Nominal diameter（nominal diameter）, also called average outer diameter（mean outside diameter）。It refers to the standardized diameter series of vessels, pipes and their accessories.The use of nominal diameter is conducive to the standardization of parts, design, manufacturing, repair and management, and reduces manufacturing costs.For vessels, the nominal diameter is expressed by the inner diameter;For pipes, it is indicated by a dimension smaller than the outer diameter and larger than the inner diameter.Corresponding to a certain nominal diameter of the pipe, the outer diameter is a certain value, and the inner diameter varies with the wall thickness.[1]
The nominal diameter of the cylinder made of rolled steel plate refers to the inner diameter.If the vessel diameter is small, the cylinder can be directly usedseamless steel tubemake.At this time, the nominal diameter refers to the outer diameter of the steel pipe.The nominal diameter of the head is the same as that of the cylinder.[2]
Pipe diameter
Generally speaking, pipe diameter can be divided into outer diameter, inner diameter and nominal diameter.The outer diameter of seamless steel pipe is indicated by the letter D, followed by the size and wall thickness of the outer diameter. For example, seamless steel pipe with an outer diameter of 108 has a wall thickness of 5MM, which is indicated by D108 * 5, and plastic pipe is also indicated by the outer diameter, such as De63, others such asreinforced concretePipe, cast iron pipeGalvanized steel pipeDN is used to represent, and the nominal diameter is generally used in the design drawings. The nominal diameter is a kind of manual specification for the convenience of design, manufacturing and maintenancestandard, also calledNominal diameter, is the specification name of the pipe (or pipe fitting).The nominal diameter of the pipe is not equal to its inner diameter and outer diameter. For example, there are 102 * 5 and 108 * 5 seamless steel pipes with a nominal diameter of 100MM. 108 is the outer diameter of the pipe and 5 is the wall thickness of the pipe. Therefore, the inner diameter of the pipe is (108-5-5)=98MM, but it is not completely equal to the outer diameter of the pipe minus twice the wall thickness. It can also be said that the nominal diameter is close to the inner diameter,The specification name of a pipe diameter that is not equal to the inner diameter. Therefore, the nominal diameter is used in the design drawings, so that pipes, pipe fittings, valvesflange, gasket and other structural dimensions and connection dimensions. The nominal diameter is represented by the symbol DN. If the outer diameter is represented in the design drawing, a pipe specification comparison table should also be made to indicate the nominal diameter and wall thickness of a certain pipe.[2]
In order to unify the connection size of pipes and pipe fittings, DN is used to represent the nominal diameter (also known asNominal diameter , nominal diameter).Chemical plants are used to transport water, gas, air, oil, heating steam and other general pressurefluidElectric welded steel pipes are often used as pipes, which are called seamed pipes.Seamed pipes can be divided into thin-walled steel pipes, ordinary steel pipes and thickened steel pipes according to thickness.The nominal diameter is neither the outer diameter nor the inner diameter, but a nominal size similar to the inner diameter of ordinary steel pipes.Each nominal diameter corresponds to an outer diameter, and its inner diameter varies with thickness.(Pipes are also divided into inch pipes and metric pipes. For pipes with a nominal diameter of DN100, if the pipe is an international general series (commonly known as inch pipes), its outer diameter is 114.3mm;If the pipe is a domestic series (commonly known as metric pipe), its outer diameter is 108mm.So there are two diameters, which are not contradictory.)Nominal diameter can be expressed in metric mm or in inch.[2]
Nominal diameter
Pipe accessories are also indicated by nominal diameter, which has the same meaning as slotted pipe.Seamless pipes used in the project, such asSeamless steel pipe for fluid transport（GB 8163-87）、Seamless steel pipe for petroleum cracking（GB 9948-88）、High pressure seamless steel pipe for fertilizer equipment(GB 6479-86), etc. The marking method is not the nominal diameter, but the outer diameter multiplied by the thickness.In the standard, the outer diameter and thickness are the nominal outer diameter andNominal thickness。Seamless steel pipes for fluid transportation and general purpose seamless steel pipes are divided into hot rolled pipes and cold drawn pipes.The maximum outer diameter of cold drawn pipe is 200mm;The maximum outside diameter of hot-rolled pipe is 630mm.In pipeline engineering, hot rolled pipes are often used when the pipe diameter exceeds 57mm.Cold drawn pipes are often used when the pipe diameter is within 57mm.[2]
