For normal operation, there must be available pressure. Preparation of specifications
Based on the results of calculating water supply networks for various water consumption modes, the parameters of the water tower and pumping units that ensure the operability of the system, as well as free pressures in all network nodes, are determined.
To determine the pressure at supply points (at the water tower, at the pumping station), it is necessary to know the required pressures of water consumers. As mentioned above, the minimum free pressure in the water supply network of a settlement with maximum domestic and drinking water supply at the entrance to the building above the ground surface in a one-story building should be at least 10 m (0.1 MPa), with a higher number of storeys it is necessary to add 4 to each floor m.
During the hours of least water consumption, the pressure for each floor, starting from the second, is allowed to be 3 m. For individual multi-storey buildings, as well as groups of buildings located in elevated areas, provide local pumping installations. The free pressure at the water dispensers must be at least 10 m (0.1 MPa),
IN external network industrial water pipelines free pressure is taken according to technical specifications equipment. The free pressure in the consumer's drinking water supply network should not exceed 60 m, otherwise for individual areas or buildings it is necessary to install pressure regulators or zoning the water supply system. When operating a water supply system, a free pressure of no less than the standard must be ensured at all points in the network.
Free heads at any point in the network are determined as the difference between the elevations of the piezometric lines and the ground surface. Piezometric marks for all design cases (for domestic and drinking water consumption, in case of fire, etc.) are calculated based on the provision of standard free pressure at the dictating point. When determining piezometric marks, they are set by the position of the dictating point, i.e., the point that has a minimum free pressure.
Typically, the dictating point is located in the most unfavorable conditions both in terms of geodetic elevations (high geodetic elevations) and in terms of distance from the power source (i.e., the sum of the pressure losses from the power source to the dictating point will be the greatest). At the dictating point they are set by a pressure equal to the normative one. If at any point in the network the pressure is less than the standard one, then the position of the dictating point is set incorrectly. In this case, they find the point with the lowest free pressure, take it as the dictating one, and repeat the calculation of the pressure in the network.
The calculation of the water supply system for operation during a fire is carried out on the assumption that it occurs at the highest points and remotest from power sources in the territory served by the water supply. According to the method of fire extinguishing, water pipelines are of high and low pressure.
As a rule, when designing water supply systems, low pressure fire water supply should be adopted, with the exception of small settlements(less than 5 thousand people). Device fire-fighting water supply high pressure must be economically justified,
In low-pressure water supply systems, the pressure is increased only while the fire is being extinguished. The necessary increase in pressure is created by mobile fire pumps, which are transported to the site of the fire and take water from the water supply network through street hydrants.
According to SNiP, the pressure at any point in the low-pressure fire-fighting water supply network at ground level during fire fighting must be at least 10 m. Such pressure is necessary to prevent the possibility of vacuum formation in the network when water is drawn from fire pumps, which, in turn, can cause penetration into network through leaky soil water joints.
In addition, a certain supply of pressure in the network is required for the operation of fire truck pumps in order to overcome significant resistance in the suction lines.
A high-pressure fire extinguishing system (usually adopted at industrial facilities) provides for the supply of water to the fire site as required by fire regulations and increasing the pressure in the water supply network to a value sufficient to create fire jets directly from the hydrants. The free pressure in this case should ensure a compact jet height of at least 10 m at full fire water flow and the location of the fire nozzle barrel at the level of the highest point of the tallest building and water supply through fire hoses 120 m long:
Nsv = N building + 10 + ∑h ≈ N building + 28 (m)
where H building is the height of the building, m; h - pressure loss in the hose and barrel of the fire nozzle, m.
In high-pressure water supply systems, stationary fire pumps are equipped with automatic equipment that ensures that the pumps start no later than 5 minutes after a signal about a fire is given. The network pipes must be selected taking into account the increase in pressure during a fire. The maximum free pressure in the combined water supply network should not exceed 60 m of water column (0.6 MPa), and during the hour of a fire - 90 m (0.9 MPa).
In case of significant differences in geodetic elevations of the object supplied with water, a large length of water supply networks, as well as when big difference in the quantities required by individual consumers of free pressure (for example, in microdistricts with different number of storeys), zoning of the water supply network is arranged. It may be due to both technical and economic considerations.
