Highway tunnel distributed fiber optic fire alarm system DTS application solution

Highway tunnel fire protection system features:

◆ Large volume of smoke, fast diffusion. Due to the restriction of the tunnel structure, the oxygen content of the air in the tunnel is relatively low compared with that outside the tunnel, and a large number of incomplete combustion products will be produced after the fire in the tunnel (such as: Carbon monoxide), the formation of rapid diffusion of smoke, the diffusion of smoke reduces visibility, and the carbon monoxide in the spread of smoke is colorless, odorless, highly toxic flammable gas, great harm.

◆ The occurrence of tunnel fire is random. Tunnel is a stationary structure, and as vehicles passing through the tunnel, it has the characteristics of diversification and random change, so the impact of external causes of tunnel fire is unpredictable, resulting in the characteristics of random occurrence.

◆ Heavy road congestion, unfavorable rescue. The tunnel is long, the road is narrow, and the vehicle flow is large. When the fire occurs, the highway management department is difficult to take effective measures to implement traffic control in time, and the blocking vehicle is difficult to evacuate in time, which is easy to cause blockage, and there are many combustions and the fire spreads along the vehicle, which is easy to form a tragic situation of conjoined fire burning.

◆ The fire develops fast and is not easy to control. The tunnel itself is a narrow and long channel, the fire in the tunnel in most cases will be affected by the longitudinal wind, of course, the development speed of the fire is also affected by the vehicle fire site, burning substances and many other factors together, the tunnel fire fire development fast, if in the initial stage failed to take good measures to fight, the fire will rapidly develop to a fierce stage, resulting in a fire

The tunnel fire itself has some characteristics:

◆ The frequency of tunnel fire is small, but the impact and damage degree is huge

◆ Relatively closed, large amount of smoke: the oxygen content in the air drops sharply. It is dark and can not see the ground at the height of the normal field of vision, resulting in fainting, disorientation and loss of escape ability;

◆ Can not intuitively determine the exact location of the fire:

◆ Fire personnel are not easy to find the ignition point, and the rescue work is difficult to start.

◆ Rapid formation: generally 8 ~ 10 minutes to complete formation, when there is a fuel leak, 1 ~ 3 minutes to form;

◆ The hot air layer above the fire point can detect that the temperature rises more than 50℃ per minute;

◆ Influence of longitudinal wind: high temperature area, thick smoke area in the longitudinal wind action offset.

Requirements for fire detection system:

◆ Quick response: Response time is the guarantee of the effectiveness of subsequent fire action. Make fire alarm (differential temperature alarm) according to the abnormal temperature gradient, and respond in time;

◆ Positioning: Determining the location of the fire is the key to quickly start the corresponding fire fighting facilities to reduce losses, and the longitudinal wind position is corrected;

The distributed optical fiber sensing technology DTS is applied to the temperature monitoring of coal yard

Necessity of coal yard temperature monitoring:

Coal yard is an indoor closed coal storage yard, which has the advantages of small area, large coal storage per unit area, high degree of automation, safe and reliable operation, not affected by external weather, little pollution to the environment, and good landscape. For the consideration of environmental protection and energy saving, the application of coal yard in domestic large thermal power plants is increasing year by year.

The accumulated coal will emit heat during the oxidation process, when the heat emitted is greater than the heat emitted, the heat will gather and the temperature will rise, and once it reaches the ignition point of the coal, it will spontaneously ignite, resulting in a large number of coal burned in vain. Smoldering coal is sent to conveying and grinding equipment, causing combustion and explosion accidents. Therefore, it is necessary to monitor the temperature of the coal yard in real time, find fire hazards and dispose of them as soon as possible.

Causes of spontaneous combustion in coal yard:

Coal is largely composed of organic and inorganic substances, the main combustible element is carbon (about 65% ~ 95%), followed by hydrogen (about 1%~2%), and contains a small amount of oxygen (about 3%~5%, sometimes up to 25%), sulfur (about 10%), the above elements together constitute combustible compounds, called coal combustible. In addition, coal also contains some non-combustible mineral ash (5% to 15%, but also up to 50%) and water (generally between 2% to 20% change), these substances are called the inert nature of coal.

The oxidation of coal by oxygen in the air is the root cause of coal spontaneous combustion. Carbon, hydrogen and other elements in coal will react at room temperature to produce combustible CO, CH4 and other alkanes. Coal oxidation is exothermic reaction, if the heat can not be dissipated in time, will make the accumulation temperature of coal rise, in turn accelerate the oxidation of coal, release more combustible and heat. When the heat accumulates and the temperature rises to a certain value, it will cause the combustion of combustible substances and spontaneous combustion.

In addition, coal particle size, moisture, ash, compaction degree, ambient temperature, humidity and other factors will affect the spontaneous combustion of coal. The finer the particle size, the larger the specific surface area, the more intense the oxidation reaction, the more prone to spontaneous combustion. Generally, spontaneous combustion of coal goes through three stages: water evaporation, oxidation and spontaneous combustion. The moisture of coal is large, the coal immersed in water can prevent the oxidation reaction between coal and oxygen directly, as long as the water is not lost, it will not affect the quality of coal; Moreover, water evaporation consumes a lot of heat, the greater the water content of coal, the longer the evaporation period, and the temperature does not rise significantly at this stage. The higher the ash content, the less prone to spontaneous combustion. Compacting the coal pile can reduce the gap between the coal blocks, reduce the amount of air penetration in the coal pile, and weaken the oxygen supply conditions. Ambient temperature and humidity will affect the time of coal spontaneous combustion. The higher the temperature and humidity, the shorter the time of coal spontaneous combustion.

