1. Generally speaking, the diameter of a pipe can be diviDed into outer diameter (d, de, φ), inner diameter (d) and nominal diameter (DN).

The meanings of DN, De, d, d and φ:

1. DN refers to the nominal diameter of the pipeline, which is the average of the outer diameter and the inner diameter. Value of DN = value of DE -0.5* wall thickness. Note: This is neither the outside diameter nor the inside diameter.

Pipe materials such as water and gas conveying steel pipes (galvanized steel pipes or non-galvanized steel pipes), cast iron pipes, steel-plastic composite pipes and polyvinyl chloride (PVC) pipes shall be marked with "DN" (such as DN15 and DN50).

2. De mainly refers to the external diameter of pipes, and the external diameters of PPR, PE pipes and polypropylene pipes are generally marked by De, and all need to be marked in the form of external diameter * wall thickness, for example, De25x3.

3, d generally refers to the pipe diameter.

4. D Internal diameter of concrete pipe. The pipe diameter of reinforced concrete (or concrete) pipes, clay pipes, acid-resistant ceramic pipes, cylinder tile pipes and other pipes should be represented by the inner diameter D (such as d230 and d380, etc.).

5. φ represents the diameter of an ordinary circle; It can also represent the outer diameter of the pipe, but it should be multiplied by the wall thickness afterwards. For example: φ Φ25x3, which means a pipe with an outer diameter of 25mm and a wall thickness of 3mm. For seamless steel pipes or non-ferrous metal pipes, "outside diameter x wall thickness" should be marked, for example, φ Φ108x4, which can be omitted. Some steel pipe standards in China, ISO and Japan use the wall thickness dimension to indicate the series of steel pipe wall thickness. The specification of this kind of steel pipe is expressed as pipe outer diameter x wall thickness. For example, φ Φ60.5x3.8.

Second, the specific expression of pipe diameter in engineering:

The outer diameter of the pipe made of seamless steel pipe is indicated by the letter D, followed by the size and wall thickness of the outer diameter. For example, the wall thickness of seamless steel pipe with an outer diameter of 108 is 5mm, which is indicated by D108*5; Plastic pipes are also expressed by outer diameter, such as De63；; Others such as reinforced concrete pipes, cast iron pipes and galvanized steel pipes are represented by DN.

Nominal diameter is generally expressed in design drawings. Nominal diameter is a standard artificially specified for the convenience of design, manufacture and maintenance, also called nominal diameter, which is the specification name of pipe (or pipe fitting). The nominal diameter of a pipe is not equal to its inner diameter and outer diameter. For example, there are several kinds of seamless steel pipes with a nominal diameter of 100mm, such as 102*5 and 108*5, where 108 is the outer diameter of the pipe and 5 represents the wall thickness of the pipe. Therefore, the inner diameter of the steel pipe is (108-5-5) = 98mm, but it is not completely equal to the difference between the outer diameter of the steel pipe minus twice the wall thickness. However, the specification name of a pipe diameter is not equal to the inner diameter. Therefore, the nominal diameter should be used in the design drawings, in order to determine the structural dimensions and connection dimensions of pipes, fittings, valves, flanges, gaskets, etc. According to the nominal diameter, the nominal diameter is represented by the symbol DN. If the outer diameter is used in the design drawings, a pipeline specification comparison table should also be made to indicate the nominal diameter and wall thickness of a certain pipeline.

Used to transport fluids with general pressure such as water, gas, air, oil and steam for heating, welded steel pipes are often used as pipelines, which are called slotted pipes. According to the thickness, slotted pipes can be divided into thin-walled steel pipes, ordinary steel pipes and thickened steel pipes. Its nominal diameter is neither the outer diameter nor the inner diameter, but a nominal size approximate to the inner diameter of ordinary steel pipes. Each nominal diameter corresponds to an outer diameter, and its inner diameter value varies with the thickness. Nominal diameter can be expressed in metric mm or English in. Pipeline accessories are also represented by nominal diameter, which means the same as slotted pipe.

The pipe diameter representation method of seamless pipes used in common projects is different from that of seamless pipes, such as seamless steel pipes for conveying fluids (GB/T 8163-2018), seamless steel pipes for petroleum cracking (GB 9948-2013) and high-pressure seamless steel pipes for fertilizer equipment (GB 6479-2013). The marking method is not nominal diameter, but external diameter multiplied by thickness (in mm). In this kind of standard, this outer diameter and thickness are called nominal outer diameter (D) and nominal thickness (S).

Knowledge required for pipeline identification:

DN refers to the nominal diameter of the pipeline. Note: this is neither the outer diameter nor the inner diameter, but the average of the outer diameter and the inner diameter, which is called the average inner diameter. Such as: DN 200 ~ φ 219× 6.

D generally refers to the inner diameter of the pipeline.

D nominal diameter of concrete pipe.

De mainly refers to the external diameter of pipes, and the external diameters of PPR, PE pipes and polypropylene pipes are generally marked by De, and all of them need to be marked in the form of external diameter × wall thickness.

φ seamless steel pipe diameter (often referred to as outer diameter), whose specification is, for example, φ Φ108mm.

Three, some commonly used standards for pipelines:

1. Welded Steel Pipe for Low Pressure Fluid Transportation GB/T 3091-2015;

2. Steel Pipe for Pipeline Transportation System in Petroleum and Natural Gas Industry GB/T 9711-2017;

3. Polypropylene Piping System for Hot and Cold Water Part 2: Pipes GB/T 18742.2-2017;

4. Seamless Steel Pipe for Fluid Transportation GB/T 8163-2018;

5. Polyethylene (PE) Piping System for Water Supply Part 2: Pipes GB/T 13663-2018

6. Rigid PVC-U Pipe for Building Drainage (GB/T5836.1-2018);

Fourth, little knowledge:

One inch equals 25.4 mm; One inch is equal to 33.33 millimeters.

(1)2 in charge: 8 mm; (2)3 in charge: 10mm；; (3)4 in charge: 15mm；; (4)5 There is no such standard series in charge; (5)6 in charge: 20mm；; (6) There is no such name for 8 in charge, (8 points =1 inch) is called 1 inch tube: 25mm；; The conversion between cents and inches is called by the city inch system, and the inch system does not need to be divided.