Corrugations are a defect during continuous pipe formation. Seamless pipes. Defects arising during the pipe production process. Pressure reduction units

In the manufacture of cold-formed steel pipes it is possible that defects may appear in the form of defects, the reasons for which may be: the use of low-quality initial workpiece (seamless or welded), violation of deformation-rate modes of rolling and drawing, modes of pipe forming and welding, violation of heat treatment modes, straightening, cutting and other finishing operations, use of worn-out technological tools.

In Fig. 83-85 show defects in cold-deformed steel pipes. If the mill is set up incorrectly, a variety of defects may appear on the pipes. Thus, with large gaps between the gauges on CPT mills, metal flows into them during the working stroke of the stand. In this case, sharp lateral protrusions (whiskers) appear on the surface of the working cone, which, during the reverse stroke of the stand, are pressed into the metal, forming deep flaws on the surface of the pipes, located in a spiral in accordance with the angle of rotation of the workpiece and called sunsets. When installing an increased supply on pipes, sunsets, waviness on the outer surface (taking the pipes beyond the tolerances in diameter and ovality), as well as variations in wall thickness are possible.

Fig. 83 – Types of defects in seamless cold-rolled steel pipes:

a – mustache; b – external waviness; c – dents

Fig. 84 – Types of pipe destruction during rolling in a CHP mill

Fig. 85 – Cracks and wrinkles on the inner surface of extra-thick-walled pipes after mandrelless drawing (section in the photo on the right, ×100)

When one gauge is displaced relative to another, dents form on the surface of the pipes. They usually appear on the surface of the pipe in a spiral according to the angle of rotation of the pipe.

Scoring on the inner surface is formed when rolling pipes made of low-alloy and corrosion-resistant steels as a result of the adhesion of metal particles to the surface of the mandrel.

Excessive compression in diameter and wall thickness (sometimes in the absence of the necessary tool) can lead to cracks on the surface of the pipes (Fig. 84).

Incorrect adjustment of the turning mechanism, as a result of which the rotation occurs too early (the pipe has not yet been freed from the gauges) or late (the gauges have already begun to roll onto the pipe), leads to transverse marks (burrs) on the outer surface of the pipes.

Incorrect installation of the mandrel in the deformation zone, when its front end gets into the pre-calibration area and causes ring-shaped imprints on the inner surface of the pipe with sharp edges, is also the cause of defects. During cold rolling, it is very important to meet the requirements for cleanliness of the workpieces and lubricant: trapped scale particles “grab” with the mandrel, and scuffs and pits form on the inner surface of the pipes. The use of a low-quality tool - one made in deviation from the requirements of the standards or one that fails during operation - also leads to defects.

For example, installing calibers on the mill with insufficient width of the strand or a mismatch between the profile of the strand of calibers and the taper of the mandrel are the cause of declines. When the calibration section of the gauges wears out, dents appear on the pipes.

In table Table 35 shows the main types of pipe defects during rolling in KhPT, KhPTR mills and measures to eliminate defects.

Table 35. Main types of defects during cold rolling of steel pipes in cold rolling mills, preventive and elimination measures

When drawing pipes, defects may occur different types, the reasons for which are: low quality of the pipe blank (conversion pipe), violation of the technological drawing process, low quality of production of technological tools (dies and mandrels), malfunction of drawing mills, etc. The main types of defects on pipes that occur during drawing are discussed below.

Pipe ends breakage occurs as a result of an incorrect drawing route (excessively large reductions), incorrect setup of the mill and calibration of the technological tool, lack of lubrication, violation of the heating mode when driving heads, high speed drawing while gripping the pipe, incorrect choice of dies and mandrels, etc. Risks and scuffing when drawing pipes - due to poor-quality chemical processing, poor preparation of pipes for drawing, poor-quality driving of heads, skew of the die, drawing misalignment, defective tools, sticking metal on the tool, solid particles entering the deformation zone, etc. During the process of setting up the mill on the first pipes, these defects are immediately identified and must be eliminated. Exceeding the pipe diameter tolerances occurs due to the incorrect choice of die or mandrel dimensions. Defects in diameter are sometimes corrected by reassigning pipes to another (smaller) size. Increasing tolerances on wall thickness is the reason for the incorrectly selected size of the technological tool (dies and mandrels). Ovality of pipes is formed when straightening pipes, as well as drawing in an oval die. This defect is corrected by additional straightening in straightening mills, however, control of the absolute size of the diameter is required, since straightening may change the diameter. The difference in the cross-section of the pipes is determined only by its presence on the workpiece. During short-mandrel drawing, the initial transverse thickness difference remains almost unchanged, but when drawing without a mandrel and on a floating mandrel, it decreases. When drawing on a long mandrel, the thickness difference is determined by the rolling conditions, therefore, in the manufacture of finished pipes after drawing on a long mandrel, mandrelless drawing is used. Transverse thickness differences also appear due to the ovality of the die or mandrel or the mismatch of the pipe axis with the drawing axis. In this case, the operation of the mill must be stopped and the reasons causing the difference in wall thickness of the pipes must be eliminated. Gaps in the form of uncompressed places on pipes subjected to reference drawing appear due to the large curvature of the workpiece, as well as incorrect settings of the mill. Ringiness on pipes appears due to elastic deformation of the rod, especially when drawing long pipes (Lr = 8...12 m) on a short mandrel. Pipe trembling occurs during drawing on a short mandrel due to poor quality lubrication and poor drying of the pipes before drawing. Shaking manifests itself most when drawing pipes of long length and with a small internal diameter, that is, when the mandrel rod is thin, but of great length, and has large longitudinal elastic deformations. The mandrel moves periodically in the deformation zone, and rings form on the pipes. This defect is not always a rejection sign, but it significantly reduces the productivity of the mill and increases pipe breakage. This can be eliminated by re-preparing the pipes or switching to another drawing method, for example, using a floating mandrel. Longitudinal cracks (cracking of pipes) are formed when drawing extra-thick-walled pipes without a mandrel, when the permissible single or total deformation is exceeded; when drawing pipes without heat treatment in several passes (see Fig. 3). This is explained by the presence of large (exceeding permissible) residual tangential tensile stresses on the outer surface of the pipes. This type of defect is typical only for arborless drawing and cannot be corrected. During mandrel drawing, there is almost no uneven deformation across the wall thickness and no cracking of pipes is observed. To avoid the appearance of this defect on pipes, you should strictly adhere to the technological route for manufacturing pipes.

