Design and research on subway dry fire hydrant system?

Urban underground railway (subway for short), as a fast, environmentally friendly, comfortable, and large-capacity urban transportation vehicle, has been widely used in major developed countries and regions in the world. In the past ten years, with the rapid development of our country's economy and the continuous enhancement of our comprehensive national strength, subway construction has made great progress in our country. In addition to the subways in Beijing, Shanghai, and Guangzhou that are already in operation, the subways in Nanjing, Hangzhou, and Chongqing are already under construction. . It is estimated that by 2008, the total length of Beijing's subway operating lines will reach 150km.

Fires that occur in the subway, due to its unique terrain conditions, bring great difficulties to personnel evacuation and fire fighting, and can easily cause death and injury and huge economic losses.

As the main facility for subway fire fighting, the fire hydrant water supply system plays a decisive role in controlling and extinguishing fires. However, the existing wet fire hydrant water supply system has some problems in actual use. When the fire water supply pipeline is installed in a location where freezing is possible, the insulation measures taken are generally: the outdoor surface-mounted parts of the elevated station and the pipes at the entrance and exit of the tunnel use an electric heating system to prevent freezing; the pipes near the air outlet of the fan room in the tunnel are The insulation material used is composite magnesium silicate tube shell, the protective layer is aluminum foil, and the combustion grade of the insulation and protective layer materials is Class A. During the joint inspection of the equipment, it was found that the aluminum foil was damaged. Since aluminum foil is a conductive material and there are many electrical equipment and power lines in the subway, there is a hidden danger of accidents and it is not suitable for subway projects. A large number of electric heat tracing and insulation measures are used in elevated stations and entrance and exit fire pipelines, which requires additional investment in some projects, increases operating costs, and also wastes energy. In addition, during the operation of subway projects, there have been accidents where fire pipes in sections have leaked and affected traffic. How to minimize project investment, save energy, and eliminate hidden dangers of accidents are issues that need to be further solved in water fire protection design. The dry water fire protection system does not require pipeline insulation and does not have the risk of accidents, which provides ideas for solving the above problems.

It is very necessary to explore the feasibility of using dry water fire-fighting systems in subway design in cold areas, especially since the subway construction in cities such as Tianjin, Shenyang, Beijing, Dalian, and Harbin is currently under intense construction. Applicable research on dry water fire protection systems is particularly important. This leads to a question: whether the subway fire water supply can use a dry system to solve the problem of frozen water pipes.

The dry fire hydrant system is bounded by manual and electric quick-opening valves. The inlet side of the valve is connected to the water source, and the pipe is filled with water; the outlet side of the valve is connected to the network, and the quick-opening valve is usually closed. The pipe network Normally empty. When a fire occurs, the valve can be opened in the following ways: button activation at the fire hydrant; automatic activation through a fire detector alarm; and manual emergency opening on site. This ensures that the valve can open water supply to extinguish fires under any circumstances.

Article 8.4.2, Paragraph 9 of the "Code for Fire Protection Design of Buildings" states: "The indoor fire hydrant system of non-heated factories, warehouses and other buildings in severe cold and cold areas can use dry fire hydrant systems, and the water inlet pipes should A quick opening and closing device should be installed, and an automatic exhaust valve should be installed at the highest point of the pipeline." This clause proposes the feasibility of using dry systems.

1 The composition, characteristics, advantages and disadvantages of the dry fire hydrant system

The main components of the dry fire hydrant system: remote-controlled rapid opening and closing devices, pipes with a certain slope, rapid Exhaust device and necessary fire hydrants and components. The main feature of the dry fire hydrant system is that there is no water in the pipeline after the quick opening and closing device, which can ensure that the pipeline will not freeze when the outside temperature is low.

In the event of a fire, the fire alarm system at the station or control center receives the alarm signal, opens the quick opening and closing valve, and simultaneously sends the pump start and alarm signals to the central control room. After the fire pump is started, the air in the pipe quickly After being eliminated, the pipeline was quickly transformed from a dry system to a wet system in a short period of time, and the fire hydrant port was connected with a hose and a water gun to achieve the purpose of extinguishing the fire.

1.1 Compared with the wet system, the main advantages of the dry system

The schematic diagram of the dry and wet fire hydrant system at the interval station is shown in Figure 1.

(1) There is usually no water in the pipes, so there is no freezing problem, so there is no need for insulation;

(2) Because there is usually no water in the pipes, even if the individual pipes If the interface is not tight enough, production operations will not be affected by water leakage;

(3) The number of segmented valves can be appropriately reduced.

1.2 Compared with the wet system, the main disadvantages of the dry system

(1) Because there is usually no water in the pipe, it requires a certain amount of water filling time, and the time to use the fire hydrant lags behind that of the wet system system;

(2) If the system is not inflated and regular inspections are not effective, there may be hidden dangers of fire pipe damage that cannot be discovered immediately;

(3) The valve must be able to open and close quickly. To achieve remote control, the reliability is slightly lower, the exhaust valve may be blocked, and the air in the pipeline cannot be effectively eliminated.

2 Classification of dry fire hydrant systems

2.1 Fixed fully automatic dry system

The main feature is that there is no water in the pipeline after the alarm valve and it is filled with water. The gas under pressure is not afraid of freezing or high ambient temperature. In the event of a fire, the fire alarm system of the station or control center receives the alarm signal, opens the quick opening and closing valve, and simultaneously sends the pump start and alarm signals to the central control room. After the fire pump is started, the air in the pipeline is quickly eliminated, and the pipeline is It is quickly converted from a dry system to a wet system, and a hose and a water gun are connected to the fire hydrant port to extinguish the fire.

