Analysis on explosion proof safety of boiling drying equipment

Two problems of electrostatic and dust explosion associated with powder boiling are discussed in theory. The importance of explosion-proof safety of boiling drying equipment is expounded, and the application of this aspect in such equipment is also introduced.

Key words: boiling drying equipment, static electricity, dust explosion, explosion-proof safety

Boiling drying equipment is based on the working principle of fluidized bed, and the basis of drying process is the boiling of powder. At the same time, under the effect of ventilation, the fine powder in the container is filtered by the trap bag and then sent to the outside. A large number of ultra-fine powder will be produced in the process of trapping. Whether in the flow boiling or in the catching operation, the whole process of boiling drying is accompanied by ultra-fine powder movement. However, the process of powder movement is often accompanied by two problems of static electricity and dust explosion, which may cause explosion under the guidance of some factors. At present, when dealing with such problems in China, it is considered that it is feasible to install explosion-proof motor. This paper discusses theoretically the electrostatic and dust explosion problems of powder boiling, and puts forward its own views on the explosion-proof safety of boiling drying equipment, with the purpose of making and applying such equipment more complete, more reliable and safer.

1. Analysis of unsafe factors of powder in boiling drying equipment

1.1 discussion on factors of electrostatic generation in powder boiling of boiling drying equipment

During the boiling drying process of materials in the boiling drying equipment, due to the intense friction between the dry gas and the surface of the material particles, the dry solid particles will also have violent collision and friction. At the same time, the solid particles also have severe collision and friction with the cylinder wall, the trap bag and the pipe wall, which makes the surface of the solid particles carry a large amount of static electricity; on the other hand, with the solid particles, the surface of the solid particles has a lot of static electricity With the continuous refinement of the particles, a large number of new surfaces are produced, and the surface charge balance of the original particles is destroyed, which makes the surface of the new particles carry a lot of charges. Due to the accumulation and adhesion to the tube wall, once the grounding condition of the whole machine is poor, the accumulated electrostatic charge can form a quite high voltage, which will lead to electrostatic discharge under certain conditions. If the materials to be dried are flammable and explosive, this kind of electrostatic spark is very easy to cause sudden combustion or explosion of dry materials.

According to the literature research, it is considered that "the electrostatic height lies in the trap bag", which is due to the operation of the fan to remove a large number of fine dust in the container, thus accumulating a large amount of charged dust in the bag. The dangerous point of spark discharge is considered to be in the trap bag and separation pipe.

1.2 discussion on factors of dust explosion in boiling drying equipment

Dust explosion is defined as the dust particles suspended in the air are in full contact with oxygen in the air, and the oxidation reaction is completed instantaneously under specific conditions. A large amount of heat is released in the reaction, resulting in the phenomenon of high temperature and high pressure. Generally speaking, combustible solid will release energy and produce a lot of gas when burning in air, and the speed of releasing energy depends on the area of solid (dust) exposed in the air. Therefore, for the same solid material, the smaller the particle size and the larger the specific area, the faster the combustion diffusion. If the particle size of the solid is so fine that it can be suspended, a large amount of energy can be released in a very short period of time once it is ignited by a ignition source. This energy has no time to dissipate to the surrounding environment, causing the gas in the space to be heated and expanded, while a large amount of gas is generated during the combustion of the other flour, which will cause the system to form local high pressure, resulting in explosion and Communication.

According to the literature, any dust explosion should have three elements: ① ignition source; ② combustible fine dust; ③ dust floating in the air to form dust cloud within the range of explosion concentration. Among them, only when three elements exist at the same time, the dust explosion can be effectively prevented as long as one of them is eliminated.

Discussion on unsafe factors of powder in 1.3 boiling drying equipment

No matter in the fluidized boiling or in the capture operation, the material boiling in the boiling drying equipment is easy to produce a large amount of dust, and the dust boiling is easy to cause the above-mentioned electrostatic sparks, which may cause ignition source elements. For the "dust cloud", as described in the literature, "when the combustible powder particles are greater than 400 μ m, the dust cloud formed will no longer be explosive." When the particle size of the powder is less than 10 μ m, there is a greater risk. When the particle size of the powder between the capture bag and the discharge pipe of the boiling drying equipment is very fine, it is very likely that the "dust cloud" mentioned above. Therefore, for general materials, it is very likely that there are dust explosion factors. If the materials contain combustible fine powder components, it means that there are three elements of dust explosion, and the possibility of dust explosion is great.

If the explosion-proof motor is only installed in the boiling drying equipment, it is only to prevent "ignition source", which is also necessary for such equipment. It can be seen from the above that static electricity and dust explosion are the key to explosion-proof safety of such equipment, which are often forgotten by people. However, the reality of drug production is to ensure the safety and reliability of production equipment and meet the corresponding process requirements. People can't just rely on the concept of "no such accidents happened in the past, so we don't have to be too serious about this" to produce such equipment. We should consider the scientific and safety.