Longitudinal folds from the pipe head are formed when thin-walled and extra-thin-walled pipes are drawn without a mandrel as a result of loss of pipe stability. To eliminate this defect, the degree of deformation during arborless drawing should be reduced or another drawing method should be used. Local narrowing of the cross section in the form of pinches is formed on the outer surface of drawn pipes due to dents on the workpiece, waviness, uneven heat treatment along the length, and poor-quality rolling during long-mandrel drawing. This defect is formed during mandrelless drawing of pipes.

Other types of defects are also possible, for example, in gas permeability, etc., the elimination of which requires best quality procurement and carrying out special additional operations.

Repair and improvement of the surface of seamless pipes is carried out by removing local defects, as well as by using the operations of turning, boring, grinding and polishing the outer surface of the pipes. Clean the inner surface of the pipes by blowing with compressed air under a pressure of 0.3...0.55 MPa. Long pipes (> 4 m) are blown with air from both sides, which ensures better cleaning of the inner surface of the pipes. After degreasing the pipes, inspect their inner surface using a periscope.

In Fig. 86 - 90 show defects in cold-deformed welded pipes.

Fig. 86 – Destruction of the ends of welded steel pipes during cold rolling

Fig. 87 – Defects in the form of burrs and scratches on the inner surface of pipes after cold rolling (a) and drawing (b). (the workpiece was obtained by induction welding)

Fig. 88 – Defect in the form of a sunset at the weld on the inner surface of cold-rolled pipes

Fig. 89 – The nature of the location of cracks on the inner surface of cold-rolled welded pipes

Fig. 90 – Defects up to 0.2 mm deep and more on welded pipes after short-arbor drawing: a – microcrack; b – sunset, which is formed due to lack of penetration and displacement of the edges on the original workpiece

Pipe quality control.

To ensure that the quality of pipes meets the requirements of GOSTs and technical specifications, pipes are subjected to control and testing, most of the methods of which are standardized. Many of them are common to all types of metal products, others are specific - they are used to control the quality of pipes for special purposes and are determined by the conditions of use of pipes and products made from them.

Some types of pipes, in accordance with the requirements of the standards, are tested for hydraulic pressure in special presses, where the ends of the pipes are fixed in clamps; Pressurized water is supplied inside the pipe. The pressure value is determined by standards depending on the purpose of the pipes.

Finished pipes are subjected, in accordance with GOST requirements, to mechanical and technological tests for strength and tensile elongation, hardness, expansion, flattening, beading, impact strength, and corrosion resistance.

Control of the dimensions of finished pipes - outer and inner diameters, wall thickness, ovality of the outer and inner surfaces, eccentricity, longitudinal and transverse wall differences, curvature, length, deviations of the actual dimensions and shape from the nominal ones is carried out using measuring instruments - thickness gauges, length gauges or ultrasonic methods.

Finished pipes are monitored for quality and chemical composition using various flaw detectors, steeloscopes and other devices.

In addition to geometric dimensions, finished pipes are also subject to requirements regarding surface roughness, chemical composition, structure (macro- and microstructure) of the metal, intergranular corrosion, and contamination of the metal with non-metallic inclusions. Control chemical composition, macrostructure of intergranular corrosion, microstructure, contamination of metal with non-metallic inclusions refers to general methods testing of metal products. Therefore, during the production of such pipes, their quality control is carried out using ultrasonic flaw detection and eddy current flaw detection, as well as the luminescent method using penetrating liquids.

The ultrasonic testing method allows you to evaluate the accuracy of geometric dimensions, the quality of the outer and inner surfaces of pipes, metal continuity, grain size and other parameters.

For the production of steam generator pipes used in nuclear power plants with water under high pressure, steels and alloys are used that have high corrosion resistance and have the least tendency to form cracks and stress corrosion.

Any pipeline structure formed in real conditions inevitably undergoes changes associated with the accumulation of defects, which leads to a decrease in reliability. The main cause of the defect is the deviation of the operating parameter from the standard value specified, as a rule, by a reasonable tolerance. Since a defect not identified during construction is a potential source of failure, and the probability of failure depends on the size of the defect and the conditions under which it changes during operation, we can assume that any defect determines the possibility of an accident leading to destruction.

Generalized classification scheme for object defects pipeline transport is shown in Figure 1.1.

Figure 1.1 - Classification of defects

When assessing the impact of a defect on the performance of a pipeline, it is necessary to take into account the operating conditions of the defect, its nature and other factors. When assessing the effect of a defect on the performance of pipe metal, it is necessary to take into account the operating mode, physical and chemical properties of the product, stress level, the possibility and nature of overloads, the degree of stress concentration, etc.

Defect in the main and process oil pipeline - this is a deviation geometric parameter pipe wall, weld seam, quality indicator of the pipe material, which does not meet the requirements of the current regulatory documents and arising during the manufacture of pipes, construction or operation of an oil pipeline, as well as unacceptable structural elements and connecting parts installed on main and technological oil pipelines and detected by in-line diagnostics, visual or instrumental inspection of the facility.

