In recent years, coal, electricity, textile, food and other dust explosion accidents happened frequently, which seriously threaten human life and production safety, causing huge casualties and property losses. Due to insufficient understanding of dust explosion, dust explosion accidents due to lack of attention and some factories have occurred, these dust explosion accidents occurred mostly in the dust removal system, dust collector and dust removal system is the most dangerous area.
With the strengthening of environmental awareness, people on the pollution gas emission to the atmosphere of the enterprise is more and more strict, and the bag type dust collector is dust, dust pollution, dust removal equipment is the nemesis of air pollution. Bag type dust collector is the biggest advantage of high dust removal efficiency, in practical application can reach 99.9%, dust emission concentration up to 10mg per m3, or even up to 1 mg per m3. Because of these advantages, the bag filter has been applied in many enterprises. However, with the popularization of the bag filter, dust explosion accident of bag filter is also on the upward trend, so the explosion proof design of the bag filter has been mentioned on the agenda.
1 dust explosion conditions and the mechanism of the occurrence of 1,2
Dust explosion requires 3 conditions:
The dust in the air reaches a certain concentration, that is, in the upper and lower limits of the explosion, the lower limit of the general explosion is most concerned about;
Ignition source with sufficient energy;
Adequate air or oxygen mixed contact.
Dust explosion is a very complex process, influenced by many factors, so the explosion mechanism is still not very clear, generally through the following process: the development of dust explosion, dust explosion is the first dust particles from the ignition source to obtain energy (heat conduction and heat radiation heating surface), dust particles to gain energy and also have the heating and melting and vaporization burst out of tiny particles or hot proton spark ignition source will form dust. Because the dust surface area is large, and the air can be fully contacted with the presence of the dust layer, the surface temperature of the dust particles increases rapidly, so that the dust particles can accelerate decomposition or gasification. When in contact with the air mixture can form a gas ignition, so that the dust in both the gas phase and solid phase, while the existence of more intense combustion. Moreover, the accumulation of static friction is also a source of ignition. When mixed with air when the dust concentration reaches the explosion limit point of fire can burn, in the initial combustion, the dust mixed with air is sufficient, dust explosion, and the pressure wave mode of energy release, so often the mechanical device to blow out the dust, the dust layer on the ground from the formation of dust the flying cloud, the dust cloud is the initial explosion burning residue instantly ignited again, then the explosion occurred second times, at the same time it can produce air turbulence. The explosion due to the dust deposition of the blow again, the explosion was much larger than the initial explosion, dust explosion is affected by many factors, the formation of the ignition source minimum ignition energy and dust concentration etc.. Can be seen from the explosion: the size of dust particle diameter is detonated the degree of difficulty of decision makers is a key factor in the initiation of ignition source, and ignite the dust cloud explosion is the biggest harm caused by two.
2 explosion proof technology
Explosion proof technology can be divided into preventive measures and design measures. Preventive measures is the accumulation of a large number of dust concentration to below the lower explosive limit and dust, and effectively eliminate the ignition source; Design Measures refers to the prevention of explosion explosion measures difficult to work and take the degree of harm to safe levels of measures, the explosion will not cause casualties, after the explosion of equipment within a short period of time can be restored the use of technology, it includes: the uprising, explosion, explosion, explosion etc..
Bag type dust collector as a collection of dust collection, so that the concentration of dust at a time lower than the explosion limit, the economy is not cost-effective, but also very difficult to do; but not to make a lot of dust accumulation can be done. Effective ignition source to distinguish the following 2 kinds of ignition sources: common sources of ignition (ignition source such as welding, grinding, smoke generated) caused by mechanical failure and ignition source (mechanical spark, hot surface, flameless combustion heat, electrostatic spark). Among them, the source of all kinds of sports friction is the most common, most of the dust explosion accidents are caused by it. In addition, the investigation and experiment, mainly in the dust clouds occur corona discharge, in industrial production has not yet occurred in the lightning discharge. In addition, industrial production equipment is also difficult to happen this lightning discharge, unless the formation of dust cloud container volume is greater than 60 3m or greater than m3 diameter, it is possible.
Antiknock is able to withstand the explosion pressure of the structure itself, the structure itself does not burst. Anti explosion structure resistant explosive pressure and resistance to blast pressure 2, explosion resistant pressure design should comply with the design and manufacture of pressure vessel standard, and resistance to blast pressure can relax appropriately. General: blast resistant pressure is 1.5 times the explosion resistance of pressure.
Is the suppression of combustible material in equipment explosion extinguished in the initial stage, so as to avoid the high explosion pressure measures, a successful explosion suppression system in the explosion pressure of 1 * 104Pa action, inhibition of maximum pressure in the equipment is less than 1 * 105Pa. The explosion suppression is divided into 2 types, one is passive, such as water and flame retardant powder device, this device is explosion pressure rise to a certain range, automatic rupture throwing water or flame retardant powder; an initial explosion detection sensor, control unit and high-speed jet high pressure driving spray (HRD) suppression intelligent composition detonator explosion suppression device.
Flameproof is to prevent the occurrence of 1 vessel explosion along the pipeline connecting the spread to the subsequent container, causing the system to explode, causing a greater loss, but the actual harm in the biggest explosion because the flameproof device can not work or not. It generally can be installed in the pipe flame probe, triggering the corresponding agencies to achieve the explosion blocking.
