Preparation of specifications. Hydraulic calculation of heating networks Calculation of natural circulation pressure

“Specification of indicators of the quantity and quality of communal resources in the modern realities of housing and communal services”

SPECIFICATION OF INDICATORS OF QUANTITY AND QUALITY OF COMMUNAL RESOURCES IN MODERN REALITIES OF HUSING AND UTILITIES

V.U. Kharitonsky, Head of Engineering Systems Department

A. M. Filippov, Deputy Head of the Engineering Systems Department,

State Housing Inspectorate of Moscow

Documents regulating the indicators of the quantity and quality of communal resources supplied to household consumers at the border of responsibility of the resource supply and housing organizations have not been developed to date. Specialists from the Moscow Housing Inspectorate, in addition to the existing requirements, propose to specify the values ​​of the parameters of heat and water supply systems at the entrance to the building, in order to maintain the quality of public services in residential apartment buildings.

A review of the current rules and regulations for the technical operation of the housing stock in the field of housing and communal services showed that currently construction, sanitary norms and regulations, GOST R 51617 -2000 * “Housing and communal services”, “Rules for the provision of utility services to citizens”, approved by Decree of the Government of the Russian Federation dated May 23, 2006 No. 307, and other current regulatory documents consider and establish parameters and modes only at the source (central heating station, boiler room, water pumping station) that produces communal resources (cold, hot water and thermal energy), and directly in the resident’s apartment, where utilities are provided. However, they do not take into account the modern realities of the division of housing and communal services into residential buildings and public utility facilities and the established boundaries of responsibility of the resource supply and housing organizations, which are the subject of endless disputes in determining the guilty party for the failure to provide services to the population or the provision of services of inadequate quality. Thus, today there is no document regulating the indicators of quantity and quality at the entrance to the house, at the border of responsibility of the resource supply and housing organization.

However, an analysis of inspections of the quality of supplied utility resources and services carried out by the Moscow Housing Inspectorate showed that the provisions of federal regulatory legal acts in the field of housing and communal services can be detailed and specified in relation to apartment buildings, which will establish the mutual responsibility of resource supply and housing management organizations. It should be noted that the quality and quantity of communal resources supplied to the boundary of the operational responsibility of the resource supplying and managing housing organization, and public services to residents, is determined and assessed based on the readings, first of all, of common house metering devices installed at the inputs

heat and water supply systems to residential buildings, and an automated system for monitoring and accounting for energy consumption.

Thus, the Moscow Housing Inspectorate, based on the interests of residents and many years of practice, in addition to the requirements of regulatory documents and in development of the provisions of SNiP and SanPin in relation to operating conditions, as well as in order to maintain the quality of utility services provided to the population in residential apartment buildings, proposed regulating when introducing heat and water supply systems into the house (at the metering and control unit), the following standard values ​​of parameters and modes recorded by general house metering devices and an automated control and accounting system for energy consumption:

1) for a central heating system (CH):

The deviation of the average daily temperature of the network water entering the heating systems must be within ±3% of the established temperature schedule. The average daily temperature of the return network water should not exceed the temperature specified by the temperature schedule by more than 5%;

The network water pressure in the return pipeline of the central heating system must be no less than 0.05 MPa (0.5 kgf/cm2) higher than the static pressure (for the system), but not higher than permissible (for pipelines, heating devices, fittings and other equipment ). If necessary, it is allowed to install pressure regulators on the return pipelines in the ITP of heating systems of residential buildings directly connected to the main heating networks;

The network water pressure in the supply pipeline of central heating systems must be higher than the required water pressure in the return pipelines by the amount of available pressure (to ensure coolant circulation in the system);

The available pressure (pressure difference between the supply and return pipelines) of the coolant at the entrance of the central heating network into the building must be maintained by heat supply organizations within the limits:

a) with dependent connection (with elevator units) - in accordance with the design, but not less than 0.08 MPa (0.8 kgf/cm 2);

b) with independent connection - in accordance with the design, but not less than 0.03 MPa (0.3 kgf/cm2) more than the hydraulic resistance of the in-house central heating system.

