Heat pipelines are laid underground or above ground. The underground method is the main one in residential areas, since it does not clutter up the territory and does not worsen the architectural appearance of the city. The above-ground method is usually used in the territories of industrial enterprises during the joint laying of energy and technological pipelines. In residential areas, the above-ground method is used only in especially difficult conditions: permafrost and soils that subside during thawing, wetlands, a high density of existing underground structures, terrain heavily indented by ravines, crossing natural and artificial obstacles.
Underground heat pipelines are currently laid in through and non-through channels (previously used semi-through channels are no longer used) or in a channelless way. In addition, in residential areas, distribution networks are sometimes laid in technical undergrounds (corridors, tunnels) of buildings, which reduces the cost and simplifies construction and operation.
When laying in channels and technical undergrounds of buildings, heat pipelines are protected from all sides from mechanical influences and loads and, to some extent, from ground and surface waters. To perceive the own weight of the heat pipe, special movable supports are installed. With channelless laying, heat pipes are in direct contact with the ground and external mechanical loads are perceived by the pipe and the heat-insulating structure. At the same time, movable supports are not installed, and heat pipes are laid directly on the ground or a layer of sand and gravel. The cost of channelless laying is 25-30% less than in channels, however, the operating conditions of heat pipelines are more difficult.
The depth of the heat pipelines from the upper level of the channels or the insulating structure (with channelless laying) to the earth's surface is 0.5--0.7 m. At a high level of groundwater, it is artificially reduced by the device of associated drainage from gravel, sand and drainage pipes under the channel or insulating structure.
Channels are currently made, as a rule, from unified prefabricated reinforced concrete parts. To protect against ground and surface waters, the outer surface of the channels is covered with bitumen with pasting with waterproof roll material. To collect moisture that gets inside the channels, their bottom should be given a transverse slope of at least 0.002 in one direction, where trays are sometimes closed (by slabs, gratings), through which water flows into prefabricated pits, from where it is discharged into drains.
It should be noted that, despite the waterproofing of the channels, the natural moisture contained in the soil penetrates into them through their outer walls, evaporates and saturates the air. When moist air cools, moisture accumulates on the ceilings and walls of the channel, which flows down and can cause dampening of the insulation.
Passing channels provide the best conditions for the operation, operation and repair of heat pipelines, however, in terms of capital costs, they are the most expensive. In this regard, it is advisable to build them only in the most critical areas, as well as when laying heat pipelines together with other utilities. When laying various communications together, the passage channels are called collectors. In cities, they are now widely used. On fig. 6.4 shows a section of a typical single-section collector.
Passage channels (collectors) are equipped with natural or forced ventilation, ensuring the air temperature in the channel is not higher than 40 ° C during periods of repairs and not higher than 50 ° C during operation, electric lighting with a voltage of up to 30 V, telephone communication. To collect moisture at low points on the route, pits are arranged that communicate with drains or are equipped with automatic or remote-controlled pumping pumps.
Rice. 6.4. Cross section of a typical city collector
1 and 2 - supply and return pipelines; 3 - condensate pipeline; 4 - telephone cables; 5 - power cables; 6 - steam pipeline; 7 - plumbing
The overall dimensions of the passage channels (collectors) are selected from the condition of free access to all elements of the heat pipelines, which allows them to be completely overhauled without opening and destroying the road surfaces. The width of the passage in the channel is taken at least 700 mm, and the height is at least 2 m (it is allowed to take a height up to the beam of 1.8 m). Every 200-250 m along the route, hatches are made, equipped with ladders or brackets for descending into the canal. In places where a large amount of equipment is located, special widenings (chambers) can be arranged or pavilions can be built.
Impassable channels are usually used for heat pipes with a diameter of up to 500-700 mm. They are made of rectangular, vaulted and cylindrical shapes from reinforced concrete slabs and vaults, asbestos-cement and metal pipes, etc. At the same time, as a rule, an air gap is left between the surface of the heat pipes and the channel walls, through which the thermal insulation dries and moisture is removed from the channels. As an example, in fig. 6.5 shows a section of a rectangular impassable channel made from unified prefabricated reinforced concrete parts.
Rice. 6.5. Cross-sections of impassable channel
1 and 2 - tray blocks, respectively, lower and upper; 3 - connecting element with cement whitening; 4 - base plate; 5 - sand preparation
The overall dimensions of impassable channels are chosen mainly depending on the distance between the heat conductors and between the surfaces of the heat-insulating structure and channels, as well as on the condition of providing convenient access to the equipment in the chambers. To reduce the distance between the heat pipes, the equipment on them is sometimes installed apart.
Channelless laying is usually used for pipes of small diameters (up to 200-300 mm), since when laying such pipes in impassable channels, their working conditions turn out to be almost more difficult (due to the entry of the air gap in the channels with dirt and the difficulty of removing moisture from them at the same time ). In recent years, due to the increase in the reliability of channelless laying of heat pipelines (through the introduction of welding, more advanced heat-insulating structures, etc.), it is also being used for pipes of large diameters (500 mm or more).
Heat pipes laid in a channelless way are divided depending on the type of heat-insulating structure: in monolithic shells, cast (prefabricated cast) and backfill (Fig. 6.6) and depending on the nature of the perception of weight loads: unloaded and unloaded.
Rice. 6.6. Types of channelless heat pipes
a - in a combined and monolithic shell; b-cast and prefabricated cast; c - backfilling
Structures in monolithic shells are usually performed in the factory. On the route, only butt welding of individual elements and isolation of butt joints are carried out. Cast structures can be manufactured both in the factory and on the route by pouring pipelines (and butt joints after pressure testing) with liquid initial heat-insulating materials, followed by their setting (hardening). Backfill insulation is performed on pipelines mounted in trenches and compressed from loose heat-insulating materials.
Unloaded structures include structures in which the heat-insulating coating has sufficient mechanical strength and unloads pipelines from external loads (soil weight, weight of vehicles passing on the surface, etc.). These include cast (prefabricated cast) and monolithic shells.
In unloaded structures, external mechanical loads are transferred through thermal insulation directly to the pipeline. These include backfill heat pipes.
On underground heat pipelines, equipment that requires maintenance (gate valves, stuffing box expansion joints, drainage devices, vents, air vents, etc.) is placed in special chambers, and flexible expansion joints are placed in niches. Chambers and niches, like channels, are constructed from precast concrete elements. Structurally, the chambers are made underground or with above-ground pavilions. Underground chambers are arranged with pipelines of small diameters and the use of manually operated valves. Chambers with elevated pavilions provide better service for large equipment, in particular, valves with electric and hydraulic drives, which are usually installed with pipeline diameters of 500 mm or more. On fig. 6.8 shows the construction of an underground chamber.
The overall dimensions of the chambers are chosen from the condition of ensuring the convenience and safety of equipment maintenance. To enter the underground chambers, hatches are arranged diagonally in the corners - at least two with an internal area of \u200b\u200bup to 6 m 2 and at least four with a larger area. The diameter of the hatch is taken at least 0.63 m. Under each hatch, ladders or brackets are installed with a step of no more than 0.4 m for descending into the chambers. The bottom of the chambers is made with a slope of > 0.02 to one of the corners (under the hatch), where pits are arranged, covered from above with a grate, to collect water with a depth of at least 0.3 m and dimensions in terms of 0.4x0.4 m. Water from the pits is discharged by gravity or with the help of pumps into drains or receiving wells.
