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Repair of defects in pipes and welds. Diagnostics of equipment of pumping and compressor stations What are minor pipeline defects

According to the Guidelines for the Technical Supervision of Ships in Service, the USSR Register provides for the survey of ship steam boilers, r.e. inspections, measurements, checks, tests in order to control the compliance of boilers with the requirements of the rules and determine the technical condition. During the technical supervision of ship steam boilers by the Register of the USSR, the following are carried out: kinds osidedetelsgioneny: internal, hydraulic test, external inspection and verification in action.

The terms, scope and nature of such surveys are regulated by the USSR Register. Thus, internal surveys of boilers are carried out annually. However, for new boilers, the first internal examination is established after 4 years, the second - after th years.

Hydraulic tests must be carried out at least once every 8 years; external examinations - at least a year later.

For internal examination, it is necessary to prepare the boilers: cool, empty from water, clean from ash, scale, soot, scale, slag of the heating surface; if necessary, remove masonry and insulation; provide access to boiler foundations, etc.

Prior to the start of the survey, it is necessary to disconnect the inspected boiler from the existing ones, close the fittings tightly, and lock the drives. Hydraulic tests are carried out after elimination defects found during internal inspection. The insulation at the collector seams is removed, and other preparatory work is carried out, including preparation for an internal survey.

Trial pressure during hydraulic testing of boilers is taken not less than 1.25 /?|>n, but not less than 0.2 MPa. After repair of the boilers, the test pressure is assumed to be at least 1.5r ra b. During the hydraulic test, the boiler must be completely filled with water, the temperature of the water and the ambient air must not be lower than 5°C. The temperature difference between the water and the outside air must prevent sweating; pumping water during holding at test pressure is not allowed.

IN during the hydraulic test, the pressure is raised to the operating pressure, then a preliminary inspection at operating pressure follows, raising the pressure to test pressure, inspection at test pressure with the pumps turned off. There must be no pressure drop during holding under test pressure. Sweating and the appearance of water at rivet holes and rivets in the form of separate non-drip drops is not considered a leak. The same phenomenon is unacceptable for welded seams.

Chasing, core and other mechanical techniques for correcting defects welds not allowed during testing. It is also unacceptable to eliminate the detected defects in the pressurized boiler, as well as welding in the presence of water in the boiler.

Each boiler on the ship has a Register Book, in which the results of an internal survey, hydraulic test and external examination are entered.

The data obtained during the survey can be considered as a preliminary fault finding, which begins during the operation of the boiler, in contrast to the working fault finding, which is performed after the ship is put into repair. This data provides information about typical failures and malfunctions (corrosion damage, tube deformation, condition brickwork boiler furnace, operation of nozzles and fittings, etc.).

The working defect in the boiler includes inspection with the use of optical instruments, measurement of deformations, metallographic and mechanical examinations of the metal. To detect defects, magnetic, x-ray, ultrasonic methods of flaw detection are used. In case of flaw detection, first of all, residual deformations, thinning of elements, ruptures, violation of impermeability, changes in the structure of the metal, degree of corrosion damage, fastening of the foundation to the ship's hull, etc.

Determination of tube defects(hot water, superheater, economizer, air heater). External examination determines the corrosion damage to the surface. For metallographic and mechanical tests of the metal of the tubes (determination of the presence of overheating, intercrystalline corrosion, etc.), several tubes are cut out for the manufacture of samples and thin sections.

During mechanical tests, the strength length, relative elongation, and impact strength are determined. Deterioration of mechanical properties compared to the original!! data is allowed no more than 5-10%.

Carefully produce a metallographic study and study the structure of the material. These studies allow you to establish the cause of the rupture of the tubes. The presence of a martsite structure may be an explanation for the fact that the rupture occurred due to overheating of the metal.

To detect cracks in the ends of tubes expanded in tube sheets, the use of a magnetic method of flaw detection is recommended.

Definition of defects collector. When defg-ktation kcujjii-i. ii>|>»m examine the internal and external surfaces and holes for hot water tubes, welded holes for cracks. Holes for hot water tubes are measured to check their tuyeres. Inspecting the inner surface of the collector, pay attention to the water level belt, where the corrosion intensity is high. Cracks in welds are detected by gammography and ultrasonic method.

