Taking into account the dependence of the number of consumers, their needs for thermal energy, as well as the requirements for the quality and uninterrupted supply of heat for certain categories of subscribers, heating networks are made radial (dead-end) or ring.
The dead-end circuit (picture) is the most common. It is used when providing thermal energy to a city, neighborhood or village from one source - a combined heat and power plant or a boiler house. As the main line moves away from the source, the diameters of heat pipes 1 decrease, the design, composition of structures and equipment on heating networks are simplified in accordance with the reduction in heat load. This scheme is characterized by the fact that in the event of a mainline failure, subscribers connected to the heating network after the accident site are not provided with thermal energy.
To increase the reliability of providing consumers 2 with thermal energy, jumpers 3 are installed between adjacent lines, which allow switching the supply of thermal energy in the event of a failure of any line. According to the design standards for heating networks, the installation of jumpers is mandatory if the power of the mains is 350 MW or more. In this case, the diameter of the lines is usually 700 mm or more. The presence of jumpers partially eliminates the main disadvantage of this scheme and creates the possibility of uninterrupted heat supply to consumers. In emergency conditions, a partial reduction in the supply of thermal energy is allowed. For example, according to the Design Standards, jumpers are designed to provide 70% of the total thermal load (maximum hourly consumption for heating and ventilation and average hourly consumption for hot water supply).
In developing areas of the city, redundant jumpers are provided between adjacent highways, regardless of the thermal power, but based on the priority of development. Jumpers are also provided between highways in dead-end circuits when supplying heat to an area from several heat sources (CHP, district and block boiler houses 4), which increases the reliability of heat supply. At the same time, in the summer, when one or two boiler houses are operating in normal mode, several boiler houses operating at minimum load can be turned off. At the same time, along with increasing the efficiency of boiler houses, conditions are created for timely preventive and major repairs of individual sections of the heating network and the boiler houses themselves. On large branches (see figure) sectional chambers 5 are provided. For enterprises that do not allow interruptions in the supply of thermal energy, heat network circuits with two-way power supply, local backup sources or ring circuits are provided.
Ring circuit(Figure) is provided in large cities. The installation of such heating networks requires large capital investments compared to dead-end ones. The advantage of the ring circuit is the presence of several sources, which increases the reliability of heat supply and requires less total reserve power of boiler equipment. As the cost of the ring main increases, capital costs for the construction of thermal energy sources decrease. Ring main 1 is connected to three thermal power plants, consumers 2 are connected to the ring main via a dead-end circuit through central heating points 6. On large branches, sectional chambers 5 are provided. Industrial enterprises 7 are also connected according to a dead-end circuit.
According to the design of thermal insulation, ductless laying of heat pipelines is divided into backfill, prefabricated, prefabricated-cast and monolithic. The main disadvantage of ductless installation is increased subsidence and external corrosion of heat pipes, as well as increased heat loss in the event of a violation of the waterproofing of the heat-insulating layer. To a large extent, the disadvantages of ductless installations of heating networks are eliminated by using thermal and waterproofing based on polymer concrete mixtures.
Heat pipes in the channels are laid on movable or fixed supports. Movable supports serve to transfer the own weight of the heat pipes to the supporting structures. At the same time, they ensure the movement of pipes, which occurs as a result of changes in their length when their length changes when the temperature of the coolant changes. Movable supports can be sliding or roller.
Sliding supports are used in cases where the base for the supports must be made strong enough to withstand large horizontal loads. Otherwise, roller bearings are installed that create smaller horizontal loads. For this reason, when laying large diameter pipelines in tunnels, on frames or masts, roller bearings should be installed.
Fixed supports serve to distribute thermal expansion of the heat pipe between compensators and to ensure uniform operation of the latter. In the chambers of underground channels and during above-ground installations, fixed supports are made in the form of metal structures, welded or bolted to pipes. These structures are embedded in foundations, walls and channel ceilings.
To absorb temperature elongations and relieve heat pipes from temperature stresses, radial (flexible and wavy hinge-type) and axial (gland and lens) compensators are installed on heating networks.
