At 1:07 am on November 14th, an explosion occurred at the south gate of the second phase of the Forest Zoo in Xigang District, Dalian City. Two LNG tankers (with a total load of 10 tons) leaked when the taxi was inflated. At present, the fire has been controlled and there were no casualties at the scene.
The explosion caused the fire of two scrap purchase vehicles in the outer wall of an abandoned factory building and a scrap purchase station. The cause of the fire was under further investigation.
Residents living nearby said that before the explosion, the fire burned for about half an hour. She was awakened by the shouts of her neighbors and woke up to see that the window was red and the room was illuminated. When I ran out to the door of the house, the first explosion sounded, and my home glass was shattered by 5 pieces.
prompt
From car refueling to industrial gas
LNG is gaining popularity
Clean energy that looks like water
But has the characteristics of flammable and explosive
What are the hazards of LNG and how to protect it?
LNG hazards and protection
What is LNG
LNG is the abbreviation of Liquefied Natural Gas. After natural gas is collected and purified (removing impurities such as CO2, sulfides, heavy hydrocarbons, and water), it is cryogenically cooled to -162 ° C at atmospheric pressure and changes from gaseous state to liquid state. It is 1/620 of the volume of natural gas of the same quality, and the weight is about 45% of the same volume of water. It is a clean, high-quality fuel. The liquefied natural gas is more conducive to long-distance transportation and storage, making the application of natural gas more flexible and wider in scope.
Basic characteristics of LNG
(1) Composition
The main component of LNG is methane, and it also contains a small amount of inert components such as ethane, propane, N2 and other substances normally contained in natural gas. According to the European standard EN1160, the methane content of LNG should be higher than 75%, and the content of nitrogen should be lower than 5%. The methane content of domestic LNG is above 87%. PS: Chromatographic detection was used during unloading, and LNG with a nitrogen content higher than 5% was suspected of adulteration.
(2) Other characteristics of LNG
Density: The density of LNG depends on its components, usually 430-470kg / m3, the higher the methane content, the lower the density.
Temperature: The temperature of LNG also depends on its composition and process. In China, the temperature of LNG is between 140 ° C and 162 ° C. Generally speaking, the high temperature liquid is 140 ° C and the low temperature cryogenic temperature is 162 ° C.
PS: The gas with a temperature lower than 162 is also mixed with liquid nitrogen. Generally, the temperature of our discharge liquid is normal between 145-155.
Evaporation of LNG: LNG is stored in an insulated storage tank. Due to the long storage time or any heat leakage into the tank, it will cause a certain amount of liquid to vaporize into a gas. This gas is called evaporated gas. Since the specific gravity of boil-off gas is lighter than air, it will normally drift with the wind.
PS: Under normal circumstances, this problem does not occur between 15 days. Except for the reason that the storage tank is not well-cooled, it will not be released under normal use.
Overflow and diffusion of LNG: When LNG is poured onto the ground, it will initially boil and evaporate violently, and then the evaporation rate will quickly decay to a fixed value. The evaporated gas forms a laminar flow along the ground, absorbing heat from the environment and gradually rising and spreading Cool the surrounding ambient air below the dew point to form a visible cloud.
Combustion and explosion of LNG: LNG has the flammable and explosive characteristics of natural gas. At a low temperature of -162 ℃, its combustion range is 5% to 15% (volume percentage); LNG ignition temperature changes with the change of composition The increase of heavy hydrocarbon content lowers the ignition temperature, and the ignition temperature of pure methane is 650 ℃.
Dangerous factors and protective measures of LNG
(1) Storage of LNG
The most common phenomenon in the LNG storage process is the tumbling phenomenon, which is mainly caused by the different components and temperature of the incoming liquid. Due to the different composition and density of LNG in the storage tank, stratification occurs, mass and heat transfer between the two layers is completed, and finally the mixing is completed, and the evaporation is performed on the surface of the liquid layer. This evaporation process absorbs the heat of the upper liquid and makes the lower liquid overheated. When the densities of the two liquids are close to equal, they will suddenly mix quickly and produce a large amount of gas in a short time, causing the pressure in the storage tank to rise sharply, and even open the safety valve.
