Analysis of fire and explosion accidents (static electricity in oil depots)

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This post was last edited by shiwendong on 2017-10-28 at 11:21. Analysis of fire and explosion accidents (static electricity in oil depots): Static electricity is generated in a medium when there are relative movements of contact and separation between a liquid phase and a solid phase, between a liquid phase and a gas phase, between a liquid phase and another incompatible liquid phase, as well as between a solid phase and a gas phase – movements caused by flow, stirring, sedimentation, filtration, scouring, spraying, pouring, splashing, violent shaking, and foaming. Many petrochemical products are high-insulation materials. These non-conductive liquids generate and accumulate large amounts of static electricity during production, storage, and transportation; when this static charge builds up to a certain level, spark discharge can occur. If explosive gases are also present in the discharge space, it may lead to ignition and explosion. Fire and explosion caused by static electricity in oil depots are serious accidents; therefore, preventing the hazards associated with static electricity in such depots is of great importance. During the storage and transportation of oil products, the main safety measures to prevent static electricity-related accidents include the following: 1. Preventing the formation of explosive gases. In areas at risk of explosions and fires, ventilation systems are used to enhance air circulation and remove explosive gases promptly, keeping their concentration below the explosive range, thereby preventing explosions caused by static electricity sparks. At the same time, the explosive concentration range is also closely related to temperature; keeping the temperature outside the explosive temperature range is another way to prevent explosions caused by static electricity. For oil storage areas, positive pressure ventilation cannot be used to prevent the formation of explosive gas mixtures; instead, inert gas blanketing (such as nitrogen blanketing) can be employed, or floating roof tanks and internal floating roof tanks can be used. Although floating roof tanks or internal floating roof tanks can eliminate the oil and gas space below the floating disk – especially since the floating roof of internal floating roof tanks contains a large amount of combustible gases – spark discharge of combustible gases above the floating disk also deserves attention. 2 Accelerate static electricity leakage to prevent or reduce static accumulation. The generation of static electricity is not dangerous in itself. The actual danger lies in the accumulation of charge, as this can store enough energy to generate a spark that ignites the flammable gases. To accelerate the leakage of charge from the oil, it can be grounded, bridged, or its electrical conductivity can be increased. 2.1 Grounding and Bonding Static grounding and bonding are used to conduct away or eliminate static electricity on conductors, and they represent one of the most effective measures for eliminating the hazards caused by static electricity. The specific method for static grounding is to connect the equipment containers and pipelines to the ground through metal wires and grounding electrodes, thereby achieving an equal potential with a minimum resistance value. Bridging refers to connecting metal devices and various pipelines with metal wires to achieve equipotentiality. Obviously, the purpose of grounding and bonding is to artificially create an equipotential surface in relation to the ground, so as to prevent harm caused by electrostatic potential differences. Another purpose of pipeline bonding is to provide a proper path for stray currents, thereby preventing sparks from occurring at the disconnections and causing accidents. The pipes in the oil tank sampling and oil product handling areas, the pipes and tanks themselves, the components on the tanks, as well as any metal objects in the vicinity that may become electrically charged, should all be grounded. According to the provisions of the \"Code for Design of Oil Depots\" (GBJ74—84) and the \"Code for Fire Protection Design of Petrochemical Enterprises\" (GB50160—92), the grounding resistance of anti-static grounding systems should not exceed 100Ω. 2.2 Adding antistatic agents: Grounding of oil containers can only eliminate the charge on the outer surface of the container. Due to the low conductivity of oil, it is difficult for the charge on the surface and within the oil to be dissipated through grounding. Adding antistatic agents can increase the conductivity of the oil, accelerating the leakage and discharge of static electricity, while also reducing the charge accumulated in the oil and lowering its electrical potential. 2.3 Installing static charge neutralizers Static charge neutralizers, also known as electrostatic neutralizers, are devices used to eliminate or reduce the charge on charged objects. Its working principle is that the electrons and ions it generates neutralize the charges of opposite sign on the charged object, thereby eliminating the risk of static electricity. 3 Preventing operators from becoming electrified: The human epidermis has a certain level of resistance, and if high-resistance shoes are worn, factors such as friction between the body and clothing can cause the body to become electrified. Therefore, those who work frequently in oil pump rooms, oil filling rooms, and are involved in loading and unloading operations should avoid wearing synthetic fiber clothing; it is best to wear cotton-based inner and outer garments as well as anti-static shoes. 4 Reducing the generation of static electricity: Given the current state of technology, it is not yet possible to completely eliminate the formation of static electricity. To prevent the hazards caused by oil static electricity, it is not possible to completely eliminate the generation of static charges; instead, technical measures must be taken to reduce such generation. 4.1 Controlling the flow rate of oil The values of the flow charge and charge density generated as oil flows through pipes are proportional to the square of the oil flow rate; therefore, controlling the flow rate (especially when oil is entering tanks, being filled, or pumped into vehicles) is an effective way to reduce static electricity generated by oil. According to the \"Code for Design of Oil Depots\" (GBJ74—84), the outlet of the loading arm can have its filling speed increased only after it is submerged by the oil; moreover, the filling speed for oils such as gasoline, kerosene, and light diesel should not exceed 4.5 m/s, while the initial filling speed should be below 1 m/s. 4.2 Controlling the oil filling method: When oil is poured into the tank from the top, it inevitably hits the tank walls, stirring the oil inside and causing a sharp increase in its static electricity level. Experiments show that the ratio of static electricity generated by oil injection from the top to that generated by oil injection from the bottom is 2:1. Furthermore, filling oil from the top also causes local charge concentration on the oil surface, making discharge more likely to occur. It can be seen that filling the tank from the bottom (or from the top along the tank wall down to the bottom) is much safer than filling it from the top. 4.3 Preventing mixing of oils with different flash points and controlling cleaning agents: Friction between different oils or between water and air contained in oils can generate static electricity. At the same time, when heavy oil is mixed into light oil, the heavy oil absorbs the vapor of the light oil, thereby reducing the concentration of oil vapor in the gas mixture present in the container’s gas space. As a result, the space that was previously filled with light oil vapor – that is, at concentrations above the explosive limit – becomes a mixture of oil vapor and air at an explosion-safe concentration. Therefore, prevent the mixing of oils with different flash points or reduce the gas and water content in the oils. The use of compressed air for the blending and cleaning of Class A and B oils is strictly prohibited. 4.4 The oil flowing through the filter must have sufficient time for leakage to occur; intense friction as the oil passes through the filter causes its charge level to increase by 10 to 100 times. To prevent a large amount of charged oil from entering the tank or tank truck, the leakage time of the oil after passing through the filter must be over 30 seconds.