The harm of the hottest static electricity and its

2022-08-06
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Generation, harm and preventive measures of static electricity although the electrostatic effect was first proved by experiments in electricity, static electricity is still regarded as "nameless fire" in modern industrial processes. Generally, the industry is quite unfamiliar with the electrostatic hazard control technology, and many misunderstandings or misuse of the control methods often occur without knowing it, so as to fail to prevent the occurrence of electrostatic hazard accidents

in most industrial processes, electrostatic charge will accumulate, which may make people feel uncomfortable, or cause harm to human body, and even cause fire and explosion accidents during the loading, unloading and transportation of flammable gas, liquid and dust. Especially in some industries with potential electrostatic hazards, such as chemical, petroleum, coating, plastic, pharmaceutical, food, printing and electronic industries, it is easy to have problems caused by electrostatic hazards

II. Generation of electrostatic hazards

appearance improvement of electrostatic appliances has a specific process, as shown in Figure 1. The structure in the figure is helpful to systematically understand the harm of electrostatic discharge ignition. In the working environment, all fire and explosion events caused by static electricity follow the same procedure, as follows: charge separation occurs first, and then charge accumulation occurs. If the charge cannot escape, electrostatic discharge will occur, which may ignite surrounding flammable substances and cause fire and explosion hazards

many industrial processes often use substances with poor conductivity, and often have surface contact, separation and movement operations, resulting in charge separation. For example: flow or filtration of high resistance liquid, absorption of powder, grinding, mixing or screening process of their work experience and R & D achievements, pneumatic transmission of powder, movement of personnel or vehicles on the insulating floor, movement of conveyor belt or sheet material on the roller, etc. Static electricity may occur in the above or similar processes

when the electric charge accumulates on the object to make the electric field reach the dielectric strength 3mv/m of air, it will produce discharge phenomenon, release all or part of its stored energy, form a discharge path with light and heat, and may ignite explosive substances. According to the minimum ignition energy (MIE) data of flammable substances, it can be inferred whether the energy of electrostatic discharge is enough to ignite the flammable substances

recently, because many equipment parts use non-conductive plastics, some metal components, assemblies, pipelines, containers or structures in the equipment form electrical insulators, resulting in the gradual accumulation of charges to a dangerous level. Typical examples include: installing a metal funnel on the plastic pipeline, insulating a section of metal pipeline due to non-conductive gasket on the metal pipeline, and insulating the human body due to wearing insulating shoes or standing on the insulating floor. When the charge accumulated on the insulated conductor generates discharge, all the energy will be released in one discharge. This kind of electrostatic discharge is called spark discharge. Generally speaking, spark discharge can ignite flammable gases, vapors and dust clouds

the charge moves very slowly on the surface of the insulating object, but the duration of electrostatic discharge is very short. Therefore, the accumulated charge of the insulating object is not easy to be released in a single electrostatic discharge, and multiple electrostatic discharges may occur in the adjacent area of the insulating object surface. Due to the difference between the charge and the geometry of the surrounding environment, the discharge patterns can be divided into three types: corona discharge, brush discharge and radial discharge. Generally speaking, the energy of brush discharge is greater than that of corona discharge. The energy of brush discharge is enough to ignite many flammable gases, solvent vapors and mixtures. When both sides of a non-conductive film are filled with positive and negative charges, a large amount of charges will be accumulated. In case of radial discharge, its energy is enough to ignite most flammable gases and dust

the radial discharge that occurs on the surface of a large silo or container filled with highly charged powder products is called a large amount of powder pile discharge. If there is flammable gas or dust cloud with low minimum ignition energy, the country leads the graphene industry to establish a strategic alliance for technological innovation of China's graphene industry, it will have great potential harm. Therefore, it is necessary to try to eliminate the generation of a large number of powder pile discharges

III. electrostatic hazard prevention methods

electrostatic hazard prevention methods can be divided into five types: grounding, increasing humidity, limiting speed, antistatic materials, and electrostatic eliminators. In the industrial manufacturing process, the electrostatic hazard prevention methods will be different due to the different working environment, process application scope, process and materials. The selection must consider the on-site process environment, conditions and restrictions, and even factors such as funds, management system and human quality. No single electrostatic hazard prevention method can be applied to all industrial processes or situations. Sometimes two or more electrostatic hazard prevention methods are used at the same time

(I). Grounding

grounding is the most effective and economical method for the prevention and control of electrostatic hazards. Static electricity is generated due to friction, induction or conduction in the process. If the charge is accumulated on metal equipment, conductive products or personnel insulated from the ground, the accumulated charge will release energy in one discharge. Such electrostatic discharge is the main cause of electrostatic hazard accidents. The prevention method is to ground all conductive objects, keep low grounding resistance, and quickly dissipate the charge accumulated on metal equipment, conductive products or personnel to the ground, so as to avoid electrostatic hazard accidents

according to relevant research, in general workplaces with flammable vapors, the potential of insulated metal equipment/components, conductive products or the body of personnel should be more than 100 V before the surrounding flammable substances may be ignited due to discharge. Therefore, the insulated metal equipment/components and conductive products shall be grounded in the factory to keep the grounding resistance less than 106 ω, It is enough to quickly dissipate the accumulated charge to the earth, and reduce its own electrostatic potential to less than 100 V, so as to avoid electrostatic hazard accidents

(II). Increasing humidity

increasing the relative humidity of the air in the working environment is also a common electrostatic hazard prevention method in the process of traditional industries at present. In high humidity (R.H. 65 ‰), if the surface of the substance is hydrophilic, it is easy to adsorb water in the air, thus reducing the surface resistance of the substance, increasing the rate of charge dissipation, and minimizing the degree of charge accumulation. Such substances include cotton, paper and cellulose acetate. In the factory process, humidifier, ground watering, or steam spraying are usually used to increase the relative humidity of the air in the working environment

if the surface of the substance is non hydrophilic, it is not easy to adsorb water in the air, so the surface resistance of the substance cannot be reduced, so the rate of charge dissipation cannot be increased. Such substances include some artificial polymers, such as ABS (acrylonitrile – butadiene - styrene), Teflon (Teflon, fluorocarbon polymer), etc. Such highly water repellent substances need to increase the relative humidity to 80%, or even more than 90%, in order to effectively reduce the surface resistance of the substance and minimize the charge accumulation

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