Cracks between holes in tube sheets can be detected by etching with nitric or hydrochloric acid or by magnetic means.

To establish the depth of crack propagation, a section of the tube sheet is cut out and examined in the laboratory; it is permissible in extreme cases to produce control drilling.

When examining the holes in the tube sheet, mechanical

damage and corrosion, the holes are measured to check the ellipticity and calculate the degree of flaring in the case of boring holes. The gap between the holes and the non-collapse of your tube should not exceed 0.3- 0,6 mm. Ovality of holes in tube sheets is allowed

no more than 0.25 mm. Local thinning is allowed in the collector due to

corrosion, but no more than 10 -12 % original thickness.

Determination of defects in nozzle partsAnd air she l leveling devices. In case of fault detection, inspect the details of the nozzles, measure the nozzle holes, inspect the nozzle spray head to detect cracks

When inspecting the atomizers, tangential grooves are controlled,

inspect the surface that ensures a snug fit of the nozzle to the head, check the condition of the working surfaces of the nozzle, on which the quality of the spray depends. The nozzle opening is checked with through and through gauges.

In the air-directing devices of the boiler, the ease of movement of the rods is checked, the lolasts are examined.

In kirlichii masonry, visually check whether there is any melting and burning of the masonry surface, cracking and falling out of bricks. In case of loss of transparency, the glass is rejected.

The main safety valves are checked in operation before the boiler is put in for repair. After disassembly, they are inspected and the condition of the parts is determined.

Any piping 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 normative value given, as a rule, by a reasonable tolerance. Since a defect not detected during construction is a potential source of failure, and the probability of failure depends on the size of the defect, the conditions for its change during operation, we can assume that any defect determines the possibility of an accident leading to destruction.

A generalized classification scheme for defects in pipeline transport facilities is shown in Figure 1.1.

Figure 1.1 - Classification of defects

When assessing the effect of a defect on the pipeline performance, 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 operation of pipe metal, it is necessary to take into account the operating mode, physicochemical characteristics product, stress level, the possibility and nature of overloads, the degree of stress concentration, etc.

Defect of the main and technological oil pipeline - this is a deviation of the geometrical parameter of the pipe wall, weld, pipe material quality index that does not meet the requirements of current regulatory documents and occurs during pipe manufacture, 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 control of the facility.

Pipe geometry defects .

These are defects associated with a change in its shape. These include:

dent - local reduction in the flow area of the pipe as a result of mechanical action, in which there is no break in the axis of the oil pipeline;

corrugation - alternating transverse convexity and concavity of the pipe wall, leading to a break in 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, scuff) - loss of metal of the pipe wall, which occurred as a result of the interaction of the pipe wall with a solid body during mutual displacement;

Figure 1.3 - Defect "loss of metal"

bundle - discontinuity of the pipe wall metal;

stratification with exit to the surface(sunset, rolling captivity) - delamination that goes to the outer or inner surface of the pipe;

delamination in the near-weld zone - delamination adjacent to the weld;

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

Figure 1.4 - Longitudinal crack along the pipe body

erosion destruction of the inner surface of the pipeline - damage to the inner surface of the pipeline wall: is a consistent destruction of the surface layer of the wall under the influence of mechanical or electromechanical action of solid particles suspended in a moving stream, as well as liquid particles. With the predominance of solid particles, mechanical erosion is observed.

Defects of corrosive origin.

General corrosion: uniform, uneven (Figure 1.5).

Figure 1.5 - Corrosion of underground piping

Uniform - corrosion covering the metal surface in an area equal to the entire surface of the pipe.

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

Local corrosion:

punctate - has the appearance of individual punctate lesions;

spots - looks like separate spots;

ulcerative - looks like separate shells.

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

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

Figure 1.6 - Stress corrosion on pipe DN1000

The exact mechanism of stress corrosion cracking and its 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, ultrasonic, radiographic, magnetographic control and in-line diagnostics.