Flexible U- and S-shaped expansion joints are made from pipes and bends (bent, steeply curved and welded) for heat pipelines with a diameter of 500 to 1000 mm. Such compensators are installed in non-passable channels, when it is impossible to inspect the installed heat pipelines, as well as in buildings with ductless installation. The permissible bending radius of pipes in the manufacture of expansion joints is 3.5...4.5 times the outer diameter of the pipe.
In order to increase the compensating capacity of bent expansion joints and reduce compensation stresses, they are usually pre-stretched. To do this, the compensator in a cold state is stretched at the base of the loop, so that when hot coolant is supplied and the heat pipe is correspondingly lengthened, the shoulders of the compensator are in a position in which the stresses will be minimal.
Stuffing box compensators are small in size and have a large compensating ability to provide little resistance to the flowing fluid. They are manufactured single-sided and double-sided for pipes with a diameter of 100 to 1000 mm. Stuffing box expansion joints consist of a housing with a flange on the widened front part. A movable glass with a flange is inserted into the compensator body for installing the compensator on the pipeline. To prevent the stuffing box compensator from leaking coolant between the rings, stuffing box packing is placed in the gap between the body and the glass. The stuffing box is pressed into the flange liner using studs screwed into the compensator body. Compensators are attached to fixed supports.
The chamber for installing valves on heating networks is shown in the figure. When laying heating systems underground, 3 rectangular underground chambers are installed to service shut-off valves. Branches 1 and 2 of the network to consumers are laid in the chambers. Hot water is supplied to the building through a heat pipe laid on the right side of the channel. The supply 7 and return 6 heat pipes are installed on supports 5 and covered with insulation. The walls of the chambers are made of bricks, blocks or panels, prefabricated ceilings are made of reinforced concrete in the form of ribbed or flat slabs, the bottom of the chamber is made of concrete. Entrance to the cells is through cast iron hatches. It is important to note that to descend into the chamber under the hatches in the wall, brackets are sealed or metal ladders are installed. The height of the chamber must be at least 1800 mm. The width is chosen so that the distance between the walls and pipes is at least 500 m.
Questions for self-control:
1. What are heat networks called?
2. How are heating networks classified?
3. What are the advantages and disadvantages of ring and stub networks?
4. What is called a heat pipe?
5. Name the methods for laying heating networks.
6. Name the purpose and types of insulation of heat pipelines.
7. Name the pipes from which heating networks are installed.
8. State the purpose of compensators.
Ticket No. 1
1. Sources of energy, including thermal energy, can be substances whose energy potential is sufficient for the subsequent conversion of their energy into other types for the purpose of subsequent targeted use. The energy potential of substances is a parameter that allows us to assess the fundamental possibility and feasibility of their use as energy sources, and is expressed in energy units: joules (J) or kilowatt (thermal) hours [kW (thermal) -h] *. All energy sources are conditional divided into primary and secondary (Fig. 1.1). Primary sources of energy are substances whose energy potential is a consequence of natural processes and does not depend on human activity. Primary sources of energy include: fossil fuels and fissionable substances heated to a high temperature in the waters of the Earth's interior (thermal waters), the Sun, wind, rivers, seas, oceans, etc. Secondary energy sources are substances that have a certain energy potential and are by-products human activity; for example, spent flammable organic substances, municipal waste, hot waste coolant from industrial production (gas, water, steam), heated ventilation emissions, agricultural waste, etc. Primary energy sources are conventionally divided into non-renewable, renewable and inexhaustible. Renewable primary energy sources include fossil fuels: coal, oil, gas, shale, peat and fossil fissile substances: uranium and thorium. Renewable primary energy sources include all possible sources of energy that are products of the continuous activity of the Sun and natural processes on the Earth's surface: wind, water resources, the ocean, plant products of biological activity on Earth (wood and other plant substances), as well as the Sun. The practically inexhaustible primary energy sources include the Earth's thermal waters and substances that can be sources of thermonuclear energy. The resources of primary energy sources on Earth are estimated by the total reserves of each source and its energy potential, i.e., the amount of energy that can be released from a unit its mass. The higher the energy potential of a substance, the higher the efficiency of its use as a primary source of energy and, as a rule, the more widespread it is in energy production. For example, oil has an energy potential of 40,000-43,000 MJ per 1 ton of mass, and natural and associated gases - from 47,210 to 50,650 MJ per 1 ton of mass, which, combined with their relatively low cost of production, made it possible their rapid spread in the 1960-1970s as primary sources of thermal energy. The use of a number of primary energy sources until recently was hampered either by the complexity of the technology for converting their energy into thermal energy (for example, fissile substances), or by the relatively low energy potential of the primary energy source, which requires large costs to obtain thermal energy of the required potential (for example, the use of solar energy, wind energy, etc.). The development of industry and the scientific and production potential of the countries of the world has led to the creation and implementation of processes for the production of thermal energy from previously undeveloped primary energy sources, including the creation of nuclear heat supply stations, solar heat generators for heating buildings, and heat generators using geothermal energy.