The division into zones is carried out based on the following conditions: at the highest point of the network the necessary free pressure must be provided, and at its lowest (or initial) point the pressure must not exceed 60 m (0.6 MPa).
According to the types of zoning, water supply systems come with parallel and sequential zoning. Parallel zoning of water supply systems is used for large ranges of geodetic elevations within the city area. To do this, lower (I) and upper (II) zones are formed, which are supplied with water by pumping stations of zones I and II, respectively, with water supplied at different pressures through separate water pipelines. Zoning is carried out in such a way that at the lower boundary of each zone the pressure does not exceed the permissible limit.
Water supply scheme with parallel zoning
1 — pumping station II lift with two groups of pumps; 2—pumps of the II (upper) zone; 3 — pumps of the I (lower) zone; 4 - pressure-regulating tanks
Incorrect data. If the amount of pressure shortage goes beyond the actual values for a given network, then there is an error when entering the initial data or an error when plotting the network diagram on the map. You should check whether the following data has been entered correctly:
Hydraulic network mode.
If there are no errors when entering the initial data, but a lack of pressure exists and is of real significance for a given network, then in this situation the determination of the cause of the shortage and the method for eliminating it is carried out by the specialist working with this heating network.
Warning There is not enough pressure at the source Delta=X m. Where Delta is the required pressure.
WORST CONSUMER: ID=XX.
Figure 283. Message about the worst consumer
This message is displayed when there is a lack of available pressure at the consumer, where DeltaH− the value of the pressure that is not enough, m, a ID (XX)− individual number of the consumer for whom the pressure shortage is maximum.
Figure 284. Message about insufficient pressure
Double-click the left mouse button on the message about the worst consumer: the corresponding consumer will blink on the screen.
This error can be caused by several reasons:
ID=ХХ "Name of consumer" Emptying the heating system (H, m)
This message is displayed when there is insufficient pressure in the return pipeline to prevent emptying of the heating system of the upper floors of the building; the total pressure in the return pipeline must be at least the sum of the geodetic mark, the height of the building plus 5 meters to fill the system. The head reserve for filling the system can be changed in the calculation settings ().
XX− individual number of the consumer whose heating system is being emptied, N- pressure, in meters of which is not enough;
ID=ХХ "Name of consumer" Pressure in the return pipeline is higher than the geodetic mark by N, m
This message is issued when the pressure in the return pipeline is higher than permissible according to the strength conditions of cast iron radiators (more than 60 m. water column), where XX- individual consumer number and N- pressure value in the return pipeline exceeding the geodetic mark.
The maximum pressure in the return pipeline can be set independently in calculation settings. ;
ID=XX "Name of consumer" Elevator nozzle cannot be selected. Set the maximum
This message may appear when there is a large heating load or when an incorrect connection diagram is selected that does not correspond to the design parameters. XX- individual number of the consumer for whom the elevator nozzle cannot be selected;
ID=XX "Name of consumer" Elevator nozzle cannot be selected. Set the minimum
This message may appear when there are very small heating loads or when an incorrect connection diagram is selected that does not correspond to the design parameters. XX− individual number of the consumer for whom the elevator nozzle cannot be selected.
Warning Z618: ID=XX "XX" The number of washers on the supply pipe to CO is more than 3 (YY)
This message means that, as a result of the calculation, the number of washers required to adjust the system is more than 3 pieces.
Because minimum diameter The default washer size is 3 mm (specified in the calculation settings “Setting up the calculation of pressure losses”), and the consumption of the consumer’s heating system ID=XX is very small, then as a result of the calculation the total number of washers and the diameter of the last washer (in the consumer database) are determined. .
That is, a message like: The number of washers on the supply pipeline for CO is more than 3 (17) warns that to set up this consumer, you should install 16 washers with a diameter of 3 mm and 1 washer, the diameter of which is determined in the consumer database.
Warning Z642: ID=XX The elevator at the central heating station is not working
This message is displayed as a result of a verification calculation and means that the elevator unit is not functioning.