Pipe geometry defects .

These are defects associated with changes in its shape. These include:

dent - local reduction in the flow area of ​​the pipe as a result of mechanical action, in which the axis of the oil pipeline does not break;

corrugation - alternating transverse convexities and concavities of the pipe wall, leading to a fracture of the axis and a decrease in the flow area of ​​the oil pipeline (Figure 1.2);

ovality - a geometry defect in which the pipe section has a deviation from roundness, and the largest and smallest diameters are in mutually perpendicular directions.

Figure 1.2 - Corrugation

Pipe wall defects.

These include:

loss of metal - change in the nominal thickness of the pipe wall, characterized by local thinning as a result of mechanical or corrosion damage or due to manufacturing technology (Figure 1.3);

risk(scratch, nick) - loss of metal from the pipe wall resulting from the interaction of the pipe wall with a solid body during mutual movement;

Figure 1.3 - Defect “loss of metal”

bundle - discontinuity of the pipe wall metal;

delamination with access to the surface(sunset, captivity rental) - delamination extending onto the outer or inner surface of the pipe;

delamination in the heat-affected zone - delamination adjacent to the weld;

crack - defect in the form of a narrow rupture in the metal of the pipe wall (Figure 1.4);

Figure 1.4 - Longitudinal crack along the pipe body

erosive destruction of the inner surface of the pipeline - damage to the inner surface of the pipeline wall: is the sequential destruction of the surface layer of the wall under the influence of mechanical or electromechanical action of solid particles suspended in a moving flow, as well as liquid particles. When solid particles predominate, mechanical erosion occurs.

Defects of corrosion origin.

Complete corrosion: uniform, uneven (Figure 1.5).

Figure 1.5 - Corrosion of underground piping

Uniform - corrosion that covers the surface of the metal over an area equal to the entire surface of the pipe.

Uneven - occurs in separate areas and occurs at different speeds.

Local corrosion:

point - has the appearance of individual point lesions;

spots - looks like separate spots;

ulcerative - looks like separate shells.

Intercrystalline corrosion - corrosion that spreads along the boundaries of metal crystals (grains).

Stress corrosion occurs under the combined influence of internal pressure and corrosion attack environment in combination with a certain microstructural susceptibility of the corresponding pipe steels (Figure 1.6).

Figure 1.6 - Stress corrosion on a pipe DN1000

The exact mechanism of stress corrosion cracking initiation and growth is still the subject of ongoing research.

Stress corrosion cracking is usually found in the base material on the outer surface of the pipe and, like fatigue cracks, has a longitudinal orientation.

Weld defects.

These are defects in the weld itself or in the heat-affected zone, the types and parameters of which are established by regulatory documents (SNiP III-42-80, VSN 012-88, SP 34-101-98), identified by visual measuring methods, ultrasonic, radiographic, magnetographic control and in-line diagnostics.

Depending on the location and type, defects are conventionally divided into external and internal.

External (external) defects are defects in the shape of the seam, as well as burns, craters, sagging, undercuts, etc. (Figure 1.7). In most cases, external defects can be determined visually.

Figure 1.7 - External defects welds:

A- uneven seam width; b- burns; V- crater; G- influxes; d- undercuts

Internal defects include pores, lack of penetration, slag and non-metallic inclusions, cracks and lack of fusion (Figure 1.8).

Figure 1.8 - Internal defects in welds:
A- pores; b- slag inclusions; V- lack of penetration at the root of the seam and along the edge; G- cracks; d- lack of fusion

Gas pores (Figure 1.8, a) are formed due to contamination of the edges of the metal being welded, the use of wet flux or damp electrodes, insufficient weld protection when welding in a carbon dioxide environment, increased welding speed and excessive arc length. When welding in a carbon dioxide environment, and in some cases, submerged arcs at high currents, through pores are formed - the so-called fistulas. The size of internal pores ranges from 0.1 to 2–3 mm in diameter, and sometimes more. Pores can be distributed in the seam in separate groups (clusters of pores), in the form of a chain along the longitudinal axis of the seam, or in the form of separate inclusions (single pores).

Slag inclusions (Figure 1.8, b) in weld metal - these are small volumes filled with non-metallic substances (slag, oxides). Their sizes reach several millimeters. These inclusions form in the seam due to poor cleaning of the welded edges from scale and other contaminants, and most often from slag on the surface of the first layers of multilayer seams before welding subsequent layers.

Slag inclusions can be various shapes: round, flat, film-shaped or oblong (in the form of elongated “tails”). The influence of single slag inclusions on the performance of structures is approximately the same as that of gas pores.

Typically, slag inclusions have a more elongated shape and a larger size compared to the pores.

Lack of penetration - discontinuities at the boundaries between the base and deposited metals (Figure 1.8, V) or unfilled cavities in the weld cross-section with metal. The reasons for the formation of lack of penetration are poor preparation of the edges of the sheets being welded, a small distance between the edges of the sheets, incorrect or unstable welding mode, etc. Lack of penetration reduces the performance of the joint by weakening the working section of the seam. In addition, sharp lack of fusion can create stress concentrations in the weld. In structures operating under static load, a lack of penetration of 10–15% of the thickness of the metal being welded does not have a significant effect on the operational strength. However, it is an extremely dangerous defect if structures operate under vibration loads.

Cracks - partial local destruction welded joint(Figure 1.9). They can arise as a result of tearing of heated metal in a plastic state or as a result of brittle fracture after the metal has cooled to lower temperatures. Most often, cracks form in rigidly fixed structures.

Figure 1.9 - Crack in the weld

The reasons for the formation of cracks may be an incorrectly selected technology or poor welding technique.

Cracks are the most dangerous and, according to existing control rules, an unacceptable defect.

