I. The three elements of combustion and explosion: The three elements that form combustion and explosion: The following conditions meet in time and space, will produce combustion or explosion.
For example, hydrogen, gasoline, etc.; oxidants, such as oxygen, air, etc.; sources of ignition, such as open flames, sparks, arcs, high temperature surfaces, etc. Therefore, at the same time
Flammable substances, air, and ignition sources are necessary and sufficient for combustion and explosion. II. Principle of explosion protection In order to prevent explosion and fire hazards, it should be on
Explosion protection measures are taken in the premises. Explosion-proof measures are divided into two major categories in engineering:
1. 1.
Explosive hazardous environment. It can be known from the three factors described above that there are explosion or burning conditions that if a flammable substance can be avoided in the environment, or that the oxidant—oxygen is avoided in the environment, the risk of fire or explosion can be fundamentally avoided. Oxygen in the air is unavoidable. A feasible solution is to avoid flammable substances. Chemical plants often use a roofless factory building to improve the natural ventilation effect, or use forced ventilation (mechanical ventilation) so that the concentration of flammable substances in the environment is lower than the lower explosion limit, so as to avoid the risk of explosion.
1. 2. Secondary explosion protection measures - Avoid sources of ignition in potentially explosive atmospheres. If the explosive atmosphere is unavoidable, remove the source of ignition in the environment. We often see signs of “no smoking†in oil depots and gas stations, which are secondary explosion-proof measures. According to national standards, explosion-proof electrical products must be used in explosion hazardous locations. This is also a secondary explosion protection measure. According to the three elements of combustion and explosion conditions, different explosion protection measures can be taken to prevent electrical equipment from becoming a source of ignition. The principle of achieving explosion-proof is to eliminate any one of the three elements so as to prevent explosion.
1. Flameproof type: If explosive gas and air are inevitably present, the explosion-proof purpose can be achieved by placing components capable of generating arcs and sparks in a flameproof enclosure with sufficient strength for normal operation of the equipment. This is explosion-proof. type. Flameproof electrical equipment is the electrical equipment with the symbol “dâ€, which has the ability to withstand the explosion pressure of the internal explosive gas mixture and prevent internal explosions from propagating to the explosive mixture around the enclosure.
2. Increased safety type: If explosive gas and air are inevitably present, increased safety equipment is to eliminate ignition sources. For ordinary motors, if some protective measures are taken in their structure, make every effort to ensure that the equipment will not cause arcs, sparks and dangerous high-temperature phenomena under normal operating conditions or approved overload conditions.
It can further improve the safety and reliability of the equipment. Therefore, this device has no ignition source during normal operation and can be used in explosion hazardous environments. Words will be explained
It is "increased security." Its flag is "e".
3, Positive Pressure Enclosure (Ventilated, Inflatable Type) “pâ€: Fresh air with a certain pressure in the housing of the device
Or inert gas, so that the surrounding flammable gas can not enter the inside of the enclosure, thus preventing the ignition source from contacting with the explosive gas, so as to prevent explosion. This measure
Shi is to eliminate the explosive gases in the three elements. The key measure of positive pressure electrical equipment is the protective gas (fresh air or inert gas) inside the equipment enclosure.
The pressure above the ambient pressure is at least 50Pa. Therefore, the equipment needs to be equipped with blowers, pipes and wind pressure relays, etc. It is generally used for large motors and control switch equipment. Positive-pressure type explosion-proof types can be divided into three levels according to the level of protection: px reduces the hazardous area within the enclosure of positive-pressure electrical equipment from zone 1 to a non-hazardous zone.
Py reduces the danger zone in the enclosure of positive pressure electrical equipment from zone 1 to zone 2. Pz reduces the hazardous area within the enclosure of positive pressure electrical equipment from zone 2 to a non-hazardous zone.
4. Non-spark type "n": an explosion-proof form of electrical equipment. This type of electrical equipment cannot ignite the surrounding environment during normal operation and under certain conditions specified in this standard (light source failure conditions of the luminaire only). Explosive environment. The non-spark type originally refers only to electrical equipment that does not generate sparks or arcs during normal operation, such as AC asynchronous motors, and takes some safety measures based on it, such as the use of non-sparking material for fan blades, enclosure protection class IP44 or IP54, and electrical clearances. Increased creepage distance and so on. Compared with the aforementioned explosion-proof type, the non-sparking type equipment has a somewhat lower level of safety, and its range of use is subject to certain restrictions. The following explosion-proof forms are mainly suitable for instruments and are not suitable for motor machines. Details are omitted here:
5. Oil-filled enclosure “oâ€---- Immerse the device in whole or in part in the oil in the enclosure so that the device cannot ignite explosive gases above or outside the enclosure.
6. Sand-filled shell “qâ€----The shell is filled with grit material so that under the specified conditions of use, arcs, flames, and the surfaces of the shell and grit produced in the shell cannot ignite the surrounding explosive mixture.
7. Encapsulant type "m" - The part where the device may generate sparks or high temperatures is encapsulated in a potting agent (resin) so that they cannot ignite the surrounding explosive mixture.
8. Intrinsic safety "i" - Under the specified experimental conditions, the EDM or thermal effect produced under the normal operation of the (equipment circuit) or the specified fault condition will not ignite the specified explosive mixture. Intrinsically safe, the main explosion protection measure is to limit the energy in the circuit so that the energy of the generated spark is less than the corresponding minimum ignition energy.
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