Schematic diagram of the thermal power plant
2. Heating point (HP) - a set of devices located in a separate room, consisting of elements of thermal power plants that ensure the connection of these plants to the heating network, their operability, control of heat consumption modes, transformation, regulation of coolant parameters and distribution of coolant by type of consumption. Main TP objectives are:
Converting the type of coolant
Monitoring and regulation of coolant parameters
Distribution of coolant among heat consumption systems
Disabling heat consumption systems
Protection of heat consumption systems from emergency increases in coolant parameters
Accounting for coolant and heat costs
The TP scheme depends, on the one hand, on the characteristics of the thermal energy consumers served by the heating point, and on the other hand, on the characteristics of the source supplying the TP with thermal energy. Further, as the most common, we consider a TP with a closed hot water supply system and an independent connection circuit for the heating system.
Schematic diagram of a heating point
The coolant entering the TP through the thermal input supply pipeline gives off its heat in the heaters of the hot water supply and heating systems, and also enters the consumer ventilation system, after which it is returned to the thermal input return pipeline and sent back through the main networks to the heat generating enterprise for reuse. Some of the coolant may be consumed by the consumer. To replenish losses in primary heating networks at boiler houses and thermal power plants, there are make-up systems, the sources of coolant for which are the water treatment systems of these enterprises.
Tap water entering the TP passes through the cold water pumps, after which part of the cold water is sent to consumers, and the other part is heated in the first stage DHW heater and enters the circulation circuit of the DHW system. In the circulation circuit, water, with the help of hot water supply circulation pumps, moves in a circle from the heating substation to the consumers and back, and consumers take water from the circuit as needed. As water circulates through the circuit, it gradually releases its heat and in order to maintain the water temperature at a given level, it is constantly heated in the second stage DHW heater.
The heating system also represents a closed loop through which the coolant moves with the help of heating circulation pumps from the heating substations to the building heating system and back. During operation, coolant leaks may occur from the heating system circuit. To make up for losses, a heating point recharge system is used, using primary heating networks as a source of coolant.
Ticket No. 3
Schemes for connecting consumers to heating networks. Schematic diagram of ITP
There are dependent and independent connection schemes for heating systems:
Independent (closed) connection diagram - a diagram for connecting a heat consumption system to a heating network, in which the coolant (superheated water) coming from the heating network passes through a heat exchanger installed at the consumer’s heating point, where it heats the secondary coolant, which is subsequently used in the heat consumption system
Dependent (open) connection diagram - a scheme for connecting a heat consumption system to a heating network, in which the coolant (water) from the heating network flows directly into the heat consumption system.
Individual heating point (ITP). Used to serve one consumer (building or part thereof). As a rule, it is located in the basement or technical room of the building, however, due to the characteristics of the building being served, it can be placed in a separate structure.
