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Explosion proof principle of explosion-proof shell

SINO Testing Services

| 2020-07-24|Return

When electrical equipment is in use, due to the thermal effect of conductor resistance, electrical components and circuits will generate heat; Due to the normal operation of electrical components such as switches and relays, electrical sparks or arcs may be generated when the circuit is turned on or off; Due to insufficient insulation or spacing between electrical components, conductive components with different potentials may experience short circuits, breakdown, and other faults, resulting in heating, electrical sparks, or arcs. These may only cause equipment failures, circuit failures, etc. in ordinary environments, but in explosive environments, they may ignite explosive substances and explode. Therefore, it is necessary to take measures to prevent electrical equipment from causing explosion accidents.

 

The most direct measure is to install electrical equipment inside a sufficiently sturdy casing, without considering the working principle and power size of the equipment. Even if the electrical equipment causes an explosion of explosive substances around it, the high temperature and high pressure generated by the explosion will not spread to the entire environment due to being wrapped in the casing, thus avoiding more serious consequences.

 

Therefore, this enclosure should first have sufficient strength to withstand the pressure generated by internal explosions. In GB3836.2 "Equipment protected by an explosion-proof enclosure" d "in explosive environments," this explosion-proof enclosure needs to withstand at least 1.5 times the internal explosion pressure. ".

 

In addition, when the equipment is used in explosive environments, the casing itself should not be a source of ignition. During long-term operation, the external surface of the equipment should not have a high temperature sufficient to cause ignition. Even if the high temperature inside the equipment causes ignition, it cannot spread to the outside. There is no need to consider its ignition risk, but it should be prevented from opening the cover immediately after power outage. The temperature of the high-temperature surface has not yet dropped below the minimum ignition temperature. Therefore, it is necessary to apply a warning sign on the outside of the casing, and the cover can only be opened after sufficient time of power outage.

 

When the shell is made of metal materials, mechanical sparks may be generated if it is subjected to external impacts and friction, causing ignition. Light metals such as aluminum, magnesium, and titanium zirconium are prone to generate high-temperature mechanical sparks after being collided with rusted iron. Therefore, the proportion of light metal components in the metal shell material should be strictly controlled. It should be noted that although in most cases, copper and copper alloys do not ignite and do not produce high-temperature mechanical sparks when collided with other metals, and are often used as explosion-proof tools, copper can form acetylene compounds with acetylene. After friction and collision, the acetylene compounds will burn violently. Therefore, if the equipment is expected to be used in an environment with acetylene, the copper content in the outer shell material should be controlled to not exceed 65%.

 

The casing should also have sufficient stability and should not corrode during daily use. High activity zinc and alloys with zinc content exceeding 80% are not suitable for manufacturing explosion-proof equipment casings.

 

When the outer shell is made of non-metallic materials other than glass and ceramics, in addition to considering the performance degradation caused by long-term operation under high and low temperatures, it is also necessary to consider the performance degradation caused by exposure to ultraviolet radiation for transparent parts of lamps used underground in coal mines and outer shells in non mining areas. In addition, non-metals will accumulate static electricity after friction, and when the static electricity is discharged at a sufficiently high potential, it will ignite combustible substances in the environment. Therefore, it is necessary to avoid the generation of static electricity or prevent the accumulation of static electricity. Anti static materials or measures to release static electricity as soon as possible can be chosen to reduce the risk of static electricity ignition.

 

During the manufacturing and use of electrical equipment, it is inevitable to open the casing for installation, debugging, maintenance, and other work. It is impossible to weld the casing into a completely enclosed whole, and two different components inevitably form a joint structure on the casing. There are gaps at the joint positions, in order to prevent explosive flames inside the equipment from spreading from the gap at the joint to the surrounding environment, Special design is required for the joint structure. According to existing research, when flames propagate along narrow gaps, the smaller the gap, the lower the energy it can propagate to the outside. When the gap is small enough, the internal flames will not be able to ignite the external environment through the joint. In addition, the longer the length of the flame channel, the better the effect on reducing flame energy. In explosion-proof standards, such flame channels are called explosion-proof joint surfaces. Based on a large number of tests, the safety values of the width of explosion-proof joint surfaces and their corresponding gaps for different explosion-proof levels have been obtained. When the width of the joint surface is greater than these minimum values and the gap is less than the maximum value, the explosion-proof shell can effectively prevent internal flames from spreading to the surrounding environment of the equipment.

