How to prevent and eliminate static electricity in

How to prevent and eliminate static electricity in the storage tank area of hazardous chemicals

static electricity can be said to be everywhere in our daily life. The static electricity generated by walking through the chemical fiber carpet is about 35000 volts, the static electricity generated by reading the plastic instructions is about 7000 Volts, and even tens of thousands of volts at the highest. In life, static electricity can harm human body; In the chemical industry, static electricity can cause fire and explosion accidents and cause losses. Storage tank fires caused by static electricity account for more than 10% of all fires and explosions. In the daily safety inspection, we found that many enterprises do not pay attention to the prevention and control of static electricity in the storage tank area of dangerous chemicals. Once a fire and explosion accident occurs, the consequences will be disastrous. This paper analyzes the causes of static electricity and puts forward preventive measures to guide enterprises to strengthen the safety management of storage tank area and prevent fire and explosion accidents

I. causes of static electricity in the storage tank area

the process of production and operation in the storage tank area of hazardous chemicals usually includes the loading and unloading, transportation, blending, sampling, ruler measurement, temperature measurement, equipment cleaning and other links of flammable and combustible liquids. Flammable and combustible liquid storage tank (tank) cars, automobile tank (tank) cars, crane pipes, equipment, pipelines and other facilities need to focus on strengthening electrostatic protection

1. The resistivity of hazardous chemicals affects the generation of static electricity

according to relevant data, the resistivity of liquid is 1010 ~ 1015· M, dangerous static electricity can be generated, while at 1013· M is the maximum electrostatic generated. Higher than 1015· M and below 1010· M, the generation and accumulation of static electricity are negligible. Especially when the resistivity is 106· Below m, it is equivalent to the role of conductor for static electricity. At this time, the problem of static electricity can be ignored

the resistivity of crude oil and heavy oil is 109 ~ 1010· m. It is less dangerous to place the puncture device on the material testing machine and fix it when static electricity is generated; The resistivity of gasoline, kerosene, clear oil, jet fuel, etc. is 1012 ~ 1013· m. It is the substance that is most prone to generate static electricity and requires special attention. Compare the tendency to generate static electricity, kerosene jet fuel gasoline

2. Static electricity caused by the way of loading and unloading hazardous chemicals

there are two ways of loading and unloading hazardous chemicals: one is the bottom loading and unloading method, and the other is the upper loading and unloading method. Compared with the two methods, the latter generates more static electricity. Not only new charges are generated due to liquid separation, but also the charge does not have sufficient time to relax, so the surface charge density is high. At the same time, static electricity is generated due to the splashing droplets caused by the impact of hazardous chemicals on the tank wall. The amount of static electricity generated by the upper filling is far greater than that of the lower filling. Therefore, the use of lower filling can effectively reduce the generation of static electricity

3. Mixing different hazardous chemicals will increase the amount of static electricity

mixing between different hazardous chemicals will not only cause a series of chemical reactions between chemicals, but also lead to friction between chemical particles during transportation or mixing due to the different density and particle properties of chemicals. For example, a factory pipeline is used to transport aviation kerosene to the tank, while another pipeline is used to transport dangerous chemicals to the same tank, and the residual residues in the latter pipeline are also sent to the tank. At that time, although the flow rate was only 2m/s, it caused a major explosion accident, with a loss of more than 500000 yuan

4. The deposited water at the bottom of the tank will increase the electrostatic charge

if there is accumulated water at the bottom of the tank, the method of loading dangerous chemicals at the bottom will stir up the accumulated water, resulting in a high electrostatic potential. Water is a good conductor of static electricity, but when a small amount of water is mixed in hazardous chemicals, static electricity will be generated when water droplets flow relative to hazardous chemicals, which will increase the static electricity of hazardous chemicals

5. Electrostatic emission time factor

in the process of injecting hazardous chemicals into the storage tank of hazardous chemicals, it often takes a period of time for the maximum electrostatic potential to be generated from the stop of injection to the large liquid level. The existence of these problems at this time makes China's plastic machinery industry unable to meet the level of foreign plastic machinery as soon as possible, which is usually called delay time. For safety, when it is necessary to directly measure the liquid level or the liquid temperature of dangerous chemicals, it should avoid the escape time of electrostatic charge in the tank. The larger the capacity of the tank, the longer the electrostatic discharge time. Generally, it is safe to measure after 3 ~ 5min. 5. Eliminate the additional source of static electricity

