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How to prevent the dust explosion proof bag type dust collector
2014/5/9 閱讀次數:[1340]

How to prevent the dust explosion proof bag type dust collector

1, the characteristics of dust explosion

Full contact with air dust particles of dust explosion is suspended in the air of oxygen, oxidation of the instantaneous under certain conditions, a lot of heat release reaction, resulting in high temperature, high pressure phenomenon. Any dust explosion must have three conditions: ignition source; combustible fine dust; the dust suspended in the air and the explosion concentration limits.

  (1) the dust explosion than combustible substance and combustible gas is complex in general, combustible dust suspended in the air of dust concentration range of the explosion cloud, in the point source function, some dust contact with ignition source was first ignited and the formation of a small fireball. Heat release effects in this small fireball combustion, makes the adjoining dust is heated, temperature, combustion phenomena, such as fireball will expand rapidly and the formation of dust explosion.
Dust explosion degree of difficulty and intensity of dust and chemical and physical properties, and the surrounding air is closely related conditions. In general, the heat of combustion is greater, the finer particles, dust activity is high, the more risk of explosion; suspension of combustible material light explosion dangerous; high oxygen content in the air, dust is easy to be ignited, the explosion also more violent. Because the water can inhibit the explosion effect, so the dust and gas drier, then the risk of explosion is bigger.
(2) dust explosion, tend to produce two explosion which is due in the first blast, there are a lot of dust deposition together, their concentrations exceed the dust explosion limit concentration values not explosion. However, when the first explosion shock waves formed or deposited dust to be raised, in the air is mixed with air, the concentration in the range of dust explosion, it may then have two times of explosive. The second explosion disasters caused by more often than the first explosion is much more serious.
The direct cause of dust explosion in a domestic aluminum products factory in 1963 occurred in the fan impeller and the suction surface friction fires. Suction fan small shrimp bending and pants at the mouth of the three through flow is not smooth, easy to dust. Especially more easily remain dust, stopping once started, the deposition of dust was raised, and soon reached the lower limit of explosion, causing dust explosion.
(3) the mechanism of dust explosion of combustible dust in the air in the combustion will release energy, generate large amounts of gas wells, and energy release speed combustion velocity and powder is exposed in the air in the relevant area. Therefore, substances with a solid powder, its particle size smaller, specific surface area is larger, the faster the spread of combustion. If the solid fine particle size. That can be suspended, once the ignition source to ignite, can release a lot of energy in a very short time. The energy is not dissipated into the surrounding environment, the space within the gas by heating and adiabatic expansion, and on the other hand, the powder combustion produces large amounts of gas, will make the system form local high pressure, resulting in explosion and propagation, which is usually called the dust explosion.
(4) solid combustible dust explosion and combustion difference large combustion is nearly parallel to the inner layer in advance, such as coal combustion. The combustion energy released more slowly. The heat generation and gas can be rapidly releases. Combustible dust heap shape combustion, form fire in good
  
Combustible dust combustion when there are several stages: the first stage, the surface powder is heated; the second stage, the surface layer of gasification, overflow volatile; the third stage, gas phase combustion volatiles.
 

The explosion is a complex process of ultrafine powder, because the scale is generally small dust clouds, and the flame propagation speed, several hundred meters per second, so the fire ignition in the dust, inflammable throughout the dust clouds in the 0.1s time. In this process, if the dust is out, it will generate the highest pressure; if the unburned, low formation pressure. Combustible particles can burn out, depends on the particle size and the depth of combustion. 

(5) according to the classification of combustible dust powder combustibility can be divided into two categories: one category is a kind of non flammable combustible. Methods and standards of classifying combustible powder is different in different countries.

America combustible powder into grade Ⅱ dangerous goods, and the metal powder, the carbon powder, grain dust in different groups. Classification method USA was developed according to the measured powder dust explosion in the standard test device when the step-up speed of classification, it is divided into three grades. Our country has not yet seen off the shelf standard about combustible dust classification.