Rice. 6.8. underground chamber
Aboveground heat pipelines they are laid on free-standing supports (low and high) and masts, on flyovers with a continuous span in the form of trusses or beams and on rods attached to the tops of masts (cable structures). At industrial enterprises, sometimes simplified gaskets are used: on consoles (brackets) along building structures and stands (pillows) along building roofs.
Supports and masts are usually made of reinforced concrete or metal. Overpass spans and anchor posts (non-movable supports) are usually made of metal. At the same time, building structures can be built one-, two- and many-tiered.
The laying of heat pipelines on free-standing supports and masts is the simplest and is usually used with a small number of pipes (two to four). At present, standard designs of free-standing low and high reinforced concrete supports have been developed in the USSR, made with one column in the form of a T-shaped support and with two separate columns or frames in the form of U-shaped supports. To reduce the number of racks, large diameter pipelines can be used as load-bearing structures for laying or hanging small diameter pipelines to them, requiring more frequent installation of supports. When laying heat pipelines on low supports, the distance between their lower generatrix and the ground surface must be at least 0.35 m with a group of pipes up to 1.5 m wide and at least 0.5 m with a width of more than 1.5 m.
Laying heat pipelines on overpasses is the most expensive and requires the highest metal consumption. In this regard, it is advisable to use it with a large number of pipes (at least five or six), as well as, if necessary, regular supervision of them. In this case, pipelines of large diameters usually rest directly on racks of overpasses, and small ones - on supports laid in the superstructure.
Laying heat pipelines on suspended (cable-stayed) structures is the most economical, as it allows you to significantly increase the distance between the masts and thereby reduce the consumption of building materials. When jointly laying pipelines of various diameters between the masts, runs are made from channels suspended on rods. Such runs allow you to install additional supports for pipelines of small diameters.
To service equipment (gate valves, stuffing box compensators), platforms with fences and ladders are arranged: stationary at a distance from the bottom of the heat-insulating structure to the ground surface of 2.5 m or more, or mobile - at a shorter distance, and in hard-to-reach places and on flyovers - through bridges. When laying heat pipelines on low supports at the equipment installation sites, the ground surface should be covered with concrete, and the equipment should be covered with metal casings.
Pipes and fittings. For the construction of heating networks, steel pipes are used, connected by electric or gas welding. Steel pipes are exposed to internal and external corrosion, which reduces the service life and reliability of heating networks. In this regard, for local hot water systems that are subject to increased corrosion, galvanized steel pipes are used. In the near future, the use of enameled pipes is planned.
From steel pipes for heating networks, currently, mainly electric-welded pipes with a longitudinal straight and spiral seam and seamless, hot-formed and cold-formed, made from steel grades St. 3, 4, 5, 10, 20 and low alloy. Electric-welded pipes are produced up to a nominal diameter of 1400 mm, seamless - 400 mm. For hot water supply networks, water and gas steel pipes can also be used.
In recent years, work has been carried out on the use of non-metallic pipes for heat supply (asbestos-cement; polymer, glass, etc.). Their advantages include high corrosion resistance, and for polymer and glass pipes and lower roughness compared to steel pipes. Asbestos-cement and glass pipes are connected using special structures, and polymer pipes are welded, which greatly simplifies installation and increases the reliability and tightness of the joints. The main disadvantage of these non-metallic pipes are the low allowable temperatures and pressures of the coolant, approximately 100°C and 0.6 MPa. In this regard, they can only be used in networks operating with low water parameters, for example, in hot water systems, condensate pipelines, etc.
The valves used in heating networks are divided into shut-off, control, safety (protective), throttling, condensate drain and control and measuring valves.
Shut-off valves are usually referred to the main general-purpose fittings, since they are most widely used directly on the route of heating networks. Other types of fittings are installed, as a rule, in heating points, pumping and throttling substations, etc.
The main types of shut-off valves for heating networks are gate valves and valves. Gate valves are usually used in water networks, valves - in steam. They are made of steel and cast iron with flanged and coupling ends, as well as ends for welding pipes of various nominal diameters.
Shut-off valves in heat networks are installed on all pipelines extending from the heat source, at branch nodes with d y > 100 mm, at branch nodes to individual buildings with d y 50 mm and branch length l> 30 m or to a group of buildings with a total load of up to 600 kW (0.5 Gcal/h), as well as fittings for draining water, venting air and starting drains. In addition, sectional valves are installed in water networks: with d y > 100 mm through l ce kts<1000 м; при d y =350...500 мм через l секц <1500 м при условии спуска воды из секции и ее заполнения водой не более чем за 4 ч, и при d y >600 mm through l c ekts<3000 м при условии спуска воды из секции и ее заполнения водой не более чем за 5 ч.
In the places of installation of sectional valves, jumpers are made between the supply and return pipelines with a diameter equal to 0.3 of the diameter of the main pipelines to create circulation of the coolant in case of accidents. On the jumper, two valves are installed in series and a control valve between them at d y \u003d 25 mm to check the tightness of the valves closing.
To facilitate the opening of valves with d y > 350 mm on water networks and with d y > 200 mm and r y > 1.6 MPa on steam networks that require high torque, bypass lines (unloading bypasses) are made with a shut-off valve. In this case, the valve is relieved from pressure forces when the valves are opened and the sealing surfaces are protected from wear. In steam networks, bypass lines are also used to start steam pipelines. Gate valves with d y > 500 mm, requiring more than 500 Nm of torque to open or close, must be used with an electric drive. With electric drive, all gate valves are also provided for remote control.
Pipes and fittings are selected from the manufactured assortment depending on the conditional pressure, operating (calculated) parameters of the coolant and the environment.
Conditional pressure determines the maximum allowable pressure that pipes and fittings of a certain type can withstand for a long time at a normal ambient temperature of + 20 ° C. As the medium temperature rises, the allowable pressure decreases.
The operating pressures and temperatures of the coolant for the selection of pipes, fittings and equipment for heating networks, as well as for calculating pipelines for strength and when determining loads on building structures, should be taken equal, as a rule, to the nominal (maximum) values in the supply pipelines or on the discharge of pumps, taking into account terrain. The values of operating parameters for various cases, as well as restrictions on the choice of materials for pipes and fittings, depending on the operating parameters of the coolant and the environment, are indicated in SNiP II-36-73.
Pipelines thermal networks can be laid on the ground, in the ground and above the ground. With any method of pipeline installation, it is necessary to ensure the greatest reliability of the heat supply system at the lowest capital and operating costs.
Capital expenditures are determined by the cost of construction and installation works and the cost of equipment and materials for laying the pipeline. AT operational include the costs of servicing and maintaining pipelines, as well as the costs associated with heat loss in pipelines and electricity consumption throughout the route. Capital costs are determined mainly by the cost of equipment and materials, while operating costs are determined by the cost of heat, electricity and repairs.
The main types of pipeline laying are underground and elevated. Underground piping is the most common. It is subdivided into laying pipelines directly in the ground (channelless) and in channels. When laying on the ground, pipelines can be on the ground or above the ground at such a level that they do not interfere with the movement of vehicles. Above-ground laying is used on suburban highways when crossing ravines, rivers, railways and other structures.
Above ground laying pipelines in channels or trays located on the surface of the earth or partially buried, are used, as a rule, in areas with permafrost soils.
The method of installing pipelines depends on the local conditions of the facility - purpose, aesthetic requirements, the presence of complex intersections with structures and communications, soil category - and should be taken on the basis of feasibility studies of possible options. Minimum capital costs are required for the installation of a heating main using underground pipe laying without insulation and channels. But significant losses of thermal energy, especially in wet soils, lead to significant additional costs and premature failure of pipelines. In order to ensure the reliability of the heat pipelines, it is necessary to apply their mechanical and thermal protection.