General principles hydraulic calculation pipelines of water heating systems are described in detail in the section Water heating systems. They are also applicable for calculating heat pipelines of heating networks, but taking into account some of their features. Thus, in the calculations of heat pipelines, the turbulent movement of water (water speed is more than 0.5 m/s, steam speed is more than 20-30 m/s, i.e. quadratic calculation area), values of the equivalent roughness of the inner surface steel pipes large diameters, mm, are accepted for: steam pipelines - k = 0.2; water network - k = 0.5; condensate pipelines - k = 0.5-1.0.
The estimated coolant costs for individual sections of the heating network are determined as the sum of the costs of individual subscribers, taking into account the connection diagram of the DHW heaters. In addition, it is necessary to know the optimal specific pressure drops in pipelines, which are previously determined by technical and economic calculations. They are usually taken equal to 0.3-0.6 kPa (3-6 kgf/m2) for main heating networks and up to 2 kPa (20 kgf/m2) for branches.
When performing hydraulic calculations, the following tasks are solved: 1) determining the diameters of pipelines; 2) determination of pressure-pressure drop; 3) determination of current pressures at various points in the network; 4) determination of permissible pressures in pipelines under various operating modes and conditions of the heating network.
When carrying out hydraulic calculations, diagrams and a geodetic profile of the heating main are used, indicating the location of heat supply sources, heat consumers and design loads. To speed up and simplify calculations, instead of tables, logarithmic nomograms of hydraulic calculations are used (Fig. 1), and in last years- computer calculation and graphic programs.
Picture 1.
PIEZOMETRIC GRAPH
When designing and in operational practice, piezometric graphs are widely used to take into account the mutual influence of the geodetic profile of the area, the height of subscriber systems, and operating pressures in the heating network. From them it is easy to determine the pressure (pressure) and available pressure at any point in the network and in the subscriber system for the dynamic and static state of the system. Let's consider the construction of a piezometric graph, and we will assume that pressure and pressure, pressure drop and pressure loss are related by the following dependencies: H = p/γ, m (Pa/m); ∆Н = ∆р/ γ, m (Pa/m); and h = R/ γ (Pa), where Н and ∆Н - pressure and pressure loss, m (Pa/m); р and ∆р - pressure and pressure drop, kgf/m 2 (Pa); γ - mass density of the coolant, kg/m3; h and R - specific pressure loss (dimensionless value) and specific pressure drop, kgf/m 2 (Pa/m).
When constructing a piezometric graph in dynamic mode, the axis of the network pumps is taken as the origin of coordinates; taking this point as a conditional zero, they build a terrain profile along the route of the main highway and along characteristic branches (the elevations of which differ from the elevations of the main highway). The heights of the connected buildings are drawn on the profile on a scale, then, having previously assumed a pressure on the suction side of the network pumps collector H sun = 10-15 m, the horizontal line A 2 B 4 is drawn (Fig. 2, a). From point A 2, the lengths of the calculated sections of heat pipelines are plotted along the abscissa axis (with a cumulative total), and along the ordinate axis from the end points of the calculated sections - the pressure loss Σ∆H in these sections. By connecting the upper points of these segments, we obtain a broken line A 2 B 2, which will be the piezometric line of the return line. Each vertical segment from the conventional level A 2 B 4 to the piezometric line A 2 B 2 indicates the pressure loss in the return line from the corresponding point to the circulation pump at the thermal power plant. From point B 2 on a scale, the required available pressure for the subscriber at the end of the line ∆H ab is plotted upward, which is taken to be 15-20 m or more. The resulting segment B 1 B 2 characterizes the pressure at the end of the supply line. From point B 1, the pressure loss in the supply pipeline ∆Н p is postponed upward and a horizontal line B 3 A 1 is drawn.
Figure 2.a - construction of a piezometric graph; b - piezometric graph of a two-pipe heating network
From line A 1 B 3 downward, pressure losses are deposited in the section of the supply line from the heat source to the end of the individual calculated sections, and the piezometric line A 1 B 1 of the supply line is constructed similarly to the previous one.
At closed systems ah TsTS and equal diameters pipes of the supply and return lines, the piezometric line A 1 B 1 is a mirror image of line A 2 B 2. From point A, the pressure loss in the boiler room of the thermal power plant or in the boiler room circuit ∆Н b (10-20 m) is postponed upward. The pressure in the supply manifold will be N n, in the return manifold - N sun, and the pressure of the network pumps will be N s.n.
