Mixed dielectric PCB board prevents the formation of explosive media
In the application of mixed-media PCB boards, substances are the basis of combustion. Trying to eliminate or replace combustibles, limit the concentration of flammable gases, vapors or dust in the air, and separate substances that interfere with each other can prevent or reduce the occurrence of fire. .
1: Use a non-combustible or non-combustible material to make a dielectric PCB board, instead of combustible or flammable materials, to improve the fire endurance.
(1) Replace the wood column of the same cross-section with a reinforced concrete column with a section of 20 cm × 20 cm, and the fire resistance limit can be increased from 1 h to 2 h.
(2) Inorganic fireproof coatings coated with water glass on wooden boards and flammable packaging, the flame resistance temperature can reach 1200 °C.
(3) Using a cellulose acetate instead of a nitrocellulose to make a film, the ignition point can be increased from 180 ° C to 320 ° C.
2: Strengthen ventilation so that flammable gases, vapors or dusts do not reach the explosion limit.
The mixed medium PCB board ventilation can be divided into natural ventilation and mechanical ventilation, and is divided into exhaust air and air supply according to the function of replacing air.
(1) The number of ventilation and ventilation should be guaranteed, and natural ventilation is insufficient. Mechanical ventilation should be added. For example, when the acid battery is charged, hydrogen can be released. When using an open battery, the number of ventilation and ventilation should be no less than 15 times per hour. When using an acid-proof explosion-proof battery, the ventilation can be based on the maximum hydrogen in the air. The amount (by volume) is not more than 0.7%. For example, the painting workshop in the woodworking workshop and the gasoline washing workshop in the machine shop should have strong local exhaust equipment.
(2) The ventilation exhaust port should be set properly. If there is a combustible gas lighter than air in the room, the exhaust vent should be located at the upper part; otherwise, the exhaust vent should be placed at the lower part.
(3) The air in the production workshops of Class A and Class B shall not be recycled due to the inclusion of flammable and explosive gases. The air exhausting equipment and air supply equipment shall be located in separate ventilator rooms. The air in the Class C production plant, if it contains combustible dust and fibers, can be recycled after being purified.
3: Closed equipment, no leakage of combustible materials and air infiltration.
Many combustible materials have fluidity and diffusibility. If the equipment and piping that contain them are not well sealed, they will escape to the outside, causing "run, run, drip, and leak", resulting in burning and explosion accidents in space. . Especially in the case of negative pressure operation, if the seal is not good, the air will enter the equipment and form an explosive mixture with the combustible materials in the equipment, which may cause a serious explosion of the equipment.
Typically, leakage occurs at various joints of the equipment, piping, and fittings, or at the joint of the equipment, the joint between the manhole cover and the body, and at the seal of the shaft and housing of the equipment. In order to ensure the tightness of the equipment system, the following methods are usually used.
(1) Use welded joints as much as possible to reduce flange connections. If flange connection is used, depending on the operating pressure, flanges of different shapes such as plane, quasi-groove surface and concave-convex surface can be used separately, and the gasket should be tight and the screws should be tightened.
(2) According to the process temperature, pressure and medium requirements, choose different sealing gaskets. Asbestos rubber sheets (also known as asbestos rubber sheets which are resistant to oil and oil) are commonly used in general processes. Plastic plates or metal washers such as Teflon are used in high temperature, high pressure or highly corrosive media. Recently, many machine pumps have been changed into end-face mechanical seals, and the leakage prevention effect is better. If the packing seal is still not up to standard, some may be sealed with water or oil.
(3) Mixed-media PCB board Note x-Ray test leak test. Before the equipment system is put into production and before the overhaul, the water pressure test should be combined with the compressed air or compressed air for air tightness test. Even if the pressure inside the equipment rises to a certain value, it is considered to be acceptable if the pressure is not lowered or the pressure is not exceeded. Or fill the equipment with inert gas. After being pressed, spray it on the joints such as welds and flanges with soapy water. If there is leakage, foam will be generated. It is also possible to adopt corresponding leak detection measures for the characteristics of the substances stored in the equipment. For example, there are chlorine gas and hydrogen chloride gas in the equipment. It can be smoked in various parts of the equipment with ammonia gas to produce white smoke, which is the leakage point. If the equipment is acidic or alkaline gas, it can be tested by pH test paper. Make the test paper discolored.
(4) Pay attention to maintenance and repair, find that the fittings and packings are damaged, and repair or replace them in time; find that the flange screws are loose and try to tighten.
4: Clean the replacement equipment system to prevent explosive mixtures from combustibles with air.
Before the start of the mixed-dielectric PCB board production or before the maintenance, the air in the pump equipment system must be replaced with inert gas. When the oxygen content is below 0.5%, the combustible material can be driven by the vehicle. Otherwise, the combustible material enters the equipment system and merges with the air to form an explosive mixture.
The inert gas must be used to replace the combustible materials in the pump equipment before the vehicle is shut down or before the maintenance. Otherwise, the combustible material leaks out (when there is pressure) or the air infiltrates (at negative pressure) to form an explosive mixture. Especially in the case of hot fire overhaul, there are combustible materials in the equipment system, and there will be explosion and injury accidents.
For barrels and tanks filled with oil and gas, when it is necessary to repair the fire, the residual liquid and sediment must be thoroughly cleaned with water or steam, otherwise it will explode.
5: For substances that are prone to spontaneous combustion in cold air, water or heat, use air storage.
For example, sodium metal is stored in kerosene, yellow phosphorus is stored in water, active nickel is stored in alcohol, aluminum alkyl is stored in nitrogen, and carbon disulfide is stored in water.
6: Filling with inert gas to protect the production process with flammable and explosive danger.
For example, nitrogenation protects the occurrence of acetylene, oxidation of methanol, ball milling of TNT, etc., and inert gases such as nitrogen should be subjected to gas analysis when used, and the oxygen content must not exceed 2%.
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