What is available pressure in a heating system. Piezometric graph of a heating network

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 heat supply system is to ensure optimal permissible pressures in all elements of the heat supply system and the necessary available pressures at the nodes of the heating network, in 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. The minimum pressure in the supply pipelines is taken according to the calculated 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. Piezometric graph is built 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 data from the hydraulic calculation of 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 of 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 done as follows:

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, the 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 point of connection 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 DH 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 return pipeline pressure line 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 total static pressure level 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 the 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 specified pressure RDDS at the point of impulse selection. 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.

Another possible solution to the 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.

The available pressure drop to create water circulation, Pa, is determined by the formula

where DPn is the pressure created by the circulation pump or elevator, Pa;

ДПе - natural circulation pressure in the calculation ring due to cooling of water in pipes and heating devices, Pa;

IN pumping systems it is allowed not to take into account DP if it is less than 10% of DP.

Available pressure drop at the entrance to the building DPr = 150 kPa.

Calculation of natural circulation pressure

The natural circulation pressure that arises in the design ring of a vertical single-pipe system with bottom distribution, adjustable with closing sections, Pa, is determined by the formula

where is the average increase in water density when its temperature decreases by 1? C, kg/(m3?? C);

Vertical distance from heating center to cooling center

heating device, m;

Water flow in the riser, kg/h, is determined by the formula

Calculation of pump circulation pressure

The value, Pa, is selected in accordance with the available pressure difference at the inlet and the mixing coefficient U according to the nomogram.

Available pressure difference at the inlet =150 kPa;

Coolant parameters:

In the heating network f1=150?C; f2=70?C;

In the heating system t1=95?C; t2=70?C;

We determine the mixing coefficient using the formula

µ= f1 - t1 / t1 - t2 =150-95/95-70=2.2; (2.4)

Hydraulic calculation of water heating systems using the method of specific pressure loss due to friction

Calculation of the main circulation ring

1) Hydraulic calculation The main circulation ring is carried out through riser 15 of a vertical single-pipe water heating system with lower wiring and dead-end movement of the coolant.

2) We divide the main central circulation system into calculation sections.

3) To pre-select the diameter of the pipes, an auxiliary value is determined - the average value of the specific pressure loss from friction, Pa, per 1 meter of pipe according to the formula

where is the available pressure in the adopted heating system, Pa;

Total length of the main circulation ring, m;

Correction factor taking into account the share of local pressure losses in the system;

For a heating system with pump circulation, the share of loss due to local resistance is b=0.35, and due to friction b=0.65.

4) Determine the coolant flow rate in each section, kg/h, using the formula

Parameters of the coolant in the supply and return pipelines of the heating system, ?C;

Specific mass heat capacity of water equal to 4.187 kJ/(kg??С);

Coefficient for taking into account additional heat flow when rounding above the calculated value;

Coefficient of accounting for additional heat losses by heating devices near external fences;

6) We determine the coefficients of local resistance in the design areas (and write their sum in Table 1) by .

Table 1

7) At each section of the main circulation ring, we determine the pressure loss due to local resistance Z, depending on the sum of the local resistance coefficients Uo and the water speed in the section.

8) We check the reserve of available pressure drop in the main circulation ring according to the formula

where is the total pressure loss in the main circulation ring, Pa;

With a dead-end coolant flow pattern, the discrepancy between pressure losses in the circulation rings should not exceed 15%.

We summarize the hydraulic calculation of the main circulation ring in Table 1 (Appendix A). As a result, we obtain the pressure loss discrepancy

Calculation of a small circulation ring

We perform a hydraulic calculation of the secondary circulation ring through riser 8 of a single-pipe water heating system

1) We calculate the natural circulation pressure due to the cooling of water in the heating devices of riser 8 using formula (2.2)

2) Determine the water flow in riser 8 using formula (2.3)

3) We determine the available pressure drop for the circulation ring through the secondary riser, which should be equal to the known pressure losses in the sections of the main circulation circuit, adjusted for the difference in natural circulation pressure in the secondary and main rings:

15128.7+(802-1068)=14862.7 Pa

4) Find the average value of linear pressure loss using formula (2.5)

5) Based on the value, Pa/m, of the coolant flow rate in the area, kg/h, and based on the maximum permissible speeds of coolant movement, we determine the preliminary diameter of the pipes dу, mm; actual specific pressure loss R, Pa/m; actual coolant speed V, m/s, according to .