It is important to note that when connecting local systems directly, the return pipeline of the heating network is hydraulically connected to the local system, and the pressure in the return pipeline is entirely transferred to the local system and vice versa.
During the initial construction of the piezometric graph, the pressure at the suction manifold of the network pumps N vs was taken arbitrarily. Moving the piezometric graph parallel to itself up or down allows you to accept any pressure on the suction side of network pumps and, accordingly, in local systems.
When choosing the position of the piezometric graph, it is necessary to proceed from the following conditions:
1. The pressure (pressure) at any point in the return line should not be higher than the permissible operating pressure in local systems, for new heating systems (with convectors) operating pressure 0.1 MPa (10 m water column), for systems with cast iron radiators 0.5-0.6 MPa (50-60 m water column).
2. The pressure in the return pipeline must ensure that the upper lines and devices of local heating systems are filled with water.
3. The pressure in the return line, in order to avoid the formation of a vacuum, should not be lower than 0.05-0.1 MPa (5-10 m of water column).
4. The pressure on the suction side of the network pump should not be lower than 0.05 MPa (5 m water column).
5. The pressure at any point in the supply pipeline must be higher than the boiling pressure at the maximum (design) temperature of the coolant.
6. The available pressure at the end point of the network must be equal to or greater than the calculated pressure loss at the subscriber input for the calculated coolant flow.
7. B summer period the pressure in the supply and return lines takes on more than the static pressure in the DHW system.
Static state of the central heating system. When the network pumps stop and water circulation in the central heating system stops, it goes from a dynamic state to a static one. In this case, the pressures in the supply and return lines of the heating network will be equalized, the piezometric lines will merge into one - the static pressure line, and on the graph it will take an intermediate position, determined by the pressure of the make-up device of the MDH source.
The pressure of the make-up device is set by the station personnel either by the highest point of the pipeline of the local system directly connected to the heating network, or by the vapor pressure of superheated water at the highest point of the pipeline. So, for example, at the design temperature of the coolant T 1 = 150 °C, the pressure at the highest point of the pipeline with superheated water will be equal to 0.38 MPa (38 m of water column), and at T 1 = 130 °C - 0.18 MPa (18 m water column).
However, in all cases, the static pressure in low-lying subscriber systems should not exceed the permissible operating pressure of 0.5-0.6 MPa (5-6 atm). If it is exceeded, these systems should be transferred to an independent connection scheme. Reducing static pressure in heating networks can be carried out by automatically disconnecting from the network tall buildings.
In emergency cases, in the event of a complete loss of power supply to the station (stopping the network and make-up pumps), circulation and make-up will stop, while the pressures in both lines of the heating network will be equalized along the line of static pressure, which will begin to slowly, gradually decrease due to the leakage of network water through leaks and cooling it in pipelines. In this case, boiling of superheated water in pipelines is possible with the formation of vapor locks. Resuming water circulation in such cases can lead to severe water hammer in the pipelines with possible damage to fittings, heating devices, etc. To avoid this phenomenon, water circulation in the central heating system should begin only after the pressure in the pipelines has been restored by replenishing the heating network at a level not lower than the static one.
To provide reliable operation heating networks and local systems, it is necessary to limit possible pressure fluctuations in the heating network to acceptable limits. To maintain the required level of pressure in the heating network and local systems, at one point of the heating network (and in difficult terrain conditions - at several points), a constant pressure is artificially maintained under all operating modes of the network and during static conditions using a make-up device.
The points at which the pressure is maintained constant are called the neutral points of the system. As a rule, pressure is secured on the return line. In this case, the neutral point is located at the intersection of the reverse piezometer with the static pressure line (point NT in Fig. 2, b), maintaining constant pressure at the neutral point and replenishing coolant leakage is carried out by make-up pumps of the thermal power plant or RTS, KTS through an automated make-up device. Automatic regulators are installed on the make-up line, operating on the principle of “after” and “before” regulators (Fig. 3).
