Ways to compensate for temperature elongations in thermal networks. Compensation for thermal deformations
190. Temperature deformations are recommended to be compensated by turns and bends of the pipeline route. If it is impossible to confine ourselves to self-compensation (in completely straight sections of considerable length, etc.), U-shaped, lens, wavy and other compensators are installed on the pipelines.
In cases where in project documentation steam purge or hot water, it is recommended to rely on these conditions for compensating capacity.
192. It is recommended to use U-shaped compensators for process pipelines of all categories. They are recommended to be made either bent from solid pipes, or using bent, sharply bent or welded bends.
In the case of preliminary stretching (compression) of the compensator, it is recommended to indicate its value in the project documentation.
193. For P- shaped expansion joints bent bends are recommended for safety reasons to be made from seamless pipes, and welded bends from seamless and welded straight-seam pipes.
194. Apply water and gas pipes for the manufacture of U-shaped expansion joints is not recommended, and electric welded with a spiral seam is allowed for straight sections of expansion joints.
195. For safety reasons, it is recommended to install U-shaped compensators horizontally with observance of the general slope. In justified cases (with a limited area), they can be placed vertically with a loop up or down with an appropriate drainage device at the lowest point and air vents.
196. U-shaped compensators are recommended to be installed on pipelines before installation together with spacers, which are removed after the pipelines are fixed on fixed supports.
197. Lens compensators, axial, as well as articulated lens compensators are recommended to be used for technological pipelines in accordance with the NTD.
198. When installing lens compensators on horizontal gas pipelines with condensing gases, it is recommended to provide for condensate drainage for each lens for safety reasons. spigot for drainage pipe it is recommended for safety reasons to be made from a seamless pipe. When installing lens compensators with an inner sleeve on horizontal pipelines, it is recommended for safety reasons to install guide supports at a distance of no more than 1.5 DN of the compensator on each side of the compensator.
199. When installing pipelines, it is recommended to pre-stretch or compress compensating devices for safety reasons. The value of the preliminary stretching (compression) of the compensating device is recommended to be indicated in the project documentation and in the passport for the pipeline. The amount of stretch can be changed by the amount of the correction, taking into account the temperature during installation.
200. The quality of compensators to be installed on process pipelines is recommended to be confirmed by passports or certificates.
201. When installing a compensator, it is recommended to enter the following data into the pipeline passport:
Technical characteristics, manufacturer and year of manufacture of the compensator;
The distance between fixed supports, compensation, amount of pre-stretching;
Ambient air temperature during installation of the compensator and date of installation.
202. Calculation of U-shaped, L-shaped and Z-shaped expansion joints It is recommended to produce in accordance with the requirements of the NTD.
Compensation for thermal deformations of steel pipelines is exclusively importance in heat transfer technology.
If there is no compensation for thermal deformations in the pipeline, then with strong heating, large destructive stresses can arise in the pipeline wall. The value of these voltages can be calculated using Hooke's law
, (7.1)
where E– modulus of longitudinal elasticity (for steel E= 2 10 5 MPa); i- relative deformation.
When the temperature rises, the pipe length l on the Dt extension should be
where a is the coefficient of linear elongation, 1/K (for carbon steel a= 12-10 -6 1/K).
If a pipe section is pinched and does not elongate when heated, then its relative compression
From the joint solution (7.1) and (7.3), one can find the compressive stress that occurs in a steel pipe during heating of a rectilinear pinched (without compensators) section of the pipeline
For steel s= 2.35 D t MPa.
As can be seen from (7.4), the compressive stress that occurs in a pinched straight section of the pipeline does not depend on the diameter, wall thickness and length of the pipeline, but depends only on the material (modulus of elasticity and coefficient of linear elongation) and temperature difference.
The compression force that occurs when a straight pipeline is heated without compensation is determined by the formula
, (7.5)
where f- square cross section pipeline walls, m 2.
By their nature, all compensators can be divided into two groups: axial and radial.
Axial compensators are used to compensate for thermal elongation of straight sections of the pipeline.
Radial compensation can be used with any piping configuration. Radial compensation is widely used on heat pipelines laid in the territories industrial enterprises, and with small diameters of heat pipelines (up to 200 mm) - also in urban heating networks. On large-diameter heat pipelines laid under city thoroughfares, they are installed mainly axial expansion joints.
