A very basic question

Φ355.6×11.0 – in this notation, Φ355.6 refers to the outer diameter, that’s correct!

Thank you for the answer from above, but I still have a question. Since it’s the outer diameter and the thickness is also given as 11, doesn’t that conflict with the nominal diameter of 350? I’m not quite sure; please give me some more guidance!

Thickness is determined by requirements such as pressure and corrosion, while the nominal diameter is the standard for interconnection

Pipes with a nominal diameter of 350 can come in various thicknesses; therefore, in addition to the outer diameter, the thickness also needs to be indicated

The nominal diameter is a nominal size; for pipes, it is generally indicated as outer diameter X wall thickness (Φ355.6×11.0) or as nominal diameter along with the pipe grade (such as DN350 Sch40). For fittings, the nominal diameter together with the pipe grade is usually used for identification (such as DN350 Sch40), while valves are typically specified by their nominal diameter and pressure class (such as DN350, PN2.5, or DN350 150#).

Additionally, the take over also has a length dimension on the equipment diagram.

Φ355.6×11.0 represents the outer diameter times wall thickness, in millimeters； DN350 is the nominal diameter.

Generally, the nominal diameter is indicated on the preliminary design drawings, while the outer diameter plus wall thickness is shown on the construction drawings

Φ355.6×11.0 represents the outer diameter times wall thickness, in millimeters； DN350 is the nominal diameter, and the nominal diameter is not necessarily equal to the inner diameter.

The nominal diameter is the integer value of a series of closely similar pipe diameters. It is a category, not referring to the inner diameter and outer diameter.

I ask in frustration: what is the actual use of the concept of nominal diameter? What was the purpose behind creating such a concept? :@

The nominal diameter is a standard established to ensure proper connection and compatibility between flanges, pipes, and fittings; it is neither the inner diameter nor the outer diameter. Generally, it is equal to the outer diameter. For example, for a pipe with a nominal diameter of 100, one can simply purchase a flange with a nominal diameter of 100 from the market in order to connect it to the pipe. Having a unified standard facilitates the development of the industry.

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 letters D or φ, 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 or φ108×5. 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 value established artificially for the convenience of design, manufacturing, and maintenance. It differs from the actual outer diameter 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. The nominal diameter is used in design drawings 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 pipes, including their nominal diameter and wall thickness, should also be provided. . Pipe series standards: For the design and construction of pressure pipelines, the selection of standard series for pressure pipelines and their components is considered first. 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-ф22mm, DN20-ф27mm DN25-ф34mm, DN32-ф42mm DN40-ф48mm, DN50-ф60mm DN60-ф76(73)mm, DN80-ф89mm DN100-ф114mm, DN125-ф140mm DN150-ф168mm, DN200-ф219mm DN250-ф273mm, DN300-ф324mm DN350-ф360mm, DN400-ф406mm DN450-ф457mm, DN500-ф508mm DN600-ф610mm, DN15-ф18mm, DN20-ф25mm DN25-ф32mm, DN32-ф38mm DN40-ф45mm, DN50-ф57mm DN65-ф73mm, DN80-ф89mm DN100-ф108mm, DN125-ф133mm DN150-ф159mm, DN200-ф219mm DN250-ф273mm, DN300-ф325mm DN350-ф377mm, DN400-ф426mm DN450-ф480mm, DN500-ф530mm DN600-ф630mm,

The nominal diameter is used to correspond to the standard; DN100 for European standards, 108 MM

Thank you all for providing so many explanations; I’m slowly starting to understand!