Mechanical protection pipes when installing pipes underground can be provided by arranging channels, and thermal protection can be confused with the use of thermal insulation applied directly to the outer surface of pipelines. Insulation of pipes and their laying in channels increase the initial cost of the heating main, but quickly pay off during operation by increasing operational reliability and reducing heat losses.
Underground laying of pipelines.
When installing pipelines of heating networks underground, two methods can be used:
- Direct laying of pipes in the ground (channelless).
- Pipe laying in channels (channel).
Laying pipelines in channels.
In order to protect the heat conduit from external influences, and to ensure free thermal elongation of the pipes, channels are intended. Depending on the number of heat pipes laid in one direction, impassable, semi-through or through channels are used.
To fix the pipeline, as well as to ensure free movement during temperature elongations, the pipes are laid on supports. To ensure the outflow of water, the trays are laid with a slope of at least 0.002. Water from the lower points of the trays is removed by gravity into the drainage system or from special pits with the help of a pump it is pumped into the sewer.
In addition to the longitudinal slope of the trays, the floors should also have a transverse slope of the order of 1-2% to remove flood and atmospheric moisture. At a high level of groundwater, the outer surface of the walls, ceiling and bottom of the channel is covered with waterproofing.
The depth of laying the trays is taken from the condition of a minimum amount of excavation and a uniform distribution of concentrated loads on the floor during the movement of vehicles. The soil layer above the channel should be about 0.8-1.2 m and not less. 0.6 m in places where vehicular traffic is prohibited.
impassable channels are used for a large number of pipes of small diameter, as well as a two-pipe gasket for all diameters. Their design depends on soil moisture. In dry soils, block channels with concrete or brick walls or reinforced concrete single or multi-cell channels are most widely used.
The channel walls can have a thickness of 1/2 brick (120 mm) for small diameter pipelines and 1 brick (250 mm) for large diameter pipelines.
The walls are erected only from ordinary brick of a grade of at least 75. Silicate brick is not recommended for use due to its low frost resistance. The channels are covered with a reinforced concrete slab. Brick channels, depending on the category of soil, have several varieties. In dense and dry soils, the bottom of the channel does not require concrete preparation, it is enough to compact the crushed stone directly into the soil. In weak soils, an additional reinforced concrete slab is laid on a concrete base. With a high level of standing groundwater, drainage is provided for their removal. The walls are erected after installation and insulation of pipelines.
For pipelines of large diameters, channels are used, assembled from standard reinforced concrete elements of the KL and KLs tray type, as well as from prefabricated reinforced concrete slabs KS.
Channels of the KL type consist of standard tray elements covered with flat reinforced concrete slabs.
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Channels of the KLS type consist of two tray elements stacked on top of each other and connected on a cement mortar using an I-beam.
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In channels of the KS type, wall panels are installed in the grooves of the bottom plate and poured with concrete. These channels are covered with flat reinforced concrete slabs.
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The bases of channels of all types are made of concrete slabs or sand preparation, depending on the type of soil.
Along with the channels discussed above, other types of them are also used.
Vaulted channels consist of reinforced concrete vaults or semicircular shells that cover the pipeline. At the bottom of the trench, only the base of the channel is made.
For pipelines of large diameter, a vaulted two-cell channel with a dividing wall is used, while the arch of the channel is formed from two semi-arches.
When installing an impassable channel intended for laying in wet and soft soils, the walls and bottom of the channel are made in the form of a reinforced concrete trough-shaped tray, and the ceiling consists of precast concrete slabs. The outer surface of the tray (walls and bottom) is covered with waterproofing from two layers of roofing material on bituminous mastic, the base surface is also covered with waterproofing, then the tray is installed or concreted. Before backfilling the trench, the waterproofing is protected by a special wall made of bricks.
Replacement of pipes that have failed, or repair of thermal insulation in such channels is possible only during the development of groups, and sometimes the dismantling of the pavement. Therefore, the heating network in impassable channels is routed along lawns or in the territory of green spaces.
semi-through channels. In difficult conditions for the intersection of existing underground devices by heat pipelines (under the roadway, with a high level of standing groundwater), semi-passage channels are arranged instead of impassable ones. Semi-through channels are also used with a small number of pipes in those places where, according to the operating conditions, opening the carriageway is excluded. The height of the semi-through channel is taken equal to 1400 mm. Channels are made of precast concrete elements. The designs of semi-through and through channels are almost the same.
through channels used in the presence of a large number of pipes. They are laid under pavements of large highways, in the territories of large industrial enterprises, in areas adjacent to the buildings of thermal power plants. Along with heat pipelines, other underground communications are also located in the passage channels - electrical cables, telephone cables, water supply, gas pipelines, etc. The collectors provide free access for maintenance personnel to pipelines for inspection and elimination of an accident.
Passage channels must have natural ventilation with three air exchanges, providing an air temperature of no more than 40 ° C, and lighting. Entrances to the passage channels are arranged every 200 - 300 m. In places where stuffing box expansion joints are located, designed to perceive thermal elongations, locking devices and other equipment, special niches and additional hatches are arranged. The height of the passage channels must be at least 1800 mm.
Their structures are of three types − from ribbed slabs, from links of a frame structure and from blocks.
Grommets made of ribbed plates, are made of four reinforced concrete panels: the bottom, two walls and the floor slab, manufactured in the factory on rolling mills. The panels are connected with bolts, and the outer surface of the channel overlap is covered with insulation. Sections of the channel are installed on a concrete slab. The weight of one section of such a channel with a cross section of 1.46x1.87 m and a length of 3.2 m is 5 tons, the entrances are arranged every 50 m.
Passage channel from reinforced concrete links of a frame structure, covered with insulation on top. Channel elements have a length of 1.8 and 2.4 m and are of normal and increased strength with a depth of up to 2 and 4 m above the ceiling, respectively. A reinforced concrete slab is placed only under the joints of the links.
The next view is collector made of reinforced concrete blocks three types: L-shaped wall, two floor slabs and a bottom. Blocks at the joints are connected by monolithic reinforced concrete. These collectors are also made in normal and reinforced.
Channelless laying.
With channelless laying, the protection of pipelines from mechanical influences is performed by reinforced thermal insulation - a shell.
Virtues channelless laying of pipelines are: a relatively low cost of construction and installation work, a decrease in the volume of earthworks and a reduction in construction time. To her shortcomings include: the complication of repair work and the difficulty of moving pipelines clamped by the ground. Channelless laying of pipelines is widely used in dry sandy soils. It finds application in wet soils, but with a mandatory device in the area where the drainage pipes are located.
Movable supports are not used for channelless laying of pipelines. Pipes with thermal insulation are laid directly on a sand cushion located on a previously leveled bottom of the trench. The sand cushion, which is a bed for pipes, has the best elastic properties and allows the greatest uniformity of temperature movements. In weak and clayey soils, the sand layer at the bottom of the trench should be at least 100-150 mm thick. Fixed supports for channelless pipe laying are reinforced concrete walls installed perpendicular to the heat pipes.
Compensation for thermal movements of pipes in any way of their channelless laying is provided with the help of bent or stuffing box compensators installed in special niches or chambers.