Figure 3. 1 - network pump; 2 - make-up pump; 3 - heating water; 4 - make-up regulator valve
The pressures of network pumps N s.n are taken equal to the sum hydraulic losses pressure (at maximum - design water flow): in the supply and return pipelines of the heating network, in the subscriber's system (including inputs to the building), in the boiler installation of the thermal power plant, its peak boilers or in the boiler room. Heat sources must have at least two network and two make-up pumps, of which one is a reserve pump.
The amount of recharge for closed heat supply systems is assumed to be 0.25% of the volume of water in the pipelines of heating networks and in subscriber systems connected to the heating network, h.
In schemes with direct water withdrawal, the amount of recharge is taken to be equal to the sum of the calculated water consumption for hot water supply and the amount of leakage in the amount of 0.25% of the system capacity. The capacity of heating systems is determined by the actual diameters and lengths of pipelines or by aggregated standards, m 3 / MW:
The disunity that has developed on the basis of ownership in the organization of operation and management of urban heat supply systems has the most negative impact on both technical level their functioning, and on their economic efficiency. It was noted above that the operation of each specific heat supply system is carried out by several organizations (sometimes “subsidiaries” of the main one). However, the specificity of district heating systems, primarily heating networks, is determined by the tight connection of the technological processes of their functioning, and uniform hydraulic and thermal regimes. The hydraulic mode of the heat supply system, which is the determining factor in the functioning of the system, is extremely unstable by its nature, which makes heat supply systems difficult to control compared to other urban engineering systems(electricity, gas, water supply).
None of the links in the district heating systems (heat source, main and distribution networks, heating points) can independently provide the required technological modes of operation of the system as a whole, and, consequently, the end result - reliable and high-quality heat supply to consumers. Ideal in this sense is organizational structure, in which heat supply sources and heating networks are under the jurisdiction of one enterprise structure.
The piezometric graph shows the terrain, the height of attached buildings, and the pressure in the network on a scale. Using this graph, it is easy to determine the pressure and available pressure at any point in the network and subscriber systems.
Level 1 – 1 is taken as the horizontal plane of pressure reference (see Fig. 6.5). Line P1 – P4 – graph of supply line pressures. Line O1 – O4 – return line pressure graph. N o1 – total pressure on the return collector of the source; Nсн – pressure of the network pump; N st – full pressure of the make-up pump, or full static pressure in the heating network; N to– total pressure in t.K at the discharge pipe of the network pump; D H t – pressure loss in the heat treatment plant; N p1 – total pressure on the supply manifold, N n1 = N k–D H t. Available supply water pressure at the CHP collector N 1 =N p1 - N o1. Pressure at any point in the network i denoted as N p i, H oi – total pressures in the forward and return pipelines. If the geodetic height at a point i There is Z i , That piezometric head at this point there is N p i – Z i , H o i – Z i in the forward and return pipelines, respectively. Available head at point i is the difference in piezometric pressures in the forward and return pipelines – N p i – H oi. The available pressure in the heating network at the connection point of subscriber D is N 4 = N n4 – N o4.
Fig.6.5. Scheme (a) and piezometric graph (b) of a two-pipe heating network
There is a loss of pressure in the supply line in section 1 - 4 
. There is a pressure loss in the return line in section 1 - 4 
. When the mains pump is operating, the pressure N The speed of the charging pump is regulated by a pressure regulator to N o1. When the network pump stops, a static pressure is established in the network N st, developed by the make-up pump.
When hydraulically calculating a steam pipeline, the profile of the steam pipeline may not be taken into account due to the low steam density. Pressure losses from subscribers, for example 
, depends on the subscriber connection scheme. With elevator mixing D N e = 10...15 m, with elevator-free input – D n BE =2...5 m, in the presence of surface heaters D N n =5...10 m, with pump mixing D N ns = 2…4 m.
Requirements for pressure conditions in the heating network:
At any point in the system, the pressure should not exceed the maximum permissible value. The pipelines of the heat supply system are designed for 16 ata, the pipelines of local systems are designed for a pressure of 6...7 ata;
To avoid air leaks at any point in the system, the pressure must be at least 1.5 atm. In addition, this condition is necessary to prevent pump cavitation;
At any point in the system, the pressure must be no less than the saturation pressure at a given temperature to avoid boiling of water.