 

The types of explosion-proof joint surfaces mainly include flat type, cylindrical type, stop type, threaded type, and bonded type. Regardless of the type, it is a channel for internal explosion flames to propagate outward, and the design should consider the most unfavorable conditions.

 

Due to the variety of fastening methods, the width and distance of the planar joint surface also vary, and the common ones include external fixation, internal fixation, and connected cavity fixation.


 

Figure 1: Tightening the bolts from the outside

Firstly, let's take a look at the most common flat cover plate structure shown in Figure 1, which is fastened to the flange by bolts from the outside. In this tightening method, the screw hole may be a through hole or a non through hole, and the shell may be on the outside or inside of the flange. Although the shape of the screw hole and the position of the shell are differentExplosion proofThe parameters are all the same. Obviously, in this case, there are two channels for flame propagation: the first propagates outward along the gap of the entire plane joint surface; The other is to propagate along the plane joint surface to the fastening bolt and outward from the bolt hole. Therefore, the explosion-proof surface parameters that need to be controlled for this structure include: the total effective explosion-proof surface joint width, and the distance from the inner side of the hole to the flange edge. When the screw hole is not a through-hole, attention should also be paid to the requirement of GB3836.2-2010 Section 11.7 for bolt head allowance to prevent the bolt from being unable to tighten when screwed in without a washer.


 

Figure 2: Bolt fixed from the inside

In some cases, it is necessary to fix the cover plate from the inside. For example, in the radiator structure of some frequency converters shown in Figure 2, due to the large heat dissipation fins on the outer sleeve of the radiator heat pipe, it is not possible to tighten the bolts from the outside. In this case, it is necessary to first fix the radiator substrate on the transition flange from the inside, and then assemble the transition flange onto the explosion-proof shell. In this structure, the propagation channel along the entire plane still exists, but when propagating from the outer edge of the substrate and flange to the bolt, both sides of the channel are inside the shell and there will be no explosion transmission. Therefore, this channel does not need to be designed according to the explosion-proof joint surface, as long as it meets the remaining thickness requirements of the explosion-proof shell wall in Chapter 11 of GB3836.2-2010; But if the flame propagates along the plane from the bolt hole to the inner side of the transition flange, it can reach the outside of the shell, which will cause an external explosion. Therefore, this channel needs to meet the requirements of the explosion-proof joint surface. Through simple analysis, this structure requires controlling the parameters of the explosion-proof surface, including the total effective explosion-proof surface joint width and the distance from the hole edge inward to the transition flange edge.


 

Figure 3: Two connected shells connected by bolts

In other cases, it is necessary to fix the two sections of the shell shown in Figure 3 together with bolts from the inside, but the two sections of the shell are not separated. For example, for multi circuit combination switches, the shell is very long. For processing convenience, manufacturing units often design it into multiple sections and then assemble it into a complete shell. In this structure, the path of flame propagation is somewhat different from the above situation. When the flame propagates from the bolt hole along the plane to the inner side of the flange, the inner side is still inside the explosion-proof shell, but when it propagates to the outer side of the flange, it will reach the outer side of the shell. This channel needs to meet the requirements of the explosion-proof joint surface to ensure that the flame does not propagate to the outside and cause explosions in the surrounding environment. Therefore, the explosion-proof surface parameters that need to be controlled for this structure are: the total effective explosion-proof surface joint width, the distance from the outer side of the hole to the edge of the flange.


 

Figure 4: Bolted Connection of Two Separated Shells

When there are some changes in this structure, if it is necessary to connect the two separated shells shown in Figure 4 with flanges, different flame channels need to be considered, such as the explosion-proof joint surface between the junction box of the explosion-proof motor and the base. In this structure, flames can propagate along the joint surface from the inner side of the flange or from the bolt hole to the outer edge of the flange, which may cause explosions in the external environment of the explosion-proof shell, When flames propagate from the inside of the flange to the bolt hole, it will cause another chamber to explode. Therefore, in this case, the entire width of the joint surface, the distance from the edge of the bolt hole to the outer edge of the flange, and the distance from the edge of the bolt hole to the inner edge of the flange all need to meet the requirements of the explosion-proof joint surface.