At the turns of the route, in order to avoid clamping the pipes in the ground and to ensure possible movements, impassable channels are arranged. As a result of uneven settlement of the soil and the base of the channel, the greatest bending of the pipelines occurs at the intersections of the drip wall with the pipeline. To avoid pipe bending, it is necessary to leave a gap in the wall hole, filling it with elastic material (for example, asbestos cord). The thermal insulation of the pipe includes an insulating layer of autoclaved concrete with a volume weight of 400 kg/m3, having steel reinforcement, a waterproofing coating consisting of three layers of brizol on bitumen-rubber mastic, which includes 5-7% rubber crumb and a protective layer , made of asbestos-cement plaster on a steel mesh.
The return lines of the pipelines are insulated in the same way as the supply lines. However, the presence of insulation of the return lines depends on the diameter of the pipes. With a pipe diameter of up to 300 mm, an insulation device is mandatory; with a pipe diameter of 300-500 mm, the insulation device must be determined by the technique of economic calculation based on local conditions; with a pipe diameter of 500 mm or more, the insulation device is not provided. Pipelines with such insulation are laid directly on the leveled compacted soil of the trench base.
To lower the groundwater level, special drainage pipelines are provided, which are laid at a depth of 400 mm from the bottom of the channel. Depending on the operating conditions, drainage devices can be made of various pipes: ceramic concrete and asbestos-cement pipes are used for non-pressure drainage, and steel and cast iron pipes are used for pressure ones.
Drainage pipes are laid with a slope of 0.002-0.003. At bends and at differences in pipe levels, special manholes are arranged like sewer wells.
Above ground piping.
Based on the ease of installation and maintenance, laying pipes above the ground is more profitable than laying underground. It also requires less material costs. However, this spoils the appearance of the environment and therefore this type of pipe laying cannot be applied everywhere.
load-bearing structures for above-ground laying of pipelines serve: for small and medium diameters - above-ground supports and masts, ensuring the location of pipes at the right distance from the surface; for pipelines of large diameters, as a rule, trestle supports. Supports are usually made of reinforced concrete blocks. Masts and flyovers can be either steel or reinforced concrete. The distance between the supports and masts during above-ground laying should be equal to the distance between the supports in the channels and depends on the diameters of the pipelines. In order to reduce the number of masts, intermediate supports are arranged with braces.
When laying above ground, thermal elongations of pipelines are compensated with the help of bent compensators, which require minimal maintenance time. Maintenance of fittings is carried out from specially arranged sites. Roller bearings should be used as movable bearings, creating minimal horizontal forces.
Also, when laying pipelines above ground, low supports can be used, which can be made of metal or low concrete blocks. At the intersection of such a route with footpaths, special bridges are installed. And at the intersection with highways, either a compensator of the required height is made or a channel is laid under the road for the passage of pipes.
The method of laying heat networks during reconstruction is chosen in accordance with the instructions of SNiP 2.04.07-86 "Heat networks". Currently, in our country, about 84% of heating networks are laid in channels, about 6% - without channels, the remaining 10% - above ground. The choice of one or another method is determined by local conditions, such as the nature of the soil, the presence and level of groundwater, the required reliability, the economy of construction, as well as the operating costs of maintenance. Ways of laying are divided into above-ground and underground.
Above-ground laying of heating networks
The above-ground laying of heating networks is rarely used, since it violates the architectural ensemble of the area, has, other things being equal, higher heat losses compared to the underground laying, does not guarantee against freezing of the coolant in case of malfunctions and accidents, and hampers driveways. When reconstructing networks, it is recommended to use it at a high level of groundwater, in permafrost conditions, with unfavorable terrain, on the territories of industrial enterprises, on sites free from buildings, outside the city or in places where it does not affect the architectural design and does not hinders traffic.
Advantages of above-ground laying: accessibility of inspection and ease of use; the ability to detect and eliminate an accident in heat pipelines as soon as possible; lack of electrocorrosion from stray currents and corrosion from aggressive groundwater; lower cost of construction compared to the cost of underground laying of heating networks. Above-ground laying of heating networks is carried out: on separate supports (masts); on flyovers with a span structure in the form of girders, trusses or suspended (cable-stayed) structures; along the walls of buildings. Freestanding masts or poles can be made of steel or reinforced concrete. With small volumes of construction of above-ground heating networks, steel masts made of profile steel are used, but they are expensive and labor-intensive and therefore are being replaced by reinforced concrete ones. It is especially advisable to use reinforced concrete masts in mass construction at industrial sites, when it is cost-effective to organize their manufacture in the factory.
For the joint laying of heating networks with other pipelines for various purposes, flyovers made of metal or reinforced concrete are used. Depending on the number of pipelines being laid at the same time, span structures of overpasses can be single-tier and multi-tier. Heat pipelines are usually laid on the lower tier of the overpass, while pipelines with a higher coolant temperature are placed closer to the edge, thereby providing a better location for U-shaped expansion joints of various sizes. When laying heating mains on the territory of industrial enterprises, the method of above-ground laying on brackets fixed in the walls of buildings is also used. Span of heat pipelines, i.e. the distances between the brackets are chosen taking into account the bearing capacity of the building structures.
Underground laying of heating networks
In cities and towns, for heating mains, underground laying is mainly used, which does not spoil the architectural appearance, does not interfere with traffic and reduces heat loss through the use of the heat-shielding properties of the soil. Soil freezing is not dangerous for heat pipelines, so they can be laid in the zone of seasonal soil freezing. The smaller the depth of the heating network, the smaller the volume of earthworks and the lower the cost of construction. Underground networks are most often laid at a depth of 0.5 to 2 m and below the surface of the earth.
The disadvantages of underground laying of heat pipelines are: the danger of moisture and destruction of insulation due to exposure to ground or surface water, which leads to a sharp increase in heat losses, as well as the danger of external corrosion of pipes due to the action of stray electric currents, moisture and aggressive substances contained in the soil. Underground laying of heat pipelines is associated with the need to open streets, driveways and yards.
Structurally, underground heating networks are divided into two fundamentally different types: channel and channelless.
The design of the channel completely unloads the heat pipelines from the mechanical impact of the soil mass and temporary transport loads and protects the pipelines and thermal insulation from the corrosive effect of the soil. Laying in channels ensures free movement of pipelines under temperature deformations both in the longitudinal (axial) and transverse directions, which allows using their self-compensating ability at the corner sections of the route.
Laying in passage channels (tunnels) is the most advanced method, since it provides constant access for maintenance personnel to pipelines to monitor their operation and carry out repairs, which in the best way ensures their reliability and durability. However, the cost of laying in through channels is very high, and the channels themselves have large dimensions (clear height - at least 1.8 m and passage - 0.7 m). Through channels are usually arranged when laying a large number of pipes laid in one direction, for example, at outlets from a thermal power plant.
Along with laying in impassable channels, channelless laying of heat pipelines is gaining more and more development. Refusal to use channels when laying heating networks is very promising and is one of the ways to reduce their cost. However, in channelless laying, the heat-insulated pipeline, due to direct contact with the soil, is under conditions of more active physical and mechanical influences (soil moisture, soil pressure and external loads, etc.) than in channel laying. Channelless laying is possible when using a mechanically strong thermal and waterproofing shell that can protect pipelines from heat loss and withstand loads transmitted by soil. Heating networks with pipe diameters up to 400 mm inclusive are recommended to be laid mainly in a channelless way.
Among channelless gaskets, the most widespread in recent years have been progressive gaskets using reinforced foam concrete, bitumen perlite, expanded clay asphalt concrete, phenolic foam plastic, foam polymer concrete, polyurethane foam and other thermal insulation materials as monolithic thermal insulation. Channelless laying of thermal networks continue to improve and are becoming more widespread in the practice of construction and reconstruction. During the reconstruction of intra-quarter heating mains, there are more opportunities for laying networks through basements than with new construction, since the construction of new sections often outstrips the construction of buildings.