Non-fusion is a defect when the deposited metal of the weld does not fuse with the base metal or with the previously deposited metal of the previous layer of the same weld (Figure 1.8, d).

Non-fusion occurs due to poor cleaning of the edges of the parts being welded from scale, rust, paint, excessive arc length, insufficient current, high speed welding, etc.

This defect is most likely to form during argon arc welding of aluminum-magnesium alloys, as well as during pressure welding. Lack of fusion is a very dangerous defect, difficult to detect modern methods flaw detection, and, as a rule, is unacceptable.

The classification of weld defects also includes welding defects.

1 Sagging (sagging).

They are formed when welding vertical surfaces with horizontal seams as a result of liquid metal flowing onto the edges of the base metal. Causes of influxes:

High welding current;

Long arc;

Incorrect electrode position;

Large angle of inclination of the product when welding up and down. In areas of sagging there are often lack of penetration, cracks, etc.

2 Undercuts.

They are depressions (grooves) formed in the base metal along the edge of the seam with a high welding current and a long arc, since in this case the width of the seam increases and the edges melt more strongly. Undercuts lead to a weakening of the base metal section and can cause destruction of the welded joint (Figure 1.7, d).

3 Burns.

Penetration of the base or deposited metal with the possible formation of through holes. They arise due to insufficient blunting of the edges, a large gap between them, high welding current or power at low welding speeds. Often, burns are observed when welding thin metal with an increase in welding duration, low compression force of the parts being welded, or in the presence of contamination on the surfaces being welded or the electrode.

4 Edge offset - assembly defect in the form of a mismatch between the midlines of the walls of the joined pipes (for a circular seam) or the joined sheets (for spiral and longitudinal seams). Classified as transverse/longitudinal/helical weld displacement (Figure 1.10).

Figure 1.10 - Edge offset

Combined defects.

Such defects include:

Geometric defect in combination with a risk, loss of metal, delamination or crack (Figure 1.11);

Geometric defect adjacent to or located on the weld;

Anomalies of welds in combination with displacements;

Delamination adjacent to a defective weld.

Figure 1.11 - Dent with a mark

Invalid structural elements.

Connecting parts that do not comply with the requirements of SNiP 2.05.06–85*/6/:

Tees (Figure 1.12);

Flat and other plugs and bottoms;

Welded sector bends;

Adapters;

Branch pipes with fittings that do not comply with current standards and regulations;

Welded patches and overhead patches of all types and sizes;

Overhead elements made of pipes (“troughs”), welded onto pipes, etc.

Figure 1.12 - Tee defect

Insulation defect .

Insulation defects (Figure 1.13) significantly reduce the effectiveness of comprehensive protection of pipelines from corrosion and, consequently, the corrosion resistance of the pipe wall decreases. As a result, the rate of premature pipeline failures increases, which can be reduced through timely detection and elimination of defects.

Figure 1.13 - Defects in insulating coating

Repair of defects in the base metal of the pipe (dents, corrugations, corrosion, loss of metal, scuffing, delamination, cracks, etc.) is proposed to be carried out using special equipment. The universal underwater chamber (caisson) is designed to repair damage to underwater passages of oil pipelines in dry conditions under normal pressure using the same repair methods as on the surface.

This camera allows you to repair defective sections of pipes different ways(installation of welded couplings, installation of composite couplings, insertion of coils, grinding, welding, etc.), repair of main gas pipeline insulation and other dry work on pipes with a diameter of up to 1420 mm. Working depth - up to 30 m.

The disassembled camera can be quickly delivered to any area by any means of transport, incl. aviation. The equipment is patented, has a GOST R certificate of conformity and permission for use from Rostechnadzor.

Welders are certified to level I in the NAKS system with permission to work on oil and gas production equipment, taking into account the additional requirements of Transneft AK

Installation of an underwater chamber (caisson) during repair of a main oil or gas pipeline:

Figure 7 Universal underwater chamber (caisson) for gas pipeline repair - an inside view

Figure 8 Classification of pipeline defects (main oil pipeline and gas pipeline)

Oil pipeline defects are divided into defects subject to repair (DSR), from which, according to the degree of danger, defects of priority repair (POR) are distinguished.

A defect subject to repair is each individual non-compliance with regulatory documents: walls, welds, geometric shapes of the pipe, as well as connecting, structural parts and welded elements on the oil pipeline or included in its composition that do not comply with regulatory documents.

A priority repair defect is a defect that limits the operation of an oil pipeline section for a period of 1 year or less and reduces the design bearing capacity of the oil pipeline, as well as a defect for which strength and durability are not determined to be repaired.

Pipe geometry defects

“Dent” is a local reduction in the flow area of ​​a pipe at a length less than 1.5 times the nominal pipe diameter D, without a break in the axis of the oil pipeline, resulting from transverse mechanical impact.

“Corrugation” is a reduction in the flow area of ​​a pipe, accompanied by alternating transverse convexities and concavities of the wall, as a result of loss of stability from transverse bending with a break in the axis of the oil pipeline.

“Narrowing” is a reduction in the flow area of ​​a pipe with a length of 1.5 times the nominal pipe diameter or more, in which the pipe cross-section has a deviation from the circle (Dн-d)/Dн, 2% or more, where Dн is the nominal outside diameter pipes, d is the minimum measured outer diameter of the pipe.

Pipe wall defects

“Metal loss” is a local decrease in the thickness of the pipe wall as a result of corrosion damage to the oil pipeline. Metal losses are divided into combined and single. Combined metal loss is a group of two or more corrosion defects combined into a single defect if the distance between adjacent defects is less than or equal to the value of 4 pipe wall thicknesses in the area of ​​the defects. A single metal loss is one metal loss defect, the distance from which to the nearest metal loss exceeds the value of 4 pipe wall thicknesses in the area of ​​the defect.