6) We determine the coefficients of local resistance in the design areas (and write their sum in Table 2) by .

7) In the section of the small circulation ring, we determine the pressure loss due to local resistance Z, depending on the sum of the local resistance coefficients Uo and the water speed in the section.

8) We summarize the hydraulic calculation of the small circulation ring in Table 2 (Appendix B). We check the hydraulic connection between the main and small hydraulic rings according to the formula

9) Determine the required pressure loss in the throttle washer using the formula

10) Determine the diameter of the throttle washer using the formula

At the site it is required to install a throttle washer with an internal passage diameter of DN=5mm

General principles of hydraulic calculation of pipelines for 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 is taken (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 of large-diameter steel pipes, mm, 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 conditional 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 laid 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.

With closed DH systems 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 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 the static pressure in heating networks can be achieved by automatically disconnecting high buildings from the network.

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 fixation is carried out 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 heater; 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 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, uniform hydraulic and thermal regimes. Hydraulic mode Heat supply systems, which is a determining factor in the functioning of the system, are extremely unstable by their nature, which makes heat supply systems difficult to manage 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, at which heat supply sources and heating network are managed by one enterprise structure.

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 of the second lift with two groups of pumps; 2—pumps of the II (upper) zone; 3 — pumps of the I (lower) zone; 4 - pressure-regulating tanks

Working pressure in the heating system - the most important parameter, on which the functioning of the entire network depends. Deviations in one direction or another from the values ​​​​provided by the project not only reduce the efficiency of the heating circuit, but also significantly affect the operation of the equipment, and in special cases can even cause it to fail.

Of course, a certain pressure drop in the heating system is determined by the principle of its design, namely the difference in pressure in the supply and return pipelines. But if there are larger spikes, immediate action should be taken.

1. Static pressure. This component depends on the height of the column of water or other coolant in the pipe or container. Static pressure exists even if working environment is at rest.
2. Dynamic pressure. It is a force that acts on the internal surfaces of the system when water or other medium moves.

The concept of maximum operating pressure is distinguished. This is the maximum permissible value, exceeding which can lead to the destruction of individual network elements.

What pressure in the system should be considered optimal?

Table of maximum pressure in the heating system.

When designing heating, the coolant pressure in the system is calculated based on the number of floors of the building, the total length of the pipelines and the number of radiators. As a rule, for private houses and cottages, the optimal values ​​of medium pressure in the heating circuit are in the range from 1.5 to 2 atm.

For apartment buildings up to five floors high connected to the system central heating, the network pressure is maintained at 2-4 atm. For nine- and ten-story buildings, a pressure of 5-7 atm is considered normal, and in taller buildings - 7-10 atm. The maximum pressure is recorded in the heating mains through which the coolant is transported from boiler houses to consumers. Here it reaches 12 atm.

For consumers located on different heights and at different distances from the boiler room, the pressure in the network has to be adjusted. To reduce it, pressure regulators are used, to increase it - pumping stations. However, it should be taken into account that a faulty regulator can cause an increase in pressure in certain areas of the system. In some cases, when the temperature drops, these devices can completely shut off the shut-off valves on the supply pipeline coming from the boiler plant.

To avoid such situations, the regulator settings are adjusted so that complete shutoff of the valves is impossible.

Autonomous heating systems

Expansion tank in an autonomous heating system.

With absence district heating In houses, autonomous heating systems are installed, in which the coolant is heated by an individual low-power boiler. If the system communicates with the atmosphere through an expansion tank and the coolant circulates in it due to natural convection, it is called open. If there is no communication with the atmosphere, and the working medium circulates thanks to the pump, the system is called closed. As already mentioned, for normal functioning In such systems, the water pressure in them should be approximately 1.5-2 atm. Such low rate due to the relatively short length of pipelines, as well as a small number of instruments and fittings, which results in relatively low hydraulic resistance. In addition, due to the low height of such houses, the static pressure in the lower sections of the circuit rarely exceeds 0.5 atm.

At the stage of launching the autonomous system, it is filled with cold coolant, maintaining a minimum pressure in closed heating systems of 1.5 atm. There is no need to sound the alarm if, some time after filling, the pressure in the circuit drops. Pressure loss in in this case are caused by the release of air from the water, which dissolved in it when filling the pipelines. The circuit should be de-aired and completely filled with coolant, bringing its pressure to 1.5 atm.