Installation of heating networks, pipe laying
Installation of pipelines and installation of thermal insulation on them is carried out using pre-insulated PPU pipes, fittings in PPU insulation (fixed supports, tees and tee branches, transitions, end elements and intermediate elements, etc.), as well as PPU shells. Thermal insulation of straight sections, branches, pipeline elements, sliding supports, ball valves are being installed, as well as butt joints are being installed using a heat-shrink sleeve, heat-shrink tape, PPU components, galvanized casings and heat-insulating shells made of polyurethane foam.
The laying of heating networks and the installation of PPU thermal insulation is carried out in several stages - the preparatory stage (earthworks, delivery of PPU pipes and elements to the route, inspection of products), laying of pipelines (installation of pipes and elements), installation of UEC system devices and installation of butt joints.
The laying depth of PPU pipes when laying heating networks should be carried out taking into account the difference in density between the PPU steel pipe and the heat-insulating layer of polyurethane foam, as well as heat transfer rates and normatively permissible heat losses.
The development of trenches for channelless laying should be carried out mechanically in compliance with the requirements of SNiP 3.02.01 - 87 "Earthworks".
The minimum depth of laying PPU pipes in a polyethylene sheath when laying heating mains in the ground should be taken at least 0.5 m outside the carriageway and 0.7 m within the carriageway, counting to the top of the thermal insulation.
The maximum laying depth of heat-insulated pipes during installation of pipelines in polyurethane foam insulation when laying heat networks should be determined by calculation, taking into account the stability of the polyurethane foam layer to the action of a static load.
PPU pipes are usually installed at the bottom of the trench. It is allowed to weld straight sections in the section on the edge of the trench. Installation of PPU pipes in a polyethylene sheath is carried out at an outdoor temperature of up to -15 ... -18 ° С.
Cutting of steel pipes (if necessary) is carried out with a gas cutter, while the thermal insulation is removed with a mechanized hand tool in a section 300 mm long, and the ends of the thermal insulation during cutting of steel pipes are covered with a moistened cloth or a hard screen to protect the thermal insulation layer of polyurethane foam.
Welding of pipe joints and control of welded joints of pipelines during the installation of PPU pipes should be carried out in accordance with the requirements of SNiP 3.05.03-85 "Heat Networks", VSN 29-95 and VSN 11-94.
When performing welding work, it is necessary to protect the polyurethane foam insulation and polyethylene sheath, as well as the ends of the wires emerging from the insulation, from sparks.
When using a heat-shrink sleeve as a protection for a welded joint, it is put on the pipeline before the start of welding. When sealing a joint using a pouring joint or a joint from a PPU shell, where a galvanized casing and heat-shrinkable tape are used as a protective layer, pipes are welded regardless of the availability of materials for sealing joints.
Before starting the construction of a heating main with channelless pipe laying, PPU pipes, fittings in PPU insulation, ball valves thermally insulated with polyurethane foam and elements of the pipeline system are subjected to a thorough inspection in order to detect cracks, chips, deep cuts, punctures and other mechanical damage to the polyethylene sheath of thermal insulation. If cracks, deep cuts and other damages are found in the coating of PPU pipes in a polyethylene or galvanized sheath, they are repaired by extrusion welding, by applying heat-shrinkable cuffs (couplings) or galvanized bandages.
Before installation of the heating main of channelless laying, pipelines in PPU insulation and fittings in PPU are laid out on the crest or the bottom of the trench using a crane or pipelayer, soft "towels" or flexible slings.
The lowering of the insulated PPU pipes into the trench should be carried out smoothly, without jerks and impacts on the walls and bottom of the channels and trenches. Before installing PPU pipes in trenches or channels, it is imperative to check the integrity of the signal wires of the operational-remote control system (SOODK system) and their isolation from the steel pipe.
PPU pipes laid on a sandy base during channelless laying, in order to prevent damage to the shell, should not be supported by stones, bricks and other solid inclusions that should be removed, and the resulting depressions should be covered with sand.
If it is necessary to perform control calculations of the laying depths of heat pipelines with polyurethane foam insulation in a polyethylene sheath for specific laying conditions, the design resistance of polyurethane foam should be taken as 0.1 MPa, polyethylene sheath - 1.6 MPa.
If it is necessary to lay underground heating networks with PPU thermal insulation in a polyethylene sheath at a depth more than permissible, they should be laid in channels (tunnels). When laying routes under the roadway, railway tracks and other objects located above the PPU pipe, pipes in PPU insulation are made with reinforcement (polyethylene overlays along the entire length of the shell) and are laid in a steel case that protects against external mechanical influences.
§ 2. Methods of underground, ground and above-ground laying and their technical and economic indicators
The arrangement of sanitary and technical communications in areas of permafrost can cause soil thawing due to the release of heat by pipelines. As a result, the stability of both the pipelines themselves and buildings may be impaired. Methods for laying sanitary and technical communications should be linked to the construction methods of buildings and structures and depend on the properties of the foundation soils and other factors, the most important of which is the location of the network route in relation to the built-up area and its architectural and planning solution.
There are the following types of laying of sanitary communications: underground, ground and above ground. These types of gaskets, in turn, can be single and combined.
Ground and overground laying due to the absence of contact between pipes and the ground and limited heat release into the soil, the bases disturb the natural thermal regime of permafrost soils to the least extent. Such gaskets clutter up the territory of populated areas, make it difficult to arrange driveways, organize snow protection and snow removal.
underground laying it is advisable to carry out within the boundaries of the development of the settlement in order to achieve maximum improvement of the territory. Water and sewer networks can be laid directly in the ground, while heating networks and steam pipelines can be laid in special channels. In the presence of such channels, it is advisable to lay water supply, sewerage and electrical cables in them.
Underground laying of heating networks is very expensive and requires special measures to preserve the thermal regime of permafrost soils at the base of the networks. So, for example, the cost 1 line m channel for heating in Norilsk is on average 300 rubles. The cost of a two-tier channel for the combined installation of a heating network, water supply, sewerage and electrical cables under the same conditions averages about 450 rubles. per 1 line m. Therefore, the underground laying of heating networks is advisable only for compact development of multi-storey (4-5 floors) buildings and in conjunction with other communications.
If the development is carried out by two- and three-story buildings with gaps, then the underground laying of heating networks is usually not economically feasible. In such cases, above-ground laying is most often used along the facades and attics of buildings, and between buildings - along flyovers, fences and fences. At the same time, water supply and sewerage can be laid in the ground without channels. If the soils of the base of the pipes are subsiding, then in order to ensure their stability, it is necessary to replace the soils with non-subsiding ones to a depth determined by the heat engineering calculation.
For small settlements, if it is possible to trace the network within blocks without crossing streets or with a minimum number of intersections, it is most economical to lay heating networks on the ground in ring insulation or in insulated boxes together with water supply. Sewerage should be laid in the ground channelless.
In subsiding soils during thawing, especially in those that turn into a fluid-plastic or fluid state during thawing, an artificial foundation is required when laying pipelines underground. The cost of such a foundation is directly dependent on the depth of thawing of the soil under the pipes.
When laying pipelines in soils that do not subside and do not lose their bearing capacity during thawing, the decisive condition is to protect them from freezing by reducing heat losses. In this case, the depth of laying is increased to 1.5-2.0 m; a large depth is undesirable, since it is difficult to detect pipeline accident sites and repair them both in summer and especially in winter.
