An acetylene generator is a device that uses acetylene to interact with water to produce acetylene.
First, the risk of acetylene combustion explosion
Acetylene is an unsaturated hydrocarbon which is very chemically active and susceptible to various reactions such as addition, polymerization and substitution. It is a high calorific value flammable and explosive gas under normal temperature and pressure. The factors affecting the combustion and explosion of acetylene are:
1. Pressure and temperature. Acetylene has an auto-ignition point of 335 ° C and is easily self-ignited by heat. At 200-300 ° C, the acetylene molecule begins to undergo an exothermic polymerization reaction. When the temperature is higher than 500 ° C, acetylene will decompose explosively. If the decomposition is carried out in a closed container, it will explode due to an increase in temperature and an increase in pressure. The effects of temperature and pressure on the polymerization of pyrethyne and the decomposition of explosives are shown in the figure below.
2. Oxidizer. Acetylene is mixed with air to form an explosive mixture. The explosion limit is 2.2-81%, the self-ignition point is 305 °C; the explosion limit is 2.8-93% with oxygen, and the self-ignition point is 300 °C; mixed with fluorine gas under sunlight or Heating will explode. Acetylene can also be combined with fluorine, bromine, etc., and a combustion explosion occurs.
3. Impurities. Acetylene often contains harmful impurities such as phosphine and thiohydrogen. Phosphine has a low self-ignition point, and spontaneous combustion occurs at 45-60 ° C, which detonates the mixed gas of acetylene and air.
4. Catalyst. Catalysts such as iron oxide, copper oxide and aluminum oxide can adsorb the molecules of acetylene on their porous surface, increase the concentration of acetylene, and promote the aggregation reaction and explosion decomposition of acetylene molecules.
5. Container volume. The smaller the container, the less likely it is to explode; on the contrary, the greater the risk of explosion.
In addition, since the ignition energy of acetylene is small (0.019 mJ), an explosive mixture or the like is formed after the metal contacts.
Second, calcium carbide combustion explosion hazard
Calcium carbide is a common name for calcium carbide. It does not have the explosive nature. The danger of combustion and explosion is mainly manifested in:
1. Burn and explode in contact with water. The contact between calcium carbide and fire immediately decomposes, producing acetylene and releasing a lot of heat.
The amount of heat can cause acetylene to explode.
2. Calcium carbide sparks. The calcium carbide generally contains ferrosilicon impurities, which can generate sparks when colliding or rubbing, and become the detonating source of acetylene; the calcium phosphide impurity contained in the calcium carbide reacts with water to form phosphine gas, which has a low natural point. Causes an explosive mixture to explode in the generator.
3. Calcium carbide particle size. The smaller the particle size of calcium carbide, the faster the decomposition speed with water, and the more heat is instantaneously released, which is likely to cause local overheating and danger. See the table below.
Third, acetylene aS explosion-proof technical measures
1. Fire resistance measures.
Table 1 Relationship between calcium carbide particle size and decomposition time
Calcium carbide particle size (mm) 2—3 8—15 15—50 50—80
Time to complete decomposition (min) 5 . 5 6 . 5 10 13
When the burning speed of the flame is greater than the airflow speed of the mixed acetylene and oxygen, the gas welding (cutting) flame will burn along the welding (cutting) torch to the hose, which is dangerous. For this purpose, a fire-stop device should be installed, usually a tempering preventer to prevent the flame from entering the gas tank and the main tank or preventing the flame from spreading in the pipe. The tempering preventer is divided into low pressure type (<0.07MPA= and medium pressure type (0.07-0.15MPa) according to pressure; according to the structure, it is divided into open type and closed type; according to the fire retardant medium, it is divided into water seal type and Dry type.
2. Pressure relief measures. Pressure relief is when the pressure of the generator rises above a certain limit, or when the pressure is generated by explosion, the pressure can be released in time to prevent the generator from rupturing. Commonly used pressure relief devices include safety valves and venting pieces.