2) For hot water supply system (DHW):

The temperature of hot water in the DHW supply pipeline for closed systems is within 55-65 °C, for open heat supply systems within 60-75 °C;

Temperature in the DHW circulation pipeline (for closed and open systems) 46-55 °C;

The arithmetic mean value of the hot water temperature in the supply and circulation pipelines at the inlet of the DHW system in all cases must be at least 50 °C;

The available pressure (pressure difference between the supply and circulation pipelines) at the calculated circulation flow rate of the hot water supply system must be no lower than 0.03-0.06 MPa (0.3-0.6 kgf/cm2);

The water pressure in the supply pipeline of the hot water supply system must be higher than the water pressure in the circulation pipeline by the amount of available pressure (to ensure the circulation of hot water in the system);

The water pressure in the circulation pipeline of hot water supply systems must be no less than 0.05 MPa (0.5 kgf/cm2) higher than the static pressure (for the system), but not exceed the static pressure (for the highest located and high-rise building) more than by 0.20 MPa (2 kgf/cm2).

With these parameters in apartments, the sanitary fixtures of residential premises, in accordance with the regulatory legal acts of the Russian Federation, must have the following values:

Hot water temperature is not lower than 50 °C (optimal - 55 °C);

The minimum free pressure for sanitary fixtures in residential premises on the upper floors is 0.02-0.05 MPa (0.2-0.5 kgf/cm2);

The maximum free pressure in hot water supply systems at sanitary fixtures on the upper floors should not exceed 0.20 MPa (2 kgf/cm2);

The maximum free pressure in water supply systems at sanitary fixtures on the lower floors should not exceed 0.45 MPa (4.5 kgf/cm2).

3) For a cold water supply system (CWS):

The water pressure in the supply pipeline of the cold water system must be at least 0.05 MPa (0.5 kgf/cm 2) higher than the static pressure (for the system), but not exceed the static pressure (for the highest located and high-rise building) by more than 0.20 MPa (2 kgf/cm2).

With this parameter in apartments, in accordance with regulatory legal acts of the Russian Federation, the following values ​​must be provided:

a) the minimum free pressure for sanitary fixtures in residential premises on the upper floors is 0.02-0.05 MPa (0.2-0.5 kgf/cm 2);

b) the minimum pressure in front of the gas water heater on the upper floors is not less than 0.10 MPa (1 kgf/cm2);

c) the maximum free pressure in water supply systems at sanitary fixtures on the lower floors should not exceed 0.45 MPa (4.5 kgf/cm2).

4) For all systems:

The static pressure at the inlet to the heat and water supply systems must ensure that the pipelines of the central heating, cold water and hot water supply systems are filled with water, while the static water pressure should not be higher than permissible for this system.

The water pressure values ​​in the DHW and cold water systems at the entrance of pipelines into the house must be at the same level (achieved by setting automatic control devices for the heating point and/or pumping station), while the maximum permissible pressure difference must be no more than 0.10 MPa (1 kgf/cm 2).

These parameters at the entrance to buildings must be ensured by resource supplying organizations by implementing measures for automatic regulation, optimization, uniform distribution of thermal energy, cold and hot water between consumers, and for return pipelines of systems - also by housing management organizations through inspections, identification and elimination of violations or re-equipment and adjustment of building engineering systems. The specified measures should be carried out when preparing heating points, pumping stations and intra-block networks for seasonal operation, as well as in cases of violations of the specified parameters (indicators of the quantity and quality of utility resources supplied to the boundary of operational responsibility).

If the specified values ​​of parameters and modes are not observed, the resource supplying organization is obliged to immediately take all necessary measures to restore them. In addition, in case of violation of the specified values ​​of the parameters of the supplied utility resources and the quality of the provided utility services, it is necessary to recalculate the payment for the provided utility services with a violation of their quality.