This system is suitable for places where the ambient temperature is below 4℃ or above 70℃ and it is not suitable to use a wet fire hydrant system. Because a set of inflatable equipment and linkage devices are added, and the air pressure in the pipe network is required to be maintained within a certain range at all times, the management is more complicated and the investment is larger.

2.2 Fixed semi-automatic dry system

Compared with the wet system, this system usually does not fill the pipe network after the pre-action valve with water, but with low-pressure air or nitrogen. Or mains. Only when a fire occurs, the fire control center or the manual alarm button near the fire hydrant issues an instruction to open the quick opening and closing valve, and at the same time sends a pump start and alarm signal to the central control room. After the fire pump is started, the air in the pipeline is quickly eliminated, and the pipeline is in a short period of time. It quickly changes from a dry system to a wet system within a short period of time, and the fire hydrant discharges water to extinguish the fire.

There is usually no water in the pipes of the system, and there will be no freezing problems. Compared with the fully automatic dry system, there is no need to set up pipeline gas boosting equipment. The pipes can be empty. The reaction speed of the system is relatively fast. It is faster, has simple equipment management and less investment, and is obviously better than the fully automatic dry system. Therefore, semi-automatic dry systems are generally used in the design of dry systems.

Applicable to places where the ambient temperature is below 4℃ or above 70℃, where wet fire hydrant systems are not suitable.

2.3 Semi-fixed dry system

The pipes of the semi-fixed dry system are dry, and the system does not have a permanent water supply source. The fire water required by the system comes from the fire truck. The fire pump supplies water to the system through the fire adapter. This system has high requirements on municipal water sources and municipal rescue fire trucks.

Comprehensive analysis shows that the fixed semi-automatic dry system is more in line with my country's national conditions.

3 Application Analysis of Fixed Semi-automatic Dry System

3.1 System Water Filling Time

The system water filling time determines the length of the pipeline using the dry system. The flow speed is considered to be a maximum of 2.0~2.5m/s. The following calculations are made based on stations and sections as examples.

(1) Station: Taking an island station with an effective length of 150m as an example, the length of the fire main pipe in the most unfavorable situation is about 600m. The water filling time of such a pipe length is about 5 minutes, that is It is said that when a station adopts a dry system, the fire hydrant system cannot guarantee fire-fighting water for the first 4 to 5 minutes of the fire.

(2) Interval: Taking an interval with a length of 1.2km as an example, considering the most unfavorable situation, if the water source supplies water from one end station, the required water filling time is 10 minutes. In other words, fire water cannot be guaranteed within 10 minutes after a fire occurs.

3.2 Analysis of the time when fire hydrants are put into use

At the station, the 4 to 5 minutes after a fire breaks out is mainly the time for passengers to evacuate. If the fire hydrant is used by station staff to extinguish the fire, it may be necessary to use the fire hydrant after 5 minutes or less. If you wait for dedicated fire fighters, it will take longer. Since subway stations are densely populated places, the amount of water used for fire fighting is larger than the interval, and the "Code for Fire Protection Design of Buildings" requires that the water filling time should be as short as possible and the fire extinguishing should be as early as possible. Therefore, for stations, if a dry system is used, there may be a problem that the fire hydrants cannot meet the fire water requirements when opened. Therefore, subway stations should use wet systems to ensure that fire hydrants can be used smoothly in the first instance of a fire.

When a train catches fire in a section, under normal circumstances, the train should travel to the next station as much as possible to put out the fire. Only when a fire breaks out in a train and cannot be towed to the station, it needs to be carried out in the section. Fire extinguishing in inter-regional tunnels requires specialized fire fighters. Even the nearest fire brigade may take more than 10 minutes from receiving the signal to entering the fire location in the tunnel. Within these 10 minutes, if the components of the dry system are operating normally, the system It has been converted from dry type to wet type. When firefighters rush to the underground section of the fire location and open the fire hydrant, the water volume and pressure can meet the fire water requirements.

3.3 Fire pressure requirements

In order to ensure the flow rate of the pipeline, the head of the fire pump should be increased. For example, when the pipeline diameter is DN150 and v=1.83m/s, the head loss along the pipeline will be 43.2m, so when the required water filling time is short, the flow rate of the pipeline needs to increase, and the head of the water pump that provides the water source needs to increase; conversely, when the head of the water pump decreases, the water filling time of the pipe will increase.

It can be seen from the above analysis that the subway fire hydrant system can be considered to adopt the following partial dry method: underground stations adopt wet systems, ground, elevated stations and inter-regional tunnels adopt dry systems, and fire protection systems are installed at stations. The pump unit provides the pressure and flow of fire water in stations and interval pipelines.

The dry system in the interval is a fixed semi-automatic dry system. The quick-opening valve of the interval dry system is installed at the end of the station close to the interval, so that each interval can obtain water sources from two adjacent stations. It is calculated as each station bears 1/2 of the two adjacent intervals. In case of fire, the fire hydrant The water filling time of the pipe can be further shortened. In addition, fire couplings are connected from the piston air shafts or interval air shafts at both ends of the station to prepare fire trucks to draw water from the municipal government to replenish the interval fire pipes.

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