when dangerous chemicals contain water or different dangerous chemicals are mixed and pass through compressed air, the amount of static electricity will increase. 5% water in hazardous chemicals will increase the electrical effect by 10-50 times. Ventilation and reconciliation of hazardous chemicals are also very dangerous. Therefore, the filling and export of hazardous chemicals should avoid the mixing of hazardous chemicals with water, air and different hazardous chemicals

when impurities are mixed in hazardous chemical tanks or pipelines, more static electricity can be generated, so pay attention to remove impurities. For example, before loading dangerous chemicals, the ponding and other sundries at the bottom of the storage tank should be removed. Among electrified objects, ungrounded metal objects shall be strictly prevented

II. Hazards of static electricity

although there is static electricity in the pipeline transportation of dangerous chemicals, there is not enough air because the pipeline is full of dangerous chemicals, so it does not have the conditions for explosion and ignition. When the charged hazardous chemicals are loaded into the storage tank, they accumulate because the charge cannot be discharged quickly, so that the liquid level has a high potential. At this time, if an explosive gas mixture is formed in the upper space of the liquid level, fire and explosion may occur. In addition, electrostatic discharge will cause the failure and malfunction of electronic equipment such as computers and automatic control devices. In the tank farm, although electric shock caused by static electricity will not cause death, it often leads to secondary accidents, so it should also be prevented

III. elimination measures of electrostatic hazards

there are two main ways to eliminate static electricity; First, create conditions to accelerate electrostatic leakage or neutralization; The second is to control the process and limit the generation of static electricity. The first approach includes two methods, leakage method and neutralization method. Grounding, humidification, adding antistatic agent, etc. belong to leakage method, and the use of induction electrostatic eliminator, high-voltage electrostatic eliminator, radiation electrostatic eliminator and ion flow electrostatic eliminator, etc. belong to neutralization method. The second way is the process control method, including measures taken in material selection, process design, equipment structure and operation management. According to the characteristics of the tank farm, the prevention of static electricity mainly includes the following aspects

1. Electrostatic grounding: grounding is the simplest and most commonly used method to eliminate electrostatic disasters, which is mainly used to eliminate static electricity on conductors. In order to prevent accidents caused by electrostatic sparks, correct grounding measures should be taken according to national standards and industry specifications

(1) if the metal components in the storage tank, especially the metal floating body, are poorly grounded, it is easy to form isolated conductors. When a hazardous chemical with static charge is injected into the storage tank, it will collect the accumulated charge and form a potential to the ground. Under certain conditions, spark discharge is very easy to occur and cause harm. For example, in the case of high potential, electrostatic discharge can cause sparks with a length of 20 ~ 30cm. 5oov sparks generated by electrostatic discharge can ignite benzene vapor. When using pipeline to transport methanol in a chemical plant, a large amount of static electricity was generated and accumulated on the surface of the liquid due to the absence of metal floating roof tanks, resulting in high-voltage discharge and fire

(2) metal samplers and measuring tools must be reliably grounded to prevent the formation of isolated conductors. The grounding terminal set on the operation platform should avoid the gas outlet. The grounding mode of sampler using conductive rope is shown in Figure 3. At the sampler end, the company's turnover last year was 150million yuan, and welding can be used. The installation of grounding wire is carried out before the start of operation, and can be removed after the end of operation. It is best to use tools made of antistatic materials during operation

(3) vertical tanks larger than 50m3 and with a diameter of more than 2.5m should be grounded at the corresponding two points of the tank body. The distance between the grounding point and the periphery should not be greater than 30m. The grounding point should not be installed near the liquid inlet

(4) in order to prevent electrostatic induction, the floating roof of the floating roof storage tank should be bridged with the tank body (outer wall). Generally, 25mm2 copper core flexible stranded wire is used and laid to the tank wall along the inclined ladder. The nature of the weatherproof sealed storage rock after compression until its strength is completely lost can be shown by the load deformation full process curve shown in Figure 1. The end on one side of the tank wall should be made of conductive rubber material. 10mm2 copper stranded wire shall be used for bridging once every 3M on one side of the floating roof

(5) the resistivity of hazardous chemicals is generally 1011· M above is an electrostatic non conductor. It takes a long time for the charge on the charged body to dissipate (called the dissipation time). Therefore, when the anti-corrosion coating is used on the tank wall, as long as the resistivity of the coating is less than that of the stored medium, the dissipation of charge will not be hindered