2. Operating principle of the MHD generator. Scheme of TPP with MHD.
Magnetohydrodynamic generator, MHD generator is a power plant in which the energy of a working fluid (liquid or gaseous electrically conducting medium) moving in a magnetic field is converted directly into electrical energy.
Just like in conventional machine generators, the operating principle of an MHD generator is based on the phenomenon of electromagnetic induction, that is, on the occurrence of a current in a conductor crossing magnetic field lines. But, unlike machine generators, in an MHD generator the conductor is the working fluid itself, in which, when moving across the magnetic field, oppositely directed flows of charge carriers of opposite signs arise.
The following media can serve as the working fluid of the MHD generator:
· Electrolytes
Liquid metals
Plasma (ionized gas)
The first MHD generators used electrically conductive liquids (electrolytes) as a working fluid; currently they use plasma, in which the charge carriers are mainly free electrons and positive ions, which deviate in a magnetic field from the trajectory along which the gas would move in the absence of a field. In such a generator, an additional electric field can be observed, the so-called Hall field, which is explained by the displacement of charged particles between collisions in a strong magnetic field in a plane perpendicular to the magnetic field.
Power plants with magnetohydrodynamic generators (MHD generators). MHD generators are planned to be built as an add-on to a IES type station. They use thermal potentials of 2500-3000 K, unavailable to conventional boilers.
A schematic diagram of a thermal power plant with an MHD installation is shown in the figure. Gaseous products of fuel combustion, into which an easily ionizable additive (for example, K 2 CO 3) is introduced, are sent to the MHD - a channel penetrated by a high-intensity magnetic field. The kinetic energy of ionized gases in the channel is converted into direct current electrical energy, which, in turn, is converted into three-phase alternating current and sent to the power system to consumers.
Schematic diagram of a IES with an MHD generator:
1 - combustion chamber; 2 – MHD - channel; 3 - magnetic system; 4 - air heater,
5 - steam generator (boiler); 6 - steam turbines; 7 - compressor;
8 - condensate (feed) pump.
Ticket No. 4
1.Classification of heat supply systems
Schematic diagrams of heat supply systems according to the method of connection to them heating systems
Based on the location of heat generation, heat supply systems are divided into:
· Centralized (the source of thermal energy production works to supply heat to a group of buildings and is connected by transport devices to heat consumption devices);
· Local (the consumer and the heat supply source are located in the same room or in close proximity).
By type of coolant in the system:
· Water;
· Steam.
According to the method of connecting the heating system to the heat supply system:
· dependent (coolant heated in a heat generator and transported through heating networks goes directly to heat-consuming devices);
· independent (the coolant circulating through the heating networks in the heat exchanger heats the coolant circulating in the heating system).
According to the method of connecting the hot water supply system to the heating system:
· closed (water for hot water supply is taken from the water supply and heated in a heat exchanger with network water);
· Open (water for hot water supply is taken directly from the heating network).
When laying underground routes in through-pass collectors, it is allowed not to provide a reserve.
When laying above ground, redundancy is provided only at tnr<-40 · С для диаметров >1200mm in size at least 70%. In addition, SNiP provides for reservation (100%) for certain types of buildings for which the technology prohibits differences in the heat supply. In this case, either 2 independent inputs into the building from various heating mains, or a network backup heat source (for example, an electric boiler) are provided.
The emergency dependence of heating networks is growing for large heat supply systems.
In large systems, 2 schemes are mainly used:
Dead end
Ring
In ring networks, several heat sources are used per network. The calculation of ring networks is performed only on a computer using Kirchhoff's laws.
Redundancy by jumpers in such networks may not be used.
If the A-t network is like a ring one, then all the valves are open and the water flows are distributed in proportion to the resistance and thermal loads, since the A-t of such networks is very complex. In practice, the sources are cut off from each other by closing the separation valves (1). In this case, the A-t network is a dead-end network. In emergency situations, the separation valves are opened and part of the heat is transferred from the first source to another. By installing backup jumpers (method 2).
Due to the device of the 1st source with redundant jumpers in small N.n. (dead-end circuit).