Axial compensation. In practice, axial expansion joints of two types are used: omental and elastic.
On fig. 7.27 shows a one-way gland compensator. Between the glass 1 and the body 2 of the compensator there is a stuffing box seal 3. The stuffing box packing, which provides tightness, is clamped between the thrust ring 4 and the bottom box 5. Usually the packing is made of asbestos rings square section impregnated with graphite. The compensator is welded into the pipeline, so its installation on the line does not lead to an increase in the number of flange connections.
Rice. 7.27. One-sided stuffing box compensator:
1 - glass; 2 - body; 3 - stuffing; 4 - thrust ring; 5 - grundbuksa
On fig. 7.28 shows a section of a double-sided stuffing box compensator. The disadvantage of stuffing box compensators of all types is the stuffing box, which requires systematic and careful maintenance in operation. The packing in the gland compensator wears out, loses its elasticity over time and begins to let the coolant through. Tightening the gland in these cases does not give positive results, therefore, after certain periods of time, the glands have to be interrupted.
Rice. 7.28. Double-sided stuffing box compensator
All types of elastic compensators are free from this disadvantage.
On fig. 7.29 shows a section of a three-wave bellows compensator. To reduce hydraulic resistance, a smooth pipe is welded inside the bellows section. Bellows sections are usually made of alloyed steels or alloys.
In our country, bellows expansion joints are made of steel 08X18H10T.
Rice. 7.29. Three-wave bellows expansion joint
The compensating capacity of bellows expansion joints is usually determined by test results or taken from manufacturers' data. To compensate for large thermal deformations, several bellows sections are connected in series.
The axial reaction of bellows expansion joints is the sum of two terms
, (7.6)
where s to- axial reaction from temperature compensation, caused by wave deformation during thermal expansion of the pipeline, N; s d- axial reaction caused by internal pressure, N.
To increase the resistance against deformation of the bellows under the action of internal pressure, the expansion joints are made unloaded from internal pressure by appropriate arrangement of the bellows sections in the body of the expansion joint, made from a pipe of a larger diameter. Such a design of the compensator is shown in Fig. 7.30.
Rice. 7.30. Balanced bellows expansion joint:
l p is the length in the stretched state; l szh - length in a compressed state
A promising method for compensating for thermal deformations can be the use of self-compensating pipes. In the production of spirally welded pipes from strip sheet metal a longitudinal groove approximately 35 mm deep is squeezed out on it with a roller. After welding of such a sheet, the groove turns into a spiral corrugation capable of compensating for the temperature deformation of the pipeline. Experimental testing of such pipes showed positive results.
radial compensation. With radial compensation, the thermal deformation of the pipeline is perceived by bends of special elastic inserts or natural turns (bends) of the route of individual sections of the pipeline itself.
The last method of compensation for thermal deformations, widely used in practice, is called natural compensation. The advantages of this type of compensation over other types: simplicity of the device, reliability, no need for supervision and maintenance, unloading of fixed supports from the forces of internal pressure. The lack of natural compensation is the transverse movement of the deformable sections of the pipeline, which requires an increase in the width of impassable channels and makes it difficult to use backfill insulation and channelless structures.
The calculation of natural compensation consists in finding the forces and stresses arising in the pipeline under the action of elastic deformation, choosing the lengths of the interacting arms of the pipeline and determining the transverse displacement of its sections during compensation. The calculation method is based on the basic laws of the theory of elasticity, which relate deformations to acting forces.
Sections of the pipeline, perceiving temperature deformations with natural compensation, consist of bends (elbows) and straight sections. Bent bends increase the flexibility of the pipeline and increase its compensating capacity. The effect of bent elbows on the compensating capacity is especially noticeable in large diameter pipelines.
The bending of curved sections of pipes is accompanied by a flattening of the cross section, which turns from round to elliptical.
On fig. 7.31 shows a curved pipe with a radius of curvature R. Select two sections ab and cd pipe element. When bending in the pipe wall, tensile forces occur on the convex side, and compressive forces occur on the concave side. Both tensile and compressive forces give the resultant T, normal to the neutral axis.