Read also:
|
In water heat supply systems, the provision of heat to consumers is carried out by appropriately distributing the estimated costs of network water between them. To implement such distribution, it is necessary to develop a hydraulic mode of the heat supply system.
The purpose of developing the hydraulic mode of the heating supply system is to ensure optimal permissible pressures in all elements of the heating supply system and the necessary available pressures at the nodes of the heating network, at group and local heating points, sufficient to supply consumers with the calculated water flows. The available pressure is the difference in water pressure in the supply and return pipelines.
To ensure reliable operation of the heat supply system, the following conditions apply:
Not exceeding permissible pressures: in heat supply sources and heating networks: 1.6-2.5 mPa - for steam-water network heaters of the PSV type, for steel hot water boilers, steel pipes and fittings; in subscriber installations: 1.0 mPa - for sectional water-water heaters; 0.8-1.0 mPa - for steel convectors; 0.6 mPa - for cast iron radiators; 0.8 mPa - for air heaters;
Security overpressure in all elements of the heat supply system to prevent pump cavitation and protect the heat supply system from air leaks. The minimum value of excess pressure is assumed to be 0.05 MPa. For this reason, the piezometric line of the return pipeline in all modes must be located above the point of the tallest building by at least 5 m of water. Art.;
At all points of the heating system, a pressure must be maintained that exceeds the pressure of saturated water vapor at the maximum water temperature, ensuring that the water does not boil. As a rule, the danger of water boiling most often occurs in the supply pipelines of the heating network. Minimum pressure in the supply pipelines is taken according to the design temperature of the supply water, table 7.1.
Table 7.1
The non-boiling line must be drawn on the graph parallel to the terrain at a height corresponding to the excess pressure at the maximum temperature of the coolant.
It is convenient to depict the hydraulic mode graphically in the form of a piezometric graph. The piezometric graph is plotted for two hydraulic modes: hydrostatic and hydrodynamic.
The purpose of developing a hydrostatic mode is to ensure the necessary water pressure in the heating system, within acceptable limits. The lower pressure limit should ensure that consumer systems are filled with water and create the necessary minimum pressure to protect the heating system from air leaks. The hydrostatic mode is developed with charging pumps running and no circulation.
The hydrodynamic mode is developed on the basis of hydraulic calculation data for heating networks and is ensured by the simultaneous operation of make-up and network pumps.
The development of a hydraulic mode comes down to constructing a piezometric graph that meets all the requirements for the hydraulic mode. Hydraulic modes water heating networks (piezometric graphs) should be developed for the heating and non-heating periods. The piezometric graph allows you to: determine the pressures in the supply and return pipelines; available pressure at any point in the heating network, taking into account the terrain; select consumer connection schemes based on available pressure and building heights; select auto regulators, elevator nozzles, throttling devices for local heat consumer systems; select network and make-up pumps.
Construction of a piezometric graph(Fig. 7.1) is produced in the following way:
a) scales are selected along the abscissa and ordinate axes and the terrain and the height of the building blocks are plotted. Piezometric graphs are constructed for main and distribution heating networks. For main heating networks the following scales can be adopted: horizontal M g 1:10000; vertical M in 1:1000; for distribution heating networks: M g 1:1000, M v 1:500; The zero mark of the ordinate axis (pressure axis) is usually taken to be the mark of the lowest point of the heating main or the mark of the network pumps.
b) the value of the static pressure is determined to ensure the filling of consumer systems and the creation of minimal excess pressure. This is the height of the highest building plus 3-5 m.water column.
After plotting the terrain and building heights, the static head of the system is determined
H c t = [N building + (3¸5)], m (7.1)
Where N rear- height of the highest building, m.