Depending on the location and type of defects, they are conditionally 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 identified visually.

Figure 1.7 - External defects in 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 non-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- non-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 protection of the seam when welding in an environment carbon dioxide, increased welding speed and increased arc length. When welding in a carbon dioxide environment, and in some cases under a flux 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. The pores can be distributed in the seam in separate groups (accumulation of pores), in the form of a chain along the longitudinal axis of the seam, or in the form of individual inclusions (single pores).

Slag inclusions (Figure 1.8, b) in the weld metal, these are small volumes filled with non-metallic substances (slags, oxides). Their sizes reach several millimeters. These inclusions are formed 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 welds before welding subsequent layers.

Slag inclusions can be of various shapes: round, flat, in the form of a film 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 are more elongated and larger than pores.

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

Cracks - partial local destruction welded joint(Figure 1.9). They can occur as a result of tearing of a 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

Cracks may be caused by incorrectly selected technology or poor welding technique.

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

Non-fusion is such 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, e).

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

The formation of this defect is most likely in argon-arc welding of aluminum-magnesium alloys, as well as in pressure welding. Non-fusion is a very dangerous defect, poorly detected by modern methods of flaw detection, and, as a rule, is unacceptable.

The classification of defects in welds can also include defects welding work.

1 Influxes (sagging).

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

High welding current;

Long arc;

Wrong position of the electrode;

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

2 undercuts.

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

3 Burn.

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, a large welding current or power at low welding speeds. Often burns are observed when welding thin metal with an increase in the duration of welding, a small compressive force of the parts to be welded, and in the presence of contamination on the surfaces to be welded or the electrode.

4 Edge offset - assembly defect in the form of a mismatch between the median lines of the walls of the joined pipes (for an annular weld) or joined sheets (for spiral and longitudinal welds). Classified as offset transverse/longitudinal/spiral weld (Figure 1.10).

Figure 1.10 - Edge displacement

Combined defects.

These defects include:

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

Geometric defect adjacent to or located on the weld;

Weld anomalies combined with displacements;

A delamination adjacent to a defective weld.

Figure 1.11 - Dent with risk

Invalid features.

Connecting parts that do not meet 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 norms and rules;

Weld-in 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 the integrated protection of pipelines against corrosion and, consequently, the corrosion resistance of the pipe wall decreases. As a result, the flow of premature failures of the pipeline increases, which can be reduced due to the timely detection and elimination of defects.

Figure 1.13 - Defects in the insulating coating

All types of defects that occur during the production of pipes can, as a first approximation, be divided into three types according to the reasons for their origin:

- mechanical damage to the outer or inner surface of the pipe as a result of the tool not meeting the requirements of the technology (excessive wear or destruction, metal sticking, incorrectly calibrated), ingress of scale and other solid foreign materials on the boundary surfaces of the tool and pipe. Such defects include scratches, risks, dents, undercuts, prints, etc.

- deformation damage associated with a violation of the pipe deformation technology, including increased metal broadening, an increase in deformation coefficients, a violation of the synchronism of operation of successively arranged stands of the installation (“mustache”, “sunset”, “squeeze”, “accordion”).

— discontinuities in the metal continuity associated with a complex stress-strain state determined by the pipe deformation scheme, the presence of tensile stresses exceeding the allowable ones (“birdhouse” during longitudinal rolling and pressing, axial or annular fracture during oblique rolling, films on the inner surface detected during calibration and reduction, etc.). It should be noted that the latter type of defects is mainly determined by the grade composition and quality of the pipe billet metal, and the main units that produce pipe deformation are a kind of “defectoscope” of the quality of the original metal. So, for example, the main unit of almost any pipe-rolling plant is a piercing cross-roller mill, which is characterized by a complex scheme of the stress state of the metal in the deformation zone, leading to high tensile stresses in the axial (for two-roll) or annular (for three-roll) zones of the rolled billet. Special technological methods make it possible to reduce the possibility of opening axial contamination of the metal in the form of captives on the inner surface, however, the quality of the pipe billet is decisive in this case. Defects in the form of films of steel-smelting origin on the inner surface of the pipes, rolled out at the second stage of deformation and not visible due to a tight fit, after rolling mills are opened (lagging behind the surface) during sizing and especially when reducing pipes, which is explained by the conditions of metal deformation near the inner surface of the pipe when the process is carried out without a mandrel.