Rice. 7.31. Pipe flattening during bending
The compensating capacity of expansion joints can be doubled by pre-stretching them during installation by an amount equal to half the thermal expansion of the pipeline. Based on the above methodology, equations were obtained for calculating the maximum bending stress and compensating capacity of symmetrical expansion joints of various types.
Thermal calculation
To the task thermal calculation includes the following issues:
determination of heat losses of the heat pipeline;
calculation of the temperature field around the heat pipeline, i.e., determination of the temperatures of the insulation, air in the channel, channel walls, soil.
calculation of the coolant temperature drop along the heat pipeline;
selection of the thickness of the thermal insulation of the heat pipe.
The amount of heat passing per unit time through a chain of series-connected thermal resistances is calculated by the formula
where q– specific heat loss heat pipeline; t– coolant temperature, °C; t o- temperature environment, °С; R- total thermal resistance of the circuit coolant - the environment (thermal resistance of the insulation of the heat pipe).
In the thermal calculation of heat networks, it is usually necessary to determine heat flows through layers and surfaces of a cylindrical shape.
Specific heat loss q and thermal resistance R usually refer to the unit length of the heat pipe and measure them, respectively, in W / m and (m K) / W.
In an insulated pipeline surrounded by outside air, heat must pass through four resistances connected in series: the inner surface of the working pipe, the pipe wall, the insulation layer and the outer surface of the insulation. Since the total resistance is equal to the arithmetic sum of the resistances connected in series, then
R \u003d R in + R tr + R and + R n, (7.8)
where R in, R tr, R and and R n- thermal resistance of the inner surface of the working pipe, the pipe wall, the insulation layer and the outer surface of the insulation.
In insulated heat pipes, the thermal resistance of the thermal insulation layer is of primary importance.
In thermal calculation, there are two types of thermal resistance:
Surface resistance
layer resistance.
Thermal resistance surfaces. The thermal resistance of the cylindrical surface is
where pd– surface area of 1 m of heat pipe length, m; a is the coefficient of heat transfer from the surface.
To determine the thermal resistance of the surface of the heat pipe, it is necessary to know two quantities: the diameter of the heat pipe and the heat transfer coefficient of the surface. The diameter of the heat pipe in the thermal calculation is given. The heat transfer coefficient from the outer surface of the heat pipe to the ambient air is the sum of two terms - the coefficient of heat transfer by radiation a l and convection heat transfer coefficient a to:
Radiant heat transfer coefficient a l can be calculated using the Stefan-Boltzmann formula:
, (7.10)
where With is the emissivity; t is the temperature of the radiating surface, °C.
The emissivity of a black body, i.e. a surface that absorbs all rays falling on it and reflects nothing, With\u003d 5.7 W / (m K) \u003d 4.9 kcal / (h m 2 K 4).
The radiation coefficient of "gray" bodies, which include the surfaces of uninsulated pipelines, insulating structures, has a value of 4.4 - 5.0 W / (m 2 K 4). Heat transfer coefficient from horizontal pipe to air under natural convection, W / (m K), can be determined by the Nusselt formula
, (7.11)
where d is the outer diameter of the heat pipe, m; t, t about– surface and ambient temperatures, °C.
With forced convection of air or wind, the heat transfer coefficient
, (7.12)
where w– air speed, m/s.
Formula (7.12) is valid for w> 1 m/s and d> 0.3 m.
To calculate the heat transfer coefficient according to (7.10) and (7.11), it is necessary to know the surface temperature. Since, when determining heat losses, the surface temperature of the heat pipe is usually unknown in advance, the problem is solved by the method of successive approximations. Pre-set by the heat transfer coefficient of the outer surface of the heat pipe a, find specific losses q and surface temperature t, check the correctness of the received value a.
When determining the heat losses of insulated heat conductors, a verification calculation can be omitted, since the thermal resistance of the insulation surface is small compared to the thermal resistance of its layer. So, a 100% error in choosing the heat transfer coefficient of the surface usually leads to an error in determining the heat loss of 3 - 5%.
For a preliminary determination of the heat transfer coefficient of the surface of an insulated heat conductor, W / (m K), when the surface temperature is unknown, the formula can be recommended
, (7.13)
where w is the speed of air movement, m/s.
The heat transfer coefficients from the coolant to the inner surface of the pipeline are very high, which determines such low values of thermal resistance of the inner surface of the pipeline, which can be neglected in practical calculations.
