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Basic knowledge of dust explosion prevention

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| 2019-12-12|Return

1、 Why do we need to do dust explosion prevention?

1. Dust explosion event. Dust, in the impression of many people, will not explode. Until the 1980s, there were nine consecutive explosions in Harbin linen factory and a dust explosion in Xingang silo On the morning of December 10, 1981, due to the installation of electronic scales requiring the use of wind welding construction, the accumulated dust inside the second floor discharge pipe was not thoroughly cleaned before construction. During the operation, the accumulated dust in the discharge pipe was ignited, causing a dust explosion and a chain explosion of silos through various connecting pipes. This caused 28 cylinder tops, including star cylinders, to be lifted out of 33 cement cylinders, and 7 people were injured, one of whom suffered severe burns, It has just begun to attract people's attention. In August 2008, a dust explosion occurred in the United States, causing the entire factory area to be paralyzed and scrapped. In mid October 2009, a feed factory in Fujian experienced two silo dust explosions in the early morning, causing system paralysis. At around 7:37 am on August 2, 2014, a suspected dust explosion occurred in the polishing workshop of Zhongrong Metal Products Co., Ltd. in Kunshan Development Zone, Jiangsu Province during the production process, causing multiple casualties. Due to the opposite nature of dust explosions and gunpowder, the most terrifying types of dust explosions are secondary and tertiary.... Explosion. The explosion caused a lot of accumulated dust to fly back up, resulting in multiple explosions.

2. What is a dust explosion? Dust explosion refers to a situation where a dust cloud encounters a heat source (open flame or temperature) within the upper and lower limits of an explosion, and the flame instantly spreads throughout the entire mixed dust space. The chemical reaction rate is extremely fast, while releasing a large amount of heat, forming high temperatures and pressures. The energy of the system is converted into mechanical work and radiation of light and heat, which has strong destructive power. (Dust cloud refers to a mixture of high concentration combustible dust and combustion supporting gas suspended in the combustion supporting gas.)

3. Three conditions for dust explosion. Dust explosion should meet three conditions, namely that the dust itself has explosiveness; Dust must be suspended in the air and mixed with the air to an explosive concentration; There is a thermal energy source (ignition source) capable of causing dust explosions. Compared to gas explosions, dust explosions require a higher minimum ignition energy. Therefore, a sufficiently strong thermal energy source is also one of the necessary conditions for forming dust explosions.

4. The process of dust explosion. The explosion of dust can be regarded as the development of the following three steps: the first step is the rapid distillation or gasification of suspended dust under the action of a heat source to produce combustible gases; The second step is to mix combustible gases with air and burn them; The third step is to transfer the heat released by the combustion of dust to nearby suspended or blown dust through thermal conduction and flame radiation. These dust particles are heated and vaporized, allowing the combustion cycle to continue. As each cycle progresses step by step, its reaction rate gradually accelerates, through intense combustion, and finally forms an explosion. This explosion reaction, as well as the speed of the explosion flame, the speed of the explosion wave, and the explosion pressure, will continue to accelerate and increase, and will develop in a jumping manner

5. The characteristics of dust explosions.

(1) Multiple explosions are the biggest characteristic of dust explosions;

(2) The minimum ignition energy required for dust explosions is relatively high, generally above ten millijoules;

(3) Compared with combustible gas explosions, dust explosions have a longer induction period and a slower pressure rise, reaching tens of seconds, which is tens of times higher than gas explosions. Due to the long duration of high pressure, it releases a large amount of energy and has strong destructive power.

6. The hazards of dust explosions.

(1) Has extremely strong destructive power.

(2) Easy to cause secondary explosions.

(3) Can produce toxic gases.


 

2、 What kind of dust can cause an explosion?

1. Explosive dust includes:

(1) Metals (such as magnesium powder, aluminum powder, titanium powder, etc.)

(2) Coal (such as coal powder)

(3) Grain (such as flour, sugar powder, corn flour, potato flour, dried milk powder, starch, etc.)

(4) Feed (such as blood meal, fish meal, etc.)

(5) Agricultural and sideline products (such as cotton, tobacco, tea dust, etc.)

(6) Forest products (such as paper powder, wood powder, fiber dust, etc.)

(7) Synthetic materials (such as plastics, dyes, etc.)

2. Production processes that are prone to dust explosions:

(1) Polishing and crushing process

(2) Gas solid separation process

(3) Dry dust removal process

(4) Drying process

(5) Pneumatic transportation process

(6) Cleaning and blowing process

3. Equipment prone to dust explosions:

(1) Dust collector

(2) Dust collector

(3) Pneumatic conveyor

(4) Drier

(5) Grinding machine

(6) Silo

 

3、 How to do dust explosion prevention?

Due to the three elements of dust combustion and explosion (combustibles, combustibles, and ignition sources), depending on the operating conditions of the system, dust as a combustible material cannot be avoided and can only be controlled to a limited extent; Air as a combustion aid cannot be controlled; The easiest to control is the ignition source. Therefore, the focus of dust explosion prevention work is to control the ignition source. The main control measures for dust explosion prevention are as follows:

1. Measures to prevent ignition sources:

(1) Smoking is strictly prohibited in the production area, carrying lighters, matches and other ignition materials is strictly prohibited, and flammable and explosive materials are strictly prohibited.

