Pipes and their connections.
Heat transport technology imposes the following basic requirements on pipes used for heat pipelines:
· sufficient mechanical strength and tightness at existing coolant pressures;
· elasticity and resistance to thermal stresses at alternating thermal mode;
· constancy of mechanical properties;
· resistance to external and internal corrosion;
· low roughness of internal surfaces;
· no erosion of internal surfaces;
· low coefficient temperature deformations;
· high heat-insulating properties of pipe walls;
· simplicity, reliability and tightness of the connection individual elements;
· ease of storage, transportation and installation.
All types of pipes known to date do not simultaneously satisfy all of the listed requirements. In particular, these requirements are not fully satisfied by steel pipes used for transporting steam and hot water. However, high mechanical properties and elasticity steel pipes, as well as the simplicity, reliability and tightness of connections (welding) ensured almost one hundred percent use of these pipes in district heating systems.
The main types of steel pipes used for heating networks:
With a diameter up to 400 mm inclusive - seamless, hot-rolled;
With a diameter above 400 mm - electric welded with a longitudinal seam and electric welded with a spiral seam.
Heat network pipelines are connected to each other using electric or gas welding. For water heating networks, preference is given to steel grades St2sp and St3sp.
The pipeline layout, placement of supports and compensating devices must be selected in such a way that the total stress from all simultaneously acting loads in any section of the pipeline does not exceed the permissible one. The weakest point steel pipelines The areas that should be used for stress testing are welds.
Supports.
Supports are critical parts of the heat pipeline. They perceive forces from pipelines and transmit them to supporting structures or soil. When constructing heat pipelines, two types of supports are used: free and fixed.
Free supports take the weight of the pipeline and ensure its free movement during temperature deformations. Fixed supports They fix the position of the pipeline at certain points and perceive the forces that arise at the fixation points under the influence of temperature deformations and internal pressure.
When installing ductless installations, they usually avoid installing free supports under pipelines in order to avoid uneven fits and additional bending stresses. In these heat pipelines, the pipes are laid on untouched soil or a carefully compacted layer of sand. When calculating bending stresses and deformations, a pipeline lying on free supports is considered as a multi-span beam.
According to the principle of operation, free supports are divided into sliding, roller, roller and suspended.
When choosing the type of supports, you should not only be guided by the value of the design forces, but also take into account the operation of the supports under operating conditions. As pipeline diameters increase, the friction forces on the supports increase sharply.
Rice. A Sliding support: 1 – thermal insulation; 2 – supporting semi-cylinder; 3 – steel bracket; 4 - concrete stone; 5 – cement-sand mortar
Fig.B Roller support. Fig.B Roller support. Fig.D Suspended support.
In some cases, when, according to the conditions of pipeline placement, relatively load-bearing structures Sliding and rolling supports cannot be installed; suspended supports are used. The disadvantage of simple suspension supports is the deformation of the pipes due to the different amplitudes of the suspensions located on different distances from a fixed support, due to different angles turn. As you move away from the fixed support, the temperature deformation of the pipeline and the angle of rotation of the hangers increase.
Compensation for temperature deformations.
Compensation for temperature deformations is carried out by special devices - compensators.
According to the principle of operation, compensators are divided into radial and axial.
Radial expansion joints allow pipeline movement in both axial and radial directions. With radial compensation, the thermal deformation of the pipeline is absorbed due to the bending of elastic inserts or individual sections of the pipeline itself.
Fig. Compensators. a) U-shaped; b) Ω-shaped; c) S-shaped.
Advantages - simplicity of the device, reliability, unloading of fixed supports from internal pressure forces. Disadvantage: lateral movement of deformed areas. This requires an increase in the cross-section of non-passable channels and complicates the use of backfill insulation and channelless installation.
Axial expansion joints allow the pipeline to move only in the direction of the axis. They are made of sliding type - stuffing box and elastic - lens (bellows).
Lens compensators are installed on pipelines low pressure– up to 0.5 MPa.
Rice. Compensator. a) one-sided stuffing box: b) three-wave lens compensator
1 – glass; 2 – body; 3 – packing; 4 – thrust ring; 5 – ground book.
Purpose of the lesson. Familiarization of students with the basic methods of connecting pipes in pipelines and relieving them from stresses arising due to temperature deformations.
