Crude oil spills and splashes are two demons in a crude oil fire. In order to avoid casualties in crude oil fires, it is necessary to understand the knowledge of boiling and splashing in fires.
The oil mined underground is called crude oil before it is processed. Crude oil is a mixture of hydrocarbons and the main component is hydrocarbons. Crude oil has a high viscosity and contains a certain amount of water to varying degrees. In a crude oil fire, the water in the crude oil is vaporized by heating to become water vapor, and bubbles are formed. The steam bubbles are surrounded by the oil film to form a large amount of oil foam, which causes the volume to expand drastically. When the tank can be accommodated, it will be like boiling. Like the porridge overflowing pot, it overflows the tank and causes boiling. When a large amount of water is rapidly vaporized into water vapor, the volume expands by 1,700 times, and the vapor pressure thereof also rapidly increases. When the water vapor is suddenly flushed out of the liquid surface with a large pressure, the fired oil is brought to the upper space to form a huge fire column. This phenomenon is called splashing. Boiling and splashing are extremely harmful in crude oil fires. Boiling can cause the spilled distance of crude oil to reach several tens of meters. When large oil tanks store oil for a long time, the overflow area can reach several thousand square meters, thus spreading the fire extensively. . When splashing, the flame of crude oil suddenly vacates, the fire column can be as high as 70-80m, and the fire column can be sprayed at a distance of up to 120m. When the flame is rolled down, it spreads to the surrounding area, easily spreads to the adjacent oil tanks, expands the disaster, and may cause the firefighting personnel to suddenly be surrounded by the flame, causing casualties.
Causes of boiling over and splashing
The role of radiant heat
When a crude oil tank fires, the radiant heat spreads around and heats the surface of the oil. As the heating time is extended, the heated liquid layer is also thicker and thicker. When the temperature is raised and the crude oil is heated to the boiling point, the burning crude oil will boil and overflow outside the tank.
2. The role of heat waves
When the crude oil is burned, the light fraction with a low boiling point becomes a vapor, and the surface of the crude oil is burned off, while the heavy fraction with a high boiling point gradually sinks and brings the heat to the underside, forming a hot front under the liquid surface. As the burning continues, the hot front gradually sinks into the lower "cold face", a phenomenon known as heat wave. Radiant heat and heat waves often work together to cause the crude oil to quickly reach its boiling point and boil.
3. The role of water vapor
The crude oil contains free water and emulsified water, and the heat wave causes the water in the crude oil to be heated and vaporized to become water vapor. Once the water becomes water vapor, its volume expands and the vapor pressure increases accordingly. When the hydraulic pressure of the crude oil is exceeded, the water vapor will escape upwards and form a large number of bubbles, which are surrounded by the oil film to form oil foam. The volume of crude oil expands violently, and when it exceeds the capacity of the tank, it overflows outwards, forming a boiling overflow. As the combustion continues, the temperature of the heat wave gradually rises and moves downwards. When the heat front encounters the water cushion (or a large amount of water), a large amount of water becomes water vapor, and the vapor pressure rapidly increases. The crude oil in the upper part of the water cushion is thrown up to form a splash.
Conditions for boiling and splashing and their differences
1. Conditions for boiling over and splashing
Boiling and splashing can occur only with oils that have the following conditions.
(1) The oil has a condition for forming a heat wave. That is, each component of the oil has a wide boiling point range. The boiling point range of each component of the crude oil is wide, and boiling and splashing can occur; and the boiling range of the gasoline component is narrow, and can only be 6 to 9 cm from the liquid surface. There is a fixed heat front, that is, the speed of the hot front is equal to the linear speed of the combustion, so there is no boiling and splashing.
(2) The oil contains a certain amount of water. Water is an important cause of boiling and splashing. Crude oil contains certain emulsified water or suspended water, and generally has a water cushion under its oil layer.
(3) The viscosity of the oil is large. The oil only has sufficient viscosity, and the water vapor does not easily escape from the bottom to the top, so that the water vapor foam is surrounded by the oil film to form an oil foam. The viscosity of crude oil is generally larger.
2. The main difference between boiling and splashing
(1) The time of occurrence is different, generally it is first splashed and then splashed.
(2) The source of water is different. The boiling overflow is the emulsified water and free water in the crude oil, and the splashing is mostly the water of the water cushion.
(2) The hazard is different, the hazard of boiling over is large, and the splash is fierce and the damage is even greater.
Boiling and splash prevention and control
Boiling and splashing in crude oil fires can be prevented and controlled.
First of all, it is necessary to grasp the type of stored crude oil, the water content and the height of the oil level in time, and then estimate the time from the fire to the occurrence of boiling and splashing at the heat wave propagation speed and the linear velocity of the combustion, so that there is a lot of heart and no heat. The wave propagation speed is related to the properties of crude oil (composition, water content), storage tank and tank diameter, oil level and other factors. Generally, there are several following rules: (1) The more low-fractions in crude oil, the more water content, the heat wave The faster the propagation speed is; (2) the heat wave propagation velocity increases with the increase of the can diameter, but when the can diameter is greater than 25 m, the heat wave propagation velocity tends to be stable; (3) the lower the liquid surface, that is, the liquid surface is away from the can The greater the distance, the smaller the heat wave propagation speed.
Secondly, in a crude oil fire, it is necessary to concentrate the force and rush to extinguish the fire before boiling and splashing. It is best to use the protein foam or fluoroprotein foam to extinguish it quickly within 0.5~1h of the fire. Otherwise, the burning time is too long. When the oil temperature in the tank exceeds 100 °C, not only boiling will occur, but also if water is used to extinguish the fire with water-containing foam, splashing may occur. At the same time, the fire should be extinguished while releasing water and oil. This is an effective measure to prevent boiling, splashing and rapid extinguishing of crude oil fires. When the fire is on, open the drain valve at the bottom of the tank and let the water in the bottom layer of the tank be drained. Generally, it can avoid splashing. Put the crude oil from the cupping tank into the storage tank far away from the fire. This is not only the bottom of the tank, but also reduces the burning liquid level and reduces the possibility of boiling.
Finally, in the fight against crude oil fires, pay attention to observe whether there is a precursor to boiling and splashing. There are generally the following signs before boiling and splashing occur; (1) The surface of the oil is generated by a large amount of oil foam, which is a phenomenon of tumbling and creeping, which occurs 2 to 4 times; (2) the flame height increases, and the color changes from deep to dark. Whitening; (3) the tank wall is violently trembled, and some slightly swell; (4) the sound of the sound is mutated, giving a strong squeak or whistling sound. If the above symptoms appear, the fire commander should immediately release Retreat orders, after boiling or splashing occurs, seize the opportunity to fire into the ground to avoid and reduce unnecessary casualties.
(Wang Min)
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