Introduction to electrostatic knowledge

Know electrostatic knowledge

In life often encounter the phenomenon of electrostatic discharge, especially in the dry winter, clothes, hair are easy to carry static electricity, but in the production and electrical operation, the protection of static electricity is particularly important, not handled well, will damage the equipment, mess up production, and even cause a disaster. Soldering iron manufacturers tell you that understanding and mastering electrostatic knowledge is very important.

I. Generation, discharge and ignition of static electricity

1. Causes of static electricity

Because of its own good electrical conductivity, static electricity will soon leak. However, such as gasoline, benzene, ether, etc., their resistivity is 1011ω.cm, -1014ω.cm, and it is easy to produce and accumulate static electricity. Therefore, resistivity is the condition for the accumulation of static electricity. The dielectric constant of a substance is the main factor that determines the electrostatic capacitance, and it also closely affects the result of electrostatic generation with the resistivity of a substance, which is usually expressed by the relative dielectric constant.

2, several forms of static electricity

A. Contact electrification

Contact electrification can occur at solid-solid, liquid-liquid, or solid-liquid interfaces. A gas cannot be charged in this way, but if there are solid particles or droplets suspended in the gas, both can be charged by contact, such that the gas can carry an electrostatic charge.

B. Break the power

Regardless of whether the internal charge distribution is uniform before the material is broken, it may lead to the separation of positive and negative charges in a macroscopic range after the breaking, resulting in static electricity. This type of electrification is called breaking electrification. The electrification in the process of solid crushing and liquid splitting belongs to breaking electrification.

C. Induction electrification

A conductor can be induced to be charged by one or more charged bodies around it. There is an electric field around any charged body, and the conductor in the electric field can change the distribution of the surrounding electric field, and under the action of the electric field, two kinds of charges with opposite polarity are separated on the conductor. If the conductor is isolated from the surrounding area, it will have a potential, which is called inductive charging. A conductor carries a potential, plus it carries a separated charge. Thus, the conductor is capable of electrostatic discharge.

D. Charge transfer

When a charged body is in contact with an uncharged body, the charge will be distributed between them to the extent that their respective conductivity allows, which is charge transfer. When charged droplets or dust strike a solid (such as electrostatic dust removal), there is a powerful charge migration. A similar charge migration occurs when a stream of gas ions is directed at an initially uncharged object.

3. Factors affecting the generation of static electricity

Electrostatic generation is affected by material type, impurities, surface state, contact characteristics, separation speed, charging history and other factors.

A. Substance type

When the materials of the two objects in contact with each other are different, the interface double electric layer and the contact potential difference are also different, and the electric strength is also different. In electrostatic sequence, the contact potential difference between two objects separated by a distance is larger.

B. impurity

Under normal circumstances, mixed impurities have the tendency to increase static electricity. However, when the addition of impurities reduces the resistivity of the original material, it is conducive to the leakage of static electricity. Since static electricity is mostly manifested as an interface phenomenon, static electricity is enhanced when the surface of the solid material is polluted by water and its dirt.

C. Surface state

Rough surface increases static electricity; Surface oxidation also increases static electricity.

D. Contact characteristics

The increase of contact area and contact pressure can increase the static electricity.

E. Separation velocity

The higher the separation speed, the stronger the static electricity generated. The static electricity generated is roughly proportional to the second square of the separation speed.

F. Charging history

The charging process changes the surface properties of the object and thus changes the charging characteristics. In general, the initial or initial charging is strong, and the repetitive or continuous charging is weak.

4. Accumulation and discharge of static electricity

A. Static accumulation

After the insulator is charged, the charge is kept on the insulator due to the high resistance of the material itself. The insulated conductor also keeps the charge on the conductor, both of which are called the accumulation of static electricity. Normally, pure gases are insulators, so suspended clouds of particles, droplets, or fogs can hold their charge for a long time regardless of their own conductivity.

In all cases, the charge leaks at a rate determined by the resistance of the insulators in the system. Therefore, the degree of system danger directly depends on the size of the resistance, resistivity, or conductivity of the system. Electrostatic leakage is carried out according to exponential law.

In many industrial processes, static electricity is generated continuously and accumulates on an isolated conductor. This is the case, for example, when a stable charged liquid or powder sinks into an isolated metal container. The potential on an isolated conductor is the result of the balance between the rate of charge input and the rate of charge leakage.

B. Electrostatic discharge.

The charge that accumulates on a liquid or solid can cause a disaster when discharged on other objects or ground conductors. Electrostatic discharge varies greatly in form and igniting ability. The following diagram plots the spark shapes of several common electrostatic discharges:

(a) Spark discharge

Spark discharge is a discharge that occurs between liquid or solid conductors. It is characterized by a bright discharge channel, a high current in the channel, and complete ionization of the gas in the entire channel. The discharge is fast and has a loud popping sound.

A spark discharge occurs when the strength of the electric field between the two conductors exceeds the breakdown strength. For the planandine or the surface with a large radius of curvature, if the gap is 10mm or more, the breakdown strength is about 3×103kV/m; If the gap is reduced, the breakdown strength will increase slightly. Because the discharge is a conductor, almost all of the charge goes into the spark, that is, almost all of the electrostatic energy consumed by the spark. If there is resistance on the discharge path between the conductor and the earth, the spark energy will be less than this value, but the spark duration will be longer.

