What is the available pressure at the connection point. Pressures in water supply systems

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

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 piezometric pressures in 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 network 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.

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

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

DPE - 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

Warning There is not enough pressure at the source Delta=X m. Where Delta is the required pressure.

WORST CONSUMER: ID=XX.

Figure 283. Message about the worst consumer




This message is displayed when there is a lack of available pressure at the consumer, where DeltaH− the value of the pressure that is not enough, m, a ID (XX)− individual number of the consumer for whom the pressure shortage is maximum.



Figure 284. Message about insufficient pressure




Double-click the left mouse button on the message about the worst consumer: the corresponding consumer will blink on the screen.

This error can be caused by several reasons:

1. Incorrect data. If the amount of pressure shortage goes beyond the actual values ​​for a given network, then there is an error when entering the initial data or an error when plotting the network diagram on the map. You should check whether the following data has been entered correctly:

   

   Hydraulic mode networks.

   If there are no errors when entering the initial data, but a lack of pressure exists and is of real significance for a given network, then in this situation the determination of the cause of the shortage and the method for eliminating it is carried out by the specialist working with this heating network.

ID=ХХ "Name of consumer" Emptying the heating system (H, m)

This message is displayed when there is insufficient pressure in the return pipeline to prevent emptying of the heating system of the upper floors of the building; the total pressure in the return pipeline must be at least the sum of the geodetic mark, the height of the building plus 5 meters to fill the system. The head reserve for filling the system can be changed in the calculation settings ().

XX− individual number of the consumer whose heating system is being emptied, N- pressure, in meters which is not enough;

ID=ХХ "Name of consumer" Pressure in the return pipeline is higher than the geodetic mark by N, m

This message is issued when the pressure in the return pipeline is higher than permissible according to strength conditions cast iron radiators(more than 60 m. water column), where XX- individual consumer number and N- pressure value in the return pipeline exceeding the geodetic mark.

The maximum pressure in the return pipeline can be set independently in calculation settings. ;

ID=XX "Name of consumer" Elevator nozzle cannot be selected. Set the maximum

This message may appear when there is a large heating load or when an incorrect connection diagram is selected that does not correspond to the design parameters. XX- individual number of the consumer for whom the elevator nozzle cannot be selected;

ID=XX "Name of consumer" Elevator nozzle cannot be selected. Set the minimum

This message may appear when there are very small heating loads or when an incorrect connection diagram is selected that does not correspond to the design parameters. XX− individual number of the consumer for whom the elevator nozzle cannot be selected.

Warning Z618: ID=XX "XX" The number of washers on the supply pipe to CO is more than 3 (YY)

This message means that, as a result of the calculation, the number of washers required to adjust the system is more than 3 pieces.

Because minimum diameter The default washer size is 3 mm (specified in the calculation settings “Setting up the calculation of pressure loss”), and the consumption of the consumer’s heating system ID=XX is very small, then as a result of the calculation the total number of washers and the diameter of the last washer (in the consumer database) are determined. .

That is, a message like: The number of washers on the supply pipeline for CO is more than 3 (17) warns that to set up this consumer, you should install 16 washers with a diameter of 3 mm and 1 washer, the diameter of which is determined in the consumer database.

Warning Z642: ID=XX The elevator at the central heating station is not working

This message is displayed as a result of a verification calculation and means that the elevator unit is not functioning.

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 ​​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 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 represents the 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, total length pipelines and 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 small 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 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 open method(not to be confused with an open type system), that is, all its pipelines, fittings and instruments 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 electrical power. IN 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.