In order to reduce heat loss and the size of taliks under pipes, underground laying of water supply and sewerage in thermal insulation is used: in boxes made of wood or reinforced concrete with sawdust or mineral wool backfill, in annular - made of foam concrete, mineral wool, felt impregnated with resin. All these types of thermal insulation do not achieve the goal when wetting the insulation material. Local malfunctions of waterproofing (and therefore thermal insulation) lead to thawing of the base and uneven precipitation of pipelines, the most undesirable. Restoration of heat and waterproofing during repairs is a complex and time-consuming process. The use of boxes creates additional difficulties in detecting and eliminating leaks. Any leakage entails a violation of thermal insulation. The cost of thermal insulation usually exceeds the cost of an artificial foundation for water supply and sewerage. Therefore, the widespread use of thermal insulation for water and sewer pipelines when laying them in the ground is impractical.
Consider some designs of pipeline foundations laid in the ground.
Ground base(Fig. IV-1). Ice-saturated local soils at the base of the heat-generating pipeline to the estimated thawing depth are replaced by non-sagging soils with a low filtration coefficient. Sandy, gravel-sandy soils in some cases are compacted by preliminary thawing. For replacement, light sandy loams and fine-grained silty sands in the thawed state are used; at the same time, an admixture of pebbles, gravel, crushed stone up to 40 ..... -45% or local dehydrated and compacted soil is desirable. A waterproofing layer of clay concrete or clay is laid under the pipe on an artificial soil base. 25-30 cm.
The width of the artificial base is taken equal to the width of the trench, and the height is determined by calculation.
In the absence of leakage, the radius of thawing from heat generation from water or sewer pipelines does not, on average, usually exceed 1.2 m. If we take into account the increased intensity of soil thawing, which replaces ice-saturated soils, then the replacement depth will not exceed 1.5 m. It must be assumed that in many cases a soil foundation will be economically viable and technically feasible.
Leg base is used to reduce the unevenness of subsidence during thawing of subsiding soils and is carried out in the form of longitudinal beds in two logs. To prevent warping of beds during subsidence, as a result of which the pipeline is destroyed, their reliable fastening is necessary.
floating base used in ice-saturated soils and is a continuous flooring of plates laid across the trench; this type of foundation is quite reliable, but cannot be widely recommended due to the high cost and consumption of a large amount of timber.
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Rice. IV-2. Pipeline on a pile foundation. 1 - pipeline; 2 - log (beam) ∅30 cm on dowels (joints apart); 3 - piles ∅30 cm through 3m with recess for 3m below the active layer; 4 - gaskets through 10 cm; 5 - backfilling with local soil
pile foundation(fig. IV-2) is applied in strongly subsidence soils. Driving piles into permafrost requires time-consuming and expensive work to steam the soil or drill wells. Piles often have to be placed, because in pipes bearing a large load from the soil, significant bending moments occur on the supports. Such bases are characterized by high cost.
underground overpasses(Fig. IV-3) due to the high cost, they are used in exceptional cases, for example, for sewerage with subsiding soils that thaw to a great depth, when passing the route near a building with large heat emissions, built according to methods I or IV and located higher in relief.
The question of the use of one or another type of foundation is decided by comparing technical and economic indicators.
To eliminate the possibility of intensive movement of the flow of supra-permafrost waters along underground pipelines, clay-concrete bridges are used across the trenches. The lintels cut into the frozen base and trench walls on 0.6-1.0 m. The distance between the jumpers is assigned depending on the longitudinal slope in such a way that the pressure at the jumper does not exceed 0.4-0.5 m; Usually this distance ranges from 50 to 200 m.
In pebbly, gravel and other well-filtering soils, the installation of jumpers is not advisable, since the flow of supra-permafrost waters easily bypasses them.
Laying in earth embankments
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Rice. IV-4. Laying pipes in earthen ridges. 1 - pipeline; 2 - a layer of clay concrete with a thickness 20 cm; 3 - local soil; 4 - sand and gravel layer; 5 - local dehydrated and compacted soil
This method of laying (Fig. IV-4) is used under fairly favorable permafrost and ground conditions, in the absence of heat-insulating materials on site, and the pipeline route must pass through an undeveloped area. This type of gasket has several advantages:
- it is not required to carry out labor-intensive earthworks for digging trenches;
- pipe leaks are easier to detect and repair;
- filtration of supra-permafrost waters along pipes is excluded;
- the presence of a talik around the pipes allows longer interruptions in the movement of water through them than with ground and above-ground laying;
- there is no need for heat and waterproofing of pipes.
The main disadvantages of this method are the excessive clutter of the territory and the complexity of the arrangement of crossings. In addition, this creates conditions for greater snow coverage of the territory.
Underground laying of pipelines in channels
Laying pipelines in underground channels is a relatively expensive type of network construction; nevertheless, in some cases, channel laying is expedient, given not only one-time capital investments, but also operating costs. The feasibility of combined laying of communications in underground channels in comparison with a single underground one should be confirmed by the cost of construction, attributed to 1 m 2 living space, and reliability in the operation of engineering networks. Combined laying is usually justified in adverse climatic and permafrost-soil conditions.
Channels can be through (semi-through) and non-through, single-tier and two-tier. In two-tier channels, the lower tier of which is a passage, the upper tier can be either semi-passage or impassable. The design of the channel with a semi-passage upper tier is cumbersome and high cost. The single-tier design of channels is the most economical and convenient in operation.
In the case of the installation of different types of channels in a populated area (which must be justified), it should be, based on the conditions of industrialization of construction, to achieve the minimum number of standard sizes of elements.
Impassable up to 0.9 m channels (Fig. IV-5) can be used in short sections (house outlets and inputs, intersections with roads, etc.) while ensuring stability conditions and operating requirements. Impassable channels should be arranged with a minimum penetration into the ground (no more than 0.5-0.7 m from floor to ground level). They must have a removable cover for cleaning channels, inspection and repair of pipelines. The longitudinal slope of impassable channels to ensure the drainage of water along the bottom must be at least 0.007.
Passage channels with a height of at least 1.8 m(Fig. IV-6) must have dimensions that provide free passage through them for inspection and repair of pipes, fittings and electrical cables.
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Rice. IV-7. Reinforced concrete two-tier passage channel. 1 - sewerage; 2 - heating system: 3 - water supply; 4 - shelves for electrical cables and communication cables; 5 - sand, δ = 10 cm; 6 - clay concrete, δ = 20 cm; 7 - replaced soil (calculated thickness)
With significant deepening of the channels and high heat generation of communications, the taliks formed under the channels can reach significant sizes. In such cases, in order to reduce the penetration of heat into the base, on the basis of a technical and economic comparison with other options, the expediency of installing two-tier channels is revealed (Fig. IV-7). In the lower passage tier of such a channel, a sewer pipeline and electrical cables are placed, in the upper - impassable or semi-passage - pipes of the heating system and water supply are laid.
When laying sewer and water pipes together, water valves must be placed in special chambers or sections isolated from the sewer pipeline.
In order to prevent the destruction of both the channels themselves and closely located buildings and structures from thawing soils in the base, it is necessary:
- thermally insulate pipelines, minimizing their heat release;
- ventilate the channels in winter to remove heat so that the soils thawed over the summer at their base (completely froze;
- arrange waterproofing along the bottom of the channel, preventing water from penetrating into the base soils. The foundations under the channels should be made of non-subsidence or low subsidence soils.