Section 1. Pipe connections in process pipelines]
Connections of individual pipe sections between each other and with fittings are made different ways. The choice of method depends on the required reliability of operation, the initial cost, the required frequency of disassembly, the material properties of the parts being connected, the availability of appropriate tools, and the skills of installation and operating personnel.
All types of connections can be divided into detachable and permanent. Detachable connections include connections on threads (using couplings, nipples), on flanges, on sockets and using special devices. Permanent connections include welding, soldering or gluing.
Threaded connections. Threaded pipe connections are used mainly in heat and water supply pipelines and gas lines for household purposes. In the chemical industry, such compounds are used in pipelines compressed air. For threaded connections, the ends of the pipes are cut from the outside with pipe threads. This thread differs from normal (metric) threads by a much smaller pitch and shallower depth. Therefore, it does not cause significant weakening of the pipe wall. In addition, pipe threads have a triangle apex angle of 55°, while metric threads have a triangle angle of 60°.
Pipe threads are made in two versions: with the top cut in a straight line, and with a rounding. Straight and rounded pipe threads manufactured to proper tolerances are interchangeable.
For connecting pipes in pipelines high pressure tapered thread is used. The conical thread connection is exceptionally tight.
The ends of the pipes are connected to each other and to the fittings using threaded couplings. Coupling threaded connections usually used for pipelines with a diameter of up to 75 mm. Sometimes this type of connection is also used when laying pipes large diameters(up to 600 mm) .
Coupling (Fig. 5.1, A And b) is a short hollow cylinder, the inner surface of which is completely cut with pipe threads. Couplings are made of ductile cast iron for nominal diameters from 6 to 100 mm and made of steel for nominal diameters from 6 to 200 mm . To connect using a coupling, the pipes to be connected are cut to half the length of the coupling and screwed together. If two previously installed pipes are joined, then a bend is used (Fig. 5.1, c). To seal the coupling joint, flax strands or asbestos cord were previously used. To increase the tightness of gas lines, the sealing material was impregnated with paint. Currently, flax strands have been practically replaced by fluoroplastic sealing material (FUM) and special paste (germeplast).
| Rice. 5.1. – Threaded fittings. a, 6– couplings; V– sogon; G– locknut. |
For branching pipelines assembled on threads, tees and crosses are used, and for transitions from one diameter to another, special couplings or inserts are used.
Flange connections. Flanges are metal discs that are welded or screwed to a pipe and then bolted to another flange (Figure 5.2). To do this, several holes are made around the perimeter of the disk. In this way, you can connect not only two sections of the pipeline, but also connect the pipe to a tank, pump, lead it to equipment or measuring device. Flange connections are used in the energy industry, oil and gas, chemical and other industries. Flanges provide ease of installation and dismantling.
Steel flanges are most commonly produced, although plastic flanges are also produced for some types of pipes. During production, the diameter of the pipe to which the fastening will be made and its shape are taken into account. Depending on the shape of the pipe, the internal hole in the flange can be not only round, but also oval or even square. The flange is attached to the pipe using welding. The paired flange is attached to another section of pipe or equipment, and then both flanges are bolted to each other through the existing holes. Flange connections are divided into non-gasketed and gasketed. In the first, tightness is ensured through careful processing and high compression. Secondly, a gasket is placed between the flanges. There are several types of gaskets, depending on the shape of the flanges themselves. If the flange has smooth surface, then the gasket can be cardboard, rubber or paronite. If one flange has a groove for the protrusion, which is located on the paired flange, then a paronite and asbestos-metal gasket is used. This is usually done when installing on high-pressure pipes.
According to the method of fitting onto the pipe, flanges are divided into welded (Fig. 5.3, f, g, h), cast integrally with the pipe (Fig. 5.3, a, b), with a threaded neck (Fig. 5.3, c), free on flanged pipe (Fig. 5.3, j) or rings (Fig. 5.3, h), the latter flat or with a neck for flanging.
According to another classification, flanges are free (Fig. 5.3, h, i, j), collar flanges (Fig. 5.3, a, b, g, h) and flat (Fig. 5.3, c, d, e, f).
Flanges have dimensions depending on the pipe diameter ( Dy) and pressure ( Py), but the connecting dimensions of all flanges are the same for the same Dy And Py.