The diameters of the reserve jumper are taken with a margin according to the calculation in order to ensure the minimum required heat supply to zone A.
Redundancy by laying a backup pipeline is used when the source is located at a distance from the consumer. In this case, the head section of the network is laid in a “three-pipe” manner.
Two pipelines - A-m for supply 1-H for return. In emergency mode, if the first pipeline fails, heat is supplied through the remaining lines.
Schematic diagram of the heating network.
Basically it consists of main and branch pipelines. Special structures are placed on these pipelines, such as heating units (CH), chambers for placing compensators, step-down and step-up substations.
The UT contains shut-off and sectional valves, devices for air removal and water discharge, and stuffing box compensators. Only stuffing box compensators are placed in the compensator chamber; it is possible to place equipment for air removal and water discharge.
The connection of m/districts and residential areas is carried out through the central heating hub.
Large buildings can be connected to heating networks through central heating stations. Connection of consumers with a load of less than 4 MW. to heating networks is prohibited. According to SNiP, heating networks must have 2 pipes. The use of 3 and 4 pipe systems is allowed during the feasibility study. The connection of consumers to heating networks should be mainly dependent. Independent connections are allowed for 12-story buildings, and depending on the piezometer.
The connection of hot water systems is mainly closed.
Determination of estimated water flow rates
Estimated water consumption is determined according to SNiP separately for each type of heat load.
o = Qo / T1р – T2р (mW), t/h
в = Qв / T1р – T2р (mW), t/h
Consumption for hot water depends on the type of system - open or closed.
- Closed
Consumption צ - depends on the scheme inclusion heaters in ITP or central heating substations. When calculating, 2 costs are determined:
- Average
- Maximum
a) Parallel circuit for connecting heaters
gv.z sr = Q gv.z sr / T1p – T2,gv (mW), t/h
Т1п – Accepted according to the reference book (70 C)
T2,gv – water temperature at the outlet of the hot water heater (30 C according to SNiP)
The average consumption for hot water supply needs is found at tnp. The maximum flow rate is determined similarly.
A heating network is a set of pipelines and devices that provide
transporting heat from the heat supply source to consumers using a coolant (hot water or steam).
Structurally, the heating network includes pipelines with thermal insulation and compensators, devices for laying and securing pipelines, as well as shut-off or control valves.
The choice of coolant is determined by an analysis of its positive and negative properties. The main advantages of a water heating system: high storage capacity of water; possibility of transportation over long distances; compared to steam, less heat loss during transportation; the ability to regulate the thermal load by changing the temperature or hydraulic mode. The main disadvantage of water systems is the high energy consumption to move the coolant in the system. In addition, the use of water as a coolant necessitates its special preparation. During preparation, carbonate hardness, oxygen content, iron content and pH are standardized. Water heating networks are usually used to satisfy heating and ventilation loads, hot water supply loads and low-potential process loads (temperatures below 100 0 C).
The advantages of steam as a coolant are the following: low energy losses when moving in channels; intense heat transfer during condensation in thermal appliances; In high potential process loads, steam can be used at high temperatures and pressures. Disadvantage: the operation of steam heating systems requires special safety measures.
The layout of the heating network is determined by the following factors: the location of the heat supply source in relation to the area of heat consumption, the nature of the heat load of consumers, the type of coolant and the principle of its use.
Heat networks are divided into:
Trunk lines laid along the main directions of heat consumption facilities;
Distribution, which are located between the main heating networks and branch nodes;
Branches of heating networks to individual consumers (buildings).
Heat network diagrams are usually used as radial ones, Fig. 5.1. From the thermal power plant or boiler house 4, the coolant is supplied through radial lines 1 to heat consumer 2. In order to provide backup heat to consumers, the radial lines are connected by jumpers 3.
The radius of action of water heating networks reaches
12 km. For small lengths of pipelines, which is typical for rural heating networks, a radial scheme is used with a constant decrease in the diameter of the pipes as they move away from the heat supply source.
Laying of heating networks can be above-ground (air) and underground.