(2) Wearing shoes with iron codes and nails is strictly prohibited from entering the dust production site; It is strictly prohibited to use iron tools to strike walls, metal equipment, pipelines, and other objects.

(3) Enter enclosed or semi enclosed spaces for work. Ventilation should be done well before homework, and anti-static work clothes and shoes should be worn, as well as dust masks, safety helmets, and safety belts; During homework, it is strictly prohibited to use mobile phones or non explosion-proof walkie talkies. Explosion proof tools such as copper, aluminum, wood, bamboo, etc. should be used for cleaning operations.

(4) Pay attention to the inspection of mechanical equipment, especially carefully check whether the protective device of the motor junction box is damaged, remove dust, and prevent loose joints or fires caused by short circuits in electrical circuits.

2. Measures to control ignition sources. The coal transportation system should avoid hot work as much as possible. When hot work is necessary for maintenance, renovation, and other projects, the hot work permit system must be implemented. During hot work operations, it is necessary to:

(1) Stop all dust cleaning operations.

(2) Within 10 meters of the homework site (including the floor), it should be cleaned thoroughly and rinsed with water. The inner walls of equipment that requires hot work welding and cutting must be cleaned of accumulated dust.

(3) All windows, entrances, and explosion vents at the homework site should be opened. A section of the pipeline connected to other sealed containers must be removed and the ports of the pipeline and sealed container must be sealed with non combustible materials to completely isolate the hot work area from the pipeline and sealed container.

(4) All combustible materials within a 10 meter range of the work site should be removed from the workplace as much as possible. Combustible buildings or objects that cannot be removed must be protected with non combustible materials, and a dedicated person should be equipped with fire-fighting equipment to monitor the entire process.

(5) All detachable equipment and pipelines should be dismantled and transported to a safe area for hot work operations.

(6) All holes on the floor and walls within a 10 meter radius of the homework site, as well as ventilation and dust removal suction ports, must be tightly blocked to prevent flames from entering.

(7) Hot work personnel must strictly follow the requirements of hot work approval and the requirements of "eight no's", "four musts", and "one clear" for hot work, implement safety measures, and strictly follow the operating procedures for work. The equipment and facilities used must comply with safety technical requirements.

(8) During homework, there must be a safety officer on site to supervise, and a dedicated person must be assigned to watch the fire. Adequate and suitable fire extinguishing equipment and water supply pipelines must be prepared, responsible for watering the work site while operating to ensure timely cooling and extinguishing of surrounding sparks.

(9) After the homework is completed, the hot work personnel and the fire watchers should jointly extinguish the residual fire marks, clean the work site, wet the surrounding area with water, check and confirm that there are no residual fire marks, and the fire watchers should stay on site for monitoring for at least one hour. Only after confirming safety through inspection can they evacuate the site, and regular inspections should be conducted within 8 to 24 hours after the hot work to ensure safety.

3. Explosion proof measures for equipment and facilities.

(1) Explosion proof measures in structural design.

1) Requirements for equipment material selection

The material of the electrical equipment casing should have good thermal stability, sufficient strength, and be able to withstand explosive pressure without damage or deformation. The explosion-proof joint surface should be able to withstand explosion without transmitting explosion

2) Requirements for maximum surface temperature of equipment

Limiting the formation of the highest surface temperature of the dust casing is one of the main factors in preventing dust ignition. On the one hand, due to the inevitable heating of electrical components, it is particularly necessary to ensure safety and limit the maximum surface temperature range of electrical equipment. On the other hand, due to the different ignition temperatures of different substances, the range of maximum surface temperature of electrical equipment should also be different. The maximum surface temperature of gas electrical equipment is consistent with the maximum surface temperature of dust electrical equipment.

3) Requirements for non-metallic components

The non-metallic components of the equipment should also meet the heat and cold resistance requirements of relevant standards.

4) Requirements for IP protection

According to the requirements of the national standard GB12476.1, the shell of dust explosion-proof electrical equipment must also meet the following two conditions:

a. Dust proof shell: Although it cannot completely prevent the entry of dust, the amount of entry is not enough to affect the normal operation of electrical equipment. The shell's ability to prevent external objects is level 5;

b. Dust tight shell: The structure of the shell is designed to be dustproof, and dust cannot enter. The shell has a level 6 ability to prevent external objects.

(2) Explosion proof measures for dust removal and dust production equipment. For dust production or dust removal systems, emphasis should be placed on eliminating ignition points, increasing wind speed, or gradually reducing dust concentration. In such conditions and environments, changing the dust concentration is neither objective nor economical. We can only increase the operating wind speed, operate under negative pressure, use explosion-proof fans and motors, eliminate ignition points, use flame-retardant and anti-static materials in the system, have good grounding, explosion-proof and cleaning ports, and have pressure, temperature, and other danger prediction and monitoring systems. When a danger occurs in the system, the main engine will automatically stop working, Each explosion-proof and explosion suppression system operates automatically. The dust removal system is started 10 minutes before the production equipment starts, and the dust removal system is stopped 10 minutes after the production equipment stops working.