Socket connections. Socket connections (Fig. 5.4) are used when laying certain types of steel, cast iron, ceramic, glass, faolite, asbestos cement pipes, as well as plastic pipes. Its advantage is its relative simplicity and low cost. At the same time, a number of disadvantages: the difficulty of connecting the connection, insufficient reliability, the possibility of a violation of the tightness when a slight misalignment of adjacent pipes occurs - limit the use of this type of connection.
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| Rice. 5.4.– Socket connection. |
1 – bell, 2 – packing To seal the socket connection (Fig. 5.4) annular space formed by the socket 1 of one pipe and the body of the other, is filled with packing 2, which is used as an oiled strand, asbestos cord or rubber rings. After which the outer portion of this space is caulked or covered with some kind of mastic. The method of carrying out this work and the type of materials used depend on the material of the pipes. Thus, the sockets of cast iron water pipes are caulked with flax strands and caulked with moistened cement, and in especially critical cases they are filled with molten lead, which is then also caulked. The sockets of ceramic sewer pipes are filled up to half with hemp resin strands. The second half is filled with white, well-washed clay. In residential construction, sealing sockets cast iron pipes
carried out with asphalt mastic. Special devices . Used a large number of
a variety of special pipe connections. However, the most common ones are easily disassembled. As an example, consider a connection using a connecting nut (Fig. 5.5.) metal parts(1, 2 and 4) and soft gasket 3. The main parts of the nut 1 and 4 are screwed onto the short threads of the pipes. Middle part - union nut 2 – pulls these main parts together. The tightness of the connection is achieved by a soft (rubber, asbestos, paronite) gasket 3. Thanks to the presence of the gasket, the union nut does not come into contact with the medium flowing through the pipes, and therefore the risk of the nut jamming is minimized.
Connecting pipes by welding, soldering and gluing. In industry, methods of connecting pipes by welding, soldering and gluing have become widespread. By welding or soldering, you can connect pipes made of ferrous metals (except cast iron), non-ferrous metals, as well as vinyl plastic.
The difference between welding and soldering is that in the first case, the same material is used to connect pipes as the one from which they are made. In the second, an alloy (solder) with a melting point significantly lower than that of the pipe material. Solders are usually divided into two groups - soft and hard. Soft solders include those with a melting point of up to 300 °C, and hard solders - above 300 °C. In addition, solders vary significantly in mechanical strength. Soft solders are tin-lead alloys (POS). A large number of tin-lead solders contain a small percentage of antimony. The most common hard solders are copper-zinc (PMC) and silver (PSr) with various additives.
The cost of preparing pipes for welding and the cost of welding itself is many times lower than the cost of a flange connection (a pair of flanges, gaskets, bolts and nuts, work on fitting the flange to the pipe). Well done welded joint it is very durable and does not require repairs and associated production stops, which occurs, for example, when tearing out gaskets at a flange connection.
On a welded pipeline, flanges are installed only in places where the fittings are installed. However, it is possible to use steel reinforcement with welded ends.
Despite the advantages of welding and soldering pipes over other types of connections, they should not be performed in three cases:
· if the product transmitted through pipes has a destructive effect on the deposited metal or on the ends of pipes heated during welding;
· if the pipeline requires frequent disassembly;
· if the pipeline is located in a workshop whose nature of production precludes working with an open flame.
When connecting carbon steel pipes, both oxygen-acetylene (gas) and electric arc welding can be used. Gas welding has the following advantages over electric arc welding:
· the metal in the weld becomes more viscous;
· work can be carried out in hard-to-reach places;
· Ceiling seams are much easier to make.
Electric arc welding, however, has its advantages:
· it is 3-4 times cheaper than gas welding;
· the parts being welded heat up less.
In preparation for welding pipes with a thickness of at least 5 mm, the edges of the pipes are filed at an angle of 30-45°. Interior the wall remains unmown at a thickness of 2-3 mm . To ensure good welding of the pipes, a gap of 2-3 mm is left between them . This gap also protects the ends of the pipes from flattening and bending. A reinforcing bead 3-4 mm high is fused along the outer surface of the seam. . To prevent droplets of molten metal from getting into the pipe, the seam is not welded by 1 mm before inner surface pipes
Connecting pipes made of non-ferrous metals by welding or soldering is carried out using one of the methods shown in Fig. 5.6.
