What to prevent and how to prevent dust explosions? How to select electrical equipment for a dust explosion environment?
Dust explosion refers to the situation where a dust cloud formed by the mixture of combustible dust and air explodes under the influence of an ignition source.
When a dust explosion occurs, the shock wave from the initial explosion stirs up the accumulated dust in other areas, forming a dust cloud and triggering a secondary explosion. The scope and power of the secondary explosion are often much larger than those of the initial explosion.
Multiple explosions are the most prominent feature of dust explosions. The first explosion wave will blow up the dust accumulated on the equipment or the ground. In a short period of time after the explosion, a negative pressure will be formed in the center area of the explosion, and the fresh air around will fill in from the outside to the inside, mixing with the raised dust and thus triggering a secondary explosion. During the secondary explosion, the dust concentration will be even higher.
Oxidizing gas: A certain oxygen content is the basis for the combustion of dust.
Flammable dust: Dust that can undergo oxidation reactions with oxidizing gases and burn.
Ignition source: A high-temperature heat source that can cause a sudden increase in the temperature of a local dust cloud and lead to combustion.
Dust cloud: A combustible dust cloud suspended in the air and reaching the lower explosive limit.
Enclosed space: The dust cloud is confined to a relatively enclosed space.
The "Regulations on Dust Explosion Safety in Industrial and Trade Enterprises" clearly stipulates that the dust explosion safety work and supervision and management of industrial and trade enterprises in metallurgy, non-ferrous metals, building materials, machinery, light industry, textile, tobacco, trade and other industries (hereinafter referred to as dust explosion-related enterprises) that have the risk of combustible dust explosion shall be subject to these regulations.
What should an enterprise's dust explosion-proof safety management system include? The "Regulations on Dust Explosion Prevention Safety for Industrial and Trade Enterprises" clearly stipulates that enterprises with dust explosion risks shall establish and implement dust explosion prevention safety management systems in light of their actual conditions.
The dust explosion-proof safety management system shall include the following contents: identification, assessment and control of dust explosion risks; Investigation and management of potential hazards of dust explosion accidents Safety Operating Procedures for Dust Operation Positions Special safety production education and training for dust explosion prevention; Dust cleaning and disposal Operation, maintenance, repair and maintenance management of dust removal systems and related safety facilities and equipment; Emergency response and rescue for dust explosion accidents.
Article 11 of the "Criteria for Judging Major Accident Hazards in Industrial and Trade Enterprises" stipulates that industrial and trade enterprises with a risk of dust explosion shall be judged as having major accident hazards if they fall under any of the following circumstances:
(1) Dust explosion hazard areas are set up in multi-story buildings (structures) without frame structures, or there are employee dormitories, meeting rooms, offices, rest rooms and other places where people gather in the dust explosion hazard areas;
(2) Different types of combustible dust, combustible dust and combustible gases and other media that are prone to aggravate the explosion risk share one dust removal system, or different buildings (structures) and different fire compartments share one dust removal system, or the dust removal systems are interconnected.
(3) The dry dust removal system has not adopted any of the explosion prevention and control measures such as explosion venting, inerting or explosion suppression;
(4) When the dust removal system for metal dust such as aluminum and magnesium adopts positive pressure dust removal, or when the dust removal system for other combustible dust adopts positive pressure blowing for dust, no measures such as spark detection and elimination to prevent ignition sources are taken;
(5) The dust removal system adopts gravity settling chamber dust removal or uses dry tunnel structures as dust removal air ducts;
(6) The dry dust removal system for metal dust such as aluminum and magnesium and wood dust is not equipped with a locked air ash discharge device;
(7) Electrical equipment in dust collectors, dust collection bins and other dust explosion hazard areas classified as Zone 20 does not meet the explosion-proof requirements;
(8) Before the process equipment such as crushing, grinding and granulation that is prone to generating mechanical ignition sources, no device for removing iron, stone and other impurities is installed, or the air duct connected to the sanding machine in the wood product processing enterprise is not equipped with a spark detection and elimination device;
(9) The collection, stacking and storage sites of metal dust that spontaneously combusted upon contact with moisture have not taken measures such as ventilation to prevent hydrogen accumulation, or the dry collection, stacking and storage sites have not taken waterproof and moisture-proof measures;
(10) Failing to implement the dust cleaning system, resulting in severe dust accumulation at the work site.
