# What you need to know about the sizes of steel pipes

If the price tag of black or galvanized steel pipes you will see the inscription DN 15 or, say, DN 40 note: this does not mean that internal pipe diameter is equal to fifteen or forty millimeters. Do the so-called «orifice diameter», conventionally used in the dimensional range of steel pipes.

Now to indicate the approximate size of the lumen inside the tube is used the concept of nominal or normal diameter (DN), however, the notion of Control still widespread (at least in the field of popular knowledge).

In this article we will talk about pipes, manufactured in accordance with GOST 3262 – 75 («Pipes steel water-gas supply»). In WC range they are represented by three series: light, ordinary and reinforced pipes the actual internal diameter which repel each other, creating, in the end, all of these pipes line Do (DN).

How? — the calculation of the average diameter of the three pipes, the same «outdoor advertising», but belonging to three different series. The result is rounded and you have DN (DN), which only rarely coincides with the actual diameter.

All the pipes of one series can withstand the same pressure as the wall thickness increases in proportion to the range of external diameters. And since they are permanent (for all three series), the changes relate to the actual inner diameters. However, the household assortment «czernina» and «galvanizing» is usually restricted to pipes of one series — ordinary, which are designed «and weld, and the thread». Also limited and relevant range of sizes: from DN65 up to du10.

It will lead to small but useful summary of the standard (conditional pass, mm — outside diameter, mm wall thickness, mm):

• 10 — 17,0 — 2,2
• 15— 21,3 — 2,8
• 20— 26,8 — 2,8
• 25— 33,5 — 3,2
• 32— 42,3 — 3,2
• 40— 48,0 — 3,5
• 50— 60,0 — 3,5
• 65— 75,5 — 4,0

Here you need to keep in mind: the standard allows for slight variations in size. As for the actual internal diameters, it is necessary to understand that over time steel pipe «overgrown», the lumen inside the tube is reduced. Therefore, the size (the same DN — DN) decided to pick up «with reserve».

Decorative rosette-plates, with which you can improve the look of the pipe, for example, not too gracefully coming out of the wall, selected outer diameter. They are often detachable, so you can «snap» even over several layers of paint.