Butt welding (Fig. 5.6, a) is widely used when connecting lead and aluminum pipes. Welding (soldering) with beading and rolling of ends (Fig. 21, b, c and d) is used when connecting lead and copper pipes. In cases where particularly high strength requirements are imposed on the connection, the weld is made as shown in Fig. 5.6, d.
To strengthen the seam when connecting aluminum pipes, the metal is welded with a roller (Fig. 5.6, a), and when connecting lead and copper pipes, the outer edges of the pipes are also slightly beaded (Fig. 5.6, b, c, d).
The connection of aluminum and lead pipes is made by surfacing metal that is the same as the base metal of the pipes, i.e. welding; connection of copper pipes - both welding and soldering (hard solder).
Faolite pipes can be connected by gluing using the methods shown in Fig. 5.6, c, d. Vinyl plastic pipes are connected according to the methods shown in Fig. 5.6, a, b and c, and the connection according to the method shown in Fig. 5.6, b, is very durable.
Section 2. Temperature expansion of pipelines and its compensation.
The normal operation temperature of pipelines differs, often significantly, from the temperature at which they were installed. As a result of temperature expansion, mechanical stresses arise in the pipe material, which, if special measures are not taken, can lead to their destruction. Such measures are called compensation for temperature expansion or simply temperature compensation of the pipeline.
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| Rice. 5.7. Pipeline bending during self-compensation |
The simplest and cheapest method of temperature compensation of pipelines is the so-called “self-compensation”. The essence of this method is that the pipeline is laid with turns so that straight sections do not exceed a certain design length. A straight section of pipe, located at an angle to another section and forming one piece with it (Fig. 5.7), can absorb its elongation due to its own elastic deformation. Typically, both pipe sections located at an angle mutually perceive thermal expansion and thus play the role of compensators. For illustration in Fig. 5.7, the solid line shows the pipeline after installation, and the dash-dotted line shows it in a working, deformed state (the deformation is exaggerated).
Self-compensation is easily carried out on pipelines made of steel, copper, aluminum and vinyl plastic, since these materials have significant strength and elasticity. On pipelines made of other materials, elongation is usually absorbed using compensators, which are described below.
Using the deformation of a straight pipe section, one can, generally speaking, perceive thermal elongation any size, provided that the compensating section is of sufficient length. In practice, however, they usually do not go beyond 400 mm. for steel pipes and 250 mm for vinyl plastic.
If self-compensation of the pipeline is insufficient to relieve temperature stresses or it cannot be carried out, then they resort to using special devices, which are used as lens and stuffing box compensators, as well as bent pipe compensators.
Lens compensators. The operation of the lens compensator is based on the deflection of round plates or wave-like broadenings that make up the body of the compensator. Lens compensators can be made of steel, red copper or aluminum.
According to the method of execution, they distinguish following types lens compensators: welded from stamped half-waves (Fig. 5.8, a and b), welded disc-shaped (Fig. 5.8, c ), welded drum (Fig. 5.8, d) and designed specifically for work on vacuum pipelines (Fig. 5.8, d) .
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| Rice. 5.8.– Lens compensators. |
The common advantages of lens compensators of all types without exception are their compactness and low maintenance requirements. These advantages are in most cases overshadowed by their significant disadvantages. The main ones are the following:
· the lens compensator creates significant axial forces acting on the fixed supports of the pipeline;
limited compensating ability (maximum deformation of the lens compensator does not exceed 80 mm):
· unsuitability of lens compensators for pressures above 0.2-0.3 MPa;
relatively high hydraulic resistance;
· complexity of manufacturing.
Due to the above considerations, lens compensators are used very rarely, namely when a number of specific conditions coincide: at low medium pressure (from vacuum to 0.2 MPa), in the presence of a pipeline large diameter(at least 100 mm), with a short length of the area served by the compensator (usually no more than 20 m), when transmitting gases and vapors through pipelines, but not liquids.
Oil seal compensators. The simplest type of stuffing box compensator (the so-called one-way unbalanced compensator) is shown in Fig. 5.9. It consists of a body 4 with a paw (with which it is attached to a fixed support), a glass 1 and an oil seal. The latter includes the stuffing box 3 and the packing box (packing seal) 2. The stuffing box is usually made from asbestos cord rubbed with graphite, laid in the form of separate rings. The glass and body are connected to the pipeline via flanges. The glass has a side (marked with the letter A), preventing the glass from falling out of the body.