The design principles for the configuration of explosion-proof electrical equipment for explosive dust environments, in addition to complying with the provisions of the general technical requirements, shall also meet the following technical requirements:
Electrical equipment that may be overloaded in explosive dust environments should be equipped with reliable overload protection
2. For the motor used for exhaust ventilation in explosive dust environments, an emergency start button should be installed at a convenient place for operation in the event of an accident in the production facility, or it should be interlocked with the accident signal and alarm device for start.
3. In explosive dust environments, fewer sockets and local lighting fixtures should be installed. If it is necessary to install, the socket should be placed in a place where explosive dust is not likely to accumulate. Local lighting fixtures should be placed in positions where gas and liquid are less likely to impact in the event of an accident.
The explosion-proof type of electrical equipment for explosive dust environments shall be selected in accordance with the provisions in the following table.
The specific selection of electrical equipment should be based on the type of dust present in the specific construction environment. For instance, in cases where the concentration of dust substances is relatively high, a "dust-tight" enclosure must be chosen to prevent dust from entering the electrical equipment and causing an explosion. In some places where the dust concentration is relatively low and there is only a dust layer but no dust cloud, a "dust-proof type" enclosure can be chosen for the dust-proof enclosure, which can prevent the occurrence of dust explosion phenomena. The main types of dust explosion-proof electrical equipment include ExtD enclosure protection type, ExiD intrinsically safe type, ExmD encapsulation protection type, and ExpD positive pressure protection type. At the same time, it is necessary to identify the marks of explosion-proof electrical equipment and correctly select explosion-proof electrical equipment.
01 Shell Requirements
There are mainly two types of dust-proof enclosures. The first type is a dust-tight enclosure of level 6, which can completely prevent all dust from entering. The second category is the 5-level dust-proof enclosure. This type of enclosure has slightly poorer dust-proof performance than the dust-tight enclosure, but it can still prevent the entry of the vast majority of dust without affecting the normal operation of the entire equipment. The entire dust-proof housing plays a very important role in preventing dust explosions. Therefore, when making the dust-proof housing, the Ex permanent mark should be engraved on its surface. At the same time, the data information, warnings, etc. of the entire dust-proof housing should be marked to prevent the dust-proof housing from being damaged.
02 Selection of Materials for electrical equipment
To prevent the entire dust-proof housing from cracking and to ensure its dust-proof performance, when choosing the material for the dust-proof housing, it is essential to select materials with greater strength, such as cast iron and copper. If in some special circumstances the volume of the entire dust-proof housing is less than 2L, plastic can be chosen for the production of the dust-proof housing at this time. However, ordinary plastic should not be selected. Instead, high-quality industrial plastic should be chosen. At the same time, it is necessary to ensure that the thickness of the entire industrial plastic is not too large to prevent situations that affect normal construction. After choosing industrial plastic, Attention should be paid to preventing the generation of static electricity. It is necessary to ensure that the resistance of the entire industrial plastic is less than 10Ω. At the same time, metal embedded seals should be set up during the process of tightening bolts, which can facilitate the discharge of static electricity and prevent dust explosions caused by static electricity.
03 The maximum allowable surface temperature of the dust enclosure
After the use of dust-proof enclosures, the entry of dust was effectively prevented, and the ignition of dust by electric arc sparks was avoided. However, the heating of electrical equipment can still lead to dust explosions. Therefore, certain methods need to be adopted to lower the temperature of the entire electrical equipment to prevent dust explosions caused by the heating of electrical equipment. Before carrying out the of limiting the surface temperature of electrical equipment, it is necessary to have an understanding of the dust in the air. The ignition temperatures of different dusts vary greatly. The heating temperature of the electrical equipment should be limited based on the ignition temperature of the dust, rather than simply lowering the temperature of the entire electrical equipment's casing. Throughout the entire working process, the dust in the construction environment should be continuously swept to lower the dust temperature in the entire environment. At the same time, regular cleaning of electrical equipment should be carried out to prevent dust accumulation.
During the entire working process, if it is necessary to turn on the electrical equipment, it is essential to ensure that the internal temperature of the electrical equipment is lower than the maximum limit temperature set on the surface to prevent dust explosion caused by the equipment being turned on.