In addition to replacing subsiding soils, it is possible to use preliminary thawing and compaction of base soils. Channels should be made of reinforced concrete, reinforced cement or other effective material. The arrangement of channels made of wood or concrete can be allowed with special justification, since concrete channels are expensive and do not meet the strength requirements for uneven subsidence of the base, and wooden ones are prone to decay, require extensive waterproofing work, and are silted up with the smallest particles of soil; in the presence of sewerage, they create unsanitary conditions for the water supply.
Channel ventilation is arranged natural and artificial (forced). Natural is carried out by arranging ventilation holes along the top of the channel at a distance 20-25 m depending on the dimensions of the channel and communications laid in it (Fig. IV-8). The efficiency of natural ventilation can be improved by installing exhaust shafts in buildings located near the canal; while the distance between the holes on the channel for air inflow can be increased up to 100-150 m.
Drainage from the canal of emergency or waste water should be carried out from its end part, using a longitudinal slope, or from intermediate water collectors (waterproofed pits) by pumping out water with pumps.
Heat pipelines and steam pipelines placed in channels should be removed as far as possible from the bottom of the channel; they must be in annular thermal insulation (for example, from foam concrete with asbestos-cement plaster and waterproofing). The use of plastics for this purpose, which have enhanced heat and waterproofing properties (polystyrene, polyethylene, etc.), has great prospects.
The technical and economic feasibility of laying sewer networks in channels together with networks for various purposes in comparison with a single underground laying is revealed based on a comparison of the cost of construction and operation, referred to 1 m 2 living space, as well as assessing the stability of networks, their durability and thermal impact on nearby buildings and structures.
Ground laying of pipelines
The ground type of laying usually includes pipelines laid on low supports. In this case, between the pipe and the ground surface there must be a blown space of at least 30 cm, which is necessary to reduce heat release into the base soils and prevent snow drifts.
Ground laying of pipelines should be used outside the construction of populated areas (as the cheapest), on low and swampy sections of the route, in places with heavily ice-saturated permafrost soils.
On the built-up area, ground laying is allowed with a small number of intersections of the pipeline with driveways and sidewalks. Pipelines are thermally and waterproofed. The use of combustible materials both for the manufacture of ducts and heat-insulating backfills for steam pipelines and heating networks at a heat carrier temperature of 90 °C and above is not recommended by fire regulations. Slag filling should also not be widely used due to the possible destruction of metal pipes by corrosion when the slag is moistened.
Wooden boxes, being in conditions of variable humidity, are deformed, the backfill is blown out, poured out and easily moistened. Waterproofing boxes with roll materials does not achieve the goal, since roll coatings are easily damaged. Therefore, reinforced concrete boxes are more reliable, but their cost with backfill is higher than the cost of ring heat and waterproofing of pipes.
In the case of combined laying, mainly for the sake of ease of use, thermal insulation is performed independently for pipelines for various purposes.
Ground pipelines can be based on bulk sand and gravel or any other non-subsidence or low subsidence soil laid without disturbing the natural moss-vegetation cover during the work. With subsiding soils of natural foundation, it is necessary to replace them with non-sagging soils to a depth determined by the calculation.
Special supports are arranged on an artificial soil base under the pipelines.
Lying supports of the transverse beds have an insignificant height, as a result of which, when the supports subside, the thermal insulation of the pipes falls on the ground, is easily moistened and deteriorates. The arrangement of common supports for several pipelines is not recommended, since under an uneven load, the beds give an uneven settlement.
City supports(Fig. IV-9) are a more advanced type of wooden supports; they make it easy to straighten the profile of pipelines in case of small subsidence of the base by wedging elements of the towns.
Reinforced concrete intermediate supports sliding and roller types (Fig. IV-10) are more economical and more durable than wooden ones. Their disadvantage is the difficulty of straightening pipelines during the settlement of embankments; to level the base, the pipeline has to be raised and the supports removed.
motionless(anchor) supports(Fig. IV-11) are made of wood, concrete and reinforced concrete. With wooden supports, the pipes are fixed to the support bars with bolts or pins.
Frame fixed supports require the performance of large volumes of work on the development and excavation of soils from pits. Therefore, they can be recommended in cases where the use of pile supports is impractical (active layer of high thickness, high-temperature frozen soils characterized by low freezing forces, boulder gravelly soils, etc.).
Massive concrete supports are arranged for pipelines of large diameters and during the construction of pipelines in 2 stages. For fastening metal parts, nests are left in the concrete mass, which, for the time being before the construction of the pipeline of the second stage, must be filled with concrete of the lowest grades. Otherwise, water accumulates in them, which, when frozen, can break the concrete mass. In order to avoid thawing of the foundation soils due to exothermy during concrete hardening, as well as from heat inflow through the support body, a sand cushion with a thickness of 20-30 cm.
In general, ground laying in the conditions of the Far North is the most economical type of laying sanitary and technical communications (excluding sewerage).
Above ground piping
Above-ground laying of pipelines is carried out on overpasses, on pile supports, towering above the terrain (Fig. IV-12), along the walls of buildings, attics and fences. The elevated type of pipeline laying is used when crossing roads, hollows, ravines and streams, in factory areas, in places with heavily ice-saturated soils of permafrost.
Similarly to ground laying, pipes are laid in annular thermal insulation or in insulated boxes.
Overpasses can be made of wood, reinforced concrete and metal. Metal overpasses are used in flammable places. The production of reinforced concrete overpasses is difficult, and their cost is high. Therefore, pile and frame wooden overpasses have received the main application.
Advantages of above-ground laying:
- pipes and boxes do not cause snow deposits and do not interfere with snow removal;
- the issue of intersections with passages and passages is successfully solved;
- pipes and their insulation are not exposed to mechanical damage from vehicles and pedestrians;
- pipelines are not subject to snow drifts, are easily accessible for inspection and repair.
Disadvantages of above-ground laying:
- high cost compared to ground laying;
- the inconvenience of installing fittings, especially fire hydrants;
- more significant than with ground laying, heat losses due to high wind speeds and the absence of snow deposits on pipes;
- pipes laid along the facades of buildings, flyovers and fences spoil the appearance of the inhabited place;
- when laying pipes along the walls of buildings, the principle of priority in the construction of sanitary communications is violated.
Technical and economic indicators for some types of gaskets are given in appendices 1 and 2.
The following types of above-ground gaskets are currently in use:
On free-standing masts and supports (Fig. 4.1);
Rice. 4.1. Laying pipelines on free-standing masts
Fig. 4.2 - on flyovers with a continuous span in the form of trusses or beams (Fig. 4.2);
Rice. 4.2. Trestle with a span structure for laying pipelines
Fig. 4.3 - on rods attached to the tops of the masts (cable-stayed structure, Fig. 4.3);
Rice. 4.3. Pipe laying with suspension on rods (cable-stayed design)
On brackets.
Gaskets of the first type are the most rational for pipelines with a diameter of 500 mm or more. In this case, pipelines of larger diameter can be used as load-bearing structures for laying or suspending several pipelines of small diameter to them, requiring more frequent installation of supports.
It is advisable to use gaskets on a flyover with a continuous flooring for passage only with a large number of pipes (at least 5 - 6 pieces), as well as if regular supervision is necessary. In terms of construction cost, the through passage is the most expensive and requires the highest metal consumption, since trusses or beam decking are usually made of rolled steel.
Laying of the third type with a suspended (cable-stayed) span structure is more economical, as it allows you to significantly increase the distance between the masts and thereby reduce the consumption of building materials. The most simple structural forms of the suspension gasket are obtained with pipelines of equal or close diameters.