Studies carried out at factories have shown that when ingots are heated for rolling into a billet or pipes, the upper layer of metal up to 4-5 mm thick burns into scale; when the rolled pipe billet is heated, the upper layer 0.8-1.1 mm thick burns out.

Consequently, the defects occurring in the surface layers of ingots and tubular blanks decrease in depth accordingly, smaller ones burn out into scale. Such defects include, for example, gas bubbles. On the surface of the workpiece (cast and rolled) there is a significantly greater number of them than remains on the surface of the pipes in the form of hair-like films. Wrinkles on the tubular billet almost completely burn out into a layer of scale. But, at the same time, defects that lie deeper approach the surface of the billet and are more easily opened during rolling, forming films on the pipes. Such defects include, for example, subcrustal gas bubbles and accumulations of exogenous non-metallic inclusions.

Student must:

Know: classification of defects in the linear part of pipelines, types of defects

v can: determine the order of repair of defects according to their parameters

Guidelines

Defects in the linear part of the main oil pipelines are divided by type: defects in insulating coatings; pipe defects; defects associated with a change in the design position of the pipeline, its deformations and stress state.

Pipe defects are classified according to the degree of danger into two categories: defects to be repaired (RDR); priority repair defects (PRD).

As a criterion for the danger of a defect, the magnitude of the breaking pressure at the level of the test pressure and geometric parameters are taken.

The parameters by which pipe defects are classified are given in Table. 1.

Tab. 1. Classification of defects according to the order of repair

Description of the defect	Defects to be repaired (RTD)	Priority Repair Defects (PRD)
Geometry defect without additional defects and adjoining to welds	Depth equal to or greater than 3.5% of the pipe diameter



Geometric defect adjacent to or located on a weld	More than 6 mm deep	Depth equal to or greater than 1% of the pipe diameter




Geometry defect in combination with risk, scuffing, crack, loss of metal	All defects	Depth equal to or more than 1% of the pipe diameter, but not less than 6 mm



Loss of metal (external and internal)	Depth equal to or greater than 20% of the pipe wall thickness	Depth equal to or
more than 50% thickness
pipes.
		Dangerous according to the results of the calculation for static strength

Risk, scratch, scuff	All defects	Depth equal to or
	more than 0.2 mm
Cracks in the pipe body or in the weld	-	All defects

bundle

Bundle in about-	More than 20 mm in size along the longitudinal and spiral welds in the zone 10 mm from the fusion line and more than 3.2 mm in size along the circumferential weld in the zone 25 mm from the fusion line	Same
seam zone
Stratification with exit to the surface	All defects	«

Anomaly transverse	The total circumferential length equal to or more than 1/6πDn with dimensions exceeding the allowable values according to SNiP III and VSN	The total circumferential length is equal to or more than 1/3 πDn Hazardous according to the results of the calculation for static strength
seam





Discontinuity of planar type transverse seam	Total circumferential length equal to or more than 1/6 πDn
Discontinuity of planar type transverse weld	Sizes exceeding the allowable values according to SNiP Sh-42-80 and VSN 012-88	Dangerous according to the results of the calculation for static strength
Seam offset	Sizes exceeding the allowable values according to SNiP Sh-42-80 and VSN 012-88	With a depth equal to or more than 25% of the pipe wall thickness and a pipe circumference equal to or more than 1/ZπDn Hazardous according to the results of the calculation for static strength
Anomaly of the longitudinal (spiral) seam	One defect with a length along the pipe axis of more than 13 mm at a length of 150 mm along the pipe axis or two defects with a length of more than 7 mm along the pipe axis at a length of 150 mm along the pipe axis	Length along the axis of the pipe equal to or more than 2√Dnt Dangerous according to the results of the calculation for static strength
Discontinuity of planar type of longitudinal (spiral) weld	Depth equal to or greater than 10% of the pipe wall thickness	Length along the axis of the pipe, equal to or more than 2√Dnt, at any depth. Dangerous according to the results of the calculation for static strength
Offset longitudinal (spiral) seam	Depth equal to or more than 10% of the pipe wall thickness	Length along the axis of the pipe, equal to or more than 3√Dnt, at any depth of displacement. Dangerous according to the results of the calculation for static strength

