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Pipes are seen every day, and their diameters are often mentioned; but when it comes to using terms like Dn, De, D, d, and Φ all together... for example: DN200, De200, D200, d200, Φ200. Generally speaking, the diameter of a pipe can be divided into the outer diameter (De), the inner diameter (D), and the nominal diameter (DN). Now let’s clarify the differences between these “dddddd” values for everyone: DN refers to the nominal diameter of the pipe. Note that this is not the outer diameter nor the inner diameter; it is the average of the outer and inner diameters, known as the average inner diameter. When the pipe diameter is specified using the nominal diameter DN in the design, there should be a table correlating the nominal diameter DN with the corresponding product specifications. Pipes such as water gas transmission steel pipes (galvanized or ungalvanized), cast iron pipes, steel-plastic composite pipes, and polyvinyl chloride (PVC) pipes should be labeled with the nominal diameter “DN” (such as DN15, DN20). De refers to the outer diameter of the pipe; it is usually indicated in the form of outer diameter X wall thickness ； It is mainly used to describe: seamless steel pipes, plastic pipes such as PVC, and other pipe materials for which the wall thickness needs to be specified. Taking galvanized welded steel pipes as an example, the two notation methods, DN and De, are as follows: DN20 De25×2.5mm, DN25 De32×3mm, DN32 De40×4mm, DN40 De50×4mm. We are accustomed to using DN to denote welded steel pipes; we rarely use De to indicate pipes when the wall thickness is not relevant ； But labeling plastic pipes is another matter altogether ； It’s still related to industry conventions; in the actual construction process, the terms 20, 25, 32, etc., which are used informally, refer to De rather than DN – there is thus a difference in specification. Failing to clarify this can easily lead to losses during the procurement and construction processes. The connection methods for the two types of piping materials are essentially thread connection and flange connection. Other connection methods are used very rarely. Both galvanized steel pipes and PPR pipes can be connected using these two methods; however, for pipes smaller than 50, threaded connections are more convenient, while flanges are more reliable for pipes larger than 50. Note: When metal pipes made of two different materials are connected, it is necessary to consider whether a galvanic cell reaction will occur; otherwise, this could accelerate the corrosion of the pipe made of the more reactive metal material. It is best to use flanges for connection, and insulating materials such as rubber gaskets should be used to separate the two metals. Even bolts should be separated by gaskets to prevent contact. D generally refers to the inner diameter of the pipe; d denotes the inner diameter of a concrete pipe, while Φ represents the diameter of a regular circle. Of course, Φ can also be used to indicate the outer diameter of a pipe, in which case the wall thickness should be multiplied by it. For example: Φ25×3 indicates a pipe with an outer diameter of 25 mm and a wall thickness of 3 mm. For seamless steel pipes or non-ferrous metal pipes, “outer diameter × wall thickness” should be indicated. For example: Φ107×4, where Φ can be omitted. In China, ISO, and Japan, some steel pipe specifications use wall thickness dimensions to indicate the series of steel pipe wall thicknesses. For this type of steel pipe, the notation is outer diameter × wall thickness. For example: the respective range of values for Φ60.5×3.8De, DN, d, and ф! De– PPR, PE pipes, polypropylene pipes; Outer diameter. DN– Polyethylene (PVC) pipes, cast iron pipes, steel-plastic composite pipes, galvanized steel pipes; Nominal diameter. d– Nominal diameter of concrete pipes. ф– For seamless steel pipes or pipes made of non-ferrous metals, “outer diameter × wall thickness” should be indicated. Now do you understand?