Aboveground pipe laying (on
free-standing masts or overpasses, on concrete blocks and is used in the territories of enterprises, when constructing heating networks outside the city limits when crossing ravines, etc.
In rural settlements, ground laying can be on low supports and supports of medium height. This method is applicable at temperatures warm
carrier no more than 115 0 C. Underground installation is the most common. There are channel and non-channel installations. In Fig. Figure 5.2 shows a channel gasket. When laying in a channel, the insulating structure of the pipelines is unloaded from the external loads of the backfill. For channelless installation (see Fig. 5.3), pipelines 2 are laid on supports 3 (gravel
or sand cushions, wooden blocks, etc.).
Backfill 1, which is used: gravel, coarse sand, milled peat, expanded clay, etc., serves as protection against external damage and at the same time reduces heat loss. When laying in a channel, the temperature of the coolant can reach 180 °C. For heating networks, steel pipes with a diameter of 25 to 400 mm are most often used. In order to prevent the destruction of metal pipes due to temperature deformation, compensators are installed along the length of the entire pipeline at certain distances.
Various designs of compensators are shown in Fig. 5.4.
Rice. 5.4. Compensators:
a – U-shaped; b– lyre-shaped; V– stuffing box; G– lens
Type compensators A (U-shaped) and b (lyre-shaped) are called radial. In them, the change in pipe length is compensated by the deformation of the material in bends. In stuffing box expansion joints V It is possible for the pipe to slip within the pipe. In such compensators there is a need for a reliable seal design. Compensator G - lens type selects a change in length due to the springing action of the lenses. Great prospects for reinforced compensators. A bellows is a thin-walled corrugated shell that allows it to absorb various movements in the axial, transverse and angular directions, reduce vibration levels and compensate for misalignment.
Pipes are laid on special supports of two types: free and fixed. Free supports ensure the movement of pipes during temperature deformations. Fixed supports fix the position of pipes in certain areas. The distance between the fixed supports depends on the diameter of the pipe, for example, with D = 100 mm L = 65 m; at D = 200 mm L = 95 m. Between the fixed supports under the pipes with compensators, 2...3 movable supports are installed.
Currently, instead of metal pipes, which require serious protection against corrosion, plastic pipes have begun to be widely introduced. The industry of many countries produces a wide range of pipes made of polymer materials (polypropylene, polyolephen); metal-plastic pipes; pipes made by winding threads from graphite, basalt, glass.
On main and distribution heating networks, pipes with thermal insulation applied in an industrial manner are laid. For thermal insulation of plastic pipes, it is preferable to use polymerizing materials: polyurethane foam, polystyrene foam, etc. For metal pipes, bitumen-perlite or phenolic-polymer plastic insulation is used.
5.2. Heating points
A heating point is a complex of devices located in a separate room, consisting of heat exchangers and elements of heating equipment.
Heating points provide connections of heat-consuming objects to the heating network. The main task of the TP is:
– transformation of thermal energy;
– distribution of coolant among heat consumption systems;
– control and regulation of coolant parameters;
– accounting for coolant and heat costs;
– shutdown of heat consumption systems;
– protection of heat consumption systems from emergency increases in coolant parameters.
Heating points are divided according to the presence of heating networks after them into: central heating points (CHP) and individual heating points (ITP). Two or more heat consumption facilities are connected to the central heating station. ITP connects the heating network to one object or part of it. According to their location, heating points can be free-standing, attached to buildings and structures, or built into buildings and structures.
In Fig. Figure 5.5 shows a typical diagram of ITP systems that provide heating and hot water supply to a separate facility.