 

Figure 5: No bolt fastening in the middle of the flange

Sometimes, flanges can be quite narrow and long. In order to reinforce them, manufacturing companies will add a reinforced flange in the middle, as shown in Figure 5. If there are no fastening bolts on this section of the flange and the cavity is not divided into two parts, then there is no need to design according to the explosion-proof joint surface.


 

Figure 6: There are bolts in the middle of the flanges of the same chamber for fastening

However, in some cases, due to the large size of the joint surface, relying solely on peripheral fixation cannot guarantee the strength of the cover plate during explosion. Therefore, there will be fastening bolts on the middle flange, as shown in Figure 6. In this case, it is necessary to consider the flame channel. Since both sides of the flange are the same cavity, the entire plane joint surface does not need to meet the width of the explosion-proof joint surface. However, if the flame propagates from the flange edge along the joint surface to the edge of the fastening bolt hole, Or propagate from the bolt hole to the outside of the shell, so this section of the joint surface needs to meet the distance of the explosion-proof joint surface.


 

Figure 7: The flanges between different chambers are fastened with bolts

In other cases, two adjacent chambers share a cover plate, as shown in Figure 7. For example, on a high-power conveyor motor, the motor chamber and wiring chamber share a cover plate above. In this structure, in addition to the distance from the flange edge to the bolt hole that needs to meet the distance of the explosion-proof joint surface, if the flame propagates along the entire joint surface, it will also propagate from one cavity to another, causing an explosion in that cavity. Therefore, the effective joint width of the flange needs to meet the width requirements of the explosion-proof joint surface.

 

In order to display information of electrical equipment or transmit light from the light source to the outside, transparent components need to be installed on the casing, unless expensive materials such as polycarbonate can be drilled or machined to meet the requirements of fixing and dimensional accuracy of the explosion-proof surface. The heavily used tempered glass cannot guarantee dimensional accuracy, and other means can only be used to meet the requirements of the explosion-proof joint surface. At present, most mining equipment is installed with pads for easy replacement, but due to the large gap between the joint surfaces, it cannot be guaranteed to pass the non explosion test of internal ignition of Class II equipment. The transparent parts of Class II equipment generally adopt the method of bonding the joint surface. The adhesive in this joint surface generally only ensures the sealing effect, and the transparent parts need to be fixed by mechanical structure. At present, there are also special fusion joints that melt glass and cast it into the shell or frame. There is almost no gap between the glass and metal components, but this process is costly and generally not used.

 

Electrical equipment usually has circuit connections with other equipment to achieve functions such as power supply, control, and communication. Explosion proof electrical equipment uses cables to achieve these functions. However, due to the fact that cables are made of elastic rubber, they cannot be processed into cylindrical shapes with fixed external dimensions like metal or plastic, and the position of the cable after installation should also have sufficient stability to prevent line failures caused by pulling during use. Based on the characteristics of cable materials and structures, cylindrical sealing rings with holes are generally used to wrap around the outside of the cable. They are installed together with metal washers into the connecting joint, and various compression devices are used to compress the sealing ring axially, causing it to expand radially. Due to the connection joint on the outer side of the sealing ring, the sealing ring can only expand inward, thus tightly combining with the cable and achieving the function of clamping the cable, This structure is called a cable entry device. For the introduction device used on the explosion-proof shell, in addition to clamping the cable, it should also prevent internal flames from spreading to the environment through the gap between the sealing ring and the cable, and between the sealing ring and the connecting joint. Therefore, additional sealing tests should be conducted to confirm its ability to prevent flame propagation. For IIC equipment or situations where the cable is too short, the risk of flame propagation from inside the cable should also be considered. It is generally recommended that the length of the cable outside the explosion-proof enclosure should not be less than 1 meter.


 

Summary of Explosion proof Enclosures

The use of explosion-proof casing protection does not require the modification or performance reduction of electrical components, and there are no restrictions on the types of electrical equipment. It is convenient to apply and is the most commonly used explosion-proof type. However, due to the need for the shell to withstand internal explosive pressure and its bulky structure, it will also increase manufacturing costs.

 

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