The main advantages of stuffing box expansion joints are their compactness and significant compensating capacity (usually up to 200 mm and higher).
Disadvantages of stuffing box expansion joints:
· large axial forces,
· the need for periodic maintenance of seals (which requires stopping the pipeline),
Possibility of passing (leakage) of the medium through the seal,
· the possibility of jamming of the seal, leading to breakage of any part of the pipeline.
Seizing of the oil seal can occur due to inaccurate laying of the pipeline in a straight line, subsidence of one of the supports during operation, curvature of the longitudinal axis of the pipeline under the influence of temperature changes in the branch, corrosion of sliding surfaces and the deposition of scale or rust on them.
Due to the listed disadvantages, gland compensators on pipelines general purpose are used extremely rarely (for example, on heating mains in cramped urban conditions). They are used on pipelines made of materials such as: cast iron (ferrosilide and antichlorine), glass and porcelain, faolite. Due to their properties, these materials require installation on rigid foundations that can provide Good work gland compensators and, due to their fragility, exclude the possibility of using self-compensation. Stuffing box expansion joints installed on pipelines made of these materials are made of corrosion-resistant materials, which prevents jamming from rusting of rubbing surfaces.
All other pipelines that require compensation for thermal elongation are recommended to be self-compensating or, if possible, equipped with compensators made of bent pipes. About them below.
Compensators bent from pipes. Compensators of this type are the most common in enterprises and on main pipelines. Bent expansion joints are made of steel, copper, aluminum and vinyl plastic pipes.
 A
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 b
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| Rice. 5.11.– Bent expansion joints a – U-shaped; b – S-shaped |
Depending on the manufacturing method, compensators are distinguished: smooth (Fig. 5.10, a), folded (Fig. 5.10, b), wavy (Fig. 5.10, c), and depending on the configuration - lyre-shaped (Fig. 5.10), P- shaped (Fig. 5.11, a) and S-shaped (Fig. 5.11, b).
The term “folded” refers to an expansion joint, the curvature of which is obtained due to the formation of folds on the inner surface of the bends; the term “wavy” refers to an expansion joint that has waves in curved sections along the entire cross-section of the pipe. The main difference between these expansion joints is their compensating capacity and hydraulic resistance. If we take the compensating capacity of a smooth compensator as one, then, other things being equal, the compensating capacity of a folded compensator will be about 3, and a wavy compensator will be about 5 - 6. At the same time, the hydraulic resistance of these devices is minimal for a smooth compensator and maximum for a wavy compensator.
The disadvantages of bent expansion joints of all types without exception include:
· significant dimensions, making it difficult to use these expansion joints in tight spaces;
· relatively high hydraulic resistance;
· the occurrence of fatigue phenomena in the compensator material over time.
Along with this, bent expansion joints have the following advantages:
· significant compensating capacity (usually up to 400 mm);
· insignificant amount of axial forces loading the fixed supports of the pipeline;
· ease of production on site;
· undemanding with regard to the straightness of the pipeline and the appearance of distortions in it during operation;
· ease of operation (does not require maintenance).
A modern way to extend service life pipeline systems is the use of compensators. They help prevent various changes that occur in pipes due to constant changes in temperature, pressure and various kinds vibrations The absence of compensators on pipes can lead to such undesirable consequences as a change in the length of the pipe, its expansion or compression, which subsequently leads to a pipeline breakthrough. In this regard, the problem of reliability of pipelines and compensators is given the closest attention and a search is carried out optimal solutions for ensuring technical safety compensation systems.
There are pipe, stuffing box, lens and bellows compensators. Most in a simple way is the use of natural compensation due to the flexibility of the pipeline itself using elbows U-shaped. U-shaped expansion joints are used for overhead and channel laying of pipelines. For above-ground installation, they require additional supports, and with channel - special cameras. All this leads to a significant increase in the cost of the pipeline and the forced alienation of areas of expensive land.
Stuffing box compensators, which until recently were most often used in Russian heating networks, also have a number of serious disadvantages. On the one hand, the stuffing box compensator can provide compensation for axial movements of any magnitude. On the other hand, there are currently no gland seals capable of ensuring the tightness of pipelines with hot water and ferry for a long time. In this regard, regular maintenance of the stuffing box expansion joints is required, but even this does not prevent coolant leaks. And since when laying heat pipelines underground, special service chambers are required to install stuffing box compensators, this significantly complicates and makes it more difficult expensive construction and operation of heating mains with compensators of this type.