Component diameter
The nominal diameter of some parts, such as flanges and supports, refers to the nominal diameter of the matching cylinder and head.DN2000 flange refers to the flange matching with DN2000 cylinder (vessel) or head.DN2000 saddle refers to theSaddle support。The nominal diameter of some components is indicated by the nominal diameter of the matching pipe.For example, DN200 pipe flange refers to the pipe flange connecting DN200mm pipes.There are also some container parts whose nominal diameter refers to an important part in the structuresize, such asMirrorSight hole, stuffing box shaft diameter, etc.The diameter of peephole of DN80 (Dg80) mirror is 80mm.[2]
Diameter of thread
Except for pipe threads, the nominal diameter of other threads is the inner diameter of the pipe (inch unit). The major diameter of all other threads is the nominal diameter (metric unit). Taking ordinary metric external threads as an example, the tooth angle of ordinary metric threads is 60 °[2]
Nominal diameter
The calculation formula is as follows:
Pitch P
Original triangle height H=0.866P
Tooth height (working height) H=0.5413P
Major diameter of internal thread D-internal thread (nominal diameter)
Major diameter of external thread d - external thread (nominal diameter)
Pitch diameter of internal thread D=D-0.6495P
Pitch diameter of external thread d=d-0.6495P
Minor diameter of internal thread D=D-1.0825P
Minor diameter of external thread d=d-1.0825P
For parameter calculation of other threads, refer to the national standard parameter manual[2]
Thread diameter conversion
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national standardAETV valveThe unit of nominal diameter is mm, and the unit of nominal diameter of American standard AETV valve is in (inch)
The conversion is as follows:
1 minute=1/8 inch=DN6
2 minutes=1/4 inch=DN8
3 minutes=3/8 inches=DN10
4 minutes=1/2 inches=DN15
6 minutes=3/4 inches=DN20
8 points=1 inch=DN25[2]
Nominal representation method
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The nominal diameter is indicated by the letter "DN" followed by a number sign as a symbol, and the size in millimeters is indicated after the symbol.The nominal diameter is the nominal diameter of cast iron pipes, jointed steel pipes, concrete pipes and other pipes, but seamless steel pipes do not use this representation.For example, DN50 refers to the pipe with a nominal diameter of 50mm.The relationship between inch NB (inch) and nominal diameter DN (mm), outer diameter OD (mm) is as follows:
NB(inch)-----DN(mm)------OD(mm)
1/2--------------15 ----------21.3
3/4--------------20 ----------26.7
1-------------- 25 ------------33.4
1 1/4 ------------ 32---------42.2
1 1/2------------40 ----------48.3
2-------------- 50 ------------60.3
2 1/2--------- 65 ------------73.0
3-------------- 80 ------------88.9
4-------------100 ------------114.3
5-------------125 ------------139.8
6-------------150 ------------168.3
8-------------200-------------219.1[2]
national standard
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1. Scope This standard specifies the definition and series of DN (nominal size).This standard is applicable to pipeline components specified in relevant standards using DN identification.Note: Other identification methods different from this standard can also be used, such as pipe elements with thread, press fit, socket weld or butt weld, and pipe elements with NPS (nominal pipe size), OD (outer diameter), ID (inner diameter) or G (pipe thread size marking).[2]
2. Definition DN: A size designation for the combination of letters and numbers used for piping system components.It consists of the letter DN followed by a dimensionless integer number.This number is directly related to the characteristic dimensions such as the hole diameter or outer diameter (expressed in mm) of the end connector.Note 1: Unless otherwise specified in relevant standards, the number after the letter DN does not represent the measured value, nor can it be used for calculation purposes.Note 2: Those standards that adopt DN identification system should give the relationship between DN and the size of pipeline elements, such as DN/OD or DN/ID.[2]