2、Influence of dust concentration and particle of explosive

  (1) lower bounds on the dust concentration of combustible dust explosion are dust concentration. This value is affected by the ignition energy, oxygen concentration, particle size, powder varieties, water etc. A simplified formula, which can estimate the explosion limit, generally lower explosive limit of dust concentration is 20 ~ 60g/m3, the upper limit of from 2 to 6kg/m3. The upper limit is affected by many factors, its value not easy to determine the concentration of lower limit, usually is not easy to reach the limit. So, the lower is more important, more useful.

  From the physical sense, dust concentration limit value reflects the distance between particles on particle combustion flame propagation effects, if the distance between particles to make the flame does not extend to the adjacent particles, burning cannot continue (Communication), the explosion will not happen; the dust concentration is less than the limit concentration of explosion value. If the distance between the particles is too small, the particle is not enough to provide sufficient oxygen combustion condition, it cannot form explosive, the particle concentration is higher than the upper limit value.

  (2) powder granularity combustible powder particles greater than 400um, dust clouds formed by no longer has the blastability. But for the superfine powder when the particle size is very dangerous to below 10um. Should pay attention to is, sometimes even if the average particle size of powder is higher than 400um, but which often contains powder, fine powder, this part also have explosive.

  Although the regularity of the influence of particle size of powder on detonation performance is not strong, but the powder size is smaller, the specific surface area is larger, the combustion is faster, pressure rise rate will increase linearly. Under certain conditions, the maximum pressure change is not big, because this is the total energy depends on the burning issue, and does not release energy at a significantly.

  3、Technical measures of dust explosion 

  The combustion reaction of combustible material and need oxygen, also need to have a certain energy ignition source. For dust explosion must have three elements: a point source; combustible fine dust; the dust suspended in the air, forming a dust cloud on the explosion concentration range. These three elements exist at the same time will explode. Therefore, as long as the elimination of one condition can prevent the explosion. Often use the following technical measures in the bag type dust collector.

  (1) the special structure design of the body structure of the explosion-proof measures, in order to prevent fouling internals combustible dust precipitator, all beams, partition board should be provided with dust plate, and a dustproof plate should be less than 70 degrees. The ash slip angle greater than 70 degrees, in order to prevent the two bucket wall angle is too small and the ash, two adjacent side plates to be welded on slide plate, eliminate dust deposition, taking into account the operation is not normal and high humidity of the dust hopper blockage occurred condensation, ash hopper design, in the panel of high temperature dust device to increase steam pipe insulation or tubular electric heater. In order to prevent the ash material expansion, each bucket also need to set the bin vibrators or air cannon arm.

  1 sets of dust collector at least 2 ~ 3 bucket, more than 5 ~ 8, deviation in use will generate wind caused by the unbalanced, the bucket of pulverized coal is uneven, the amount of ash H behind the big.

  Take the following measures to solve the problem of uneven wind in the structure can be: ① plus wind deflector in air duct inclined partition, as shown in Figure 5 - 168 shows. Baffle size required air volume and pressure according to the principle of determining; II and considering the actual situation of the change of scene, between the lifting valve stem and valve plate with adjustable, so that export to change value, in order to further correction; the air inlet branch pipe with air adjusting valve, equipment operation on the chamber volume adjust. The chamber volume difference is controlled within 5%.

     
(2) the use of anti-static bag in the dust colle, ctor, due to the high concentration of dust in the process of flowing friction with each other, the dust and the filter is also have mutual friction can produce electrostatic, electrostatic spark combustion product assembly. For the pulse dust cleaning method, using polyester needle felt filter bag, easy to produce static electricity shortage in order to eliminate the terylen needled felt filter bag cloth, spinning into the metal wire or carbon fiber conductive, installed in the bag, the bag is connected through a steel skeleton and porous plate, through the shell into the workshop of grounding grid. For the back blowing dust bag, has developed a variety of MP922 antistatic products. The effect is very good.