Defectiveness of insulating coatings according to the degree of danger is regulated in accordance with GOST. As an integral criterion for the limiting state of insulating coatings, the minimum value of the transition resistance of the insulation Rn = 103 Ohm-m2 is used. In addition, performance parameters are evaluated: thickness of insulating coatings, moisture permeability, water absorption, continuity, resistance to peeling under the action of cathodic current, adhesion, heat resistance and durability, which must be within the limits of regulatory requirements.

A main oil pipeline defect is a deviation of the geometrical parameter of a pipe, a weld, the quality of a pipe material that does not meet the requirements of current regulatory documents and occurs during the manufacture of a pipe, the construction or operation of an oil pipeline, as well as unacceptable structural elements and connecting parts installed on main oil pipelines and detected by in-line diagnostics, visual or instrumental control or by analysis executive documentation object.

According to the current NTD, all defects are divided into the following groups: pipe geometry defects; pipe wall defects; weld defects; combined defects; invalid features.

Defects in the geometry of the pipe are associated with a change in its shape. These include the following: dent - a local decrease in the flow area of the pipe as a result of mechanical action, in which there is no break in the axis of the pipeline; corrugation - alternating transverse bulges and concavities of the pipe wall, leading to a break in the axis and a decrease in the flow area of the pipeline; ovality - a defect in which the pipe section deviates from the cylindrical shape, and the largest and smallest diameters are in mutually perpendicular directions.

Pipe wall defects include: metal loss - a 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; risk (scratch, scratch) - loss of metal that occurred as a result of the interaction of the pipe wall with a solid body during mutual movement; delamination - discontinuity of the metal of the pipe wall; delamination with access to the surface (sunset, rolling captivity) - delamination that goes to the outer or inner surface of the pipe; delamination in the heat-affected zone - delamination adjacent to the weld; crack - a defect in the form of a narrow gap in the metal of the pipe wall.

Weld defects are defects directly in the weld or in the near-weld zone, the types and parameters of which are established by regulatory documents and detected by any methods of external and in-line diagnostics. Weld defects include: cracks, lack of penetration, non-fusion, pores, slag inclusions, undercuts, excess penetration, etc.

Combined defects are various combinations of the defects listed above.

Inadmissible structural elements are elements or fittings that do not meet the requirements of the current NTD: tees, flat plugs and bottoms, welded sector bends, adapters, weld-in and overhead patches of all types and sizes.

Questions for self-control

1. Types of defects in the linear part of pipelines

2. Classification of defects according to the order of repair

3. Types of pipe geometry defects

4. Types of pipe wall defects

5. Weld defects

6.Combined defects

7. Invalid construction elements

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Introduction

Timely maintenance of the gas pipeline, preventive maintenance of the gas pipeline is the key to its long, uninterrupted and reliable operation. The operation of the gas pipeline provides for periodic holding 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 gas contamination of chambers, wells, underground structures, identifying and eliminating blockages, checking and running repairs of pipes and gas fittings.

Current repair of gas pipelines and gas equipment should 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 from the gas supply side.

If necessary, the gas pipeline is subjected to a major overhaul.

Overhaul of the gas pipeline is necessary in the event of sufficiently serious malfunctions that threaten the safety of the operation of the entire system as a whole. At overhaul completely replace damaged sections of the gas pipeline, repair or replace fittings, restore or replace broken insulation systems, repair wells, protective equipment, etc. Often, cast-iron gas pipelines that have become unusable during major repairs are replaced with modern steel pipelines.

