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A variety of profiles of rolled metal pipes are widely used in industry, construction, architecture, and urban utilities. They are also used in everyday life. The correct choice of assortment and dimensions of a pipe of circular cross section is carried out in two stages. First, mass and technological characteristics of rolled products are established. Then, from the range of pipes, the expected assessment of the strength and corrosion resistance of the product is calculated, taking into account the conditions of its operation.

## Classification of steel profiles of circular cross section

The considered rental is distinguished by the following parameters:

- By production technology. These products can be obtained by rolling on mills (hot and cold), drawing on round mandrels, pressing through round dies, as well as rolling from metal tapes or strips, followed by electric or flame welding. Accordingly, they speak of rolled, drawn, extruded and welded pipes.
- According to the dimensions of the cross section — with a constant or variable (socket connections) section.
- Based on source material.
- For dimensional accuracy.

Seamless pipes are the most durable. They, in turn, can be divided into:

- Hot rolled seamless.
- Cold-drawn (hot drawing is not used in modern metallurgical production).
- Hot and cold pressed.
- Precision steel of especially high accuracy.

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Seamless steel pipes show good performance in pressure pipelines and gas pipelines of main lines.

Welded steel pipes are technologically easier to manufacture and require less energy consumption for manufacturing.

They differ:

- Welding method (flame, electric, resistance welding).
- The direction of the relative movement of the welding head (only in relation to electric welded workpieces!) — in a straight line or in a spiral.

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Welded steel pipe is formed by welding a steel plate rolled into a tubular shape with a seam that runs along the entire product. Such profiles are used in main water pipelines of medium pressure, in internal gas pipelines, heating and air conditioning systems, and also as a housing for laying electrical networks.

For domestically produced pipe steel products, the following standards apply:

- GOST 8732-78, which establishes technical requirements for hot-formed seamless pipes.
- GOST 10705-91 concerning electric-welded longitudinal pipes.
- GOST 3262-75, defining the assortment and technical requirements for steel round pipes intended for the installation of water supply networks.
- GOST 10704-91, the norms of which apply to thin-walled pipe products (see Fig. 8).
- GOST 20295-85, which presents the standard sizes of round pipes for main pipelines.

Some of the special types of profiles, in particular, drill pipes or stainless steel pipes, are produced according to industry standards and specifications. The domestic assortment of round steel pipes is metric, the assortment of foreign ones is often inch.

## The main geometric characteristics of the section of pipe metal

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To assess the operational capabilities of round pipes, such cross-sectional parameters as the circular moment of resistance, the moment of inertia and the radius of inertia are important.

Under moment of resistance W, mm^{3}, understand the force factor, which is caused by internal loads that occur in a pipe subjected to external elastic deformations. In the resistance of materials, this parameter depends on the moment of inertia of the flat section I, mm^{four}and from the distance between the outer outer diameter and the axis of the pipe e, mm:

* W = I/e*

The moment of resistance characterizes the ability of the section to withstand external force factors. For a ring (a flat figure that defines the cross section of an ordinary, not thin-walled, round pipe), the moment of resistance does not depend on the direction of the coordinates, and is set according to the dependence

* W = πD ^{3}/32 (1-s^{four}),*

where:

- D is the outer diameter of the profile, mm;
- c \u003d d / D — the ratio of the internal d and external D diameters of the section.

The pipe profile is characterized by a higher moment of resistance. This allows him to successfully cope with external force factors than, for example, a solid profile with the same cross-sectional area. Therefore, such pipes are used in such mechanical and hydraulic systems, which are subjected to significant bending stresses during operation. Often these voltages change in sign and time.

Moment of inertia is a term used to measure or quantify the amount of mass located at the most distant points of an object. The moment of inertia of a symmetrical section is calculated with respect to a hypothetical axis of rotation, and therefore will be the same for both the x-axis and the y-axis. In this case, choosing the axis of rotation of the ring, the moment of inertia of its section will be equal to

* I = πD ^{four}/64(1 – s^{four})*

The moment of inertia is considered to be an energy property of a section: when calculating how much energy will be stored in a rotating object, the energy is proportional to the moment of inertia. Thus, always try to choose the axis of rotation and the shape of the object, which would provide the largest moment of inertia with the maximum stored energy. For a ring, this condition is satisfied automatically. Therefore, from the strength point of view, the moment of inertia of the ring is the maximum counteraction of the object when trying to turn it along the axis.

The radius of gyration i is the distance from the axis of rotation of the annular section to the point where the mass of the material of this ring is concentrated. The radius of gyration is determined by the formula i = (I/F)^{0.5}, where F is the cross-sectional area. The radius of gyration characterizes the flexibility and stability of the pipe under the action of external loads. The considered characteristics are taken into account in the calculations for torsional stiffness. The corresponding formulas are summarized in the table:

Cross section shape |
Torsional moment of inertia |
Torsional moment |
Position of the point at which the greatest torsional stresses occur |

Solid thick wall pipe | I_{k} = 0.1d^{four}(1-c^{four}) |
W_{k} = 0.2d^{3}(1-c^{four}) |
Perimeter of the outer contour of the pipe |

Solid thin wall pipe | I_{k} = πd^{3}t/4 (t — wall thickness) |
W_{k} = πd^{2}t/2 (t — wall thickness) |
The stresses are the same throughout |

Welded thin wall pipe | I_{k} = πdt^{3}/3 |
W_{k} = πdt^{2}/3 |
The greatest stress occurs along the line opposite the weld |

Note!Thin-walled pipes are those for which the ratio D / t > 40 is met, or profiles with a wall thickness of less than 1.5 mm.