Hazards caused by spontaneous coal combustion:

Spontaneous combustion of coal storage often occurs in coal yard, which brings a series of problems and hazards to factory safety production:

(1) The surface concrete of the circumferential reinforced concrete side wall of the coal yard is burned and peeled off by spontaneous combustion of the coal pile. After a long time, the circumferential loaded steel bars are exposed, which damages the side wall of the circumferential coal yard, reduces the bearing capacity of the wall, and makes the whole wall of the circular coal yard have serious safety risks of fracture and collapse.

(2) In the process of treating spontaneous combustion of coal pile and cooling by spray, a large amount of water vapor with acidic heating is generated, which condenses on the entire closed spherical grid steel structure at the upper part of the circular coal yard, causing corrosion. If corroded for a long time, the upper part of the circular coal yard is easy to fail, which further causes the collapse of the whole grid structure.

(3) The spontaneous combustion of coal piles produces a large amount of toxic acid oxide gas, which permeates the entire hemispherical closed circular coal yard space and is difficult to disperse, and the gas escaping to the outside of the coal yard also causes secondary air pollution to the factory. On-site operators rely on wearing heavy gas masks to enter the coal yard for normal production operations, and staying in such a working environment for a long time is easy to cause physical injury, and even personal accidents of gas poisoning and suffocation may occur in serious cases.

(4) The high-temperature coal with hidden danger of spontaneous combustion is transported to the original coal bunker of the boiler, and the coal also has the risk of spontaneous combustion causing fire again, which also poses a threat to the normal and safe operation of the unit.

(5) The closed circular coal yard stores tens of thousands of tons of coal, coal spontaneous combustion volatilizes flammable gases, and when it encounters water, it will produce a large amount of water gas to cause dedetonation. It is very serious to prevent and control fire in coal yard.

(6) The calorific value loss of coal is serious. The spontaneous combustion oxidation of coal storage presents a serious phenomenon of coal ash, which hindering the development of energy conservation and emission reduction in the factory and affecting the production and operation efficiency of the factory.

One of the traditional monitoring technologies is to monitor the temperature of the coal pile by inserting a thermistor or electric coupler in the coal pile to see whether the early warning temperature is reached, so as to determine whether there is spontaneous combustion. However, this method requires a large amount of engineering, many monitoring points are needed, and the spontaneous combustion point is difficult to accurately locate.

The second traditional monitoring technology is to detect the temperature of the coal pile through the handheld infrared thermal imager to see whether it reaches the early warning temperature, so as to determine whether there is spontaneous combustion. However, the harmful gases such as CO and SO2 produced by coal oxidation in this way will pose a threat to the health of the operator

Significance of distributed optical fiber temperature monitoring

Due to the special structure of the circular coal yard, spontaneous combustion often occurs near the coal retaining wall, and the space of the circular coal yard is relatively large and the operating environment is relatively harsh, so the traditional monitoring means can not meet the requirements, and the linear distributed optical fiber measurement means is needed for temperature monitoring.

Distributed optical fiber temperature measurement system DTS (also known as linear optical fiber temperature sensing fire detector) is an internationally advanced linear temperature measurement technology, which can realize real-time temperature measurement along the optical fiber distribution, timely detection of hot spots, and prevention of non-" fire ", which is of great significance to ensure the safe operation of large industrial and mining enterprises.

The distributed optical fiber temperature measurement system has the following advantages for the monitoring of circular coal yard:

◆ The distributed temperature condition of circular coal yard wall is obtained in real time, and the measurement covers a wide area;

◆ Find hot spots in time, and automatically send alarm signals;

The distributed optical fiber temperature measurement system is set in the transfer station or the control room, and realizes the fire alarm after real-time processing of the temperature signal collected by the optical cable laid on the monitoring site and comparing with the set alarm value. The detector can be used independently to realize temperature acquisition and fire alarm. It can also be connected with the fire alarm control system to complete more monitoring functions through the linkage logic set. When the temperature of a certain optical fiber is abnormal, the coal at that location should be taken first. When the situation is more serious, the operator will manually determine whether to start the fire water cannon

On the firebrick wall of the inner wall of the coal retaining wall, three rings of armored temperature sensing optical fiber are installed 0.5 meters, 1 meters and 1.5 meters from the ground; If I have to, I can

In order to increase the temperature sensing cable at the bottom of the coal yard, the way of circular spacing can be laid.

The armored temperature measuring cable is laid in a ring layout

In addition, in order to prevent the abnormal use of temperature measuring cable (excavator accidentally pulled) caused by fiber breaking, the temperature measuring cable is laid in a loop, that is, the head and tail of the temperature measuring cable are connected back to the temperature measuring host. In this way, once a point in the middle of the temperature measurement cable is broken, the optical signal can be incident from both ends of the cable, which will not cause the loss of the monitoring area and provide the reliability of the system.