The static head H st is parallel to the x-axis, and it should not exceed the maximum operating pressure for local systems. The maximum operating pressure is: for heating systems with steel heating devices and for air heaters - 80 meters; for heating systems with cast iron radiators - 60 meters; for independent connection schemes with surface heat exchangers - 100 meters;
c) Then the dynamic mode is constructed. The suction pressure of network pumps H sun is arbitrarily selected, which should not exceed the static pressure and provides the necessary supply pressure at the inlet to prevent cavitation. The cavitation reserve, depending on the size of the pump, is 5-10 m.water column;
d) from the conditional pressure line at the suction of network pumps, pressure losses in the return pipeline DН return of the main heating line are successively deposited ( line A-B) using the results of hydraulic calculations. The amount of pressure in the return line must meet the requirements specified above when constructing the static pressure line;
e) the required available pressure is set aside at the last subscriber DN ab, based on the operating conditions of the elevator, heater, mixer and distribution heating networks (line B-C). The amount of available pressure at the connection point of distribution networks is assumed to be at least 40 m;
e) starting from the last pipeline node, pressure losses are deposited in the supply pipeline of the main line DN under ( line C-D). Pressure at all points of the supply pipeline based on its conditions mechanical strength should not exceed 160 m;
g) pressure losses are delayed in the heat source DН it ( line D-E) and the pressure at the outlet of the network pumps is obtained. In the absence of data, the pressure loss in the communications of a thermal power plant can be assumed to be 25 - 30 m, and for a district boiler house 8-16 m.
The pressure of the network pumps is determined
The pressure of the charging pumps is determined by the pressure of the static mode.
As a result of this construction, the initial form of a piezometric graph is obtained, which allows one to estimate pressures at all points of the heat supply system (Fig. 7.1).
If they do not meet the requirements, change the position and shape of the piezometric graph:
a) if the pressure line of the return pipeline crosses the height of the building or is less than 3¸5 m from it, then the piezometric graph should be raised so that the pressure in the return pipeline ensures filling of the system;
b) if the maximum pressure in the return pipeline exceeds the permissible pressure in heating devices, and it cannot be reduced by shifting the piezometric graph down, it should be reduced by installing booster pumps in the return pipeline;
c) if the non-boiling line intersects the pressure line in the supply pipeline, then boiling of water is possible beyond the intersection point. Therefore, the water pressure in this part of the heating network should be increased by moving the piezometric graph upward, if possible, or by installing a booster pump on the supply pipeline;
d) if the maximum pressure in the equipment of the heat treatment plant of the heat source exceeds the permissible value, then booster pumps are installed on the supply pipeline.
Division of the heating network into static zones. The piezometric graph is developed for two modes. Firstly, for static mode, when there is no water circulation in the heating system. It is assumed that the system is filled with water at a temperature of 100°C, thereby eliminating the need to maintain excess pressure in the heat pipes to avoid boiling of the coolant. Secondly, for hydrodynamic mode - in the presence of coolant circulation in the system.
The development of the schedule begins with the static mode. The location of the full static pressure line on the graph should ensure the connection of all subscribers to the heating network according to a dependent scheme. To do this, the static pressure should not exceed what is permissible based on the strength of subscriber installations and should ensure that local systems are filled with water. The presence of a common static zone for the entire heating system simplifies its operation and increases its reliability. If there is a significant difference in geodetic elevations of the earth, establishing a common static zone is impossible for the following reasons.
The lowest position of the static pressure level is determined from the conditions of filling local systems with water and ensuring that at the highest points of the systems of the tallest buildings located in the area of the highest geodetic marks, an excess pressure of at least 0.05 MPa. This pressure turns out to be unacceptably high for buildings located in that part of the area that has the lowest geodetic elevations. Under such conditions, it becomes necessary to divide the heat supply system into two static zones. One zone is for part of the area with low geodetic marks, the other - with high ones.
In Fig. 7.2 shows the piezometric graph and circuit diagram heat supply systems for an area with a significant difference in geodetic ground level marks (40m). The part of the area adjacent to the heat supply source has zero geodetic marks; in the peripheral part of the area the marks are 40 m. The height of the buildings is 30 and 45 m. To be able to fill building heating systems with water III and IV, located at the 40 m mark and creating an excess pressure of 5 m at the upper points of the systems, the level of the total static pressure should be located at the 75 m mark (line 5 2 - S 2). In this case, the static head will be equal to 35m. However, a head of 75m is unacceptable for buildings I And II, located at the zero mark. For them, the permissible highest position of the level of total static pressure corresponds to 60 m. Thus, under the conditions under consideration, it is impossible to establish a common static zone for the entire heat supply system.
A possible solution is to divide the heat supply system into two zones with different levels of total static heads - the lower one with a level of 50 m (line S t-Si) and the upper one with a level of 75m (line S 2 -S 2). With this solution, all consumers can be connected to the heat supply system according to a dependent scheme, since the static pressures in the lower and upper zones are within acceptable limits.
