On the significance of introducing the concept of nominal diameter in engineering (newbie question)
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Everyone, I’m a newcomer to piping work in design firms, and I have some questions. During the design process, I’ve noticed that no matter what the material is, pipes of the same pressure class always have the same nominal diameter as well as the same outer diameter. So what’s the purpose of using the nominal diameter in engineering? Wouldn’t it be better to just use the outer diameter instead?
“You think that as long as the nominal diameter is the same, the outer diameter will also be the same. The information found online is as follows: There are two ways to denote pipes – one is by nominal diameter, and the other is by outer diameter * wall thickness. Welded steel pipes used for transporting low-pressure fluids are generally specified using the nominal diameter method, as per GB/T3091. Seamless steel pipes, on the other hand, are usually denoted by their outer diameter * wall thickness, as specified in standards such as GB/T8163 ； For welded steel pipes used for low-pressure fluid transport, the nominal diameter and outer diameter are in one-to-one correspondence, but for seamless steel pipes, the nominal diameter is not in one-to-one correspondence with their outer diameter and wall thickness. Nominal diameter is just a way of naming pipes. The nominal diameters of the flanges connecting the roots to the pipes and those of the valves correspond one-to-one. The pipes I’ve come across are all small, and their diameters are determined based on their outer diameter; a 3/4 outer diameter corresponds to 19.05, while DN20 pipes purchased in China have a diameter of around 26. I haven’t fully understood the system yet
I believe the introduction of the nominal diameter is purely for the convenience of engineering calculations. When selecting pipes in practice, there are two ways of representing them: one is the nominal diameter plus pressure rating (which indicates the wall thickness), and the other is the outer diameter times the wall thickness.
These two are not the same; it still depends on the standards
There are several standard pipes in hgt20553; take a close look – those with the same nominal diameter may not necessarily have the same wall thickness or outer diameter
So, what are the common characteristics of pipes with the same nominal diameter, and on what basis is this concept established?
Thank you for your advice. Could you explain in detail how it can be calculated more easily? Is it to calculate the fluid flow velocity or something else?
It is possible to directly estimate the flow rate of the fluid in the pipeline for each nominal diameter
Could you explain what the advantage of using the nominal diameter is? I remember that in my studies, the internal diameter was used in Bernoulli’s equation
For precise calculations, of course, the Bernoulli equation is used, as well as the inner diameter. However, it is convenient and fast to use the nominal diameter directly in engineering calculations. For a DN20 pipe, the nominal diameter is simply taken as 20 for the inner diameter. If a precise calculation is required, using 26.75×2.75 gives an inner diameter of 21.25. Which approach is more convenient?
hg20553: There are still such new newcomers in chemical engineering design, which indicates that the industry is recovering