“Reduction in wall thickness” is a gradual thinning of the wall formed during the manufacturing process of a hot-rolled pipe or a technological defect in the rolled product.

“Delamination” is an internal violation of the continuity of the pipe metal in the longitudinal and transverse directions, separating the metal of the pipe wall into layers of technological origin. “Delamination with access to the surface” (sunset, rolling film) - delamination extending to the outer or inner surface of the pipe. "Delamination in the heat-affected zone" - delamination adjacent to the weld (the distance of the seam transition line to the base metal to the edge of the delamination is less than or equal to the value of 4 pipe wall thicknesses).

“Crack” is a defect in the form of a rupture in the metal wall of an oil pipeline pipe.

“Surface defect” is a rolled defect on the pipe surface (rolled out contamination, rippling, scaliness, overheating of the surface, rolled scale, scale shells, indentation shells), which does not bring the pipe wall thickness beyond maximum dimensions according to GOST 19903-74.

Defects in the welded joint (seam)

Crack, lack of penetration, lack of fusion - defects in the form of discontinuity of metal along the weld. Pores, slag inclusions, sink marks, undercuts, excess penetration, sagging, scaliness, deviations of weld dimensions from the requirements of regulatory documents - “anomalies” of a transverse, longitudinal, spiral weld.

Edge displacement is a discrepancy between the levels of location of the internal and external surfaces of the walls of welded (welded) pipes (for a transverse weld) or sheets (for spiral and longitudinal seams) in butt welded joints.

An oblique joint is a welded butt joint between a pipe and a pipe (with a spool, with a connecting part of a main oil pipeline), in which the longitudinal axes of the pipes are located at an angle to each other. A connection with an angle of 3 degrees or more between the pipe axes to each other is classified as a defect in the “oblique joint” of the transverse weld.

The procedure for repairing a main pipeline (oil pipeline, gas pipeline). Elimination of defects in a pipeline (oil pipeline, gas pipeline) subject to repair is carried out by selective repair of individual defects in accordance with the methods regulated by this document, and during major repairs with replacement of pipes and replacement of insulation. During a major overhaul involving replacement of gas pipeline insulation, all existing defects in the area that need to be repaired must be repaired, followed by replacement of the insulation. More information about the technology for replacing the insulation of a main pipeline (oil pipeline, gas pipeline)

It is necessary, first of all, to detect damage and defects on the inside and outside of the pipe. They are a kind of “beacons” that show specialists weak points in the operation of the gas pipeline. There is a classification of such flaws. All damage and defects on a metal gas pipe are divided into the following groups:

• axial pipe deviations from design solutions;
• defects and damage affecting the cross-sectional shape of the metal pipe;
• mechanical damage etc.

The axial deviations of the pipe, in turn, include the following route objects: pop-up, bulges and arched outbursts, as well as subsidence and sagging.

If part of the gas main pipeline is located in waterlogged soil and at the same time has access to the surface, then it is classified as a surfaced section. Technical diagnostics of such objects are described in detail in the relevant regulatory documentation.

Gas pipeline sections in which the axis deviated from the design solutions and the pipe reached the surface are called arched. Their shape can correspond to the following types:

• asymmetrical and symmetrical (one half-wave sinusoid);
• axis displaced in a vertical position (on a slope);
• horizontal “snake” (more than two half-waves).

At the moment of severe freezing of the gas pipeline network, a process of soil heaving occurs. This is typical in places where thawed soils are exposed to cold temperatures.

Classified as sagging, they have exposed areas that are not in contact with the ground. This, as a rule, occurs during thawing of soils located in the permafrost zone and during karst processes.

In forest areas, as well as in clayey areas, so-called subsidence of the gas pipeline often occurs below the level required by the project. This process is associated with soil moisture above standard or thawing in cold regions.

There are factors that influence the cross-section of gas pipes and change its shape. As a result, it becomes oval, with corrugations or dents.

The oval cross-section of a pipeline is a defect that results from a mechanical change in the annular cross-section of the pipe into an ellipse. The reason for this process is significant radial pressure on the metal surface of the object.

Dents of various shapes and lengths may also appear on the pipe. They appear due to the contact of the object with the external body of a solid base without sharp corners or edges. Pressure on the pipe surface can be applied both dynamically and statically. This damage, as a rule, has a smooth contact with the mating sections of the pipe and does not lead to high stresses of the section in the affected area.

The technical condition of the linear part of the main gas pipeline requires a more careful inspection of the lower surface of the pipe. It is in this place that dents most often appear during pipeline installation and operation.

Folds on the metal surface of a gas pipeline are called corrugations. They appear as a result of cold bending of pipes, as well as during their installation and insulation work. Sometimes they are formed directly during operation in places where the gas pipeline route bends, in combination with weak-bearing soil rocks, high temperature conditions and pressure.

There is another group of damage and defects of pipes - this time their walls, including places of welded joints and seams. They arise as a result of unregulated transportation, laying of the gas pipeline, as well as its operation. Damage to the walls of a gas pipeline can be as follows:

1. Small damage (both through and non-through) of a narrow shape in the form of cracks. They usually have an angle close to 90 degrees and are directed towards the surface of the pipe wall.
2. Metal delamination and formation of parallel layers.
3. Lack of continuity of long-length metal in the rolling direction (sunset).
4. Metal peeling of varying thickness and size. It passes towards the rolling side and on one side is connected to the base metal (film).
5. A metal rupture that has a different open shape. It is oxidized and is located on top or at an angle towards the rolling (flaw).
6. Content of non-metallic substances in the pipe (liquation).
7. A groove on the metal surface of a pipe, having a longitudinal shape. It is formed as a result of contact between pipe metal and sharp protrusions during the rolling process.

