# Representation of DN, De and Φ

## DN is the nominal diameter of the pipe

Usually used to describe: steel pipe
The nominal diameter can be expressed in metric mm or inch. Pipe accessories are also represented by nominal diameter, meaning the same as slotted pipe. Its correspondence with English units is as follows:

Nominal diameter, also known as mean meanoutside diameter. This is because the wall of the metal pipe is very thin, and the outer diameter of the pipe is almost the same as the inner diameter of the pipe, so the average value of the outer diameter and the inner diameter of the pipe is taken as the diameter of the pipe.
DN is the nominal diameter. The nominal diameter is the common diameter of all kinds of pipes and pipeline accessories. Nominal diameter is a convenient round integer for reference, which is not strictly related to machining size. The same nominal diameter of the pipe and pipe accessories can be connected with each other, with interchangeability. It is not the actual outer diameter or inner diameter of the pipe, although its value is close to or equal to the inner diameter of the pipe.
For example, welded steel pipe can be divided into thin-walled steel pipe, ordinary steel pipe and thickened steel pipe according to thickness. Its nominal diameter is not the outer diameter, nor the inner diameter, but a nominal size similar to the inner diameter of ordinary steel pipe. Each nominal diameter corresponds to an outer diameter, and the inner diameter varies with the thickness.

## De mainly refers to the outer diameter of the pipe

Mainly used to describe: seamless steel pipe, PVC and other plastic pipes, and other pipes with clear wall thickness.
Generally, those marked with De need to be marked in the form of outer diameter x wall thickness.
Taking galvanized welded steel pipe as an example, the two marking methods of DN and De are as follows:

• DN20 De25×2.5mm
• DN25 De32×3mm
• DN32 De40×4mm
• DN40 De50×4mm

We are used to using DN to mark the welded steel pipe, and rarely use De to mark the pipe when the wall thickness is not involved. However, marking plastic pipe is another matter. It is also related to the industry habit. In the actual construction process, the pipes 20, 25, 32 and so on we call simply refer to De, not DN.
In addition, according to the practical experience on site:

• a. Two kinds of pipe materials are connected in the same way: screw connection and flange connection.
• b. Galvanized steel pipe and PPR pipe can adopt the above two kinds of connection, but it is more convenient to use screw thread for pipe less than 50, and more reliable to use flange for pipe more than 50.

If two metal pipes of different materials are connected, it is necessary to consider whether galvanic cell reaction will occur, otherwise the corrosion rate of active metal pipes will be accelerated. It is better to use flange connection, and separate the two metals with rubber gasket insulation material, including bolts, to avoid contact.
Φ can also represent the outer diameter of the pipe, but it should be multiplied by the wall thickness behind it.

## Φ is the diameter of an ordinary circle

Of course, Φ can also represent the outer diameter of the pipe, but at this time, it should be multiplied by the wall thickness behind it. Such as: Φ 25 × 3, it means the pipe with outer diameter of 25 mm and wall thickness of 3 mm.
For seamless steel pipe or non-ferrous metal pipe, “outer diameter × wall thickness” shall be marked. For example: Φ 107 × 4, where Φ can be omitted.
Some steel pipe labels in China, ISO and Japan adopt wall thickness dimension to represent steel pipe wall thickness series. For this kind of steel pipe, the expression method is outer diameter × wall thickness. For example: Φ 60.5 × 3.8
Comparison table of nominal diameter and outer diameter of each pipe

 Serial number Nominal diameter DN OutsiDe diameter De Welded steel pipe Seamless steel tube Spiral steel pipe UPVC pipe PP-R pipe PB pipe Aluminum plastic pipe Cast iron drain pipe HDPE pipe 1 15 21.3 20 20 20 2 20 26.8 28 25 25 25 3 25 33.5 32 32 32 32 4 32 42.3 38 40 40 40 5 40 48 48 50 50 50 6 50 60 57 50 63 63 63 50 50 7 65 75.5 76 75 75 75 75 8 80 88.5 89 75 90 90 90 75 9 100 114 108 110 110 110 110 100 110 10 125 140 133 125 11 150 165 159 160 150 160 12 200 219 219 200 200 200 13 250 273 273 14 300 325 325 15 350 377 377 16 400 426 426 17 450 480 480 18 500 530 530 19 600 630 630

## The difference of DN, De, DG and Φ

• De is the diameter of the outer wall of the pipe.
• DN is De minus half the wall thickness.
• DG is usually not used.
• Φ is the diameter of an ordinary circle.

The pipe diameter shall be in mm, and the expression of pipe diameter shall meet the following requirements:

• The pipe diameter of water gas transmission steel pipe (galvanized or non Galvanized) and cast iron pipe should be expressed by nominal diameter DN;
• The diameter of seamless steel pipe, welded steel pipe (straight seam or spiral seam), copper pipe and stainless steel pipe should be expressed as outer diameter × wall thickness Φ;
• The diameter of reinforced concrete (or concrete) pipe, clay pipe, acid resistant ceramic pipe, cylinder tile pipe, etc. should be expressed by inner diameter D;
• The diameter of plastic pipe should be expressed according to the product standard;
• When nominal diameter DN is used to represent pipe diameter in design, there should be a comparison table between nominal diameter DN and corresponding product specifications.

The specification of unplasticized polyvinyl chloriDe pipes for building drainage is expressed by De (nominal outer diameter) × e (nominal wall thickness) (GB 5836.1-92), and the specification of polypropylene pipes for water supply is expressed by De × e (nominal outer diameter × wall thickness). The marking of plastic pipes on engineering drawings.

### Metric dimension size

DN is generally referred to as “nominal size”, which is neither the outer diameter nor the inner diameter of the pipe. It is the average value of outer diameter and inner diameter, which we call average inner diameter.
For example, the metric mark of plastic pipe with outer diameter of 63mm is DN50.

### ISO metric dimension size

Da is the outer diameter of PVC pipe and ABS pipe.
De is the outer diameter of PP pipe and PE pipe.
For example: the metric mark of plastic pipe with outer diameter of 63mm (mm dimension size), da63 for PVC pipe and ABS pipe.

## The respective ranges of De, DN, D and ф.

De — outer diameter of PPR, PE and PP pipes.
DN – nominal diameter of steel plastic pipe and PVC pipe.
D — nominal diameter of concrete pipe.
ф – seamless steel pipe or non-ferrous metal pipe shall be marked with “outer diameter × wall thickness”.

Source: Network Arrangement – China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)