What exactly is the relationship between DN, SDR, and Sch?
The selection of pipelines is a complex issue that usually involves multiple factors such as pipeline material, diameter, wall thickness, and pressure. Here, we will not delve into them in detail, but only discuss three basic terms related to diameter and wall thickness: DN, SDR, and Sch. Understanding their concepts and scope of application is crucial for pipeline selection.
1. DN: Basic dimensions of the pipeline
Definition:
DN (nominal diameter) is the "nominal size" of a pipeline, typically used to indicate the size range of the pipeline, but does not equal the actual outer or inner diameter. It is an approximate value mainly used for engineering design and selection, facilitating the connection between different standards.
● Function:
Standardize the specifications of pipes, fittings, and valves to ensure compatibility, for example, DN50 pipes must be matched with DN50 tees and flanges.
Common expressions of DN:
——Plastic pipes (PE, PVC): DN is usually close to the outer diameter, such as DN110, which means the outer diameter is about 110mm.
——Metal pipes (steel pipes, stainless steel pipes): DN does not directly equal the outer diameter, and standards need to be queried. For example, the outer diameter of a DN50 steel pipe may be 60.3mm, while the outer diameter of a DN50 PE pipe may be 50mm or 63mm.
2. SDR: Pressure resistant password for plastic pipes
——Definition:
SDR stands for Standard Dimension Ratio, which is the ratio of the outer diameter to the wall thickness of a pipeline. The formula is: SDR=outer diameter ÷ wall thickness.
For example, SDR 11 represents an outer diameter that is 11 times the wall thickness; SDR 17 represents an outer diameter that is 17 times the wall thickness. It is evident that for the same pipe diameter, the wall thickness of SDR11 is thicker than that of SDR17.
——Characteristics:
The larger the SDR value, the thinner the wall thickness and the lower the pressure bearing capacity of the pipeline; The smaller the SDR value, the thicker the wall thickness and the stronger the pressure bearing capacity.
SDR is usually related to pressure rating (PN). Taking HDPE water supply pipelines as an example, SDR 11 corresponds to a pressure of 1.6 MPa, and SDR 17 corresponds to a pressure of 1.0 MPa.
——Application scope:
SDR is mainly used for plastic pipelines (such as PE, PVC, etc.), especially for pressure pipeline systems, such as municipal water supply, gas transmission, industrial pipeline networks, etc.
3.Sch: The pressure resistant core of metal pipelines
——Definition:
Sch (Schedule) is a representation of pipeline wall thickness grades, mainly used for metal pipelines. Sch grades correspond to different wall thicknesses, but their calculation is not simply based on the outer diameter/wall thickness ratio.
——Characteristics:
Under the same material and specification, the larger the Sch value, the thicker the wall thickness, such as Sch 40<Sch 80<Sch 160.
The Sch grade of stainless steel pipes is marked with "S", such as Sch 40S, Sch 80S, etc; The Sch grade of carbon steel and alloy steel pipes does not have an "S", such as Sch 5, Sch 10, etc; Sch XXS is an ultra thick walled pipe commonly used in high-pressure pipeline systems;
Metal pipes of different materials, with the same Sch grade, may not have the same wall thickness. Stainless steel pipes can be found in ASME B36.19, while carbon steel and alloy steel pipes can be found in ASME B36.10.
When DN ≤ 300, Sch 40 and Sch 80 are most commonly used, while thin-walled pipes such as Sch 10 and Sch 20 are usually used for large diameter pipelines (DN ≥ 350).
——Application scope:
Sch is mainly used for metal pipelines (such as carbon steel, stainless steel pipes, alloy steel pipes, etc.), suitable for pressure pipelines, industrial pipelines, oil and gas transportation and other fields.
Finally, to summarize, DN is the basic size of a pipeline, but the DN of different pipes may correspond to different outer or inner diameters. SDR and Sch are both used to specify the wall thickness of pipelines, with the difference being that SDR is mainly used for plastic pipelines, while Sch is mainly used for metal pipelines.
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