Working principle of dry powder fire truck

Dry powder fire engines rely on the physical and chemical effects that occur when the dry powder mist ejected by high-pressure oxygen contacts and mixes with the flame. Dry powder fire extinguishing agents are generally divided into BC dry powder fire extinguishing agents and ABC dry powder, such as sodium bicarbonate dry powder, modified sodium salt dry powder, potassium salt dry powder, ammonium dihydrogen phosphate dry powder, diammonium hydrogen phosphate dry powder, phosphoric acid dry powder and amino dry powder fire extinguishing agents.

The fire extinguishing efficiency of unit fire extinguishing agent is closely related to the particle size of fire extinguishing agent. The larger the critical particle size of fire extinguishing component and the larger the fraction of fire extinguishing particles smaller than the critical powder size, the better the fire extinguishing effect. The fire extinguishing particles of dry powder fire extinguishing agent have an upper limit critical particle size. All particles smaller than the critical particle size play the role of fire extinguishing. The fire extinguishing efficiency of particles larger than the critical particle size decreases sharply, but its motion is large. The aerodynamic tension is generated on small particles through the air, forcing small particles to follow closely and attack the flame center, rather than being blown away by hot air before reaching the flame, resulting in the reduction of fire extinguishing efficiency. The particle size of common dry powder fire extinguishing agent is between 10 ~ 75 PM, which has poor dispersion and relatively small specific surface area. Suffocation, cooling and chemical inhibition of flaming combustion are the concentrated embodiment of the fire extinguishing efficiency of ten powders. Among them, chemical inhibition is the basic principle of dry powder fire extinguishing agent and plays the main role of fire extinguishing. The fire extinguishing component in the dry powder fire extinguishing agent is the inactive substance of the combustion reaction. When it enters the flame in the combustion area, the free radical generated by decomposition reacts with the H and OH radicals generated in the flame combustion reaction to capture and terminate the free radical generated by the combustion reaction and reduce the rate of the combustion reaction. When the dry powder concentration in the flame is high enough, the contact area with the flame is large enough, and the base stop rate is greater than the rate generated by the combustion reaction, the chain combustion reaction is terminated, so as to extinguish the flame. The dry powder fire extinguishing agent decomposes endothermically in the combustion flame. Because each decomposition reaction is endothermic reaction, it has a good cooling effect. In addition, when some dry powder fire extinguishing agents decompose at high temperature, a layer of glass film will be formed on the surface of solid substances, and the residual cover will cover the surface to prevent combustion and prevent re combustion. In recent years, some special dry powder foam combined fire engines will load ultra-fine dry powder extinguishing agent and mix ultra-fine dry powder with foam for fire extinguishing. Superfine fire extinguishing particles have large specific surface area, fire extinguishing agent particle size is less than the critical particle size, high activity, can be suspended in the air for several minutes to form a relatively stable aerosol, fast thermal decomposition speed and strong ability to capture free radicals. Therefore, the fire extinguishing efficiency is greatly improved, the dosage is significantly reduced, the sediment after fire extinguishing is not obvious, and there is little pollution to the fire site. However, due to the small critical particle size of ultra-fine dry powder fire extinguishing agent particles and small momentum when sprayed alone, it is easy to be blown away by the hot air flow in the fire site and cannot reach the key area in the center of the fire source. By mixing the ultra-fine dry powder with the foam mixture, the ultra-fine dry powder can be brought to the center of the fire with the help of the wrapping of water particles in the foam, so as to achieve better fire extinguishing effect. Such fire engines are common in petrochemical fire fighting.

The dry powder fire truck is composed of chassis, cab, dry powder system, additional control system and equipment box. The typical dry powder system equipped with dry powder fire truck is shown in Figure 8-2, which is composed of nitrogen cylinder group, safety, high-pressure air supply pipeline, decompression, low-pressure air inlet pipeline, dry powder tank, pressure relief, dry powder gun, dry powder roll, dry powder gun and purging pipeline disc. During operation, first open the bottle mouth of the high-pressure nitrogen cylinder group to stop, and the high-pressure nitrogen is collected from each steel cylinder to the high-pressure pipeline (the pressure of high-pressure nitrogen is generally 12 ~ 15 MPa), and then depressurize through decompression (the pressure of nitrogen after decompression is generally 1.2 ~ 1.7 MPa). The depressurized nitrogen is filled into the dry powder tank through the dry powder tank. As the depressurized gas enters the bottom of the dry powder tank and is strongly stirred and mixed with the dry powder, the pressure of the dry powder tank gradually increases to the rated depressurization pressure, and the depressurization is in a pressure balance state at this time. When necessary, turn on the dry powder gun or the dry powder gun at the end of the dry powder reel, and the nitrogen with the wrapped dry powder will be sprayed out at a high speed to extinguish the fire. With the injection of dry powder, the pressure of the dry powder tank decreases. At this time, the decompression breaks the balance and continues to depressurize the high-pressure nitrogen and deliver it to the dry powder tank to promote the injection of the remaining dry powder until the dry powder pressure drops to 0.5 MPa. When the nitrogen cylinder group is fully equipped, observe that only gas is ejected from the injection port. At this time, the dry powder has been ejected. Close the dry powder tank into the balloon, and then turn on the purge switch to purge the injection pipeline to prevent the dry powder from corroding the injection pipeline.