2. Preventive measures for powder safety in boiling drying equipment

In this paper, the potential safety hazards caused by the boiling of powder in the boiling drying equipment are described. The prevention of these two kinds of hidden dangers is described in some literatures. As the boiling drying equipment, it can be used for reference as follows.

2.1 anti static measures for boiling drying equipment

The following measures can be used for reference to prevent static electricity from boiling powder in boiling drying equipment

1) The trap bag is made of anti-static fiber material to eliminate the static electricity on the powder entering the bag in time, so as to avoid the stripping electrification when the powder agglomerates and falls off.

2) The conductor material is used as the connecting part of the air supply pipe and the connecting parts, and the whole system is carefully electrified, so that the static electricity on the powder can be conducted away in time.

3) Other methods, such as increasing pipe diameter, reducing elbows, cleaning ash and discharging regularly, wearing anti-static clothing for operators, are also effective in restraining, reducing and eliminating static electricity in the process of powder boiling.

2.2 measures to prevent dust explosion of boiling drying equipment

The following measures can be used for reference to prevent dust explosion from boiling in boiling drying equipment

1) To remove the ignition source, explosion suppression system shall be installed for equipment and devices that may produce fire source, and explosion-proof membrane or pressure relief valve shall be set in dust removal or dust filtering equipment and pipeline.

2) The dust in the pipe and the trap bag shall be removed regularly, and there shall be no dust accumulation.

3) Electrical equipment to use explosion-proof devices, such as explosion-proof motor, installation of ground wire, etc.

4) The design of the workshop should meet the requirements of dust explosion prevention, according to the anti explosion design, and should have enough explosion-proof pressure relief area.
3. Application of explosion-proof safety in our company's boiling drying equipment

In view of some domestic boiling drying equipment explosion-proof consciousness is not perfect, only take simple explosion-proof measures, can not ensure the safety of operators and workshops, there are certain hidden dangers in the production. It is often fixed in the concept of "there was no such accident in the past, so it is not necessary to be too serious about it". With the enhancement of people's safety awareness and the complexity of drying materials in the future, the explosion-proof requirements of such equipment have been raised to an urgent level. Therefore, Changzhou Changhong drying equipment Co., Ltd. has taken necessary preventive measures in structure according to the production experience of boiling granulator for many years and the characteristics of similar foreign equipment, especially the boiling drying equipment. Now take one of the explosion-proof boiling drying equipment developed by our company as an example to illustrate the application of explosion-proof structure.

The new explosion-proof pelletizing and coating machine is shown in the figure. Its inlet air has two to three-stage filtration, and the induced draft fan is equipped with a sound attenuation device. The whole granulating and coating process is in a closed state from the air inlet, atomization and exhaust, so as to achieve the effect of high efficiency and energy saving. In addition to the anti-static fiber material used in the trap bag, the pressure difference between the two ends of the dust-proof bag filter can be automatically measured and controlled to keep the bed in a good fluidization state. Among them, the equipment is divided into left and right capture chambers, with a total of two capture cloth bags. The lower edge of each bag is fixed with the bottom edge of the bag by using an air bag structure. When the bag rack and airbag structure are pulled to an appropriate position, the air bag inflates and expands to seal the lower edge of the bag with the inner wall of the upper cylinder. The structure is easy to operate, and the operator does not need to climb to the high place to install and remove the cloth bag.

The explosion-proof device is set on the upper part of the main engine. When the positive pressure is generated inside the main engine, the pressure warning mechanism in the explosion-proof door f will send out gas signal, and the whole system will be cut off through the gas electric conversion, and the quick Valves a and B will close quickly. If the cylinder pressure continues to rise, the explosion door f will be forced to open automatically, and the explosion gas will be discharged from the main engine through the explosion relief channel, so as to ensure the safety of personnel and equipment. The explosion relief test was carried out by Tianjin Pharmaceutical Machinery Testing Center of State Economic and Trade Commission. When the positive pressure inside the main cylinder is greater than 0.8kg/cm2, the explosion relief door will open automatically.

In addition, the electrical part of the equipment should be explosion-proof measures, fan explosion-proof, observation lamp explosion-proof. In addition, safety grounding and other installation requirements are emphasized in the operation manual and electrical installation, which are related to the general grounding facilities of the equipment use plant.

The explosion-proof pelletizing and coating machine with the above structure has been put into the market. The equipment has the functions of granulation, coating, drying and explosion-proof performance. It is suitable for powder granulation and surface coating coating of powder, granule and pill in Chinese and Western medicine, food, chemical industry and other industries.

4. Summary

The two problems of electrostatic and dust explosion associated with the boiling of powder are discussed in theory, so as to reiterate the importance of explosion-proof safety of boiling drying equipment.