Dust explosion will produce up to 0.7 1MPa pressure range and venting can be upward pressure limit can bear in strength of component material, dust explosion is shown in Figure 1, the curve of a said explosion occurred in the sealed container, Pmax represents the maximum explosion pressure, Pn said the maximum explosion pressure rise rate curve; B said explosion happened, Pred said Pn said the maximum explosion pressure and the maximum explosion pressure rise. It can be seen that the explosion of the explosion hazard has its important role.
Fig 1 dust explosion curve
Explosion proof measures of 3 bag filter [3]
3.1 control the dust accumulation in bag filter
Industrial combustible dust hazard concentrations are generally: 20 - 6000g/m3. Usually the bag type dust collector is the last part of the process system, dust contained gas through the pipeline into the baghouse trapped dust formation layer, and through the pulse back blowing into ash hopper. In these processes, the concentration of dust in the bag type dust collector is likely to be in the range of the above dangerous concentration. Therefore, to strengthen the ventilation system, in particular, the cleaning should be timely, so that the dust concentration in the bag filter and pipe is lower than the lower limit of the dangerous range. Timely to carry away the funnel of dust, is crucial to the bag type dust collector the normal operation of the safety aspects of baghouse dust treatment is flammable dust, if not promptly removed the dust hopper, the dust in the heat accumulation of dust, easy to spontaneous combustion. The accumulation of dust is not a tight overall, which is air, the accumulation of dust is mixed colloidal air and dust, it will become the two source of dust explosion, safety hidden troubles. Double pneumatic unloading valve, can ensure timely discharge, reduce leakage rate, and can avoid the imprecision of dust, to prevent dust pollution two.
3.2 clear ignition source
In the bag type dust collector in the ignition source is mainly the following: common sources of ignition, impact or friction, and the shell temperature electrostatic cremation cremation etc..
The common source of ignition is directly brought in by external fire, especially cutting and welding spark flame, because the bag type dust collector is generally weldment, is mainly produced in the repair instrument for flame cutting and welding spark, so can strengthen management, improve the control of civil air defense work explosion awareness. In the instrument repair but also remove dust around the repair site.
The impact or friction sparks is usually made of iron or other metal objects nut suction bag type dust collector collision caused by sparks, the elimination method is mainly in the dust hood at the metal net is arranged properly, the electromagnetic iron removing device, to prevent the metal in the dust collecting pipe and a bag type dust remover, maintenance after promptly remove the metal material into the pipeline. The second best fan arranged in the bag filter after the clean air side, to prevent Spark metal foreign body and fan of high speed rotating blade collision, and can prevent the inflammable and explosive dust and high-speed rotating blade friction heat of combustion. The wind speed inside the pipe should be reasonable, high speed can be accelerated wear of dust pipe, test shows that the wear rate of the cubic relationship with air crash, resulting in more metals for dust collector.
The mechanism of static electricity generation is not yet clear, but it is necessary to take the necessary measures to prevent static electricity. Selection of substances with similar sequence, or the use of the material combined with the opposite sequence, as far as possible to reduce the contact potential difference, then the maximum degree of inhibition of the generation of static electricity. The positive and negative charge of the belt is neutralized, and the purpose of eliminating static electricity is achieved. Secondly grounding is one of the most important and common leakage measures. Equipment, containers, pipes, etc. should be maintained, such as potential. According to the actual situation, the grounding resistance of the grounding system is in the range of 2 to 1. In addition to increase the conductivity, with the conductor or conductive material instead of high insulation material. In particular the use of antistatic filter material, which is mainly test shows that the electrostatic filter is the most concentrated area, in addition, the dust layer stacked on the bag to the space electric field strength increases, there may occur air breakdown spark discharge. In the end, we can use the method to improve the environmental humidity to leak static electricity.
The dust collector shell to maintain the temperature is not too high, because a large amount of dust is the inner wall of the outer shell surface heated dust adsorption, high temperature, energy generated after melting and evaporation burst out of hot particles or micro proton spark ignition source, but also the formation of dust. So control the ambient temperature can not put the bag type dust collector placed outside.
3.3 installation of venting film
For the bag filter, the most effective and most economical explosion protection measure is to install the venting film, the installation of the key is to vent the explosion area of the calculation, the vent area is too small to play a role, too large and will increase the cost. In the world the most commonly used method is the calculation method of the blasting area Kst, St. Bag type dust explosion venting location is also very important, mainly close to the easy point of outbreak, the venting effect is better. Figure 2 Schematic diagram of bag type dust collector is shown.
Figure 2 Schematic diagram of bag type dust collector
3.4 explosion and explosion suppression
The explosion proof device can adopt an emergency shutdown valve, which is realized by the quick start of a pneumatic spring valve by an infrared flame sensor. Can trigger in an emergency shut off valve far distance sensor installation, prevent flame or blast explosives to other places spread form the two explosion, explosion and specific area of accident control, to avoid the deterioration of the situation, resulting in more serious consequences. If the small bag type dust collector to use passive pressure bag or flame retardant powder dust device, hydrophilic material easy to use pressure bag, the other used flame retardant powder device; if the big bag type dust collector the intelligent high-pressure spraying device.
3.5 other
The vast majority of dust explosion is caused by the negligence of personnel, to strengthen the management of personnel is the most fundamental and important. The theory and practice of knowledge and training of operating personnel, the operation personnel aware of dust explosion hazard and how to effectively prevent the dust explosion protection; strengthen the labor discipline, work must always pay attention to the operation condition of bag type dust collector; the absolute prohibition in the bag type dust collector working time of welding or cutting operation and etc. fire in or near the work area.