Thus, compliance with these indicators will ensure comfortable living for citizens, the effective functioning of engineering systems, networks, residential buildings and public utility facilities that provide heat and water supply to the housing stock, as well as the supply of utility resources in the required quantity and standard quality to the boundaries of the operational responsibility of the resource supply and managing housing organization (at the entrance of utilities into the house).

Literature

1. Rules for the technical operation of thermal power plants.

2. MDK 3-02.2001. Rules for the technical operation of public water supply and sewerage systems and structures.

3. MDK 4-02.2001. Standard instructions for the technical operation of municipal heating systems.

4. MDK 2-03.2003. Rules and regulations for the technical operation of housing stock.

5. Rules for the provision of public services to citizens.

6. ZhNM-2004/01. Regulations for the preparation for winter operation of heat and water supply systems of residential buildings, equipment, networks and structures of fuel, energy and public utilities in Moscow.

7. GOST R 51617 -2000*. Housing and communal services. General technical conditions.

8. SNiP 2.04.01 -85 (2000). Internal water supply and sewerage of buildings.

9. SNiP 2.04.05 -91 (2000). Heating, ventilation and air conditioning.

10. Methodology for checking violations of the quantity and quality of services provided to the population by accounting for heat energy consumption, cold and hot water consumption in Moscow.

(Energy Saving Magazine No. 4, 2007)

Q[KW] = Q[Gcal]*1160;Converting load from Gcal to kW

G[m3/hour] = Q[KW]*0.86/ ΔT; where ΔT– temperature difference between supply and return.

Example:

Supply temperature from heating networks T1 – 110˚ WITH

Supply temperature from heating networks T2 – 70˚ WITH

Heating circuit flow G = (0.45*1160)*0.86/(110-70) = 11.22 m3/hour

But for a heated circuit with a temperature curve of 95/70, the flow rate will be completely different: = (0.45*1160)*0.86/(95-70) = 17.95 m3/hour.

From this we can conclude: the lower the temperature difference (temperature difference between supply and return), the greater the coolant flow required.

Selection of circulation pumps.

When selecting circulation pumps for heating, hot water, ventilation systems, you need to know the characteristics of the system: coolant flow,

which must be ensured and the hydraulic resistance of the system.

Coolant flow:

G[m3/hour] = Q[KW]*0.86/ ΔT; where ΔT– temperature difference between supply and return;

Hydraulic The system resistance should be provided by specialists who calculated the system itself.

For example:

We consider the heating system with a temperature graph of 95˚ C /70˚ With and load 520 kW

G[m3/hour] =520*0.86/25 = 17.89 m3/hour~ 18 m3/hour;

The heating system resistance wasξ = 5 meters ;

In the case of an independent heating system, you need to understand that the resistance of the heat exchanger will be added to this resistance of 5 meters. To do this, you need to look at its calculation. For example, let this value be 3 meters. So, the total resistance of the system is: 5+3 = 8 meters.

Now it’s quite possible to choose circulation pump with flow rate 18m3/hour and a head of 8 meters.

For example this one:

In this case, the pump is selected with a large margin, it allows you to ensure the operating pointflow/pressure at the first speed of its operation. If for any reason this pressure is not enough, the pump can be “accelerated” to 13 meters at third speed. The optimal option is considered to be a pump that maintains its operating point at second speed.

It is also quite possible, instead of an ordinary pump with three or one operating speed, to install a pump with a built-in frequency converter, for example this one:

This pump version is, of course, the most preferable, since it allows the most flexible adjustment of the operating point. The only downside is the cost.

It is also necessary to remember that for the circulation of heating systems it is necessary to provide two pumps (main/backup), and for the circulation of the DHW line it is quite possible to install one.

Recharge system. Selection of the charging system pump.

Obviously, a make-up pump is necessary only in the case of using independent systems, in particular heating, where the heating and heated circuit

separated by a heat exchanger. The make-up system itself is necessary to maintain constant pressure in the secondary circuit in case of possible leaks

in the heating system, as well as for filling the system itself. The make-up system itself consists of a pressure switch, a solenoid valve, and an expansion tank.