Thermal resistance of the layer. The expression for the thermal resistance of a homogeneous cylindrical layer is easily derived from the Fourier equation, which has the form
where l is the thermal conductivity of the layer; d 1 , d 2 - inner and outer diameters of the layer.
For thermal calculation, only layers with high thermal resistance are essential. Such layers are thermal insulation, channel wall, soil massif. For these reasons, in the thermal calculation of insulated heat pipes, the thermal resistance of the metal wall of the working pipe is usually not taken into account.
Thermal resistance of insulating structures of above-ground heat pipelines. In aboveground heat pipelines between the coolant and the outside air, the following thermal resistances are connected in series: inner surface working pipe, its wall, one or more layers of thermal insulation, the outer surface of the heat pipe.
The first two thermal resistances are usually neglected in practical calculations.
Sometimes thermal insulation perform multilayer, based on various allowable temperatures for the insulating materials used or for economic reasons in order to partial replacement expensive materials insulation is cheaper.
The thermal resistance of multilayer insulation is equal to the arithmetic sum of the thermal resistances of successively superimposed layers.
The thermal resistance of cylindrical insulation increases with an increase in the ratio of its outer diameter to the inner one. Therefore, in multilayer insulation, it is advisable to lay the first layers from a material having a lower thermal conductivity, which leads to the most efficient use of insulating materials.
Temperature field of the above-ground heat pipeline. The calculation of the temperature field of the heat pipeline is carried out on the basis of the heat balance equation. In this case, the condition is based on the condition that, in a steady thermal state, the amount of heat flowing from the coolant to a concentric cylindrical surface passing through any point of the field is equal to the amount of heat leaving this concentric surface to the external environment.
The surface temperature of the thermal insulation from the heat balance equation will be equal to
. (7.15)
Thermal resistance of soil. In underground heat pipelines, soil resistance is involved as one of the thermal resistances connected in series.
When calculating heat losses for ambient temperature t about take, as a rule, the natural temperature of the soil at the depth of the axis of the heat pipeline.
Only at small depths of laying the axis of the heat pipe ( h/d < 2) за температуру окружающей среды принимают естественную температуру поверхности грунта.
The thermal resistance of the soil can be determined by the Forchheimer formula (Fig. 7.32)
, (7.16)
where l is the thermal conductivity of the soil; h is the depth of the heat pipe axis; d is the diameter of the heat pipe.
When laying underground heat pipelines in channels that have a shape other than cylindrical, in (7.16) the equivalent diameter is substituted for the diameter
where F is the cross-sectional area of the channel, m; P– channel perimeter, m.
The thermal conductivity of the soil depends mainly on its moisture content and temperature.
At soil temperatures of 10 - 40 ° C, the thermal conductivity of soil of medium humidity lies in the range of 1.2 - 2.5 W / (m K).
During operation, pipelines change their temperature due to changes in the temperature of the environment and the pumped liquids. Fluctuations in the temperature of the pipeline wall leads to a change in its length.
The law of change in the length of the pipeline is expressed by the equation
Δ=α · l(t y - t o ),
where Δ - lengthening or shortening of the pipeline; a - coefficient of linear expansion of pipe metal (for steel pipes α = 0.000012 1/°С); 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 compression stresses arise in it from temperature effects, the magnitude of which is determined according to Hooke's law
where E- modulus of elasticity of the pipe material (for steel) E= 2.1 10 6 kg / cm 2 \u003d 2.1 10 5 MPa).
These stresses cause forces at the pipeline fixing points, directed along the axis of the pipeline, independent of the length, and equal to
where σ - compressive and tensile stress that has arisen in the pipe due to temperature changes; F - area of the living section of the pipe material.
Value N can be very large and lead to the destruction of pipelines, fittings, supports, as well as 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 against the ground, which prevents changes in length.
If the efforts from thermal stresses do not depend on the length of the pipeline, then the friction force of the pipe against the ground is directly proportional to the length of the pipeline. There is such 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, on which it is possible to move the pipeline in the ground, is determined by the equation
where δ is the pipe wall thickness, cm; k - soil pressure on the pipe surface, kg / cm 2; μ - coefficient of friction of the pipe on the ground.