Two pipes are connected from the heating network to the shut-off valves of the heating point: supply (high-temperature coolant enters) and
return (cooled coolant is removed). Parameters of the coolant in the supply pipeline: for water (pressure up to 2.5 MPa, temperature - not higher than 200 0 C), for steam (p t 0 C). At least two heat exchangers of a recuperative type (shell-and-tube or plate) are installed inside the heating point. One ensures the transformation of heat into the heating system of the facility, the other into the hot water supply system. In both systems, devices for monitoring and regulating parameters and coolant supply are installed in front of the heat exchangers, which allows for automatic recording of consumed heat. For the heating system, the water in the heat exchanger is heated to a maximum of 95 0 C and pumped through the heating devices by a circulation pump. Circulation pumps (one working, the other standby) are installed on the return pipeline. For hot water supply
The water pumped through the heat exchanger by a circulation pump is heated to 60 0 C and supplied to the consumer. The water flow is compensated into the heat exchanger from the cold water supply system. To account for the heat expended on heating water and its consumption, appropriate sensors and recording devices are installed.
5.2. Determination of the diagram and configuration of heating networks.
When designing heating networks, choosing a scheme is a complex technical and economic task. The layout of the heating network is determined not only by the location of heat sources in relation to consumers, but also by the type of coolant, the nature of heat loads and their calculated value.
The main criteria by which the quality of the designed heating network is assessed should be its economic efficiency. When choosing the configuration of heating networks, you should strive for the simplest solutions and, if possible, shorter pipeline lengths.
In heating networks, both water and steam can be used as coolants. Steam as a coolant is used mainly for process loads of industrial enterprises. Typically, the length of steam networks per unit of design heat load is small. If, due to the nature of the technological process, short-term (up to 24 hours) interruptions in the steam supply are permissible, then the most economical and at the same time quite reliable solution is to lay a single-pipe steam pipeline with a wire.
It must be borne in mind that duplication of steam networks leads to a significant increase in their cost and consumption of materials, primarily steel pipelines. When laying, instead of one pipeline designed for full load, two parallel ones designed for half load, the surface area of the pipelines increases by 56%. Accordingly, metal consumption and the initial cost of the network increase.
Choosing the design of water heating networks is considered a more difficult task, since their load is usually less concentrated. Water heating networks in modern cities serve a large number of consumers, often measured in thousands and even tens of thousands of connected buildings located in areas often measured in many tens of square kilometers.
Water networks are less durable than steam networks, mainly due to the greater susceptibility to external corrosion of steel pipelines laid in underground channels. In addition, water heating networks are more sensitive to accidents due to the higher density of the coolant. The emergency vulnerability of water heating networks is especially noticeable in large systems with dependent connection of heating installations to the heating network, therefore, when choosing a scheme for water heating networks, special attention must be paid to the issues of reliability and redundancy of heat supply.
Water heating networks must be clearly divided into current and distribution. TO ny networks usually include heat pipelines connecting heat sources with areas of heat consumption, as well as with each other.
The coolant enters from the distribution networks and is supplied through the distribution networks through group heat substations or local heat substations to the heat consuming installations of subscribers. Direct connection of heat consumers to these networks should not be allowed, with the exception of cases of connection of large industrial enterprises,
New heating networks are divided into sections 1–3 km long using valves. When a pipeline opens (ruptures), the location of the failure or accident is localized by sectional valves. Thanks to this, losses of network water are reduced and the duration of repairs is reduced due to a decrease in the time required to drain water from the pipeline before repairs and to fill the pipeline section with network water after repairs.
The distance between the sectional valves is selected so that the time required for repairs is less than the time during which the internal temperature in the heated rooms, when heating is completely turned off at the design outside temperature for heating, drops below 12 - 14 ° C. This is the minimum limit value that is usually accepted in accordance with the heat supply contract.
The distance between sectional valves should generally be smaller for larger pipeline diameters and at lower design outside temperatures for heating. The time required to carry out repairs increases with increasing pipeline diameter and the distance between sectional valves. This is due to the fact that as the diameter increases, the repair time increases significantly.