A make-up pump is installed only when the coolant pressure in the return is not enough to fill the system (the piezometer does not allow it).

Example:

Return coolant pressure from heating networks P2 = 3 atm.

The height of the building taking into account technical requirements. Underground = 40 meters.

3atm. = 30 meters;

Required height = 40 meters + 5 meters (at spout) = 45 meters;

Pressure deficit = 45 meters – 30 meters = 15 meters = 1.5 atm.

The pressure of the feed pump is clear; it should be 1.5 atmospheres.

How to determine consumption? The pump flow rate is assumed to be 20% of the volume of the heating system.

The operating principle of the recharge system is as follows.

A pressure switch (pressure measuring device with a relay output) measures the pressure of the return coolant in the heating system and has

pre-setting. For this particular example, this setting should be approximately 4.2 atmospheres with a hysteresis of 0.3.

When the pressure in the heating system return drops to 4.2 atm, the pressure switch closes its group of contacts. This supplies voltage to the solenoid

valve (opening) and make-up pump (switching on).

Make-up coolant is supplied until the pressure rises to a value of 4.2 atm + 0.3 = 4.5 atmospheres.

Calculation of a control valve for cavitation.

When distributing the available pressure between the elements of a heating point, it is necessary to take into account the possibility of cavitation processes inside the body

valves that will destroy it over time.

The maximum permissible pressure drop across the valve can be determined by the formula:

ΔPmax= z*(P1 − Ps) ; bar

where: z is the cavitation onset coefficient, published in technical catalogs for equipment selection. Each equipment manufacturer has its own, but the average value is usually in the range of 0.45-06.

P1 – pressure in front of the valve, bar

Рs – saturation pressure of water vapor at a given coolant temperature, bar,

Towhichdetermined by the table:

If the calculated pressure difference used to select the valve Kvs is no more

ΔPmax, cavitation will not occur.

Example:

Pressure before valve P1 = 5 bar;

Coolant temperature T1 = 140C;

Valve Z according to catalog = 0.5

According to the table, for a coolant temperature of 140C we determine Рs = 2.69

The maximum permissible pressure drop across the valve will be:

ΔPmax= 0.5*(5 - 2.69) = 1.155 bar

You cannot lose more than this difference on the valve - cavitation will begin.

But if the coolant temperature was lower, for example 115C, which is closer to the actual temperatures of the heating network, the maximum difference

pressure would be greater: ΔPmax= 0.5*(5 – 0.72) = 2.14 bar.

From here we can draw a quite obvious conclusion: the higher the temperature of the coolant, the lower the pressure drop possible across the control valve.

To determine the flow rate. Passing through the pipeline, it is enough to use the formula:

;m/s

G – coolant flow through the valve, m3/hour

d – nominal diameter of the selected valve, mm

It is necessary to take into account the fact that the flow velocity of the pipeline passing through the section should not exceed 1 m/sec.

The most preferable flow speed is in the range of 0.7 - 0.85 m/s.

The minimum speed should be 0.5 m/s.

The criterion for choosing a hot water supply system is usually determined from the technical conditions for connection: the heat generating company very often prescribes

type of DHW system. If the type of system is not specified, a simple rule should be followed: determination by the ratio of building loads

for hot water supply and heating.

If 0.2 - necessary two-stage hot water system;

Respectively,

If QDHW/Qheating< 0.2 or QDHW/Qheating>1; necessary single-stage DHW system.

The very principle of operation of a two-stage hot water system is based on heat recovery from the return of the heating circuit: return coolant of the heating circuit

passes through the first stage of the hot water supply and heats up cold water from 5C to 41...48C. At the same time, the return coolant of the heating circuit itself cools down to 40C

and already cold it merges into the heating network.

The second stage of the hot water supply heats up the cold water from 41...48C after the first stage to the required 60...65C.

Advantages of a two-stage DHW system:

1) Due to heat recovery from the heating circuit return, cooled coolant enters the heating network, which sharply reduces the likelihood of overheating

return lines This point is extremely important for heat generating companies, in particular heating networks. Now it is becoming common to carry out calculations of heat exchangers of the first stage of hot water supply at a minimum temperature of 30C, so that even colder coolant is drained into the return of the heating network.