5.2. Compensators
Unloading of pipelines from thermal stresses is carried out by installing compensators. Compensators - devices that allow pipelines to freely lengthen or contract with temperature changes without damaging the connections. Lens, gland, bent compensators are used.
When choosing a pipeline route, it is necessary to strive to ensure that the temperature elongations of some sections could be perceived by deformations of others, i.e. strive for self-compensation of the pipeline, using for this all its turns and bends.
Lens compensators(Fig. 5.5) are used to compensate for extensions of pipelines with a working pressure of up to 0.6 MPa with a diameter of 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 in order to avoid buckling. The compensating capacity of lens compensators is up to 350 mm.
L 
Lens compensators are characterized by tightness, small dimensions, ease of manufacture and operation, but their use is limited by their unsuitability for high pressures. Gland compensators (Fig. 5.6) are axial compensators 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 bowl and the body is created by a stuffing box. The compensating capacity of the gland compensation ditch is from 150 to 500 mm.
Gland compensators are installed on the pipeline with precise laying, since possible distortions can lead to jamming of the glass and destruction of the compensator. Stuffing box compensators are unreliable in terms of tightness, require constant supervision of the sealing of the stuffing boxes and, therefore, are of 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 made at the installation site from the pipes from which the pipeline is assembled. These compensators are suitable for all pressures, balanced and tight. Their disadvantages are significant dimensions.
12.1. One of the conditions for maintaining strength and reliable operation pipelines - full compensation of temperature deformations.Temperature deformations are compensated for by turns and bends of the pipeline route. If it is impossible to limit ourselves to self-compensation (for example, in completely straight sections of considerable length), U-shaped, lens or wavy expansion joints are installed on the pipelines.
12.2. It is not allowed to use stuffing box compensators on process pipelines transporting media of groups A and B.
12.3. When calculating the self-compensation of pipelines and the design dimensions of special compensating devices, the following literature can be recommended:
Designer's Handbook. Design of thermal networks. M.: Stroyizdat, 1965. 396 p.
Reference book on the design of power stations and networks. Section IX. Mechanical calculations of pipelines. M.: Teploelektroproekt, 1972. 56 p.
Wavy compensators, their calculation and application. M.: VNIIOENG, 1965. 32 p.
Guidelines for the design of fixed pipelines. Issue. II. Calculations of pipelines for strength taking into account compensation stresses, No. 27477-T. All-Union State Design Institute "Teploproekt", Leningrad branch, 1965. 116 p.
12.4. Thermal elongation of a pipeline section is determined by the formula:
where l- thermal elongation of the pipeline section, mm; - average coefficient of linear expansion, taken according to tab. eighteen depending on temperature; l- length of the pipeline section, m; t m - Maximum temperature environment, °С; t n- design temperature of the outside air of the coldest five-day period, °С; (for pipelines with negative ambient temperature t n- maximum ambient air temperature, °C; t m - minimum temperature environment, °С).
12.5. U-shaped compensators can be used for technological pipelines of all categories. They are made either bent from solid pipes, or using bent, sharply bent or welded bends; the outer diameter, steel grade of pipes and bends are taken the same as for straight sections of the pipeline.
12.6. For U-shaped compensators, bent bends should be used only from seamless pipes, and welded bends from seamless and welded pipes. Welded bends for the manufacture of U-shaped expansion joints are allowed in accordance with the instructions clause 10.12.
12.7. Use water pipes GOST 3262-75 for the manufacture of U-shaped expansion joints is not allowed, and electric welded with a spiral seam, specified in tab. 5, are only recommended for straight sections of expansion joints.
12.8. U-shaped expansion joints must be installed horizontally with the required overall slope. As an exception (if space is limited) they can be placed vertically with a loop up or down with an appropriate drain at the lowest point and air vents.
12.9. U-shaped compensators before installation must be installed on pipelines together with spacers, which are removed after fixing the pipelines to fixed supports.
12.10. Lens compensators, axial, manufactured according to OST 34-42-309-76 - OST 34-42-312-76 and OST 34-42-325-77 - OST 34-42-328-77, as well as articulated lens compensators, manufactured according to OST 34-42-313-76 - OST 34-42-316-76 and OST 34-42-329-77 - OST 34-42-332-77 are used for process pipelines transporting non-aggressive and low-aggressive media at pressure R at up to 1.6 MPa (16 kgf / cm 2), temperatures up to 350 ° C and a guaranteed number of repeating cycles not more than 3000. The compensating capacity of lens compensators is given in tab. nineteen.