Lens compensators are used mainly on heat and gas mains, water and oil pipelines. The rigidity of these expansion joints is such that significant effort is required to deform them. However, lens compensators have a very low compensating ability compared to other types of compensators; moreover, the labor intensity of their manufacture is quite high, and a large number of welds (which is caused by the manufacturing technology) reduces the reliability of these devices.
Considering this circumstance, the use of bellows-type expansion joints, which do not leak and do not require maintenance, is currently becoming relevant. Bellows expansion joints are small in size, can be installed anywhere in the pipeline using any method of laying it, and do not require the construction of special chambers or maintenance during the entire service life. Their service life, as a rule, corresponds to the service life of pipelines. The use of bellows expansion joints ensures reliable and effective protection pipelines from static and dynamic loads arising from deformations, vibration and water hammer. Thanks to the use of high-quality stainless steels in the manufacture of bellows, bellows expansion joints are capable of operating in the most severe conditions with operating fluid temperatures ranging from " absolute zero» up to 1000 °C and can withstand operating pressures from vacuum to 100 atm, depending on the design and operating conditions.
The main part of the bellows expansion joint is the bellows - an elastic corrugated metal shell that has the ability to stretch, bend or shift under the influence of temperature changes, pressure and other types of changes. They differ from each other in such parameters as dimensions, pressure and types of displacements in the pipe (axial, shear and angular).
Based on this criterion, compensators are divided into axial, shear, angular (rotary) and universal.
The bellows of modern expansion joints consist of several thin layers of stainless steel, which are formed using hydraulic or conventional pressing. Multi-layer expansion joints neutralize the effects of high pressure and various kinds vibrations without causing reaction forces, which in turn are provoked by deformation.
Kronstadt Company (St. Petersburg), official representative Danish manufacturer Belman Production A/S, supplies to Russian market bellows expansion joints specially designed for heating networks. This type of compensator is widely used in the construction of heating networks in Germany and Scandinavia.
The design of this compensator has a number of distinctive features.
Firstly, all layers of the bellows are made of high-quality stainless steel AISI 321 (analogue 08Х18Н10Т) or AISI 316 TI (analogue 10Х17Н13М2Т). Currently, in the construction of heating networks, expansion joints are often used in which the inner layers of the bellows are made of a material of lower quality than the outer ones. This can lead to the fact that for any reason, even minor damage outer layer, or with a slight defect weld, water, which contains chlorine, oxygen and various salts, will get inside the bellows and after some time it will collapse. Of course, the cost of a bellows in which only the outer layers are made of high-quality steel is somewhat lower. But this difference in price cannot be compared with the cost of work in the event of an emergency replacement of a failed compensator.
Secondly, Belman compensators are equipped with both an external protective casing that protects the bellows from mechanical damage, and an internal pipe that protects the internal layers of the bellows from the effects of abrasive particles contained in the coolant. In addition, the presence internal protection bellows prevents the deposition of sand on the bellows lenses and reduces flow resistance, which is also important when designing a heating main.
Ease of installation is another distinctive feature Belman compensators. This compensator, unlike its analogues, is supplied completely ready for installation in the heating network: the presence of a special fixing device allows the compensator to be mounted without resorting to any preliminary stretching and does not require additional heating of the heating network section before installation. The compensator is equipped with a safety device that protects the bellows from twisting during installation and prevents excessive compression of the bellows during operation.
In cases where the water flowing through the pipeline contains a lot of chlorine or may enter the compensator groundwater, Belman offers a bellows in which the outer and inner layers are made of a special alloy that is particularly resistant to aggressive substances. For ductless laying of heating mains, these expansion joints are produced in polyurethane foam insulation and are equipped with an operational remote control system.
All of the above advantages of compensators for heating networks produced by Belman, coupled with high quality manufacturing, allow us to guarantee trouble-free operation of the bellows for at least 30 years.
Literature:
- Antonov P.N. “On the peculiarities of using compensators”, magazine “ Pipeline accessories", No. 1, 2007.