Solving the problem of ensuring the trouble-free operation of pipelines, especially gas pipelines, is an extremely important task. During the operation of gas pipelines, there are many problems associated with ensuring safe operation. Various defects occur in pipelines: material delamination, dents, corrosion caverns, stress corrosion cracks, erosion wear, scratches, etc. To solve a particular problem, of course, one must have an idea of the state of affairs in this direction.

This paper will consider the causes of defects in pipelines, classifications and methods for eliminating defects in the pipeline.

1. Defects in pipeline structures and their causes

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

Technical diagnostics is carried out in order to determine the technical condition of the gas pipeline and establish the resource for its further operation, based on the examination.

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, it is necessary to periodically monitor their parameters, both structural and functional (during operation).

A defect is any non-compliance with regulated standards. The main reason for the appearance of defects is the 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.

There are the following pipe defects:

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

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

Construction - due to the imperfection of the technology of construction and installation works, violations of technological and design solutions for transportation, installation, welding, insulation and laying works (scratches, scuffs, dents on the pipe surface).

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 manufacture of low-quality pipes. There are frequent cases of destruction due to delamination of the metal.

At pipe plants, raw material input control is imperfect or completely absent. This leads to raw material defects becoming pipe defects.

In the manufacture of pipes, it is necessary to subject the metal to loads under which it operates beyond the yield point. This leads to hardening, micro-delamination, 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 “trigger” already during the operation of the MT.

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

Mechanical effects during loading and unloading, transport and installation operations lead to the appearance of dents, scratches, scratches, scuffs on the pipes.

When cleaning pipelines with cutter scrapers, plastic deformation defects of local sections of the pipe surface occur - risks, undercuts, etc. These stress concentrators are potential centers for the development of corrosion-fatigue cracks. Cleaning pipelines with wire brushes eliminates damage to pipes in the form of undercuts, but under certain processing conditions, it leads to deformations of the metal surface, which reduces its corrosion resistance.

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

Also, in addition to defects in metallurgical, construction and technological defects of pipes, 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 the destruction of the entire structure. The most common are defects in the shape and size of welds, defects in macro- and microstructure, deformation and warping of welded structures.

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

Influxes - most often formed when welding vertical surfaces with horizontal seams, as a result of liquid metal leaking 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 sagging are a large welding current, a long arc, incorrect position of the electrode, a large angle of inclination of the product when welding up and down.

Undercuts - are recesses formed in the base metal along the edge of the seam. Undercuts are formed due to the increased power of the welding torch and lead to a weakening of the cross section of the base metal and the 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, a large welding current or torch power at low welding speeds. Especially often burns are observed in the process of welding thin metal and during the first pass of a multilayer weld, as well as with an increase in the duration of welding, low compressive force and the presence of contamination on the surfaces of the parts to be welded or electrodes (spot and seam contact welding).

Unwelded craters - formed when the arc abruptly breaks at the end of welding. They reduce the cross section of the seam and can be centers of cracking.

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

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

Figure 2 - gas pores

Such a defect is observed with 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, the use of wet or damp flux.

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

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

Picture 3 - slag inclusions

Lack of penetration - local non-fusion of the base metal with welding, as well as non-fusion of individual layers of the seam 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 fusion

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

Cracks - depending on the temperature, the formations 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 interlayers 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 weld has cooled or after a long period of time. The main reason for their formation is the significant stress that occurs 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 come to the surface of the seam and are clearly visible.

Welded joint microstructure defects include

micropores,

microcracks,

Nitride, oxygen and other non-metallic inclusions,

Coarse grain,

Areas of overheating and burnout.

Insulation defects - discontinuity; adhesion; underestimated thickness; corrugations; wrinkles; badass; 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 a primer and mastic - careless dosage of components; non-observance of the boiler heating mode; insufficient mixing of bitumen during the preparation of the primer;

3) when applying a primer and bituminous mastic - thickening of the primer; the formation of bubbles on the surface of the pipeline; dust settling on the surface of pipes; gaps 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; extrusion of a layer of mastic; insufficient immersion of fiberglass in mastic;

5) when applying polymeric 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 of the temperature regime for applying the tape; poor cleaning of the pipe surface;

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

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

Thus, in connection with the growth of pipeline networks for natural gas, which have an increased risk of various kinds of emergencies, it becomes actual problem safety and reliability of operation of gas pipelines. Various research units are being set up to address pipeline safety issues.