2) The two-stage hot water system allows for more precise control of the temperature of hot water, which is used for analysis by the consumer and temperature fluctuations

at the exit from the system is significantly less. This is achieved due to the fact that the control valve of the second stage of DHW, during its operation, regulates

only a small part of the load, and not the whole thing.

When distributing loads between the first and second stages of DHW, it is very convenient to do the following:

70% load – 1st DHW stage;

30% load – DHW stage 2;

What does it give?

1) Since the second (adjustable) stage is small, in the process of regulating the DHW temperature, temperature fluctuations at the outlet

systems turn out to be insignificant.

2) Thanks to this distribution of the DHW load, in the calculation process we obtain equality of costs and, as a consequence, equality of diameters in the heat exchanger piping.

The consumption for DHW circulation must be at least 30% of the consumption for DHW disassembly by the consumer. This is the minimum number. To increase reliability

system and stability of DHW temperature control, circulation flow can be increased to 40-45%. This is done not only to maintain

hot water temperature, when there is no analysis by the consumer. This is done to compensate for the “drawdown” of DHW at the time of peak DHW withdrawal, since the consumption

circulation will support the system while the heat exchanger volume is filled with cold water for heating.

There are cases of incorrect calculation of the DHW system, when instead of a two-stage system, a single-stage one is designed. After installing such a system,

During the commissioning process, the specialist is faced with extreme instability of the hot water supply system. Here it is even appropriate to talk about inoperability,

which is expressed by large temperature fluctuations at the outlet of the DHW system with an amplitude of 15-20C from the set setpoint. For example, when the setting

is 60C, then during the regulation process, temperature fluctuations occur in the range from 40 to 80C. In this case, changing the settings

an electronic regulator (PID - components, rod stroke time, etc.) will not give a result, since the DHW hydraulics are fundamentally incorrectly calculated.

There is only one way out: limit the consumption of cold water and maximize the circulation component of the hot water supply. In this case, at the mixing point

a smaller amount of cold water will be mixed with a larger amount of hot (circulation) and the system will work more stable.

Thus, some kind of imitation of a two-stage DHW system is performed due to the circulation of DHW.

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 for 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 recent years, computer calculation and graphic programs are used.

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 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.

With closed PZT systems and equal pipe diameters 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) the operating pressure is 0.1 MPa (10 m of 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. In summer, 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 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 ensure reliable operation of 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 heater; 4 - make-up regulator valve

The pressures of the network pumps N s.n are taken equal to the sum of the hydraulic pressure losses (at the 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 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 the technical level of 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, 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 operating modes of the system as a whole, and, consequently, the end result - reliable and high-quality heat supply to consumers. Ideal in this sense is an organizational structure in which heat supply sources and heating networks are under the jurisdiction of one enterprise structure.

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 ДРр = 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 inlet pressure difference =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 of the main circulation ring is carried out through riser 15 of a vertical single-pipe water heating system with bottom wiring and dead-end movement of the coolant.

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

3) To pre-select the pipe diameter, 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 main circulation circuit sections, 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

Based on the results of calculating water supply networks for various water consumption modes, the parameters of the water tower and pumping units are determined to ensure the operability of the system, as well as free pressures in all network nodes.

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 lowest 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, local pumping installations are provided. The free pressure at the water dispensers must be at least 10 m (0.1 MPa),

In the external network of industrial water pipelines, free pressure is taken according to the technical characteristics of the 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 that has 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. Depending on the method of fire extinguishing, water supply systems are divided into high and low pressure.

As a rule, when designing water supply systems, low-pressure fire-fighting water supply should be used, with the exception of small settlements (less than 5 thousand people). The installation of a high-pressure fire-fighting water supply system 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).

When there are significant differences in the geodetic elevations of the object supplied with water, a large length of water supply networks, as well as when there is a large difference in the values ​​of free pressure required by individual consumers (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