Many oils are flammable, explosive, accumulating static electricity and other dangerous characteristics. It is easy to accumulate static electricity during oil flow, loading, unloading and transportation. The electrostatic field accumulates to a certain extent, and electrostatic discharge occurs, which leads to fuel vapor, causing combustion and explosion. Waiting for a security incident. According to statistics, there were 65 electrostatic accidents in China's petrochemical industry from 1990 to 2010, of which 41% were electrostatic accidents during loading and unloading operations. This paper focuses on the loading and unloading operations of automobile tank trucks, and analyzes the factors affecting the static electricity of oil and protective measures.
Factors affecting the static electricity of oil
Impurities contained in the oil. The reason why the liquid is electrified is because there are positive and negative ions that have been dissociated in it. Most of the oils are neutral media with a low dielectric constant and are generally not directly ionizable. The source of ions in the oil is mainly impurities, some of which can be directly dissociated into positive and negative free ions.
The impurities contained in the oil are naturally present and added in the refining. Impurities include various oxides, asphaltenes, naphthenic acids, and metal salts of sulfonic acids. These active compounds can charge the liquid medium at very low concentrations, typically from one part per million to one part per billion. Pure oil is not easy to generate static electricity. When the content of impurities is within a certain range, the amount of static electricity of the oil is large. However, when there are too many impurities, the amount of static electricity of the oil is also reduced. This is not the amount of static electricity generated, but impurities. Too much, the conductivity is too large, and the static charge is more likely to scatter.
Oil conductivity. Since the main component of the oil is a non-polar hydrocarbon, its electrical conductivity is greatly affected by impurities. The higher the conductivity, the more impurity ions, and the less the impurity ions. The effect of the electrical conductivity of the oil on the static electricity of the oil is similar to the effect of the impurity ions on the static electricity of the oil. For a medium with a conductivity less than 10-13 S/m, it is difficult to form an electric layer because it contains few "impurity ions", so it is difficult to charge; and when the conductivity is large, the number of "impurity ions" is larger. It is easy to form an electric layer, the amount of static electricity is increased, and the amount of charge flow is also increased. Experiments show that when the conductivity is between 10-9 and 10-8S/m, the amount of static electricity generated is greater than the amount of dispersion, and the static electricity is easy to accumulate. The medium with conductivity greater than 10-8S/m is almost equivalent to the conductor, although it is easy to form the electrical layer. The amount of static electricity generated is large, but the charge is also scattered quickly and it is difficult to accumulate, so it is difficult to charge a substance with a large conductivity.
Moisture. There are three reasons for the influence of moisture on the oil. First, the water makes the impurities in the oil easier to ionize. This is the main factor that affects the static electricity of the oil. The second is the contact and separation between water droplets and oil. This results in partial static electricity; the third is the increase in moisture, which increases the electrical conductivity of the oil and increases the dissipative capacity, which is usually the most dangerous when 1% to 5% moisture is mixed into the oil.
Loading and unloading pipeline material and pipe wall roughness. The flow currents of different material pipelines are different, and the conductivity of the material is better, and the flow current is smaller. In addition, the roughness of the inner wall of the pipeline also has an effect on the generation of static electricity. The rougher the inner wall of the pipeline, the larger the contact area between the oil and the inner wall of the pipeline, the more opportunities for impact and separation, and the greater the flow current.
Flow rate. An increase in the flow rate will increase the rate at which the substance contacts and separates, thereby increasing the amount of static electricity generated. Therefore, limiting the flow rate is an important method for controlling the static electricity of the oil.
Anti-static measures in loading and unloading operations
Control the flow rate. The higher the flow rate of the oil, the greater the amount of static electricity. The test shows that the oil surface potential is 2300V when the average flow rate is 2.6m/s, and the oil surface potential is 580V when the average flow rate is 1.7m/s. It can be seen that controlling the flow rate is an effective measure to reduce the static electricity generated by the oil.
When the light oil is into the tank, when the inlet immersion depth is less than 200mm, the flow rate should be limited to 1m/s; when the inlet immersion depth is greater than 200mm, the maximum flow rate should be less than 7m/s. If the oil in the pipeline has obvious moisture, air and impurities, the flow rate should be limited to 1m/s. The filling speed of the railway tanker can increase the flow rate after the inlet port is immersed, but the railway tanker should meet V2D≤0.5 (V is the oil flow rate m/s, D is the diameter of the crane tube m), the automobile tanker Should meet V2D ≤ 0.8, the flow rate of the large crane tube should not exceed 5m / s. It is strictly forbidden to transport light oil products in car tankers without internal buffer baffles.
