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An explanation is provided here on how to determine the pressure rating corresponding to seamless steel pipes of Sch grade: The wall thickness series for Sch was specified in 1938 by the American Standard Association’s ANSI B36.10 standard (for welded and seamless steel pipes). The pipe series used by Chinese petrochemical companies (SH3405) also uses pipe designations to indicate the wall thickness series. The pipe schedule number (Sch) is the ratio of the design pressure to the allowable stress of the material at the design temperature, multiplied by 1000 ； And the rounded value. That is, Sch = (P/σ) × 1000, where P is the design pressure (MPa), and σ is the allowable stress of the material at the design temperature (MPa). Remember: The Sch designation for pipes does not refer to the wall thickness, but rather to the wall thickness series. For the same pipe diameter, the wall thickness varies among different pipe designations! Generally, the diameter of a pipe can be divided into outer diameter, inner diameter, and nominal diameter. For pipes that are seamless steel tubes, the outer diameter is denoted by the letter D, followed by the value of the outer diameter and the wall thickness. For example, a seamless steel tube with an outer diameter of 108 mm and a wall thickness of 5 MM is indicated as D108*5. Plastic pipes are also specified using their outer diameter, such as De63. Pipes made of reinforced concrete, cast iron, galvanized steel, etc., are denoted by DN. In design drawings, the nominal diameter is generally used for representation; this is a standard arbitrarily established for the convenience of design, manufacturing, and maintenance. It differs from the nominal bore and serves as the specification name for pipes (or pipe fittings). The nominal diameter of a pipe is not equal to its inner or outer diameter. For example, for a seamless steel pipe with a nominal diameter of 100 MM, there are several variations such as 102*5 and 108*5. Here, 108 represents the outer diameter of the pipe, while 5 indicates the wall thickness. Thus, the inner diameter of this pipe is (108*5 – 5) = 98 MM. This value is not exactly equal to the outer diameter minus twice the wall thickness. In other words, the nominal diameter is a specification name for the pipe diameter that is close to, but not exactly equal to, the inner diameter. In design drawings, the nominal diameter is used so that the structural and connection dimensions of pipes, fittings, valves, flanges, gaskets, etc., can be determined based on it. The nominal diameter is denoted by the symbol DN. If the outer diameter is used in the design drawings, a table showing the specifications of the pipe, including its nominal diameter and wall thickness, should also be provided. . Pipe series standards for the design and construction of pressure pipelines: the first consideration is the selection of standard series for pressure pipelines and their components. Although there are many standard systems in use around the world, they can generally be divided into two major categories. The standards for pressure pipelines are shown in Table 3. Flange standards are shown in Table 4. Table 3 Standards for Pressure Pipes – Classification: Large Outer Diameter Series, Small Outer Diameter Series. Specifications: DN – Nominal Diameter; Ф – Outer Diameter. DN15–ф22 mm, DN20–ф27 mm; DN25–ф34 mm, DN32–ф42 mm; DN40–ф48 mm, DN50–ф60 mm; DN60–ф76(73) mm, DN80–ф89 mm; DN100–ф114 mm, DN125–ф140 mm; DN150–ф168 mm, DN200–ф219 mm; DN250–ф273 mm, DN300–ф324 mm; DN350–ф360 mm, DN400–ф406 mm; DN450–ф457 mm, DN500–ф508 mm; DN600–ф610 mm. DN15–ф18 mm, DN20–ф25 mm; DN25–ф32 mm, DN32–ф38 mm; DN40–ф45 mm, DN50–ф57 mm; DN65–ф73 mm, DN80–ф89 mm; DN100–ф108 mm, DN125–ф133 mm; DN150–ф159 mm, DN200–ф219 mm; DN250–ф273 mm, DN300–ф325 mm; DN350–ф377 mm, DN400–ф426 mm; DN450–ф480 mm, DN500–ф530 mm; DN600–ф630 mm. Last edited by kingberg on 2009-2-2 at 13:07.]