• De: Denotes the outside diameter of PPR pipes, PE pipes, and polypropylene pipes.
• DN: Denotes the nominal diameter of polyvinyl chloride (PVC) pipes, cast iron pipes, steel-plastic composite pipes, and galvanized steel pipes.
• d: Denotes the nominal diameter of concrete pipes.
• ф: Denotes the nominal diameter of seamless steel pipes, specified in the format of outside diameter × wall thickness (e.g., ф100 corresponds to 108×4).
Differences Between Pipe Sizes De and DN
1. DN (Nominal Diameter) refers to a standardized dimension that is neither the outside diameter nor the inside diameter of a pipe. Its origin is linked to imperial units used in the early development of pipeline engineering, and it is typically applied to describe galvanized steel pipes. The conversion between DN and imperial units is as follows:
◦ 1/2-inch pipe (4/8 inch): DN15
◦ 3/4-inch pipe (6/8 inch): DN20
◦ 1-inch pipe: DN25
◦ 1-1/4-inch pipe: DN32
◦ 1-1/2-inch pipe: DN40
◦ 2-inch pipe: DN50
◦ 3-inch pipe: DN80 (also marked as DN75 in some regions)
◦ 4-inch pipe: DN100
2. De (Outside Diameter) is used to specify the external diameter of pipes. For pipes labeled with De, the notation must take the form of outside diameter × wall thickness. It is primarily used for seamless steel pipes, PVC and other plastic pipes, and any pipe requiring explicit wall thickness specification.
Take galvanized welded steel pipes as an example, the two labeling methods using DN and De are shown below:
• DN20 → De25×2.5mm
• DN25 → De32×3mm
• DN32 → De40×4mm
• DN40 → De50×4mm
The DN designation is conventionally used for welded steel pipes, while De is rarely adopted unless wall thickness needs to be specified.
However, the labeling convention differs for plastic pipes. In practical construction, abbreviated references to pipe sizes (e.g., 20, 25, 32) always refer to De rather than DN.
Additionally, based on on-site construction experience:
a. The common connection methods for these two types of pipes are threaded connection and flange connection.
b. Both galvanized steel pipes and PPR pipes can be joined using these two methods. Threaded connection is more convenient for pipes with a size smaller than DN50, while flange connection offers greater reliability for pipes larger than DN50.
c. When connecting metal pipes of different materials, it is critical to consider the risk of galvanic corrosion. To prevent accelerated corrosion of the more reactive metal, flange connection is preferred, with insulating gaskets (e.g., rubber gaskets) installed to separate the two metals—including the bolts, which should also be isolated with gaskets to avoid direct contact.
Differences Between DN, De, and Dg
• DN: Nominal diameter
• De: External diameter
• Dg: An obsolete designation derived from the Chinese Pinyin "gōng" (meaning "metric"). It is a domestically developed standard unique to China and is no longer in use today.
a. Labeling Standards for Different Pipe Types
1. For water-gas steel pipes (galvanized or ungalvanized) and cast iron pipes, the pipe size should be indicated by nominal diameter (DN) (e.g., DN15, DN50).
2. For seamless steel pipes, welded steel pipes (straight-seam or spiral-seam), copper pipes, and stainless steel pipes, the size should be specified as outside diameter × wall thickness (e.g., D108×4, D159×4.5).
3. For reinforced concrete pipes, concrete pipes, clay pipes, acid-resistant ceramic pipes, and vitrified clay pipes, the size should be marked by inside diameter (d) (e.g., d230, d380).
4. For plastic pipes, the size should be labeled in accordance with the relevant product standards.
5. If the design uniformly uses nominal diameter (DN) for all pipe sizing, a conversion table correlating DN values with corresponding product specifications must be provided.
b. Relationship Between DN, De, and Dg
• De measures the outside diameter of the pipe wall.
• DN is approximately calculated as De minus half of the pipe wall thickness.
• Dg is generally obsolete and no longer in use.
Additional Specifications for Pipe Sizing
1. All pipe sizes shall be expressed in millimeters (mm).
2. The expression of pipe sizes shall comply with the following provisions:
1. Water-gas steel pipes (galvanized or ungalvanized) and cast iron pipes shall be labeled by nominal diameter (DN).
2. Seamless steel pipes, welded steel pipes (straight-seam or spiral-seam), copper pipes, and stainless steel pipes shall be labeled as outside diameter × wall thickness.
3. Reinforced concrete pipes, concrete pipes, clay pipes, acid-resistant ceramic pipes, and vitrified clay pipes shall be labeled by inside diameter (d).
4. Plastic pipes shall be labeled in accordance with product standards.
5. If DN is uniformly used in the design, a conversion table between DN and product specifications is required.
Special Notes on Plastic Pipe Labeling in Construction Drawings
• Metric Dimension Size: Marked with DN, generally referred to as the "nominal size". It represents neither the outside nor the inside diameter, but the average inside diameter (the mean value of outside and inside diameters).
• ISO Metric Dimension Size:
◦ Da denotes the outside diameter of PVC pipes and ABS pipes.
◦ De denotes the outside diameter of PP pipes and PE pipes.
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