- Polyakov V. “Localization of pipe deformation using bellows expansion joints”, “Industrial Vedomosti” No. 5-6, May-June 2007
- Logunov V.V., Polyakov V.L., Slepchenok V.S. “Experience in using axial bellows expansion joints in heating networks”, “Heat Supply News” magazine, No. 7, 2007.
During operation, pipelines change their temperature due to temperature changes environment and pumped liquids. Fluctuations in the temperature of the pipeline wall lead to changes in its length.
The law of change in pipeline length is expressed by the equation
Δ=α · l(t y - t o ),
where Δ is the lengthening or shortening of the pipeline; a is the coefficient of linear expansion of pipe metal (for steel pipes α = 0.000012 1/°C); l - pipeline length; t y - pipeline laying temperature; t 0 - ambient temperature.
If the ends of the pipeline are rigidly fixed, then thermal tensile or compressive stresses arise in it due to temperature influences, the magnitude of which is determined by Hooke’s law
Where E- modulus of elasticity of the pipe material (for steel) E= 2.1·10 6 kg/cm 2 =2.1·10 5 MPa).
These stresses cause forces at the points where the pipeline is fixed, directed along the axis of the pipeline, independent of the length, and equal to
where σ - compressive and tensile stress arising in the pipe due to temperature changes; F - open cross-sectional area of the pipe material.
Magnitude N can be very large and lead to the destruction of the pipeline, fittings, supports, as well as cause damage to equipment (pumps, filters, etc.) and tanks.
Changes in the length of underground pipelines depend not only on temperature fluctuations, but also on the friction force of the pipe on the ground, which prevents changes in length.
If the forces from thermal stresses do not depend on the length of the pipeline, then the friction force of the pipe on the ground is directly proportional to the length of the pipeline. There is a length at which the frictional forces can balance with the thermal force, and the pipeline will not have a change in length. In sections of shorter length, the pipeline will move in the ground.
The maximum length of such a section 1 max, at which the pipeline can move in the ground, is determined by the equation
where δ is the pipe wall thickness, cm; k - soil pressure on the surface of the pipe, kg/cm 2 ; μ - coefficient of friction of the pipe on the ground.
5.2. Compensators
Relief of pipelines from thermal stresses is carried out by installing compensators. Compensators are devices that allow pipelines to freely lengthen or contract with temperature changes without damaging the connections. Lens, stuffing box, and bent compensators are used.
When choosing a pipeline route, it is necessary to strive to ensure that the temperature extensions of some sections can be perceived by deformations of others, i.e. strive for self-compensation of the pipeline, using all its turns and bends for this.
Lens compensators(Fig. 5.5) are used to compensate for elongations of pipelines with operating pressures up to 0.6 MPa and diameters from 150 to 1,200 mm.
Rice. 5.5. Lens compensators with two flanges
Compensators are made of conical plates (stamped), each pair of plates welded together forms a wave. The number of waves in the compensator is made no more than 12 to avoid longitudinal bending. The compensating capacity of lens compensators is up to 350 mm.
L 
Insulation compensators are characterized by tightness, small dimensions, ease of manufacture and operation, but their use is limited by their unsuitability for high pressures. Stuffing box expansion joints (Fig. 5.6) are axial expansion joints and are used for pressures up to 1.6 MPa. Compensators consist of a cast iron or steel body and a glass included in it. The seal between the glass and the body is created by an oil seal. The compensating capacity of stuffing box compensation ditch ranges from 150 to 500 mm.
Stuffing box compensators are installed on the pipeline with precise installation, since possible distortions can lead to jamming of the sleeve and destruction of the compensator. Stuffing box compensators are unreliable in terms of tightness, require constant supervision of the sealing of the seals and, therefore, have limited use. These compensators are installed on pipelines with a diameter of 100 mm and above for non-flammable liquids and on steam pipelines.
Bent expansion joints have a U-shaped (Fig. 5.7), lyre-shaped, S-shaped and other shapes and are manufactured at the installation site from the pipes from which the pipeline is assembled. These compensators are suitable for all pressures, balanced and sealed. Their disadvantages are their significant dimensions.
Thermal elongation of pipelines at a coolant temperature of 50 °C and above must be absorbed by special compensating devices that protect the pipeline from the occurrence of unacceptable deformations and stresses. The choice of compensation method depends on the parameters of the coolant, the method of laying heating networks and other local conditions.