When laying large and small diameter pipelines together, a slightly modified cable-stayed structure with purlins made of channels suspended on rods is used. Runs allow you to install pipeline supports between masts. However, the possibility of laying pipelines on overpasses and with suspension on rods in urban areas is limited and applicable only in industrial areas. The most widely used is the laying of water pipelines on free-standing masts and supports or on brackets. Masts and supports are usually made of reinforced concrete. Metal masts are used in exceptional cases with a small amount of work and reconstruction of existing heating networks.
Masts according to their purpose are divided into the following types:
§ for movable supports of pipelines (the so-called intermediate);
§ for fixed supports of pipelines (anchor), as well as those installed at the beginning and at the end of the route section;
§ installed on the turns of the route;
§ serving to support expansion joints of pipelines.
Depending on the number, diameter and purpose of the pipelines being laid, the masts are made in three different structural forms: single-column, two-column and four-column spatial structures.
When designing air gaskets, one should strive to increase the distance between the masts as much as possible.
However, for unhindered water flow when pipelines are turned off, the maximum deflection should not exceed
f = 0,25∙i∙l,
where f- deflection of the pipeline in the middle of the span, mm; i- pipeline axis slope; l- distance between supports, mm.
Precast concrete mast structures are usually assembled from the following elements: racks (columns), crossbars and foundations. The dimensions of prefabricated parts are determined by the number and diameter of the laid pipelines.
When laying from one to three pipelines, depending on the diameter, single-column free-standing masts with consoles are used, they are also suitable for cable-stayed pipe suspension on rods; then a top device for fastening the rods is provided.
Solid rectangular masts are acceptable if the maximum cross-sectional dimensions do not exceed 600 x 400 mm. For larger dimensions, to facilitate the design, it is recommended to provide cutouts along the neutral axis or use factory-made centrifuged reinforced concrete pipes as racks.
For multi-pipe laying, the masts of intermediate supports are most often designed with a two-column design, single-tier or two-tier.
Prefabricated two-post masts consist of the following elements: two posts with one or two consoles, one or two crossbars and two glass-type foundations.
The masts, on which the pipelines are fixed, are loaded by horizontally directed forces transmitted by the pipelines, which are laid at a height of 5 - 6 m from the ground surface. To increase stability, such masts are designed in the form of a four-post spatial structure, which consists of four posts and four or eight crossbars (with a two-tier arrangement of pipelines). The masts are mounted on four separate glass-type foundations.
When laying above-ground pipelines of large diameters, the bearing capacity of the pipes is used, and therefore no span structure is required between the masts. Suspension of large-diameter pipelines on rods should not be used either, since such a design will practically not work.
Fig. 4.4 As an example, the laying of pipelines on reinforced concrete masts is shown (Fig. 4.4).
Two pipelines (direct and return) with a diameter of 1200 mm are laid on roller supports on reinforced concrete masts installed every 20 m. The height of the masts from the ground is 5.5 - 6 m. Prefabricated reinforced concrete masts consist of two foundations interconnected by a monolithic joint, two rectangular columns 400 x 600 mm and a crossbar.
Rice. 4.4. Laying pipelines on reinforced concrete masts:
1 - column; 2 - crossbar; 3 - connection; 4 - foundation; 5 - connecting joint; 6 - concrete preparation.
The columns are interconnected by metal diagonal ties made of angle steel. The connections with the columns are made with scarves welded to the embedded parts, which are embedded in the columns. The crossbar, which serves as a support for pipelines, is made in the form of a rectangular beam with a section of 600 x 370 mm and is attached to the columns by welding embedded steel sheets.
The mast is designed for the weight of the pipe span, horizontal axial and lateral forces arising from the friction of the pipelines on the roller bearings, as well as for the wind load.
Rice. 4.5. Fixed support:
1 - column; 2 - transverse crossbar; 3 - crossbar longitudinal; four - cross connection; 5 - longitudinal connection; 6 - foundation
The fixed support (Fig. 4.5), designed for a horizontal force from two pipes of 300 kN, is made of prefabricated reinforced concrete parts: four columns, two longitudinal crossbars, one transverse support beam and four foundations connected in pairs.
In the longitudinal and transverse directions, the columns are connected by metal diagonal braces made of angle steel. On the supports, the pipelines are fixed with clamps covering the pipes, and scarves in the lower part of the pipes, which abut against a metal frame of channels. This frame is attached to reinforced concrete crossbars by welding to embedded parts.
The laying of pipelines on low supports has found wide application in the construction of heating networks in the unplanned territory of new urban development areas. It is more expedient to cross rough or wetlands, as well as small rivers, in this way using the bearing capacity of pipes.
However, when designing heat networks with laying pipelines on low supports, it is necessary to take into account the period of the planned development of the territory occupied by the route for urban development. If after 10 - 15 years it will be necessary to put pipelines into underground channels or reconstruct the heating network, then the use of air laying is inappropriate. To justify the application of the method of laying pipelines on low supports, feasibility studies must be performed.
When laying above-ground pipelines of large diameters (800-1400 mm), it is advisable to lay them on separate masts and supports using special factory-made prefabricated reinforced concrete structures that meet the specific hydrogeological conditions of the heating main route.
Design experience shows the cost-effectiveness of using pile foundations for foundations of both anchor and intermediate masts and low supports.
Above-ground heating mains of large diameter (1200-1400 mm) of considerable length (5-10 km) were built according to individual projects using high and low supports on a pile foundation.
There is experience in the construction of a heating main with pipe diameters D= 1000 mm from the CHPP using pile-pillars in swampy sections of the route, where rocky soils lie at a depth of 4-6 m.
The calculation of supports on a pile foundation for the combined effect of vertical and horizontal loads is carried out in accordance with SNiP II-17-77 "Pile Foundations".
When designing low and high supports for laying pipelines, the structures of unified prefabricated reinforced concrete detached supports designed for process pipelines can be used [3].
The project of low supports of the type of "swinging" foundations, consisting of a reinforced concrete vertical shield installed on a flat foundation slab, was developed by AtomTEP. These supports can be used in various soil conditions (with the exception of heavily watered and subsiding soils).
One of the most common types of aerial laying of pipelines is the laying of the latter on brackets fixed in the walls of buildings. The use of this method can be recommended when laying heating networks on the territory of industrial enterprises.
When designing pipelines located on the outer or inner surface of the walls, one should choose such a placement of pipes so that they do not cover window openings, do not interfere with the placement of other pipelines, equipment, etc. The most important thing is to ensure that the brackets are securely fixed in the walls of existing buildings. The design of piping along the walls of existing buildings should include a survey of the walls in kind and a study of the projects on which they are built. With significant loads transmitted by pipelines to the brackets, it is necessary to calculate the overall stability of the building structures.
Pipelines are laid on brackets with welded sliding bearing housings. The use of roller movable bearings for external laying of pipelines is not recommended due to the difficulty of their periodic lubrication and cleaning during operation (without which they will work as sliding ones).
In case of insufficient reliability of the walls of the building, constructive measures should be taken to disperse the forces transmitted by the brackets by reducing the spans, bracing, vertical racks, etc. The brackets installed in the places where the pipelines are fixed supports should be designed for the forces acting on them. Usually they require additional fastening by means of struts in the horizontal and vertical planes. On fig. 4.6 shows a typical design of brackets for laying one or two pipelines with a diameter of 50 to 300 mm.
Rice. 4.6. Laying pipelines on brackets.
