Available pressure of the heating network. Hydraulic calculation of a water heating system

To task hydraulic calculation includes:

Determination of pipeline diameter;

Determination of pressure drop (pressure);

Determination of pressures (pressures) at various points in the network;

Linking all network points in static and dynamic modes in order to ensure permissible pressures and required pressures in the network and subscriber systems.

Based on the results of hydraulic calculations, the following problems can be solved.

1. Determination of capital costs, metal (pipes) consumption and the main volume of work on laying a heating network.

2. Determination of the characteristics of circulation and make-up pumps.

3. Determination of operating conditions of the heating network and selection of subscriber connection schemes.

4. Selection of automation for the heating network and subscribers.

5. Development of operating modes.

a. Schemes and configurations of heating networks.

The layout of the heating network is determined by the location of heat sources in relation to the area of ​​consumption, the nature of the heat load and the type of coolant.

The specific length of steam networks per unit of design heat load is small, since steam consumers - usually industrial consumers - are located at a short distance from the heat source.

A more difficult task is the choice of the scheme of water heating networks due to the large length, large quantities subscribers. Water vehicles are less durable than steam vehicles due to greater corrosion, and are more sensitive to accidents due to the high density of water.

Fig.6.1. Single-line communication network of a two-pipe heating network

Water networks are divided into main and distribution networks. The coolant is supplied through main networks from heat sources to areas of consumption. Through distribution networks, water is supplied to GTP and MTP and to subscribers. Subscribers very rarely connect directly to backbone networks. At the points where distribution networks are connected to the main ones, sectioning chambers with valves are installed. Sectional valves on main networks are usually installed every 2-3 km. Thanks to the installation of sectional valves, water losses during vehicle accidents are reduced. Distribution and main vehicles with a diameter of less than 700 mm are usually made dead-end. In the event of an emergency, a break in the heat supply to buildings for up to 24 hours is acceptable for most of the country. If a break in heat supply is unacceptable, it is necessary to provide for duplication or loopback of the heating system.

Fig.6.2. Ring heating network from three thermal power plants Fig.6.3. Radial heat network

When supplying heat to large cities from several thermal power plants, it is advisable to provide for mutual interlocking of thermal power plants by connecting their mains with interlocking connections. In this case, a ring heat network with several power sources is obtained. Such a scheme has higher reliability and ensures the transmission of redundant water flows in the event of an accident on any part of the network. When the diameters of the mains extending from the heat source are 700 mm or less, a radial heating network diagram is usually used with a gradual decrease in the pipe diameter as the distance from the source increases and the connected load decreases. This network is the cheapest, but in the event of an accident, the heat supply to subscribers is stopped.

b. Basic calculation dependencies

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 temperature chart 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 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 some reason this pressure is not enough, the pump can be “accelerated” to 13 meters at third speed. The best option A pump version is considered that maintains its operating point at the 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:

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 concrete 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 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.

Selection criterion DHW systems, as a rule, is determined from technical specifications 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 hot water 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.

The operating pressure in the heating system is the most important parameter on which the functioning of the entire network depends. Deviations in one direction or another from the values ​​specified in the design 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 more significant 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 the working medium 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 a central heating system, the pressure in the network 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 at different heights and at different distances from the boiler room, the pressure in the network must be adjusted. Pressure regulators are used to reduce it, and pumping stations are used to increase it. 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.

In the absence of a centralized heating supply, autonomous heating systems are installed in houses, 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 the normal functioning of such systems, the water pressure in them should be approximately 1.5-2 atm. This low figure is 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 losses in this case are caused by the release of air from the water, which dissolved in it when the pipelines were filled. 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 heating systems, in order to save money, a small safety margin is included, 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, an expansion tank is installed in a closed heating system. Unlike a similar device in an open type system, it does not communicate 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 boiler automation;
• due to incomplete opening of shut-off valves after repairs or maintenance work;
• due to the appearance of an 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 closed-type heating systems, 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 emergency situations are those in which the pressure gradually or sharply decreases. 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 inspected, you should seek the help of specialists. They have special equipment that will help detect leaks and fix them if the home owner is unable to do this themselves. 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 the pressure in the system drops is the incorrect selection of the 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, the safety valve is activated. 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, pointer 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.

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); p and ∆p - 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 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 an organizational structure in which heat supply sources and heating networks are under the jurisdiction of one enterprise structure.

The piezometric graph shows the terrain, the height of attached buildings, and the pressure in the network on a scale. Using this graph, it is easy to determine the pressure and available pressure at any point in the network and subscriber systems.

Level 1 – 1 is taken as the horizontal plane of pressure reference (see Fig. 6.5). Line P1 – P4 – graph of supply line pressures. Line O1 – O4 – return line pressure graph. N o1 – total pressure on the return collector of the source; Nсн – pressure of the network pump; N st – full pressure of the make-up pump, or full static pressure in the heating network; N to– total pressure in t.K at the discharge pipe of the network pump; D H t – pressure loss in the heat treatment plant; N p1 – total pressure on the supply manifold, N n1 = N k–D H t. Available supply water pressure at the CHP collector N 1 =N p1 - N o1. Pressure at any point in the network i denoted as N p i, H oi – total pressures in the forward and return pipelines. If the geodetic height at a point i There is Z i , then the piezometric pressure at this point is N p i – Z i , H o i – Z i in the forward and return pipelines, respectively. Available head at point i there is a difference in piezometric pressures in the forward and return pipelines - N p i – H oi. The available pressure in the heating network at the connection point of subscriber D is N 4 = N p4 – N o4.

Fig.6.5. Scheme (a) and piezometric graph (b) of a two-pipe heating network

There is a loss of pressure in the supply line in section 1 - 4 . There is a pressure loss in the return line in section 1 - 4 . When the mains pump is operating, the pressure N The speed of the charging pump is regulated by a pressure regulator to N o1. When the network pump stops, a static pressure is established in the network N st, developed by the make-up pump.

When hydraulically calculating a steam pipeline, the profile of the steam pipeline may not be taken into account due to the low steam density. Pressure losses from subscribers, for example , depends on the subscriber connection scheme. With elevator mixing D N e = 10...15 m, with elevator-free input – D n BE =2...5 m, in the presence of surface heaters D N n =5...10 m, with pump mixing D N ns = 2…4 m.

Requirements for pressure conditions in the heating network:

At any point in the system, the pressure should not exceed the maximum permissible value. The pipelines of the heat supply system are designed for 16 ata, the pipelines of local systems are designed for a pressure of 6...7 ata;

To avoid air leaks at any point in the system, the pressure must be at least 1.5 atm. In addition, this condition is necessary to prevent pump cavitation;

At any point in the system, the pressure must be no less than the saturation pressure at a given temperature to avoid boiling of water.