The cable tunnel project is basically in the central area of the city, and once a fire occurs, it will endanger the safety of the city, so active measures should be taken to prevent the occurrence of cable tunnel fires. "Fire Prevention, Fire Protection and Ventilation Design of Urban Cable Tunnels (Part I)" has introduced the fire prevention and fire protection measures of urban cable tunnels in detail, and this article will talk to you about what are the precautions for the ventilation design of urban cable tunnels.
In general, urban power cable tunnel ventilation mainly solves the following three problems: temperature, humidity and freshness of the air. If the heat generated during the operation of the cable in the cable tunnel cannot be taken away in time, the air temperature in the tunnel will be too high, the current carrying capacity of the cable will be reduced, and the full load operation will not be possible, which will directly cause losses; Excessive air humidity causes the protective layer of cable armor to rust and fall off, reducing the life of the cable insulation layer and causing losses; The cable tunnel is a closed underground structure, the deposition of exhaust gas, the activities of personnel and microorganisms will cause the decline of oxygen content in the ditch, and the urban cable tunnel inspection and maintenance times are more, so the ventilation system can inspect the maintenance personnel to provide fresh air.
There are four commonly used cable tunnel ventilation methods: natural air inlet and natural exhaust; Natural air intake, mechanical exhaust; mechanical air intake, natural exhaust; Mechanical air intake, mechanical exhaust. These 4 ventilation methods are applicable to different forms. The ventilation mode of natural air inlet and natural exhaust can be used in occasions with short ventilation areas, and there should be a certain height difference between the air inlet and exhaust outlets; The ventilation methods of natural air intake, mechanical exhaust and mechanical air inlet and natural exhaust can be used in occasions with short ventilation areas, and there is no height difference requirement for air inlet and exhaust outlets, but it may be necessary to set up an inlet fan room or an exhaust fan room according to the situation; The ventilation mode of mechanical air inlet and mechanical exhaust can be used in the occasion of a long ventilation area, and there is no height difference requirement for the inlet and exhaust outlets.
The above four ventilation methods have been applied in previous projects, but the most widely used are natural air intake, mechanical exhaust and mechanical air intake, mechanical exhaust these two forms.
Power Engineering Cable Design Standard GB50217-2018 in 56.8. Provisions: natural ventilation should be adopted for cable tunnels. Long-distance tunnels should be divided into sections to implement independent ventilation; "Technical Regulations for the Design of Urban Power Lines" DL T5221-2016 in 45.15.4. Provisions: The ventilation system in the cable tunnel can be in the form of natural ventilation or mechanical ventilation. However, judging from the actual experience of the current domestic urban power cable tunnels, the form of natural ventilation, mechanical ventilation or all mechanical ventilation is basically adopted.
The main reason is that the urban power cable tunnel is long, and it must be divided into independent ventilation sections. According to the natural ventilation design alone, it will inevitably cause the ventilation section to be divided too short and too many vents reserved; This can be achieved in factories or substations, but in cities where the environmental impact is high, and vents need to be minimized. At the same time, too many vents are set up to increase the cost of civil construction, and reducing the vent setting is to reduce the one-time investment.
The lower the exhaust air temperature of the cable tunnel, the safer it is, but at the same time, the required ventilation will also increase. Therefore, there should be a reasonable and economical temperature control range. "Technical Regulations for the Design of Urban Power Cable Lines" DL T5221-2016 in 45.15.4. Provisions: 1) The inlet air temperature should be selected according to the dry bulb temperature calculated outside the summer ventilation room, the exhaust air temperature should not exceed 40, and the temperature difference between the inlet and exhaust air should not exceed 10; 2) The wind speed at the minimum section in the tunnel should not be greater than 5m s; 3) The air inlet and exhaust outlet should be set in a clean outdoor area, and the lower edge should not be less than 0 from the outdoor floor5m (The exhaust fumes of the car on the road generally settle on the ground of the road 0.)places below 5m); 4) The ventilation system should be started and stopped by temperature control, when the ambient temperature in the tunnel reaches 40, the ventilation system starts to run, and when the ambient temperature is lower than 35, the ventilation system stops running; 5) The noise emitted by the ventilation system should meet the requirements of national environmental protection; 6) A metal mesh should be added at the air inlet to prevent small animals from entering the tunnel;
Design Code for Power Cable Tunnels DL T5484-2013 in 91.3. Provisions: 1) The amount of waste heat ventilation should be eliminated according to the maximum ampazare passing capacity of the tunnel cable tunnel under normal operation; 2) Personnel maintenance fresh air volume, should be calculated as 30m (h·person); 3) The accident ventilation volume of each ventilation section should be calculated according to the minimum number of air exchanges of 6 hours;
Due to the relatively long length of the urban power cable tunnel, it is necessary to divide the ventilation section independently. The shorter the ventilation section, the more vents are reserved, which is ideal in a plant or substation, but in cities it is necessary to reduce the number of vents as much as possible in order to reduce the environmental impact. The reduction of ventilation openings also correspondingly reduces civil construction costs. On the other hand, if the length of the ventilation section is blindly increased, the power of the fan will be too large, the noise emitted by the fan will affect the environment, and the daily operating cost will also increase, and the ventilation machine room needs to be set up, which increases the civil construction cost. Therefore, it is necessary to reasonably determine the value of the length of the ventilation zone, which not only meets the planning requirements, but also is safe and economical.