(b) Corona discharge

Corona discharge occurs when a tip with a small radius of curvature is present on the conductor. Corona discharges may or may not be directed in a particular direction. When corona discharge, the field intensity near the tip is very strong, the gas near the tip is ionized, and the charge can leave the conductor. Far from the tip, the field strength weakens sharply, and the ionization is incomplete, so only a small current can be established. Corona discharges are characterized by a "hiss" sound and sometimes a faint glow.

Corona discharge can be continuous discharge or discontinuous pulse discharge. The energy density of corona discharge is much smaller than that of spark discharge. In some cases, if the potential of the tip conductor is raised, the corona can develop into a spark to another object.

(c) Brush discharge

A brush discharge occurs between a conductor and a non-conductor and is a discharge that emits short sparks from many points on the non-conductor. Each spark is determined by the amount of electricity that can flow into a non-conducting surface. Its discharge population often has a brush-like shape. If the conductor is sharp, the discharge at the conductor will have the characteristic of spreading forward like a corona discharge.

(d) Field emission discharges are discharges that emit electrons from the surface of an object. Its energy is small, so it needs attention only when it involves highly sensitive explosive items.

(e) Lightning discharge

When charged particles suspended in the air form a large area; When a space charge cloud with a high charge density, a thunder-like discharge called a lightning discharge can occur. Thunder-shaped discharge may occur when pressurized liquid and liquefied gas are ejected at high speed. The energy of thunder-shaped discharge is very large, and the risk of ignition is also very great.

5. Electrostatic ignition

Whether electrostatic discharge can ignite flammable and explosive mixtures depends on the composition and temperature of the mixture, the discharge energy, and the distribution of energy over time and in space. The ignition source is often a spark discharge from a conductor. Therefore, spark discharge is often used to test the ignition energy. Generally, the selected test voltage is 10kV. The minimum ignition energy of most organic vapors and hydrocarbon gases is between 0.0l and 0.1mJ. The minimum ignition energy of acetylene and hydrogen in air is about 0.02 mJ, while the minimum ignition energy of explosives can be as low as 0.001 mJ. Acetone is 1.15

mJ, isopropyl fermentation is 0.65 mJ.

Second, the basic measures to prevent electrostatic hazards

To prevent static electricity, we must first try not to produce static electricity; The generated static electricity should be limited as much as possible so that it does not reach the dangerous level. Secondly, make the generated charge leak or neutralize as soon as possible, thus eliminating a large accumulation of charge.

1. Reduce triboelectrification

In the transmission device, the skid phenomenon on the belt and other transmission parts should be reduced. For example, the belt should be properly relaxed, maintain a certain tension, and avoid overload operation. The selected belt should use conductive tape or conductive triangle tape with high transmission efficiency as far as possible. In the transport of flammable gases, flammable liquids and flammable and explosive objects on the equipment, should be used direct shaft (or coupling) transmission, generally not suitable for belt drive; If belt drive is required, effective anti-static measures must be taken.

Limiting the flow rate of flammable and combustible liquids can greatly reduce the generation and accumulation of static electricity. When the liquid advection, the static electricity generated is proportional to the flow rate, and has nothing to do with the inner diameter of the pipe. When the liquid is turbulent, the static energy generated is proportional to the 1.75 square of the flow rate and is proportional to the o.75 square of the inner diameter of the pipe. At present, the standards for controlling the flow rate in the world are not uniform. In short, when determining the flow rate, it is necessary to consider not only the inner diameter of the pipeline, but also the influence of various factors such as the nature of the fluid, the composition and amount of impurities contained, and the material of the pipeline.

The flammable liquid flowing in the pipeline, even if it has a high average charge density, often does not show a high voltage because of the large capacitance in the pipeline, and because there is no air in the pipeline, it will not cause combustion and explosion. In this case, although static electricity does not pose a danger inside the pipeline, its serious harm is mainly at the exit of the pipeline, which must be paid attention to.

2. Electrostatic grounding

The function of electrostatic grounding is to discharge the charge that may be gathered on the conductor, so that the conductor and the earth are equal potential, and the potential difference between the conductors is zero.

A. Electrostatic grounding principle

Electrostatic grounding is the method of grounding to provide a static charge leakage channel. In fact, the generation and leakage of static electricity are parallel, and are the processes of input and output of charges to the charged body. The accumulated electrostatic potential on the object, when the capacitance to the ground is fixed, completely depends on the difference between the starting power of the object and the leakage power. The rate of electrostatic electrification is a random variable. It can increase very slowly over time, or it can change dramatically. In order to ensure the electrostatic safety of the object, it is solved by leakage (grounding).

