Nominal diameter is also known as mean outside diameter, which denotes the symbol DN, which is the general diameter of various pipe and pipe accessories. The pipe and pipe accessories of the same nominal diameter can be connected with each other and have interchangeability. It is not the outer diameter or inner diameter of the pipe in practical sense, although its value is close to or equal to the inner diameter of the pipe. In order to unify the connecting dimensions of the pipe and pipe fittings, the nominal diameter (also known as nominal diameter and nominal diameter) is adopted. For example, welded steel pipes can be divided into thin wall steel tubes, ordinary steel pipes and thickened steel pipes according to their thickness. The nominal diameter is not the outer diameter, nor the inner diameter, but a nominal size similar to the inner diameter of ordinary steel pipe. Each nominal diameter corresponds to an outer diameter, and its inner diameter varies with thickness. The nominal diameter can be expressed in metric mm or in English in. The pipe accessories are also expressed in nominal diameters, meaning the same as those with seams.

Nominal pressure represents the symbol PN and the maximum allowable operating pressure at a given temperature. For carbon steel valve body, the maximum allowable working pressure for applications is below 200 C. For cast iron valve body, the maximum allowable working pressure for applications is below 120 C, and for stainless steel valve body, the maximum allowable working pressure for applications is below 250 C. When the working temperature rises, the pressure of the valve body will decrease. For example, a carbon steel control valve with nominal pressure of 6.4 MPa (PN64) has a pressure of 5.2 MPa at 300 C, 4.1 MPa at 400 C and 2.9 MPa at 450 C. Therefore, the nominal pressure of the control valve should be determined not only according to the highest working pressure, but also according to the highest working temperature and material characteristics, not only to meet the nominal pressure that is greater than the working pressure.