2. Methods for eliminating defects in the pipeline

The procedure for assigning a defective pipe repair method begins with the formation of initial data used to check the conditions for the maintainability of defective pipe sections, and the conditions under which the defective pipe section is not repaired. After the formation of the initial data, the conditions for the interaction of defects are checked, according to the results of which, for each defective pipe, a list of single and combined defects is formed.

In-pipe inspection allows obtaining a qualitative 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 in selective and major repairs. Elimination of defects during overhaul is carried out at a pressure in the oil pipeline not higher than 2.5 MPa.

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

1. Grinding

Grinding is used to repair sections and fittings (bends, tees, adapters, plugs, etc.) with defects up to 20% of the nominal thickness of the pipe wall such as metal loss (corrosion defects, risks), delamination with access to the surface, small cracks, as well as defects such as "weld anomalies" (scaling, pores emerging on the surface) with a residual height of reinforcement of at least the values specified in RD 08.0 0-60.30.00-KTN-050-1-05.

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

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

When grinding by removing the metal, the smooth shape of the surface should be restored, and the stress concentration should be reduced. The maximum allowable pressure in the pipe during selective repair by grinding is no more than 2.5 MPa. The polished area must be subjected to visual, magnetic particle or color flaw detection.

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

When grinding cracks before installation, the depth of the selected metal should exceed the crack depth 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 of repairing defects in pipelines.

There are several methods for eliminating defects in the pipeline:

Sanding repair:

It is used for corrosion defects, risks, delaminations with access to the surface, with small cracks;

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

nominal wall thickness;

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

2. Weldingdefects

Welding is allowed to be used to repair pipe wall defects of the "loss of metal" type (corrosion pits, risks) with a residual pipe wall thickness of at least 5 mm, as well as defects of the "transverse weld anomalies" type (pores protruding to the surface, undercuts of the weld, insufficient or absent reinforcement, insufficient width of the seam) 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 values. The distance between adjacent damages must be at least 100 mm. Distance from welded defects to welds, incl. to spiral, must be at least 100 mm.

Weld repair:

It is 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 is allowed only on a completely filled oil pipeline;

The maximum allowable 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 tres is the residual wall thickness at the place of welding, mm; the coefficient 0.4 has the dimension of MPa/mm.

Performed by manual arc welding;

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

Installation of repair structures

For permanent repairs:

Composite clutch

crimp welded sleeve;

several types of dumbbell clutches;

Welded pipe with elliptical bottom

For temporary repairs:

Welded non-compression sleeve;

Welded socket with conical transitions

Technological schemes for repairing pipelines with the replacement of insulation

in a trench without raising the pipeline with a dig and support for the repaired area;

in a trench with the pipeline section under repair raised by pipelayers to a height that allows cleaning and insulating machines to pass through the raised section without digging under the pipeline;

· on the edge (berm) of the trench with its rise to the height necessary to pass the cleaning machine.

Characteristics of the main methods for repairing defects in pipelines

1. Methods emergency repair

Methods of emergency repair of oil pipelines (applying patches, clamps, clamping devices, clogging of chopsticks) can only be considered as emergency, temporary methods for eliminating emergency situations.

2. Banding with winding structures

There are several ways to repair pipes by winding with preload:

winding of steel wire or tape;

winding of fiberglass materials with their impregnation with a binder composition; winding tapes from composite materials

Conclusion

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

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

Timely maintenance of the gas pipeline, preventive maintenance of the gas pipeline is the key to its long, uninterrupted and reliable operation. The operation of the gas pipeline provides for 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 gas contamination of chambers, wells, underground structures, identifying and eliminating blockages, checking and running repairs of pipes and gas fittings. Maintenance main pipeline has 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.

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Repair of defects in pipes and welds. Diagnostics of equipment of pumping and compressor stations What are minor pipeline defects

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