The application range of electrostatic grounding is conditional, and not all objects charged can be solved by means of grounding. Generally speaking, metal conductors and equipment that may cause fire and explosion to endanger the safety of the place, non-gold chips that belong to electrostatic conductors, and the human body must be electrostatic grounded. At the same time, it is also necessary to consider the grounding of the whole system, otherwise grounding will cause electrostatic discharge. For example, when the electrically charged body (or object) in the state of insulation is close to or in contact with the ground body, a discharge spark is generated. Conversely, when a grounded human body (or object) is close to an electrically charged isolated conductor, a spark discharge will also occur.

B. Electrostatic grounding system

(a) Direct grounding Direct grounding is electrical grounding, that is, metal wires are directly connected to the live body and the ground trunk.

(b) Indirect grounding

Indirect grounding is to make objects other than metal electrostatic ground, the whole or part of its surface is closely connected with the grounding body. In other words, the charged body and the ground body are connected by an electrostatic conductor with a certain resistance value.

3, reduce the resistivity

When the resistivity of the substance is less than 106 ω.cm, the accumulation of static charge can be prevented.

A. Addition of conductive filler

The resistivity is reduced by adding a substance with good conductivity. For example, in the refining process of rubber, a certain graphite powder is added to make it conductive rubber; In plastic production, a small amount of metal powder and graphite powder is added to make it a low-resistance plastic; In industrial oil, mixed with a small amount of alcohol or a small amount of acetic acid; The resistivity of benzene can be reduced by injecting some metal soap such as magnesium oleate into benzene.

B. Use antistatic agent

Antistatic agent with oil as raw material, the main component of quaternary amine salt, its role is chemical fiber, rubber, plastic and other objects on the surface of the adsorption of water in the air, increase the conductivity. For example, SN cationic antistatic oil agent, in the process of polyethylene chemical fiber textile and polyvinyl alcohol synthetic fiber drawing, as long as a small amount is applied, the electrostatic voltage can be limited to tens of volts. The cationic PK antistatic oil used in the production of polyester fiber and the MOA3 and KP oil used in long fiber also have good antistatic properties. In the production of anti-static conveyor belt, that is, in the raw material butadiene rubber, add anti-static agent; On polyester film or other plastic products, adding or coating SM anti-static oil agent also has a certain effect. Propylene oxide hydrophilic gene was added into the spinning of chemical fiber. Add ASA3 anti-static additive to aviation kerosene and other liquids; Coating anti-static agents on the sensitive film can greatly reduce the surface resistivity or volume resistivity and reduce the accumulation of static electricity.

4. Increase air humidity

When the relative humidity of the air is more than 65-70%, the surface of the object often forms a very thin water film. The water film can dissolve CO2 in the air, so that the surface resistivity is greatly reduced, and the static charge is not easy to accumulate. If the relative humidity of the surrounding air drops to 40-50%, static electricity is not easy to escape, and it is possible to form a high potential. The common way to increase the humidity of the air is to spray fog into the air, generally choose a rotary fan sprayer, but the machine is not explosion-proof, must be blown into the wall.

5, air ionization method

The use of electrostatic eliminator to ionize oxygen and nitrogen ions in the air, so that the air becomes a conductor, can effectively eliminate the electrostatic charge on the surface of the object. Commonly used static eliminator are:

(a) Induction static eliminator

It can also be divided into steel grounding induction type, brush type induction type, needle point induction type and so on. Mainly used in papermaking, rubber, textile, plastics and other production and processing industries.

(b) High voltage static eliminator

It mainly has external type, power frequency alternating current, thyristor, AC high frequency and high voltage type. In chemical, textile and other industries can be selected according to different requirements. In addition, there are high-pressure ion currents, radioactive radiation and other forms, suitable for other special places.

Third, the common electrostatic discharge spark risk control and elimination

1. The solid is charged

Solid insulation materials are increasingly used in chemical production equipment and components. Because the solid insulation has no free electrons, its surface is often adsorbed and oxidized by impurities, forming a thin layer with electron transfer ability, so it can generate static electricity in the case of friction, rolling, extrusion, stripping, etc. Solid static electricity can eliminate or reduce the discharge sparks caused by the accumulation of static electricity by reducing the resistivity (such as adding an appropriate amount of additive - carbon black in products made of polymeric materials, which can be made into conductive products, increasing humidity, ionization, grounding, grounding metal mesh applications, etc.).

(1) In the production of rubber products in the calender process, the rubber material under the pressure of the calender drum, due to the high pressure, the pressure area is large, the charge transfer is fast, the electrostatic voltage can be as high as several hundred thousand V. Generally, local humidification is used to make the relative humidity above 75% to reduce static electricity.

(2) Transport and transmission equipment is also very easy to produce static electricity. Such as rubber flat belt, plastic belt, synthetic fiber belt, cattle (pig) belt high-speed transmission and transmission equipment are often electrostatic generation. A hard polyethylene idler in operation on a belt conveyor was tested with a static voltage of up to 45000V. It can be seen that in the case of flammable and explosive gas or dust, the transmission shaft and roller of the conveyor belt shall not use insulating materials with high resistivity, so as to avoid combustion and explosion caused by electrostatic discharge.

(3) The static voltage generated by different abrasives when they rub against each other is also different. According to the actual measurement, the speed is fixed, the temperature is 20 soil 3℃, and the relative humidity of the air is 65 5% of the bar