I have been working as a plumber for a long time, in my work I have to work with pipes of different diameters. And also from different materials, metal and polymer (PPR, PVC, LDPE, HDPE).
When I started working with polypropylene pipes, I wondered why metal pipes have different diameters compared to polypropylene pipes. Began to think and analyze. And he came to certain conclusions. Let's start in order:
For clarity, I took a caliper and took measurements. Let's start with a steel, ordinary gas pipeline.
Digression, guys, I deliberately left the designation of diameters in inches. I explain that less and less often in the conversations of plumbers and sellers in stores, as well as in the documentation, you can hear and see this English measure, so the conversation goes in millimeters.
The outer diameter of the pipes, the so-called 20 - ki... equals 26.9. It’s strange, because it’s not close to 20 mm. The catch is this:
The diameter of steel pipes is calculated from the inner section of the pipe. And it is approximately equal to 20 mm. Therefore, often in the designations you can see or hear such a concept as DN 20 (Conditional diameter)
What about plastic pipes?
The so-called 20 has an outer diameter close to 20 mm. But his internal one can vary, for a PPR without reinforcement 13.19 mm, and for a reinforced pipe 14.24 mm.
Why is this happening, I will tell you my reasons:
An important feature of the installation of steel pipes is versatility. Firstly, these pipes can be connected by electric and gas welding. Non-removable connection. And the second, a collapsible connection using threads cut at the installation site using a die (die). The pipe wall allows threading. The conditional passage remained as it was.
Why, then, are polymer pipes counted by their outer diameter?
The connection of polypropylene pipes is carried out using socket welding, the inner diameter of these nozzles is welding machines (irons, soldering irons) of absolutely all manufacturers have a certain diameter of 20, 25, 32,... mm. Accordingly, pipe and fitting manufacturers adhere to these dimensions. Therefore, PPR is measured by the outer diameter of the pipes.