All these defects are associated with metallurgical manufacturing defects. But defects also form as a result of pipe transportation, installation and operation. They are classified as follows:

1. Excessive reduction in metal wall thickness over a significant area of ​​the pipeline.
2. Single and local defects on the surface of a gas pipeline.
3. Extended linear defects.

Thinning of the metal walls on the pipeline is usually caused by corrosion damage, which is continuous, uniform and uneven in nature. The critical criterion in the technical assessment of a gas pipeline zone affected by corrosion is not so much the size of the damaged area of ​​the object, but rather the fixation of the minimum thickness of the metal wall.

Pipe defects that have a linear-extended shape are damage in which the length is greater than the width and depth. These include nicks and scratches, which are usually caused by mechanical influences to the object. The possibility of efficient and safe operation of a gas pipeline with such damage depends on the tension of the metal in the defect area.

The indicated defects and damage to the metal surface of the pipeline are considered from the point of view of a qualitative assessment, and not a quantitative one, which also has its own classification and is based on specially developed regulatory standards.

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Introduction

Timely maintenance of the gas pipeline and preventive repairs of the gas pipeline are the key to its long, uninterrupted and reliable operation. The operation of a gas pipeline involves periodic inspections, preventive maintenance and repairs. All these operations are necessary primarily for safety - timely detection and elimination of possible gas leaks. These works include checking the pressure inside the gas pipeline system, checking the gas contamination of chambers, wells, underground structures, identifying and eliminating blockages, checking and Maintenance pipes and gas fittings.

Routine repairs of gas pipelines and gas equipment must be carried out at least once every 12 months on disconnected equipment and gas pipelines with the installation of plugs at the boundaries of the disconnected section on the gas supply side.

If the need arises, the gas pipeline is subjected to major repairs.

Major repairs of a gas pipeline are necessary when sufficiently serious malfunctions occur that threaten the safe operation of the entire system as a whole. During a major overhaul, damaged sections of the gas pipeline are completely replaced, fittings are repaired or replaced, broken insulation systems are restored or replaced, wells are repaired, protective equipment, etc. Often, cast iron gas pipelines that have become unusable are replaced with modern steel pipelines during major repairs.

Solving the problem of ensuring trouble-free operation of pipelines, especially gas pipelines, is an extremely important task. During the operation of gas pipelines, many problems arise related to ensuring safe work. Various defects occur in pipelines: material delamination, dents, corrosion cavities, stress-corrosion cracks, erosive wear, scratches, etc. To solve a particular problem, of course, you need to have an idea of ​​the state of affairs in this direction.

This work will discuss the causes of defects in pipelines, classifications and methods for eliminating defects in pipelines.

1. Defects in pipeline structures and the reasons for their occurrence

In order to determine the presence of defects in the pipeline, it is necessary to carry out technical diagnostics.

Technical diagnostics are carried out in order to determine the technical condition of the gas pipeline and establish the service life of its further operation, based on the examination performed.

The appearance of operational defects in pipelines is caused by a variety of factors, well studied and predictable, as well as random (for example, damage to the pipeline by third parties, etc.). To ensure the reliability of pipelines, periodic monitoring of their parameters, both structural and functional (during operation), is necessary.

A defect is any non-compliance with regulated standards. The main reason the appearance of defects is a deviation of the operating parameter from the standard value, justified by the tolerance.

Defects in pipeline structures are divided into:

Pipe defects;

Defects in welded joints;

Insulation defects.

The following pipe defects are distinguished:

Metallurgical - defects in sheets and strips from which pipes are made, i.e. various types of delamination, rolled film, rolled scale, non-metallic inclusions, etc.

Technological - associated with the imperfection of pipe manufacturing technology, which can be divided into welding defects and surface defects (work hardening during expansion, displacement or angularity of edges, ovality of pipes)

Construction - due to imperfect technology of construction and installation work, violations of technological and design solutions for transportation, installation, welding, insulation and installation work (scratches, scuffs, dents on the surface of pipes).

Causes of pipe defects:

The existing technology of metal rolling, the technology of continuous casting of steel at individual metallurgical plants is one of the reasons for the production of low-quality pipes. There are frequent cases of destruction due to metal delamination.

At pipe factories, incoming control of raw materials is imperfect or completely absent. This leads to raw material defects becoming pipe defects.

When making pipes, the metal has to be subjected to loads under which it operates beyond its yield point. This leads to the appearance of work hardening, micro-delaminations, tears and other hidden defects. Due to the short duration of subsequent factory tests of pipes (20...30 s), many hidden defects are not detected and are “triggered” already during the operation of the MT.

The geometric shape of the pipes is also insufficiently controlled by factories. Thus, on pipes with a diameter of 500...800mm, the displacement of the edges reaches 3mm (at the norm for spiral-seam pipes 0.75...1.2mm), ovality - 2%

Mechanical impacts during loading and unloading, transport and installation operations lead to the appearance of dents, marks, scratches, and burrs on pipes.

When cleaning pipelines with pig-cutters, plastic deformation defects occur in local areas of the pipe surface - scratches, undercuts, etc. These stress concentrators are potential sites for the development of corrosion fatigue cracks. Cleaning pipelines with wire brushes eliminates damage to the pipes in the form of undercuts, but under certain processing conditions it leads to deformations of the metal surface, reducing its corrosion resistance.

Corrosion damage to pipes (external - in places where the insulation continuity is broken, and internal - in places where water accumulates).

Also, in addition to metallurgical, construction and technological defects of pipes, the following defects are distinguished:

A defect in a welded joint is a deviation of various kinds from established standards and technical requirements, which reduce the strength and operational reliability of welded joints and can lead to destruction of the entire structure. The most common defects are the shape and size of welds, macro- and microstructure defects, deformation and warping of welded structures.