(3) The main explosion-proof measures taken from the production process, selection and layout of electrical equipment.

4. Explosion relief, suppression, and isolation measures. For dust explosions: On the one hand, it is to eliminate the factors that form dust explosions and prevent the occurrence of dust explosions. On the other hand, it is to control the explosion range and reduce the losses caused by the explosion. In situations where explosion conditions have formed due to certain environments or explosion factors cannot be eliminated temporarily, venting, suppression, and isolation will reduce or control the losses caused by explosions.

5. Strengthen the management of equipment and facilities. In daily life, it is necessary to strengthen the inspection of mechanical equipment such as equipment and facilities, check for aging, heating, wear, impact sparks, strong vibration, poor contact, poor grounding, leakage, and reduced dust removal efficiency. If hidden dangers are found, they should be rectified in a timely manner and daily maintenance work should be done well, Ensure that the safety devices of the mechanical equipment are in good technical condition (especially the inspection of the dust removal bag inside the dust collector to prevent loose friction and collision at the bag opening connection, which may cause sparks).

(1) During homework, it is necessary to strengthen inspections and pay attention to understanding the operation of machine electrical equipment and circuits. If any abnormal situations such as metal collisions, abnormal noises, temperature rise, sparks, or obvious deformations are found in the equipment, it should be immediately shut down for timely maintenance and elimination of hidden dangers. It is not allowed to continue working at risk.

(2) Various friction components of mechanical equipment should be lubricated on time to prevent overheating and fire explosions. The temperature of the bearings used on the conveyor should be controlled, and once the temperature exceeds 200 ℃, the machine should be stopped in advance.

(3) Timely handle equipment oil leaks and other flammable materials. It is not allowed to store oil and other flammable materials in the production area.

6. Remove accumulated dust to prevent explosions. The harm of dust accumulation cannot be underestimated, as it is the most important and dangerous dust source for explosions. For dust explosions, the dust accumulation state is more dangerous than suspension, and its explosive pressure rises at the fastest speed, causing the greatest destructive force. So effectively eliminating dust accumulation is one of the main measures to prevent dust explosions. Require on-site team cleaning and weekly cleaning. When cleaning, it is advisable to mainly use vacuum suction and flushing, but it is not advisable to use wet cleaning methods such as spraying, as combustible dust can easily produce combustible gases at a certain temperature and evaporate in the space. It is prohibited to use cleaning methods that may cause dust and gas to fly in enclosed spaces, such as blowing with compressed air.

7. Dust control.

The methods of dust control generally include dust suppression, sealing, dust removal, and dust unloading. The selection and treatment of dust sources in coal transportation in power plants mostly adopt three methods: dust suppression, dust sealing, and dust suction. Generally, a combination of these three methods is used.

(1) Vacuum cleaning: Vacuum cleaning is the most commonly used method to reduce dust by using a fan to extract dusty air from the coal guide groove, forming a slight negative pressure inside the coal guide groove. The most commonly used method to reduce indoor dust content in coal conveying systems is vacuum cleaning. A dust removal point is set up at the guide groove of the belt conveyor and the raw coal hopper between the coal bin to form a negative pressure inside the guide groove and the raw coal hopper, and to suppress dust from flying out. The equipment used for vacuum cleaning mainly includes bag filters, high-voltage electrostatic precipitators, etc.

(2) Dust suppression: When air containing dust encounters water and increases humidity, water and dust particles adhere together to form larger particles that fall. Water spraying is one of the effective methods for dust suppression in coal transportation systems. However, the amount of water sprayed should not be too much and should be appropriate, otherwise it will cause side effects such as belt slipping and coal sticking. Usually installed at the outlet of the conveyor guide groove, the wheel bucket of the bucket wheel machine, the inlet of the impeller coal feeder, and the inlet of the coal chute.

(3) Dust sealing: Coal dust needs to spread and diffuse through the flow of air. Dust sealing is to isolate the air, separating clean air from air with dust. Thermal power plants generally install protective covers at the head of the belt conveyor and coal guide grooves at the tail to prevent a large amount of coal dust from flying when coal falls into the coal chute. In order to seal the dust, the volume of the protective cover and coal guide groove should be as large as possible, and the upper cover should be in a circular shape with uniform pressure distribution, which can reduce the air flow inside the cavity. However, due to the limitation of belt width, uneven pressure inside the coal guide groove is inevitable. The pressure is highest at the coal drop point, and the further away from the point, the lower the pressure. To reduce the pressure difference, a circulating bypass pipe is generally installed.


 

4、 Fire emergency measures

1. Firefighting equipment and facilities should be equipped as required.

2. Once a fire or explosion occurs, the equipment should be stopped immediately and the power supply should be disconnected.

3. When extinguishing a fire, avoid raising accumulated dust. It is not advisable to use water to extinguish fires, as water can easily react with heat and burning coal powder, producing explosive mixtures.



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