1. Water and gas transportation steel pipes (galvanized or non-galvanized), cast iron pipes and other pipes, the pipe diameter should be expressed by the nominal diameter DN;

2. Seamless steel pipe, welded steel pipe (straight or spiral seam), copper pipe, stainless steel pipe and other pipes, the pipe diameter should be expressed by outer diameter × wall thickness;

3. Reinforced concrete (or concrete) pipes, clay pipes, acid resistant ceramic pipes, cylinder tile pipes and other pipes, the pipe diameter should be expressed by the inner diameter d;

4. For plastic pipes, the pipe diameter should be expressed according to the method of product standard;

5. When the design uses the nominal diameter DN to represent the pipe diameter, there should be a comparison table of the nominal diameter DN and corresponding product specifications.