(1) Safety valve. Also known as pressure relief valve, its function is to ensure that the pressure of the acetylene generator exceeds the safety specified pressure (0.215MPa), it can automatically open and vent some gas; when the pressure falls to the safe range, it will automatically close to ensure the generator does not Overpressure damage. In order to ensure the safety and reliability of the safety valve, the exhaust test should be carried out regularly to prevent the exhaust pipe and valve body from being stuck and blocked. In addition, the safety valve should be checked frequently for leaks or continuous exhaust, and should be repaired in time.
(2) Explosive film. Explosive sheet materials for generators include aluminum foil and rubber sheets. In contrast, aluminum foil is ideal. The venting piece should have sufficient strength to withstand the working pressure (generally below 0.15 MPa): good heat resistance and corrosion resistance; at the same time it should be brittle; easy to rupture; thickness should be as thin as possible. For tanks with a volume greater than 300L, the choice of venting area should be determined by blasting test. The materials and specifications specified in the venting sheet application cannot be replaced at will.
3. Monitoring measures.
The role of monitoring is to control the pressure of acetylene, the temperature of water and acetylene, and the amount of water. For fixed acetylene generators, all of the above parameters must be monitored; for small capacity mobile generators, temperature monitoring is not necessary.
(1) Pressure monitoring. The medium pressure acetylene generator must be equipped with a pressure gauge to directly display the acetylene pressure inside the tank. In order to keep the pressure gauge sensitive and accurate, care and maintenance should be taken during use. The pressure gauge should be kept clean. If the dial glass is broken or the scale is blurred, it should be stopped. The connecting pipe of the pressure gauge should be periodically purged to prevent clogging. Always check if the pointer returns to zero normally after the pointer is turned. The pressure gauge must be inspected regularly, and the pressure gauge exceeding the expiration date should be stopped.
(2) Water level control. The water level can be indicated by a water level gauge or a water level faucet. Water should be added to each tank of the generator according to the water level indicator or the water level indicated by the faucet. The indicator scale of the water level gauge should be kept clear and the water level faucet should not be rusted.
(3) Temperature monitoring. An alcohol thermometer is used to measure the temperature of the acetylene temperature and the temperature of the water in the electrolytic decomposition zone of the generator. It is forbidden to use a mercury thermometer. The glass tube of the thermometer should be scrubbed frequently to make the scale of the thermometer clear.
4. The layout of the acetylene generator.
(1) Mobile generators are prohibited from being placed in hot working workshops such as blacksmiths, casters and heat treatments, and in operating boiler rooms.
(2) The fixed generator shall be arranged in a separate room, and when placed outdoors, there shall be a special shed.
(3) The horizontal distance between acetylene generator and open flame, sparking place, high voltage power line and other heat sources should be kept above 10m, and it is not allowed to be placed on the working platform and equipment with severe vibration.
5. Preparation before acetylene is used.
(1) Check if the safety device of the generator is complete and the working performance is normal.
(2) Irrigation according to the specified water loading.
(3) Quantitatively installed calcium carbide according to the requirements of various types of generators shall not be overfilled.
(4) If the winter effect generator is found to be frozen, it can only be thawed with hot water or steam. It is strictly forbidden to bake with open flame or red iron, and it is not allowed to use objects such as irons that are prone to sparks.
6. Use of acetylene generators.
(1) Check the water level of the tempering preventer before starting the generator. Wait until everything is normal before opening the water supply valve to send water to the calcium carbide.
(2) After the water is delivered, check whether the pressure gauge, safety valve and joints are normal.
(3) If the pressure gauge reading rises too fast after starting, or if gas escapes from the safety valve, or the pressure gauge pointer still stops at zero, stop the gas immediately. The fault can only be restarted after the fault has been eliminated.
7. Explosion protection during the work process.
(1) The acetylene and air mixture remaining in the generator should be held before the gas supply.
(2) All parts of the generator should be inspected at any time during the operation. If problems such as air leakage, water level mismatch or malfunction of the safety device are found, measures should be taken in time.
(3) The work of removing carbide slag during operation must be carried out after the safe decomposition of calcium carbide.
(4) When the water temperature in the generator exceeds 80 °C, cold water should be poured or temporarily stopped, and cooling measures should be taken to cool it.
(5) Do not open the generator and drain water to prevent fire and explosion caused by overheating of calcium carbide.
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