Today, communications from polymeric materials have supplanted most metal counterparts. However, steel still remains the common alloy from which pipelines are made. One of the main characteristics of steel pipes, affecting the operational accessories, is the diameter. This indicator is mandatory in the preparation of construction schemes. The diameters of steel pipes are regulated by the relevant State Standards.

## Types of steel pipes by the method of their production

Steel pipes are used when laying water supply systems and gas transport communications. Such products have many positive qualities, among which one can distinguish high strength, resistance to linear expansion. A significant disadvantage of such products is that they have a low resistance to corrosion. All steel pipes are classified according to the method of manufacture and purpose.

One of the most common types of steel pipes are electric welded products. They are also called straight-seam. Made of sheet steel. The use of electric welding equipment allows you to get a small flat seam. Such pipes are used when laying water communications and gas transmission systems. The diameters of pipelines of this type are fairly easy to determine with the help of regulatory documentation (GOST 10704-91). The range of dimensions of the cross section in this case is 10-1420 mm.

The next type of steel pipes – spiral. Such products are made of steel, which is produced in rolls. Like the previous type, these parts have a seam, but it does not have a minimum width. Thus, spiral-seam pipes are not able to withstand high pressure values (unlike electrowelded ones). Because of this, they are not used for laying gas transmission systems. These products are governed by GOST 8696-74.

Modern industry allows to produce structures that do not have a seam at all. Seamless pipes are made of special blanks. It should be noted that such products are made in two ways: hot and cold. In GOST, the diameters of steel pipes are prescribed in special tables containing other geometric characteristics of products, which simplifies the search. Seamless details regulate GOST 8732-78 and GOST 8734-75.

The range of diameters of products that do not have a seam varies from 10 to 550 mm. The absence of a seam significantly increases the strength characteristics of these pipes, which allows their use in responsible communications.

Note!Regulatory documentation controls the seamless products, but it applies only to parts whose diameter does not exceed 250 mm.

Knowing the exact value of the diameter, which has a water or gas pipe, it is possible to calculate the volume of the substance transported by communications. The use of such pipes in construction requires a clear definition of the dimensional characteristics necessary for the calculation of economic systems.

An example is the heating system. The diameter of the pipes in such communications should be clearly calculated so that in the winter period the system ensures uniform heating of the living quarters.

To date, there are several common methods to calculate the diameter of steel pipes. The size chart, which can be found in the regulatory documentation, is the simplest of these. This parameter can also be determined using online calculators. Such programs are located on specialized sites on the network, so finding them is easy.

Independent calculation of the diameter of communication is made by means of mathematical expressions. The type of formula depends on the operational purpose of the communication. For example, the following equation is used to determine the diameter of the heating pipe:

**D = sqrt ((3.14 x Q) / (V x DT)), where:**

D is the diameter (internal);

Q is the heat flux rate, calculated in kW;

V is the velocity of the substance transported through the pipeline (measured in m / s);

DT is the temperature difference at the starting and ending point of the system (input / output);

sqrt – square root.

This formula allows you to quite accurately determine the diameter of the pipe. The designation of this indicator in the diagrams makes it possible to correctly calculate the required pressure and the amount of the substance transported per unit of time.

Today, diameters are subdivided into several types depending on what exactly this quantity characterizes. To use this parameter in the calculations it is recommended to study the types of diameters.

Conditional passage of pipes (DN). Indicator of the internal space of the pipe. This parameter is calculated in mm or inches. In the second case, the value is rounded off. Knowledge of this parameter allows you to select components (fittings) of the desired size to the pipes.

Nominal diameter. This indicator is very similar to the previous type, but it has some differences. For example, the main characteristic of a nominal parameter is its high accuracy, which does not tolerate rounding.

Internal diameter (DN). This indicator is a physical quantity that is measured in mm. The inner diameter is used in calculating the permeability of the pipeline structure. Do not confuse this value with the nominal diameter of pipes.

Note!To calculate this parameter, there is a special formula: Dy = Dн – 2S.

Outer diameter (DH). According to this parameter, all pipes made of steel are divided into 3 main groups: small, medium and large. Each group has its own range of sizes and its own purpose. It is easiest to determine this indicator by the steel pipe table. GOST standards regulating such products can be easily found on the Internet using the search box of your browser.