Combine 21. The division of fire partitions, for open-cut cable tunnels, the maximum ventilation partition should not exceed 200m; For the cable tunnel constructed by the underground excavation method (including the shallow buried underground excavation, pipe jacking method and shield method), it is necessary to reserve the working well in combination with the construction technology to design the ventilation section, and the ventilation distance should not be limited, but generally not more than 500m (this value is not required by the online data), and the fire partition is 21 division. Personally, it is suggested that when the ventilation partition is less than or equal to the fireproof partition, firewalls or fireproof doors can be used at both ends of the fireproof partition, and when the ventilation partition is larger than the fireproof partition, fireproof doors should be set at both ends of the fireproof partition.
This calculation formula is from the "Q GDW 11186-2014 Design Guidelines for Underground Cable Tunnels" and "Q GDW 11187-2014 Design Guidelines for Exposed Cable Tunnels"; No specification was found for the amount of heat dissipated by the cable.
The ventilation volume of the cable is calculated through the above formula, and the natural ventilation is judged according to the ventilation volume. The code requires that the wind speed at the minimum section in the tunnel should not be greater than 5m s; The above calculation formula determines the lower limit of exhaust airflow, and the wind speed required by the specification determines the upper limit of exhaust airflow.
4.4 Fire prevention measures for tunnel ventilation
The ventilation and fire protection of cable tunnels is diametrically opposed to that of general civil buildings, because the purpose is different. The ventilation and fire protection of general civil buildings is aimed at immediately evacuating the crowd at the beginning of the fire, so in the smoke-proof stairwell and the front room of the fire elevator, the fan should be immediately started to pressurize the air supply to ensure that there is no smoke infiltration in these places, and in other places, the smoke and fire should be immediately discharged to avoid the spread of smoke and fire, so that the crowd is easy to withdraw immediately. The cable tunnel is the opposite, because there is no personnel entering and exiting the tunnel, and the purpose of ventilation and fire protection is to immediately extinguish the fire and ensure that the cables in other fire protection zones are not damaged. Therefore, when the tunnel fires, the ventilation is stopped immediately, and the fire and smoke exhaust valves in front of all fans should be closed immediately, so that the tunnel fire section is extinguished due to lack of oxygen, and the loss of other cables is reduced. Wait until the fire is confirmed to be extinguished, reopen the fire damper and start the fan accident ventilation.
1) The reasonable value of fire partition and ventilation partition should comprehensively consider many factors such as planning, cable calorific value, civil engineering and ventilation equipment investment, daily operating costs and noise. In tunnel construction, the length of ventilation partition and fire partition should be determined according to the specific structure and construction plan of the cable tunnel.
2) The requirements for ventilation and fire protection of cable tunnels (including the length of fire protection zones, ventilation and fire protection purposes, etc.) must be coordinated with the local fire audit department and clarified.
3) The value of fire partition and ventilation partition of urban power tunnel not only involves fire protection design, but also involves cable tunnel safety and ventilation system; It is necessary to cooperate with multiple majors such as civil engineering, electrical primary, and secondary. The current available codes have limited support for tunnel ventilation and fire protection design, and a higher level of literature support is needed for in-depth research.
References. 1 DL T 5484-2013 Code for the design of power cable tunnels;
2 GB 50217-2018 Standard for the design of cables for power engineering.
3 DL T 5221-2016 Technical Regulations for the Design of Urban Power Cable Lines.
4 GB 50016-2014 Code for fire protection in building design.
5 GB 50229-2019 Fire protection standard for the design of thermal power plants and substations.
6 DL T 5035-2004 Technical specification for the design of heating and air conditioning of thermal power plants.
7 GB 50116-2013 Code for the design of automatic fire alarm systems.
8 GB 23864-2009 Firestop Blocking Material.
9 GB 50019-2015 Design Code for Heating, Ventilation and Air Conditioning of Industrial Buildings.
10 GB 12955-2008 Fire Door.
11 GB 50140-2005 Code for design of fire extinguishing configuration in buildings.
12 Q GDW 11186—2014 Design Guidelines for Underground Cable Tunnels.
13 Q GDW 11187—2014 Design Guidelines for Cut-and-Cover Cable Tunnels.
14 State Grid Corporation of China Power Transmission and Transformation Engineering General Design Cable Line Fascicle (2017 Edition).
15 Zhao Hui, Ventilation Design of Urban Power Tunnels, North China Electric Power Technology, 2009
16 Zhao Hui, Design Values of Fire Protection Partition and Ventilation Partition of Urban Power Cable Tunnel, Power Supply and Consumption, 2009
17 Zhao Hui, Fire Protection and Fire Protection Design of Urban Power Cable Tunnels, North China Electric Power Technology, 2010
18 Peng Chen, Fire Protection and Fire Protection Design of Urban Power Cable Tunnels, Electric Power Technology and Management, 2014
19 Gao Yi, Analysis of Fire Protection Technical Scheme for Power Cable Tunnels, Power and Energy, 2013