After heating the coolant in the heating system, its pressure will increase slightly, reaching the calculated operating values.

Precautionary measures

A device for measuring pressure.

Since when designing autonomous systems In heating systems, in order to save money, a small safety margin is laid down; even a low pressure surge of up to 3 atm can cause depressurization of individual elements or their connections. In order to smooth out pressure drops due to unstable pump operation or changes in coolant temperature, in closed system heating system, install an expansion tank. Unlike a similar device in the system open type, it has no communication with the atmosphere. One or more of its walls are made of elastic material, due to which the tank acts as a damper during pressure surges or water hammer.

The presence of an expansion tank does not always guarantee that pressure is maintained within optimal limits. In some cases it may exceed the maximum permissible values:

• if the expansion tank capacity is incorrectly selected;
• in case of malfunction of the circulation pump;
• when the coolant overheats, which is a consequence of malfunctions in the operation of the boiler automation;
• due to incomplete opening of shut-off valves after repairs or maintenance work;
• due to the appearance air lock(this phenomenon can provoke both an increase in pressure and a drop);
• when the throughput of the dirt filter decreases due to its excessive clogging.

Therefore, in order to avoid emergency situations when installing heating systems closed type, it is mandatory to install a safety valve that will release excess coolant if the permissible pressure is exceeded.

What to do if the pressure in the heating system drops

Pressure in the expansion tank.

When operating autonomous heating systems, the most common are the following: emergency situations, in which the pressure decreases smoothly or sharply. They can be caused by two reasons:

• depressurization of system elements or their connections;
• problems with the boiler.

In the first case, the location of the leak should be located and its tightness restored. You can do this in two ways:

1. Visual inspection. This method is used in cases where the heating circuit is laid in an open manner (not to be confused with an open-type system), that is, all its pipelines, fittings and devices are visible. First of all, carefully inspect the floor under the pipes and radiators, trying to detect puddles of water or traces of them. In addition, the location of the leak can be identified by traces of corrosion: characteristic rusty streaks form on radiators or at the joints of system elements when the seal is broken.
2. Using special equipment. If a visual inspection of the radiators does not yield anything, and the pipes are laid in a hidden way and cannot be examined, you should seek the help of specialists. They have special equipment, which will help detect a leak and fix it if the home owner is not able to do it himself. Localizing the depressurization point is quite simple: water is drained from the heating circuit (for such cases, a drain valve is installed at the lowest point of the circuit during the installation stage), then air is pumped into it using a compressor. The location of the leak is determined by the characteristic sound that leaking air makes. Before starting the compressor, the boiler and radiators should be insulated using shut-off valves.

If the problem area is one of the joints, it is additionally sealed with tow or FUM tape and then tightened. The burst pipeline is cut out and a new one is welded in its place. Units that cannot be repaired are simply replaced.

If the tightness of pipelines and other elements is beyond doubt, and the pressure in a closed heating system still drops, you should look for the reasons for this phenomenon in the boiler. You should not carry out diagnostics yourself; this is a job for a specialist with the appropriate education. Most often the following defects are found in the boiler:

Installation of a heating system with a pressure gauge.

• the appearance of microcracks in the heat exchanger due to water hammer;
• manufacturing defects;
• failure of the make-up valve.

A very common reason why system pressure drops is wrong selection expansion tank capacity.

Although the previous section stated that this may cause increased pressure, there is no contradiction here. When the pressure in the heating system increases, it triggers safety valve. In this case, the coolant is discharged and its volume in the circuit decreases. As a result, the pressure will decrease over time.

Pressure control

For visual monitoring of pressure in the heating network, dial pressure gauges with a Bredan tube are most often used. Unlike digital instruments, such pressure gauges do not require an electrical power connection. Automated systems use electrical contact sensors. A three-way valve must be installed at the outlet to the control and measuring device. It allows you to isolate the pressure gauge from the network during maintenance or repair, and is also used to remove an air lock or reset the device to zero.

Instructions and rules governing the operation of heating systems, both autonomous and centralized, recommend installing pressure gauges at the following points:

1. Before the boiler installation (or boiler) and at the exit from it. At this point the pressure in the boiler is determined.
2. Before and after the circulation pump.
3. At the entrance of the heating main into a building or structure.
4. Before and after the pressure regulator.
5. At the inlet and outlet of the coarse filter (mud filter) to control its level of contamination.

All control and measuring instruments must undergo regular verification to confirm the accuracy of the measurements they perform.