Choose the right way to refuel. When the tanker is sprayed from the top, the oil will inevitably hit the tank wall and agitate the oil in the tank, causing the static electricity to increase sharply and the oil to evaporate. From the bottom or the crane pipe to the bottom of the tanker to load the oil, it can effectively reduce the risk of electrostatic ignition and detonation, because the bottom oil reduces the oil splash, which helps to reduce the static charge generation; Atomization and evaporation, thereby reducing the probability of oil and gas reaching the range of ignition and explosion concentrations; not only can the oil flow through the middle of the tank with the smallest capacitance, but also generate a large oil surface potential; The concentrated drop of the column forms a higher oil surface charge density. When the oil surface potential reaches the maximum value at the late stage of oil loading, the ground metal is provided with a protruding metal on the upper part of the oil to avoid an increase in local electric field and no fire.
A 500m3 tank was tested in a factory: the diesel was sprinkled from the top at a flow rate of 2.6m/s. After 5min, the oil level in the tank increased from 190V to 7000V. If oil is loaded from the bottom of the tank (the flow rate is the same), the oil level potential drops from 6000V to 3300V. Tests have shown that the ratio of the amount of static electricity generated by splashing oil from the top to the amount of static electricity generated by the bottom oil is 2:1. It can be seen that loading oil from the bottom of the tank (or from the top to the bottom of the tank) is much safer than loading the top.
During the filling process, the scattering of oil should be prevented. The end of the oil filling pipe from the bottom or the upper part of the tank should be designed to be in the shape of an inverted T shape that is not easy to cause the liquid to scatter, or an additional baffle; or When installed, the liquid is slowly flowed down the side wall.
China's relevant regulations stipulate: When loading oil for railway tank trucks and automobile tank trucks, the inlet of the crane pipe should be no more than 200mm from the bottom.
Do the grounding and bridging of the tanker. Electrostatic grounding refers to connecting the facility equipment to the ground through a wire grounding device or the like to form a path with a small resistance value, so that the electric charge is introduced into the earth. The jumper refers to an equipotential bonding between adjacent conductors, which can effectively prevent discharge between the conductors.
The ground end plate shall be provided on the lower side of the body of the automobile tank truck. The flexible grounding wire shall not be metal chain, and the conductive rubber tow zone shall be used. In the case of electrostatic hazards, the grounding wire should be connected before operation. The grounding line must be removed after the specified rest time has elapsed after the operation. The connection point of the grounding wire should be avoided in the dangerous area, and should not be in the downwind direction of the wind direction in the loading and unloading work place. Automobile tank trucks shall be connected to the grounding branch and trunk by means of a standard specification connector, and shall not be connected by unreliable methods such as winding. For mobile equipment, a copper core stranded cable with a cross-sectional area of ​​not less than 10 mm2 should be used as the grounding connection line. In an explosion-hazardous location, the grounding device should be connected to the protected object through an explosion-proof switch. When using a hose to transport light oil, use an electrostatically conductive hose. When using a plastic (plastic) tube with a wire (net) attached, the wire must be placed at both ends of the plastic (plastic) tube. Reliably connect to the device and ground.
Add static elimination equipment. The static elimination device commonly used in automobile tank trucks is mainly a static eliminator, also known as an electrostatic neutralizer, which is a device for eliminating or reducing the charge of a charged body. The electrons and ions generated by the static eliminator neutralize the charge of the opposite sign on the charged body, thereby eliminating the danger of static electricity. According to the working principle and structure, it can be divided into four types: inductive, applied voltage, radiation and ion current. At present, inductive static eliminators are mainly used in automobile tank cars.
The results of the domestic fuel tank vehicle fueling test show that the inductive static eliminator can effectively reduce the oil surface potential, and generally reduces the oil level potential of the tanker vehicle from 20000V to 300-400V. From the test results, the performance of the thick-walled static eliminator is similar to that of the thin-walled hollow static eliminator.
In addition to the static electricity caused by the oil itself, the static electricity caused by the human body itself, as well as the oil tank gauge and temperature measurement can not be ignored. Therefore, in the process of loading and unloading the oil, the oil depot staff must strictly follow the implementation requirements of the explosion hazardous area. Always alert to static safety hazards, improve safety awareness, and minimize the occurrence of electrostatic ignition accidents in tank trucks.
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