If the repair time is longer than permissible, it is necessary to provide for system backup of heat supply in the event of failure of a section of the heating network. One of the redundancy methods is to block adjacent highways. Sectional valves are conveniently placed in connection points between distribution networks and heating networks. In these nodal chambers, in addition to sectional valves, there are also head valves of distribution networks, valves on blocking lines between adjacent mains or between mains and backup heat supply sources, for example, district ones (chamber 4 in Fig. 5.1). There is no need to section steam lines, since the mass of steam required to fill long steam lines is small. Sectional valves must be equipped with an electric or hydraulic drive and have a telemechanical connection with the central control center. Distribution networks must be connected to the main line on both sides of sectional valves so that uninterrupted service to subscribers can be ensured in case of accidents on any sectioned section of the main line.
Rice. 5.1. Principal single-line communication diagram of a two-pipe water heating network with two mains
1 - collector; 2 - network; 3 - distribution network; 4 - sectioning chamber; 5 - sectional valve; 6 - ; 7 - blocking connection
Interlocking connections between highways can be made using single pipes. An appropriate scheme for connecting them to the network may provide for the use of blocking connections for both the supply and return pipelines.
In buildings of a special category that do not allow interruptions in heat supply, provision must be made for backup heat supply from gas or electric heaters or from local heaters in the event of an emergency interruption of centralized heat supply.
According to SNiP 2.04.07-86, it is allowed to reduce the heat supply in emergency conditions to 70% of the total design consumption (maximum hourly for ventilation and average hourly for hot water supply). For enterprises in which interruptions in the heat supply are not allowed, duplicate or ring circuits of heating networks should be provided. Estimated emergency heat consumption must be taken in accordance with the operating mode of enterprises.
In Fig. Figure 5.1 shows a basic single-line diagram of a two-pipe water heating network with an electrical power of 500 MW and a thermal power of 2000 MJ/s (1700 Gcal/h).
The radius of the heating network is 15 km. Heat consumption is transmitted to the final area via two two-pipe transit mains 10 km long. The diameter of the outlet lines is 1200 mm. As water is distributed into associated branches, the diameters of the lines decrease. Heat consumption is introduced into the final area through four mains with a diameter of 700 mm, and then distributed over eight mains with a diameter of 500 mm. Interlocking connections between main lines, as well as redundant substations, are installed only on lines with a diameter of 800 mm or more.
This solution is acceptable in the case when, with the accepted distance between sectional valves (2 km in the diagram), the time required to repair a pipeline with a diameter of 700 mm , less time during which the internal temperature of heated buildings, when the heating is turned off at the outside temperature, will drop from 18 to 12 ºС (not lower).
Interlocking connections and sectioning valves are distributed in such a way that in the event of an accident on any section of a main line with a diameter of 800 mm or more, all subscribers connected to the heating network are provided with. subscribers is violated only in case of accidents on lines with a diameter of 700 mm or less.
In this case, subscribers located behind the accident site (along the heat path) are terminated.
When supplying heat to large cities from several, it is advisable to provide for mutual interlocking by connecting their mains with interlocking connections. In this case, a combined ring can be created
Blocking connections between large-diameter mains must have sufficient capacity to ensure the transmission of redundant water flows. In necessary cases, substations are built to increase the capacity of blocking connections.
Regardless of the blocking connections between the mains, it is advisable in cities with a developed hot water supply load to provide jumpers of a relatively small diameter between adjacent heat distribution networks to reserve the hot water supply load.
When the diameters of the mains emanating from the heat source are 700 mm or less, a radial (radial) heating network diagram is usually used with a gradual decrease in diameter as the distance from the station increases and the connected heat load decreases.
Such a network is the cheapest in terms of initial costs, requires the least metal consumption for construction and is easy to operate. However, in the event of an accident on the backbone of the radial network, the subscribers connected to the accident site are terminated. If an accident occurs on the main line near the station, then all consumers connected to the main line are interrupted. This solution is acceptable if the repair time for pipelines with a diameter of at least 700 mm satisfies the above condition.
The question of what diameters of heat pipelines and which heating network scheme (radial or ring) should be used in district heating systems should be decided based on the specific conditions dictated by the heat supply of heat consumers: whether they allow a break in the supply of coolant or not, what are the costs of redundancy and so on. Therefore, in a market economy, the above regulation of diameters and diagrams of heating networks cannot be considered the only correct solution.