Compensation for thermal elongation of pipelines through the use of route turns (self-compensation) can be used for all methods of laying heating networks, regardless of pipeline diameters and coolant parameters at an angle of up to 120°. When the angle is more than 120°, and also in the case when, according to strength calculations, the rotation of the pipelines cannot be used for self-compensation, the pipelines at the turning point are secured fixed supports.
To ensure proper operation of compensators and self-compensations, pipelines are divided by fixed supports into sections that are independent of one another with respect to thermal elongation. On each section of the pipeline, limited by two adjacent fixed supports, installation of a compensator or self-compensation is provided.
When calculating pipes to compensate for thermal expansion, the following assumptions were made:
fixed supports are considered absolutely rigid;
the resistance of the friction forces of the movable supports during thermal elongation of the pipeline is not taken into account.
Natural compensation, or self-compensation, is the most reliable in operation, and therefore is widely used in practice. Natural compensation for thermal expansion is achieved at turns and bends of the route due to the flexibility of the pipes themselves. Its advantages over other types of compensation are: simplicity of design, reliability, lack of need for supervision and maintenance, and unloading of fixed supports from internal pressure forces. The installation of natural compensation does not require additional consumption of pipes and special building structures. The disadvantage of natural compensation is the lateral movement of deformed sections of the pipeline.
Let us determine the total thermal elongation of the pipeline section
For trouble-free operation of heating networks, it is necessary that compensating devices be designed for maximum pipeline extensions. Therefore, when calculating elongations, the coolant temperature is taken to be maximum and the ambient temperature to be minimum. Complete thermal expansion of a pipeline section
l= αLt, mm, Page 28 (34)
where α is the coefficient of linear expansion of steel, mm/(m-deg);
L – distance between fixed supports, m;
t – design temperature difference, taken as the difference between the operating temperature of the coolant and the design temperature of the outside air for heating design.
l= 1.23*10 -2 *20*149 = 36.65 mm.
l= 1.23* 10 -2 * 16* 149 = 29.32 mm.
l= 1.23*10 -2 *25*149 = 45.81 mm.
Similarly we find l for other areas.
The forces of elastic deformation that arise in the pipeline when compensating for thermal elongation are determined by the formulas:
Kgs; , N; Page 28 (35)
where E is the elastic modulus of pipe steel, kgf/cm2;
I- moment of inertia of the cross-section of the pipe wall, cm;
l– length of the smaller and larger section of the pipeline, m;
t – calculated temperature difference, °C;
A, B - auxiliary dimensionless coefficients.
To simplify the determination of the force of elastic deformation (P x, P v) Table 8 gives an auxiliary value for various pipeline diameters.
Table 11
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Pipe outer diameter d H, mm | |||||||||||||||
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Pipe wall thickness s, mm | |||||||||||||||
During the operation of the heating network, voltages appear in the pipeline, which create inconvenience for the enterprise. To reduce the stresses that arise when the pipeline is heated, axial and radial steel expansion joints (stuffing, U- and S-shaped, and others) are used. Wide Application found U-shaped compensators. To increase the compensating capacity of U-shaped expansion joints and reduce the bending compensation stress in the operating state of the pipeline for sections of pipelines with flexible expansion joints, the pipeline is pre-stretched in a cold state during installation.
Pre-stretching is done:
at coolant temperatures up to 400 °C inclusive by 50% of the total thermal elongation of the compensated section of the pipeline;
at a coolant temperature above 400 °C by 100% of the total thermal elongation of the compensated section of the pipeline.
Estimated thermal expansion of the pipeline
Mm Page 37 (36)
where ε is a coefficient that takes into account the amount of pre-stretching of compensators, possible inaccuracy of calculation and relaxation of compensation stresses;
l– total thermal elongation of the pipeline section, mm.
1 section х = 119 mm
According to the application, at x = 119 mm, we select the expansion joint offset H = 3.8 m, then the compensator arm B = 6 m.
To find the force of elastic deformation, we draw a horizontal line H = 3.8 m, its intersection with B = 5 (P k) will give a point, lowering the perpendicular from which to the digital values of P k, we get the result P k - 0.98 tf = 98 kgf = 9800 N.
Figure 3 – U-shaped compensator
7 section х = 0.5*270 = 135 mm,
N = 2.5, V = 9.7, R k – 0.57 tf = 57 kgf = 5700 N.
We calculate the remaining sections in the same way.