Violation of the shape and size of the seam indicates the presence of defects such as sagging (sagging), undercuts, burns, and unwelded craters.

Sagging - most often formed when welding vertical surfaces with horizontal seams, as a result of liquid metal flowing onto the edges of the cold base metal. They can be local (in the form of individual frozen drops) or extended along the seam. The causes of influxes are great strength welding current, long arc, incorrect position of the electrode, large angle of inclination of the product when welding up and down.

Undercuts are depressions formed in the base metal along the edge of the weld. Undercuts are formed due to the increased power of the welding torch and lead to a weakening of the base metal section and destruction of the welded joint.

Burn-through is the penetration of the base or deposited metal with the possible formation of through holes. They arise due to insufficient blunting of the edges, a large gap between them, high welding current or torch power at low welding speeds. Burn-throughs are observed especially often during the welding of thin metal and when performing the first pass of a multilayer seam, as well as with increasing welding duration, low compression force and the presence of contamination on the surfaces of the parts being welded or electrodes (spot and seam resistance welding).

Unwelded craters are formed when the arc suddenly breaks at the end of welding. They reduce the cross-section of the seam and can become sources of crack formation.

Macrostructure defects include defects: gas pores, slag inclusions, lack of penetration, cracks, detected using optical means (magnification no more than 10 times).

Gas pores - formed in welds due to the rapid solidification of gas-saturated molten metal, during which the released gases do not have time to escape into the atmosphere. (Fig. 2)

Figure 2 - gas pores

Such a defect is observed when there is an increased carbon content in the base metal, the presence of rust, oil and paint on the edges of the base metal and the surface of the welding wire, or the use of wet or damp flux.

Slag inclusions are the result of careless cleaning of the edges of the welded parts and welding wire from scale, rust and dirt, as well as (in multi-layer welding) incomplete removal of slag from previous layers.

They can occur when welding with a long arc, incorrect tilt of the electrode, insufficient welding current, or excessive welding speed. Slag inclusions vary in shape (from spherical to needle-shaped) and size (from microscopic to several millimeters). They can be located at the root of the weld, between individual layers, and also inside the deposited metal. Slag inclusions weaken the weld cross-section, reduce its strength and act as stress concentration zones.

Figure 3 - slag inclusions

Lack of penetration is a local lack of fusion of the base metal with the deposit, as well as failure of fusion of individual layers of the weld with each other during multilayer welding due to the presence of a thin layer of oxides, and sometimes a coarse slag layer inside the seams.

Figure 4 - lack of penetration

The reasons for lack of penetration are: poor cleaning of the metal from scale, rust and dirt, small gap in the joint, excessive blunting and small bevel angle of the edges, insufficient current or burner power, high welding speed, displacement of the electrode away from the axis of the weld. Lack of penetration along the cross-section of the seam can occur due to forced breaks in the welding process.

Cracks - depending on the temperature of formation, are divided into hot and cold.

Figure 5 - Cracks

Hot cracks appear during the crystallization of the weld metal at a temperature of 1100 - 1300 C. Their formation is associated with the presence of semi-liquid layers between the crystals of the deposited weld metal at the end of its solidification and the action of tensile shrinkage stresses in it. The increased content of carbon, silicon, hydrogen and nickel in the weld metal also contributes to the formation of hot cracks, which are usually located inside the weld. Such cracks are difficult to detect.

Cold cracks occur at temperatures of 100 - 300 C in alloy steels and at normal (less than 100 C) temperatures in carbon steels immediately after the seam has cooled or after a long period of time. The main reason for their formation is the significant stress that arises in the welding zone during the decomposition of the solid solution and the accumulation of molecular hydrogen under high pressure in the voids present in the weld metal. Cold cracks appear on the surface of the seam and are clearly visible.

Defects in the microstructure of a welded joint include

Micropores,

Microcracks,

Nitride, oxygen and other non-metallic inclusions,

Coarseness,

Areas of overheating and burnout.

Insulation defects - loss of continuity; adhesion; reduced thickness; corrugations; wrinkles; bullies; scratches; punctures.

The main reasons for the formation of defects in the insulating coating on pipelines:

1) during storage and preparation of materials - clogging of bitumen and watering of the finished mastic and its components;

2) when preparing primer and mastic - careless dosage of components; non-compliance with the boiler heating mode; insufficient mixing of bitumen when preparing the primer;

3) when applying primer and bitumen mastic - thickening of the primer; formation of bubbles on the surface of the pipeline; dust settling on the surface of the pipes; omissions of primer and mastic on the surface of the pipeline and especially near welds; uneven application of mastic; mastic cooling; design flaws of the insulating machine;

4) when applying reinforcing and wrapping roll materials - violation of the uniformity of the coating; squeezing out a layer of mastic; insufficient immersion of fiberglass in mastic;

5) when applying polymer tapes - through holes in the tape; non-continuous adhesive layer; uneven thickness of the tape in the roll; incorrect adjustment of the winding machine; violation temperature regime applying tape; poor cleaning of pipe surfaces;

6) when laying a pipeline - violation of laying technology, especially with a separate laying method; gripping insulated pipes with a cable; friction of the pipeline against the walls of the trench during installation; lack of preparation of the trench bottom; absence of backfill of at least 10 cm at the bottom of the trench in areas with rocky and gravelly soils; poor loosening frozen soils and especially the lack of regulation of insulating machines;

7) during pipeline operation - soil action; pipeline weight; soil waters; microorganisms; plant roots; temperature effects; soil aggressiveness.

Thus, due to the growth of pipeline networks for natural gas, which have an increased risk of various types of emergency situations, it becomes actual problem safety and reliability of gas pipeline operation. Various research units are being established to address pipeline safety issues.