12.11. When installing lens compensators on horizontal gas pipelines with condensing gases, condensate drainage must be provided for each lens. The branch pipe for the drainage pipe is made from a seamless pipe according to GOST 8732-78 or GOST 8734-75. When installing lens compensators with an internal sleeve on horizontal pipelines, guide supports must be provided on each side of the compensator.
12.12. To increase the compensating ability of expansion joints, their preliminary stretching (compression) is allowed. The value of preliminary stretching is indicated in the project, and in the absence of data, it can be taken equal to no more than 50% of the compensating capacity of the expansion joints.
12.13. Since the ambient air temperature during the installation period most often exceeds the lowest temperature of the pipeline, the pre-expansion of expansion joints must be reduced by popr, mm, which is determined by the formula:
Where - coefficient of linear expansion of the pipeline, taken according to tab. eighteen; L 0 - length of the pipeline section, m; t mont- temperature during installation, °С; t min - minimum temperature during operation of the pipeline, °C.
12.14. The limits for the use of lens compensators for operating pressure, depending on the temperature of the transported medium, are set according to GOST 356-80; the limits of their application according to cyclicity are given below:
| The total number of operation cycles of the compensator for the period of operation | Compensating ability of the lens with wall thickness, mm |
||
| 2,5 | 3,0 | 4,0 |
|
| 300 | 5,0 | 4,0 | 3,0 |
| 500 | 4,0 | 3,5 | 2,5 |
| 1000 | 4,0 | 3,5 | 2,5 |
| 2000 | 2,8 | 2,5 | 2,0 |
| 3000 | 2,8 | 2,2 | 1,6 |
12.15. When installing hinged compensators, the axis of the hinges must be perpendicular to the plane of the pipeline bend.
When welding joints of a hinged compensator, the maximum deviations from alignment should not exceed for the nominal diameter: up to 500 mm - 2 mm; from 500 to 1400 mm - 3 mm; from 1400 to 2200 mm - 4 mm.
The asymmetry of the hinge axes with respect to the vertical plane of symmetry (along the axis of the pipeline) should be no more than for the nominal diameter: up to 500 mm - 2 mm; from 500 to 1400 mm - 3 mm; from 1400 to 2200 mm - 5 mm.
12.16. The quality of lens compensators to be installed on process pipelines must be confirmed by passports or certificates.
12.17. Bellows axial expansion joints KO, angular KU, shear KS and universal KM in accordance with OST 26-02-2079-83 are used for process pipelines with conditional bore D y from 150 to 400 mm at pressure from residual 0.00067 MPa (5 mm Hg) to conditional R at 6.3 MPa (63 kgf / cm 2), at operating temperature from - 70 to + 700 °С.
12.18. The choice of the type of bellows compensator, the scheme of its installation and the conditions for its use must be agreed with the author of the project or with VNIIneftemash.
Variants of material execution of bellows expansion joints are given in tab. 20, and their technical specifications- in tab. 21 - 30.
12.19. Bellows expansion joints must be mounted in accordance with the installation and operating instructions included in the scope of delivery of the expansion joints.
12.20. In accordance with OST 26-02-2079-83 average term service life of bellows compensators before decommissioning - 10 years, average life before decommissioning - 1000 cycles for compensators KO-2 and KS-2 and 2000 - for compensators of other types.
The average life until the write-off of compensators KS-1 with vibration with an amplitude of 0.2 mm and a frequency not exceeding 50 Hz is 10,000 hours.
Note. The operation cycle of the compensator is understood as the “start-stop” of the pipeline for repair, survey, reconstruction, etc., as well as each fluctuation temperature regime operation of the pipeline, exceeding 30 °C.
12.21. At repair work in sections of pipelines with compensators, it is necessary to exclude: loads that lead to twisting of the compensators, ingress of sparks and splashes on the compensator bellows during welding, mechanical damage to the bellows.
