Regarding the issue of different pipe designations for pipes with the same pressure rating but different diameters
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As the title suggests, I’d like to ask the experts here. In the projects I have worked on in the past, the pressure level was indicated by wall thickness; it was rare to use the pipe size designation to represent the wall thickness. But recently, I have some doubts regarding the use of pipe designations to indicate wall thickness: 1. Why is such a straightforward way of expressing wall thickness chosen, when an less intuitive method like pipe designations is used? 2. It is found that under the same pressure level, the pipe designations vary for pipes of different diameters. According to the definition of pipe designation numbers, when the design pressure is the same, why are the pipe designation numbers different? 3. The pipe designation number increases as the pipe diameter decreases. For example, for class150 with a nominal diameter of 40, SCH80 is used; the wall thickness corresponding to a diameter of 40 is 5. In the past, however, the maximum wall thickness for a diameter of 40 was 3. A greater wall thickness means a smaller flow area. Why is the pipe designation number so high for smaller diameters? 4. The pipe grade for DN50 is SCH40, and its wall thickness is actually smaller than that of DN40; whereas in the past, the wall thickness increased as the diameter increased. What is the reasoning behind this? If the reason is the small pipe diameter, such a thick wall thickness allows for butt welding, and there is no need to use socket welding. But the pipe grade table states that socket welding is to be used
1. The use of the pipe size as an indicator of wall thickness is adopted because it not only reflects the wall thickness of the pipe but also takes into account other factors such as the pressure and temperature it must withstand, thereby making pipe selection more standardized and systematic. 2. At the same pressure level, the nominal pipe sizes for pipes with different diameters vary; this is mainly because, when subjected to the same pressure, larger-diameter pipes require a relatively smaller wall thickness due to their larger surface area ； Small-diameter tubes, due to their smaller stress-bearing area, require a thicker wall thickness to withstand the same pressure. 3. Regarding the issue of the pipe identifier changing with the pipe diameter, it is true that for smaller diameters, the pipe identifier is larger; this is because pipes with smaller diameters require thicker walls to maintain structural stability and safety. A reduction in the fluid flow area is a negative effect, but it is necessary from the perspective of overall safety and pressure resistance. 4. In the case where the specification for DN50 pipes is SCH40 while that for DN40 is SCH80, this indicates that the operating conditions or requirements for DN40 demand a higher pressure resistance or greater load-bearing capacity. Possible scenarios include working conditions with higher temperatures or greater pressures. The choice of welding method depends on various factors such as installation and maintenance costs, as well as technical requirements. In general, the selection of pipe size is closely related to the operating conditions, safety requirements, and economic factors in practical applications, and it is not determined solely by the wall thickness. .
The thicker wall thickness in smaller diameters is due to the requirements of threaded connections; the machining of pipe threads reduces the effective wall thickness.
In addition, for socket weld pipe fittings, the wall thickness of the steel pipe must be at least Sch80.
Yes, it refers to socket welding. Why must the standard for socket welding be no less than SCH80? The reason why socket welding is used for small-diameter pipes is not because of the thin wall thickness; if butt welding were used, it would be easy to achieve full penetration, which is why socket welding is employed. But your pipe walls are thicker than those of larger-diameter pipes, so butt welding would be completely sufficient – why then use socket welding?
The question is, isn’t the wall thickness a reflection of various conditions such as temperature, pressure, and corrosion margin? Moreover, at the same pressure level, why could the operating conditions for dn40 be more stringent than those for dn50? Finally, when we use metric pipes to represent wall thickness, the wall thickness is smallest for smaller pipe diameters and largest for larger pipe diameters; moreover, according to the wall thickness calculation formula, the wall thickness is also smallest for smaller pipe diameters and largest for larger pipe diameters. If, as you say, smaller pipe diameters require greater wall thickness, then that wall thickness calculation formula must be incorrect
Socket weld pipe fittings are made of forged materials, with a minimum pressure rating of Class3000 = PN50; if the wall thickness of the fitting is Sch40, then it is below PN50.
It is recommended that the original poster learn about the origin of the pipe designation Sch, so as to understand its purpose and the difference between it and the actual wall thickness of the pipes.