It is necessary to note such a parameter as the wall thickness. This value refers to the physical and affects the quality characteristics of the part. For example, the volume of the product and its mass depend on the wall thickness. Calculated wall thickness in mm. To define it, the following simple formula is often used:

**t = Dn – Dу**

Products whose diameter is calculated in inches (for example, 5?), Are used when laying water pipelines and gas transmission structures. On the Internet you can find tables containing this value in millimeters and in inches. Some of the schemes combine both measurement systems, which is very convenient. The five-inch pipe complies with a standard internal diameter of 125 mm.

Measuring the diameter of pipes in inches is applied during the installation of water and gas transmission lines, as this simplifies the overall calculation. One inch is equal to 25.4 mm. It is important to remember that when measuring a pipe, 1 inch has a different size, namely 33.5 mm. The reason is quite simple: the dimensions of the pipes are calculated by the internal diameter, and not by the external one. In drawing up the installation plan and scheme, it is necessary to take into account this discrepancy. Such information allows us to answer the question of how to measure the diameter of the pipe and prevent errors.

**Table 1. Du in millimeters and inch system of designation of the sizes of steel pipes:**

D, mm | Thread diameter, inches |

150 | 6? |

40 | 1 1/2? |

80 | 3? |

15 | 1/2? |

100 | four? |

32 | 1 1/4? |

50 | 2? |

125 | five? |

25 | one? |

Useful information!As a rule, no problems arise when installing only steel products, as they are measured in inches. However, if it becomes necessary to replace the old steel communication with plastic, confusion may occur. Therefore, it is worth remembering that the actual and metric dimensions of an inch differ.

In most cases, the laying of inch pipes does not cause any difficulties. The nature of the discrepancy is in the designation of steel products (water and gas pipelines), which are realized and marked with a nominal diameter, while their real cross-section has other dimensions. As an example, a simple calculation of the dimensions of a pipe, the outer diameter of which is 140 mm and the wall thickness is 5.5 mm.

To determine the actual diameter, a simple equation is used:

**D = Dn – t x 2**

After introducing the required values, this formula takes the following form: D = 140 – 5.5 x 2 = 129 mm. This indicator corresponds to the real diameter of the pipe, in which the cross-section of the outer wall is 140 mm. However, the conditional passage or internal diameter of the inch pipe (or millimeter) is the dominant value. In this case, this value is 125 mm, it is for it that most construction calculations are made.

For connecting steel and plastic pipes, special transition elements are used – fittings. Such adapters allow you to connect two pipes with different cross-section indicators, made of different materials. In order not to be mistaken when installing a communication or replacing it, it is recommended to take into account both the external and internal diameters of steel pipes.

## Outer diameter of pipes (DN): classification of steel products

As mentioned above, the inner and outer diameters are different from each other. The first of them is used in the designation of individual elements of pipeline structures. The implementation of steel products is also performed on the internal diameter. This indicator is important in the event that it is necessary to carry out installation calculations of the water supply system or gas line. In turn, the outer diameter is used to determine the strength characteristics of the pipeline and its resistance to mechanical loads.

The outer diameter of steel pipes is a characteristic by which all products from this material are classified. Depending on this parameter, there are three main types of pipelines:

- small;
- average;
- big ones.

Pipes that belong to a group of small, have a range of diameters from 10 to 102 mm. Medium-sized products can have a cross section of 102 to 426 mm. The diameter range of large steel pipes starts at 426 mm. In turn, the definition of internal diameter is recommended to do the table.

Small pipes are used when installing communications in residential buildings. By means of medium-sized products, urban water transport lines are laid, and also they are actively exploited by companies that are engaged in oil production (crude). The main areas in which large steel products are used are the oil and gas transportation areas. From large parts collect trunk lines. The diameter of the gas pipes can reach 1220 mm.

When finding the inner diameter, the indicator corresponding to the size of the nominal passage is rounded to 0 or 5. Thus, when determining this characteristic, the size is standardized to the nearest parameter of the metric system.

Thus, the diameter of water pipes and gas transport products has an impact on their cost. Parts that have a cross section of more than 100 mm are practically not used in everyday life. The cost of such pipes is quite high, since 1 m of a part can weigh over 10 kg. The heaviest pipe has a diameter of 1220 mm and a wall thickness of 16 mm. The weight of a meter of such a detail is approximately 475 kg.

Diameter is a necessary characteristic, without which laying a pipeline structure is impossible. When determining this parameter, it is necessary to clearly understand that there are several varieties of it. In the tables that can be found in the relevant regulatory documentation, the internal diameter of steel products is most often indicated. If you decide to use a special formula for determining the section, it is recommended to pay attention to the examples containing the solution.