12.22. When running 500 cycles for expansion joints KO-2 and KS-2 and 1000 cycles for bellows expansion joints of other types, it is necessary:
when operating in fire-explosive and toxic environments, replace them with new ones;
when operating in other media, the technical supervision of the enterprise to decide on the possibility of their further operation.
12.23. When installing a compensator, the following data is entered in the pipeline passport:
technical characteristics, manufacturer and year of manufacture of the compensator;
distance between fixed supports, necessary compensation, pre-stretching;
ambient air temperature during installation of the compensator and date.
Thermal elongation of pipelines at a coolant temperature of 50 ° C and above should be taken up 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 due to the use of turns in the route (self-compensation) can be used for all methods of laying heating networks, regardless of the diameters of the pipelines and the parameters of the coolant at an angle of up to 120 °. If the angle is more than 120°, and also in the case when, according to the strength calculation, the rotation of the pipelines cannot be used for self-compensation, the pipelines at the turning point are fixed with fixed supports.
To ensure the correct operation of compensators and self-compensation, pipelines are divided by fixed supports into sections that do not depend on each other in terms of thermal elongation. Each section of the pipeline, limited by two adjacent fixed supports, provides for the installation of a compensator or self-compensation.
When calculating pipes for thermal elongation compensation, 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, therefore it is widely used in practice. Natural compensation of temperature elongations is achieved at the turns and bends of the route due to the flexibility of the pipes themselves. Its advantages over other types of compensation are: simplicity of device, reliability, lack of need for supervision and maintenance, unloading of fixed supports from the forces of internal pressure. The natural compensation device does not require additional consumption of pipes and special building structures. The disadvantage of natural compensation is the transverse movement of the deformable sections of the pipeline.
Determine the total thermal elongation of the pipeline section
For trouble-free operation of heating networks, it is necessary that compensating devices are designed for maximum elongation of pipelines. Therefore, when calculating elongations, the temperature of the coolant is assumed to be maximum, and the ambient temperature - minimum. Total 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 is the distance between fixed supports, m;
t is the calculated temperature difference, taken as the difference between the operating temperature of the coolant and the calculated outdoor temperature for heating design.
l\u003d 1.23 * 10 -2 * 20 * 149 \u003d 36.65 mm.
l\u003d 1.23 * 10 -2 * 16 * 149 \u003d 29.32 mm.
l\u003d 1.23 * 10 -2 * 25 * 149 \u003d 45.81 mm.
Similarly, we find l for other areas.
The forces of elastic deformation arising in the pipeline when compensating for thermal elongation are determined by the formulas:
kgs; , N; Page 28 (35)
where E - the modulus of elasticity of pipe steel, kgf / cm 2;
I- moment of inertia of the cross section of the pipe wall, cm;
l- the length of the smaller and larger section of the pipeline, m;
t – calculated temperature difference, °C;
A, B are auxiliary dimensionless coefficients.
To simplify the determination of the elastic deformation force (P x, P v) table 8 gives an auxiliary value for various pipeline diameters.
Table 11
|
Outer pipe diameter d H , mm | |||||||||||||||
|
Pipe wall thickness s, mm | |||||||||||||||
During the operation of the heating network, stresses 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 compensators (gland, U- and S-shaped, and others) are used. U-shaped compensators have found wide application. To increase the compensating capacity of the U-shaped compensators and reduce the bending compensation stress in the working condition of the pipeline for sections of pipelines with flexible compensators, the pipeline is pre-stretched in a cold state during installation.
Pre-stretching is done:
at a coolant temperature 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.
Calculated thermal elongation of the pipeline
mm Page 37 (36)
where ε is a coefficient that takes into account the pre-stretching of expansion joints, possible inaccuracy in the 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 of the compensator H = 3.8 m, then the shoulder of the compensator B = 6 m.
To find the force of elastic deformation, we draw a horizontal line H \u003d 3.8 m, its intersection with B \u003d 5 (P k) will give a point, lowering the perpendicular from which to digital values \u200b\u200bP k, we get the result P k - 0.98 tf = 98 kgf = 9800 N.
Picture 3 - U-shaped compensator
7 plot x = 0.5 * 270 = 135 mm,
H \u003d 2.5, B \u003d 9.7, P k - 0.57 tf \u003d 57 kgf \u003d 5700 N.
The rest of the sections are calculated in the same way.