The valve nominal diameter, or average diameter, is the size of all the piping line system represented by numbers. The nominal diameter is a convenient round integer for reference and is only in a strict relationship with the machining size . The nominal diameter is followed by the letter " DN " followed by a numeric sign. If the nominal diameter is 200mm, it should be marked as DN200 . The nominal diameter DN of the valve is marked on the valve body . Generally speaking, the nominal diameter of the valve directly determines the size of the valve and the size of the pipe.

The nominal diameter is only a mark. The nominal size cannot represent the measured valve diameter. The actual diameter of the valve is specified by the relevant standards. The general measured value (unit mm ) must not be less than 95% of the nominal size .

The nominal diameter is the common diameter of various pipe and pipe accessories. Pipes and pipe fittings of the same nominal diameter can be connected to each other and are interchangeable. It is not the outer diameter or inner diameter of the pipe in the actual sense , although its value is close to or equal to the inner diameter of the pipe. For example: nominal diameter of 100mm seamless steel pipe has types of 102 × 5 , DN108 × 5,and others. 108 is the outer diameter of the tube while 5 denotes the pipe wall thickness, and therefore, the inner diameter of the pipe (108-5-5 ) = 98mm, but it is not exactly equal to the value that outer diameter of the steel pipe deducts twice of the wall thickness.

Normally, the channel diameter of the valve is the same as the nominal diameter . However, when the valve body is welded, the actual channel diameter of the valve is not equal to the nominal number DN . For example, using 54mm × 3mm seamless pipes, the valve nominal diameter is DN50 , but the actual inner diameter D was 48mm .

Common nominal diameters are 

DN1 , DN2 , DN3 , DN4 , DN5 , DN6 , DN8 , DN10 , DN15 , DN20 , DN25 , DN32 , DN40 , DN50 , DN65 , DN80 , DN100 , DN125 , DN150 , DN175 , DN200 , DN225 , DN250 , DN300 , DN350 , DN400 , DN450 , DN500 , DN600 , DN700 , DN800 , DN900 , DN1000 , DN1100 , DN1200 , DN1300 , DN1400 , DN1500 , DN1600 , DN1800 , DN2000 , DN2200 , DN2400 , DN2600 , DN2800 , DN3000 , DN3200 , DN3400 , DN3600 , DN3800 , DN4000, etc.

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