2. Methods for eliminating defects in the pipeline

The procedure for assigning a method for repairing a defective pipe begins with the formation of initial data used to check the conditions for the repairability of defective pipe sections, and the conditions under which the defective pipe section is not repaired. After generating the initial data, the interaction conditions of defects are checked, based on the results of which a list of single and combined defects is generated for each defective pipe.

In-line inspection allows you to obtain a high-quality picture of the technical condition of gas pipeline sections, which is the initial information for planning repair work.

This section provides the main provisions of oil pipeline repair technologies used for selective and major repairs. Elimination of defects during major repairs is carried out at a pressure in the oil pipeline no higher than 2.5 MPa.

Each repair must be reflected in the pipeline passport. Repair structures must be manufactured in a factory according to technical specifications and design documentation developed in the prescribed manner and have a passport. The use of couplings and other repair structures manufactured in the field (in highway conditions) is prohibited.

1. Grinding

Grinding is used to repair sections and connecting parts (bends, tees, adapters, plugs, etc.) with defects up to 20% of the nominal pipe wall thickness such as loss of metal (corrosion defects, risks), delamination reaching the surface, small cracks, as well as defects such as “weld seam anomalies” (flaking, pores extending to the surface) with a residual height of reinforcement not less than the values ​​specified in RD 08.00-60.30.00-KTN-050-1-05.

Grinding is used to repair additional defects in dents - scratches, metal losses, cracks, delaminations that reach the surface.

Welded connections (places of old welding of control and measuring columns, places of welding of shunt jumpers and other metal deposits) adjacent to a defect-free transverse or longitudinal weld are ground flush with the surface of the pipe. pipeline defect lack of insulation penetration

When grinding by removing metal, the smooth shape of the surface should be restored and stress concentration reduced. The maximum permissible pressure in the pipe when carrying out selective repairs by grinding is no more than 2.5 MPa. The sanded area must be subjected to visual, magnetic particle or color flaw detection inspection.

After grinding, the remaining thickness of the pipe wall should be checked using ultrasonic thickness gauging. The residual thickness must be at least 80% of the nominal wall thickness.

When grinding cracks before installation, the depth of the selected metal must exceed the depth of the crack by at least 5% of the nominal wall thickness. The remaining wall thickness after grinding cracks must be at least 5 mm.

Characteristics of the main methods for repairing pipeline defects.

There are several methods for eliminating defects in a pipeline:

Repair by grinding:

Used for corrosion defects, risks, delaminations that reach the surface, and small cracks;

The maximum depth of the sanded area should be no more than 20%

nominal wall thickness;

The sanded area must be subjected to visual, magnetic particle or color flaw detection inspection.

2. Tea leavesdefects

Welding may be used to repair pipe wall defects such as “loss of metal” (corrosion pits, risks) with a residual pipe wall thickness of at least 5 mm, as well as defects such as “transverse weld anomalies” (pores exposed to the surface, weld undercuts, insufficient or missing reinforcement, insufficient seam width) on welds.

Welding is allowed if the depth and maximum linear size of a single defect (length, diameter) or its area do not exceed the following values. The distance between adjacent damage must be at least 100 mm. Distance from welded defects to welded seams, incl. to spiral ones, must be at least 100 mm.

Welding repair:

Used to repair defects such as “loss of metal” (corrosion pits, risks) with a residual wall thickness of at least 5 mm;

The maximum linear size of the defect should not exceed three nominal pipe wall thicknesses;

Welding may only be carried out on a completely filled oil pipeline;

The maximum permissible pressure in the pipe during welding should be determined from the conditions:

Rzav 0.4 tost MPa at tost 8.75 mm;

Rzav 3.5 tost MPa at tost 8.75 mm,

where tost is the residual wall thickness at the welding site, mm; coefficient 0.4 has the dimension MPa/mm.

Performed by manual electric arc welding;

The number of surfacing layers (excluding the contour weld) is at least three.

Installation of repair structures

For permanent repairs:

· composite coupling;

· crimp welded coupling;

· several types of dumbbell muffs;

· welded pipe with elliptical bottom

For temporary repairs:

· welded non-crimp coupling;

· welded coupling with conical transitions

Technological schemes for pipeline repair with insulation replacement

· in a trench without lifting the pipeline with undermining and support for the area being repaired;

· in a trench with the pipeline section being repaired being lifted by pipe layers to a height that allows cleaning and insulation machines to pass through the raised section without digging under the pipeline;

· on the edge (berm) of the trench with its elevation to the height required for the passage of the cleaning machine.

Characteristics of the main methods for repairing pipeline defects

1. Emergency repair methods

Methods for emergency repair of oil pipelines (application of patches, clamps, clamping devices, driving in plugs) can only be considered as emergency, temporary methods for eliminating emergency situations.

2. Banding using winding structures

There are several ways to repair pipes using preload winding:

· winding steel wire or tapes;

· winding of glass fiber materials impregnated with a binder composition; winding of tapes made of composite materials

Conclusion

Thus, main pipeline transport is the most important component of the Russian fuel and energy complex.

One of the most important problems pipeline transport is to maintain the normal state of the linear part of field and main pipelines.

Timely maintenance of the gas pipeline and preventive repairs of the gas pipeline are the key to its long, uninterrupted and reliable operation. The operation of a gas pipeline involves periodic inspections, preventive maintenance and repairs. All these operations are necessary primarily for safety - timely detection and elimination of possible gas leaks. These works include checking the pressure inside the gas pipeline system, checking the gas contamination of chambers, wells, underground structures, identifying and eliminating blockages, checking and routine repair of pipes and gas fittings. Maintenance The main pipeline is of great importance, since not only profit and production volume, but also the economy as a whole will depend on the integrity and performance of the pipeline.

Posted on Allbest.ru

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