PS: In order to avoid this danger, the method of treatment should be taken: high temperature LNG enters the liquid from the bottom of the tank, low temperature LNG enters the liquid at the top of the tank, performs secondary liquefaction, and enters the storage tank after mixing in the pipeline conditionally;
(2) Low temperature frostbite
Since LNG is a cryogenic liquid at -162 ℃, direct skin contact with the surface of low-temperature objects can cause serious injuries (cold burns). When in direct contact, the moisture on the skin surface will condense and stick to the surface of low temperature objects. The skin and tissues below the skin freeze and can easily tear and leave wounds. After bonding, the meat can be thawed by heating and then uncovered. At this time, if the skin is torn from the low temperature surface, it will tear this part of the skin, so you don't need to wear warm gloves to work.
Low temperature liquids have lower viscosity, they will penetrate into textiles or other porous clothing faster than other liquids (such as water). Wear non-absorbent gloves (made of PVC or leather) when handling anything in contact with or contact with cryogenic liquids or vapors. The gloves should be loose so that if liquid splashes on the gloves or penetrates into the gloves At that time, you can easily take off your gloves. If intense spraying or splashing may occur, use a face shield or goggles to protect your eyes.
(3) Leakage of LNG
Due to the low temperature operation, the metal parts will shrink obviously. Any part of the piping system, especially the welds, valves, flanges, fittings, seals and cracks may leak and evaporate. It is recommended to check with soapy water regularly.
PS: Welding seams, valves, flanges, fittings, seals and cracks where frost is fastest should be inspected.
(4) Low temperature anesthesia
Without adequate protection measures, there will be a danger of hypothermic anesthesia after a long time at below 10 ℃. With the decrease of body temperature, physiological function and intellectual activity decline, heart failure, and further decline will cause death. For those who are obviously affected by hypothermia, they should be quickly transferred from cold areas and bathed in hot water to restore body temperature.
PS: Dry heat should not be used to raise body temperature.
(5) Suffocation
Breathing LNG low-temperature vapor is detrimental to health. In a short time, it causes difficulty in breathing. For a long time, it will have serious consequences. Although LNG vapor is not toxic, its low oxygen content makes it suffocating.
PS: If you inhale pure LNG vapor without quickly leaving, you will soon lose consciousness and die in a few minutes. When the oxygen content in the air gradually decreases, the operator has no feeling and no warning.
(6) Cold explosion
In the case of LNG leakage encountering water, there is a very high heat transfer rate between water and LNG. LNG will boil violently, accompanied by a loud noise, spraying water mist, causing LNG vapor to explode. This phenomenon is similar to what happens when water falls on a red-hot steel plate, which can cause the water to evaporate immediately. To avoid this danger, the rainwater in the sump should be regularly discharged.
(7) Fire
When LNG gasification encounters a fire source, the flame will spread to the place where oxygen reaches. The explosion limit of natural gas is 5% -15% (volume fraction), the minimum ignition energy is only 0.28mJ, the combustion speed is fast, the combustion calorific value is high (average calorific value is 33440kJ / m3), the relative density to air is 0.55, diffusion The coefficient is 0.196, which is extremely easy to burn and explode, and has strong diffusivity and rapid fire spread.
PS: In case of fire, some equipment needs a lot of water for protection in order to avoid heat radiation. When dealing with LNG fires, it is recommended to use dry powder (preferably potassium carbonate) fire extinguishers. Note that in any case, do not use water in the fire of the LNG storage tank. Water will increase the gasification rate and increase the flame height by 6 times. Radiation The heat increases 3 times.
(8) Personal safety protection
If you want to contact low-temperature gas and liquid, you must wear a protective mask, leather gloves, bagless trousers and high boots (putting the trousers on the outside of the boot), and long-sleeved clothes. Under hypoxic conditions, respiratory equipment is required. The face mask is required not to be broken at low temperature, and the clothes are made of special synthetic fibers or fiber cotton, and the size is large to prevent the low temperature liquid from splashing on the clothes and freezing the skin.
PS: Personnel are never allowed to enter the LNG tank or LNG injection, because these protective equipment cannot ensure safety. Only enter the LNG vapor when there is no ignition source and emergency operation is required.
(9) First aid for low temperature frostbite
When frostbite occurs, the skin frostbite should be washed with a large amount of warm water (41 ~ 46 ℃). Dry heating method should not be used. The wounded should be moved to a warm place (about 22 ℃). If immediate medical treatment is not available, the injured person should be taken to the hospital without delay.