  (3) sets the security hole (valve) is limited to the explosion of bag type dust collector and are not extended to other aspects, fire equipment fire set the security hole and essential, it is important. Set the security hole is not intended for security holes to prevent an explosion, but it limits the explosion scope and reduce the number of bursts. A majority of explosive dust precipitator are running in setting the security hole conditions. Because of this, the design of security holes shall ensure that in case of explosion accident, can effectively play a role; usually to strengthen the maintenance and management of security holes.

 Rupture type safety hole is made of ordinary thin metal plate. Because the precipitator box bag not bear great pressure, so the design of rupture strength of the plate should be to the board in the lower pressure is destroyed. Sometimes due to prolonged pressure make aluminum box fatigue deformation and rupture phenomenon, plate even if normal is not allowed to change the high strength.

  Spring door security hole is through the increase or decrease the spring tension regulating opening pressure. In order to ensure accident gate hole to safety, must regularly test the action.

  The security hole area should be determined according to the relationship between the increase of maximum pressure, dust explosion pressure and the speed of the compressive strength, but at present there is no precise data. According to the form, structure of bag type dust collector to determine the security hole the size of the area, we think that the small dust collector security hole and the dust volume is 1/10 ~ 1/30, for medium and large dust collector the ratio is 1/30 ~ 1/60 is appropriate. Encounter difficulties, the appropriate reference to other device for determining safety explosion-proof hole.

  The explosion-proof plate proof board is emergency pressure relief device consists of a pressure difference to drive, non automatic closed, mainly used for pipeline or dust removal equipment, so they avoid the overpressure or vacuum failure. Compared with the safety valve, bursting disc has large discharging area, sensitive action, high accuracy, corrosion resistance and not easy to jam etc.. Bursting disc can be used alone, can also be used in combination with safety valve.

  Bursting disc device consists of bursting disc and holder of two parts, the gripper is made by Q235, 16Mn or OCr13 materials, its role is clamped and protect the explosion-proof plate, in order to ensure the stable pressure blasting. Explosion proof plate is made of aluminum, nickel, stainless steel or graphite and other materials, with different shapes: concave arched proof plate toward the pressure side, blasting tensile or shear failure occurs; convex counter arched proof plate toward the pressure side, blasting due to instability was suddenly turning blade cut or tear along the slit plane; proof board blasting also occurred when the tensile or shear failure.

  Filter selection pressure resistant explosion-proof plate should be precipitator work according to the pressure. Because the dust remover body pressure 8000 ~ 18000Pa according to the set pressure requirements check data to determine the venting valve membrane rupture pressure.

  There are two main types of explosion-proof valve design and safety design: one is the proof plate; the other is the valve hammer. Before a rupture need to be replaced after the new board, production to interrupt, in high negative pressure, easily billet and not easy to heat preservation. After a relatively before an advanced some, in the off state by heavy pressure, poor tightness. The above two methods are not using high voltage pulse cleaning. In order to solve the problems in rigor, hammer type fat blasting valve can be designed explosion proof safety lock. Its characteristics are: in closing, locking safety door is mainly through the lock, in the case of explosion can be automatically opened to release, the release force (An Quanli) and can be adjusted by spring. In order to make the security door stress equilibrium, according to the general security door area set up 4 ~ 6 lock range. In order to make the proof doors tight without leakage can be designed into double structure proof plate and a safety lock.

  (4)Detection and fire protection measures for preventive, in dust cleaning system may take the necessary fire protection measures.

  Fire protection facilities. There are water, CO2 and inert fire extinguishing agent. For the cement factory main production rejection, CO2, and steel with nitrogen.

  The temperature detection. In order to change, remove dust collector temperature control ignition point, generally in the precipitator entrance, hopper are respectively installed several thermometer.

  The detection of CO. For large dust removal equipment due to large volume, the installation is very limited, and sometimes in the thermometer measuring point distance is difficult to reflect the combustion phenomena from the thermometer. In the outlet dust remover to install a CO detection device, to help detect, as long as any place in cyclone combustion phenomenon, CO in the flue gas will increase, and the dust control system and interlock alarm elevated CO concentration, in order to stop the system running.