So that when water circulation in the system stops, the static pressure levels are established in accordance with the accepted two zones, a separating device is placed at the point of their connection (Fig. 7.2 6 ). This device protects heating network from increased pressure when the circulation pumps stop, automatically cutting it into two hydraulically independent zones: upper and lower.
When the circulation pumps are stopped, the pressure drop in the return pipeline of the upper zone is prevented by the pressure regulator “towards itself” RDDS (10), which maintains a constant set pressure RDDS at the point where the pulse is taken. When the pressure drops, it closes. A pressure drop in the supply line is prevented by a check valve(11), which also closes. Thus, the RDDS and the check valve cut the heating network into two zones. To feed the upper zone, a feed pump (8) is installed, which takes water from the lower zone and supplies it to the upper one. The pressure developed by the pump is equal to the difference between the hydrostatic heads of the upper and lower zones. The lower zone is fed by the make-up pump 2 and the make-up regulator 3.
Figure 7.2. Heating system divided into two static zones
a - piezometric graph;
b - schematic diagram of the heat supply system; S 1 - S 1, - line of total static pressure of the lower zone;
S 2 – S 2, - line of total static pressure of the upper zone;
N p.n1 - pressure developed by the feed pump of the lower zone; N p.n2 - pressure developed by the top zone make-up pump; N RDDS - pressure to which the RDDS (10) and RD2 (9) regulators are set; ΔН RDDS - pressure activated on the RDDS regulator valve in hydrodynamic mode; I-IV- subscribers; 1-make-up water tank; 2.3 - feed pump and feed regulator for the lower zone; 4 - pre-switched pump; 5 - main steam-water heaters; 6- network pump; 7 - peak hot water boiler; 8 , 9 - make-up pump and top zone make-up regulator; 10 - pressure regulator “towards you” RDDS; 11- check valve
The RDDS regulator is set to the pressure Nrdds (Fig. 7.2a). The make-up regulator RD2 is set to the same pressure.
In hydrodynamic mode, the RDDS regulator maintains the pressure at the same level. At the beginning of the network, a make-up pump with a regulator maintains the pressure of H O1. The difference in these pressures is spent on overcoming the hydraulic resistance in the return pipeline between the separating device and circulation pump heat source, the rest of the pressure is activated in the throttle substation at the RDDS valve. In Fig. 8.9, and this part of the pressure is shown by the value ΔН RDDS. The throttle substation in hydrodynamic mode makes it possible to maintain the pressure in the return line of the upper zone not lower than the accepted level of static pressure S 2 - S 2.
Piezometric lines corresponding to the hydrodynamic regime are shown in Fig. 7.2a. Highest pressure in the return pipeline at the consumer, IV is 90-40 = 50m, which is acceptable. The pressure in the return line of the lower zone is also within acceptable limits.
In the supply pipeline, the maximum pressure after the heat source is 160 m, which does not exceed what is permissible based on the strength of the pipes. The minimum piezometric pressure in the supply pipeline is 110 m, which ensures that the coolant does not boil over, since at a design temperature of 150 ° C the minimum permissible pressure is 40 m.
The piezometric graph developed for static and hydrodynamic modes provides the ability to connect all subscribers according to a dependent circuit.
To others possible solution hydrostatic mode of the heating system shown in Fig. 7.2 is the connection of some subscribers according to an independent scheme. There may be two options here. First option- set the general level of static pressure at 50 m (line S 1 - S 1), and connect the buildings located at the upper geodetic marks according to an independent scheme. In this case, the static pressure in water-water heating heaters of buildings in the upper zone on the side of the heating coolant will be 50-40 = 10 m, and on the side of the heated coolant will be determined by the height of the buildings. The second option is to set the general level of static pressure at 75 m (line S 2 - S 2) with the connection of the buildings of the upper zone according to a dependent scheme, and the buildings of the lower zone - according to an independent one. In this case, the static pressure in water-water heaters on the side of the heating coolant will be equal to 75 m, i.e. less than the permissible value (100 m).
Main 1, 2; 3;
add. 4, 7, 8.