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## materials

For the production of the products in question are used:

- High-quality carbon steels according to GOST 1050-90.
- Structural alloy steels according to GOST 4543-91 (with the exception of those that contain an increased percentage of zinc, an element that increases brittleness).
- Stainless steels according to GOST 5632-89.
- Some grades of building steels according to GOST 27772-2015.

The choice of material is determined by the limitations of the relevant standard and the operating conditions of the pipeline. For example, when pumping chemically aggressive media, when working in high humidity or when laying underground utilities, pipes must be subjected to anti-corrosion treatment. Due to the increasingly complex requirements of operators, the assortment of stainless pipes of round cross-section is constantly changing.

As the percentage of carbon increases, the strength of the pipes increases, and the ability to withstand dynamic loads decreases. With a decrease in the percentage of carbon, the cost of production decreases, and the conditions for plastic deformation of workpieces without cracking improve.

Ordinary carbon steel pipes are used to supply drinking water, and therefore are widely used in plumbing, fire fighting, heating, ventilation and air conditioning. Such pipes are also ideal for use in other industries if they are pre-coated with paints, varnish or other metals (in particular nickel, chromium, zinc). This helps not only to protect the profiles from rust, but also to make them durable when working in critical conditions.

We list the main positive features of low-carbon steel pipes:

- Sufficiently high values of tensile strength/rupture;
- Plasticity, which is important when forming complex pipeline lines;
- Low cost;
- Good weldability;
- Wide nomenclature range of assortment;
- Long service life (with surface anti-corrosion treatment — up to 100 years).

Since any steel has a high thermal conductivity, steel pipes for pumping hot liquid or gaseous media need thermal insulation. In addition, at elevated temperatures, the resistance of rolled products made from conventional steel grades is sharply reduced; in such cases, pipelines are made of stainless pipes or heat-resistant steels.

## Design calculations of pipelines

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They are divided into mechanical and hydraulic. The first are associated with certain restrictions on this type of rental. In particular, steel is distinguished by an increased weight per linear meter (compared to plastic or aluminum profiles of the same section). Therefore, in the process of calculation and design work, it is always necessary to establish as accurately as possible the mass of the pipeline section that will affect the supporting elements of metal structures. The mass of the product can be set in several ways:

- By the value of the mass of a running meter of a pipe (such a table is always given in the text of the corresponding standard);
- By calculation, multiplying the cross-sectional area F = ρπ(D
^{2}– d^{2})/4 for the length of the pipeline; - According to the online mass calculator, which are abundantly presented on the Internet.

The need for hydraulic calculation of pipelines is associated with the variability of the mode of movement of the working media inside. In addition, along the length L, the diameter of the section and the location of its axis, as well as the wall thickness, can vary. The main cross-sectional shapes of pipelines:

- Bell-shaped.
- Stepped.
- Periodic.
- Bellows (with corrugations).
- Spiral.
- With radiator.

With individual design, there are other options. For complex pipelines, the total hydraulic head loss is required, which is taken into account by the so-called Reynolds number.

The Reynolds number is a dimensionless parameter, which is determined by the dynamic pressure ratio ρu^{2} and shear stress μu/L (ρ is the density of the pumped medium, u is its velocity).

The Reynolds number can be used to determine if the flow is laminar, transient, or turbulent. So, flow:

- Laminar at Re < 2300;
- Transient process at 2300 < Re < 4000;
- Turbulent at Re > 4000.

The calculation of this parameter is performed according to the dependence:

* Re = (ru ^{2})/(μu/L)*

For a pipe or duct, the Reynolds number is

* Re = ruD _{at}/µ,*

where D_{at} — hydraulic diameter in the estimated section of the pipeline.

When assembling several dissimilar sections, two main options are used: detachable and one-piece connections. In the first case, various connecting fittings are used (flanges, connectors, sockets), in the second — welding (electric or flame).

Pipe fittings and adapters must provide a tight, gas-tight seal that is convenient for installation, disassembly, and installation. The main requirements for them are sufficient fatigue strength, vibration resistance, durability under high pressures and extreme temperatures.

Industrially produced connecting fittings are made of steels, aluminum alloys, brass, copper, and then optimized in terms of strength, corrosion resistance, weldability, ductility. By configuration, it is subdivided into spurs, elbows, tees, plugs, adapter sleeves and tips.

The use of steel pipes makes it possible to reduce the weight of structures and save up to 40% of metal, as well as to use mechanized installation methods more often. As a result, construction is simplified, investment and operating costs are reduced.

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