  (5)The equipment grounding measures of anti explosion duster for running safety need to often open layout. Even open layout of steel structure in tall, according to the equipment grounding requirements, equipment grounding lightning protection has become a necessary, but not a lightning arrester duster.

  Dust all flange are provided better performance of conductor, conductor can be made into card. Also can be made into line type. The line conductor figure. Card type conductor figure. No matter which kind of conductor, connection must be reliable, and be buried at the surface, a certain corrosion function. Otherwise it will affect the equipment grounding lightning protection effect.

(6) supporting parts in dust explosion-proof explosion-proof measures in the selection of explosion-proof component is essential. Anti explosion duster taboo operation condition of dust into the electrical load induced by induced explosion hazard. Dust collector operation electrical load, element in the current transfer contact, even conduction is also produce shock cremation, explosive dust source gas discharge spark induced over the limit concentration is easy to happen, the electrical load components must all use explosion-proof comp, onent, stop explosion induced factors. Ensure the equipment running and safe operation. For example, the electromagnetic valve pulse dust collector, the pulse valve poppet valve for all should use explosion-proof products.

(7) prevent Mars into measures in the bag type dust collector processing sawdust, rice husk boiler, boiler furnace and smelting furnace of aluminum recycling exhaust gas in the stove, burned dust may wind pipe flow into the box, and the accumulation of dust in the filter cloth is on fire, the accident.
In order to prevent the Mars enters the bag type dust remover, should take the following measures:

The setting of pre filter and cooling pipe. Diagram for a cyclone or inert filter as a pre filter, to capture the coarse dust and mars. Mars in this way is too fine particles is not easy to capture, in most cases, the dust particles can burn out before entering the dust collector. The arrangement of the cooling pipe after the pre dust remover, and control the flow velocity in the tube, to as low as possible. This is a more reliable measure, it can make the gas residence time is sufficient in the tube.

But Ling spray tower. Gas cooling water spray directly in advance. In order to ensure the dust gas fire safety in bag filter, cooling water supply is controlled. Dust most burning with a micro water contact can be cooling, but the water is easy to gasification, for the burning of dust has not been in contact with water droplets can cooling, the necessary space and time.

Under special circumstances, the spray tower, cooling tube and pre filter combined with, thoroughly prevent Mars into.
The Mars trapping device is shown in figure. In the pipeline installation Mars trapping device is a simple and feasible method. There through the trap at the instant of Mars, which sends electrical signals, alarm. At the same time, stop operation or changing the gas circuit etc.

Mars catcher design requirements are as follows:

A, spark trap for trapping spark particle in high temperature flue gas, using the equipment the main material 15Mo3 or 16Mo, the beam, column and a platform ladder is used Q235, spark collector as a flue gas pre separator with rotating blades generally use the 15Mn, other materials can be used Q235;

B, equipment import and export speed generally between 18 ~ 25m/s;

C, considering the separation effect of dust. Blade should wear certain measures and the rotating angle of the appropriate;

D, structure design of equipment to take into account the high temperature caused by the deformation. 
          

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8) entrance dust concentration control and join the incombustible powder bag type dust collector in the process of operation, the internal concentration distribution will inevitably make some parts in the explosion limits, in order to improve the safety, dust explosion avoid pipeline between the lower concentration. For example, for pneumatic conveying and crushing dust collection work, from the design should be noticed, which operate in high concentration exceeds the limit; in the local collection and so on, are in the pipeline to keep the dust concentration in the lower concentration limit below.

         A case of fire prevention by dilution method. In the collection of explosive dust, because the dust hood, diluted with air dust, in the pipeline was far below the lower explosive limit. From the system continuously provide incombustible powder into the pipeline, such as clay, bentonite, the dust collector of internal diluted explosive dust, so as to prevent the risk of explosion and fire.

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