressure PN:
Nominal pressure is a grade of pressure assigned for the purpose of establishing a standard. 14 - see table 2.
3. Maximum allowable working pressure:
The nominal pressure in pressure vessel flange standard is determined under the condition of flange material 16Mn (or 16MnR) and design temperature 200oC. When the flange material and temperature change, the maximum allowable working pressure of the flange will increase or decrease. For example, the maximum allowable working pressure of a long-neck butt welding flange is shown in Table 14-3.