Static electricity (according to GOST 12.1.018) is a set of phenomena associated with the emergence, conservation and relaxation of a free electric charge on the surface (or in the volume) of dielectrics or on insulated conductors.
The generation of static electricity. Static electricity charges are formed under a wide variety of industrial conditions, but most often when one dielectric is rubbed against another or dielectrics against metals. On rubbing surfaces, electric charges can accumulate, easily flowing into the ground, if the physical body is a conductor of electricity and is grounded. Electric charges are held on dielectrics for a long time, as a result of which they are called static electricity.
Static electricity arises as a result of complex processes associated with the redistribution of electrons and ions when two surfaces of inhomogeneous liquid or solid substances come into contact, having different atomic and molecular forces of surface attraction.
A measure of electrization is the charge possessed by a given substance. The intensity of the formation of charges increases with an increase in the speed of movement of materials, their resistivity, contact area and interaction force. The degree of electrification of a charged body characterizes its potential relative to the earth.
In production, the accumulation of static electricity charges is often observed when: friction of drive belts on pulleys or conveyor belts on shafts, especially with slipping; pumping flammable liquids through pipelines and filling oil products into containers; movement of dust through the air ducts; crushing, mixing and screening of dry materials and substances; compression of two dissimilar materials, one of which is a dielectric; mechanical processing of plastics; transportation of compressed and liquefied gases through pipes and their flow through openings, especially if the gases contain a finely atomized liquid, suspension or dust; movement of vehicles, trolleys on rubber tires and people on a dry insulating coating, etc.
The electrification current strength of the flow of oil products in pipelines depends on the dielectric properties and kinematic viscosity of the liquid, flow velocity, pipeline diameter and length, pipeline material, roughness and condition of its internal walls, liquid temperature. With turbulent flow in long pipelines, the current strength is proportional to the velocity of the liquid and the diameter of the pipeline. The degree of electrification of moving dielectric tapes (for example, conveyor belts) depends on physical and chemical properties materials in contact, their contact density, movement speed, relative humidity, etc.
Danger of static electricity discharges. Spark discharges of static electricity are a great fire and explosion hazard. Their energy can reach 1.4 J, which is quite enough to ignite vapor, dust and gas-air mixtures of most combustible substances. For example, the minimum ignition energy of acetone vapor is 0.25 10-3 J, methane 0.28 10-3, carbon monoxide 8 10-3, wood flour 0.02, coal 0.04 J. Therefore, in accordance with GOST 12.1.018, the electrostatic safety of an object is considered achieved only if the maximum energy of discharges that can occur inside the object or from its surface does not exceed 40% of the minimum ignition energy of substances and materials.
The electrostatic charge that occurs during some manufacturing processes can reach several thousand volts. For example, when sand and dust particles rub against the bottom of the body when the car is moving, a potential of up to 3 kV is generated; when pumping gasoline through the pipeline - up to 3.6 kV; when pouring electrifying liquids (ethyl alcohol, gasoline, benzene, ethyl ether, etc.) into ungrounded tanks in the case of a free fall of a liquid jet into a vessel being filled and a high outflow rate - up to 18 ... 20 kV; during friction of the conveyor belt on the shaft - up to 45 kV; when transmission belts rub against pulleys - up to 80 kV.
In this case, it should be borne in mind that a potential of 300 V is sufficient for the explosion of gasoline vapors; at a potential difference of 3 kV, combustible gases ignite, and at 5 kV, most combustible dusts.
Static electricity can also accumulate on the human body when wearing clothes made of wool or artificial fiber, moving on a non-conductive floor covering or in dielectric shoes, in contact with dielectrics, reaching a potential of 7 kV or more in some cases. The amount of electricity accumulated on people can be quite sufficient for a spark discharge on contact with a grounded object. The physiological effect of static electricity depends on the energy released during the discharge and can be felt in the form of weak, moderate or strong pricks, and in some situations - in the form of mild, moderate and even acute convulsions. Since the current strength of the discharge of static electricity is negligible, in most cases such exposure is not dangerous. However, the reflex movements of a person arising from this phenomenon can lead to serious injuries due to a fall from a height, capture of overalls or individual parts of the body by unprotected moving parts of machines and mechanisms, etc.
Static electricity can also interfere with normal flow technological processes, create interference in the operation of electronic devices of automation and telemechanics, radio communications.
Measures to protect against static electricity are carried out in explosion and fire hazardous rooms and areas of open installations related to classes B-I, B-I6, B-II and B-IIa. In rooms and areas that do not belong to the specified classes, protection is carried out in those areas of production where static electricity adversely affects the normal course of the technological process and product quality.
Measures of protection against static electricity are aimed at preventing the occurrence and accumulation of static electricity charges, creating conditions for the dissipation of charges and eliminating the danger of their harmful effects.
Prevention of the accumulation of static electricity charges is achieved by grounding the equipment and communications where they can appear, and each system of interconnected machines, equipment and structures made of metal (pneumatic dryers, mixers, gas and air compressors, mills, closed conveyors, filling and draining devices liquids with low electrical conductivity, etc.), are grounded at least in two places. Pipelines located in parallel at a distance of up to 10 cm are connected to each other by metal jumpers every 25 m. All mobile containers temporarily located under the filling or draining of liquefied combustible gases and flammable liquids are connected to the ground during filling. Refueling trucks and tank trucks are grounded with a metal chain, keeping the length of contact with the ground at least 200 mm.
Reducing the intensity of the occurrence of static electricity charges is achieved by appropriate selection of the speed of movement of substances, excluding splashing, crushing and spraying of substances, removal of an electrostatic charge, selection of friction surfaces, purification of combustible gases and liquids from impurities. Safe transport speeds of liquid and pulverulent substances depend on their specific volumetric electrical resistance ρv. So, for liquids with ρv ≤ 105 Ohm m, the allowable velocity should be no more than 10 m/s, at 105 Ohm m< pv < 109 Ом· м — до 5 м/с, а при ρv >109 ohm m speeds are set for each liquid separately, but, as a rule, not more than 1.2 m/s. When supplying liquids to tanks, it is necessary to exclude their splashing, spraying and violent mixing. The filling tube must be extended to the bottom of the vessel with the direction of the jet along its wall. During the initial filling of the tanks, the liquid is supplied at a speed not exceeding 0.5 ... 0.7 m / s.
The best way to reduce the intensity of the accumulation of static electricity in belt drives is to increase the electrical conductivity of the belts, for example, by stitching the inner surface of the belt with thin copper wire in the longitudinal direction or by lubricating its inner surface with conductive compounds (containing, for example, soot and graphite in a ratio of 1: 2 ,5 by weight, etc.). Attention should also be paid to adjusting the tension of the belts and, if possible, reducing their speed to 5 m/s.
If it is not possible to prevent the accumulation of static electricity charges by grounding, then measures should be taken to reduce the volume and surface dielectric resistances of the materials being processed. This is achieved by increasing the relative humidity of the air to 65 ... 70%, chemical surface treatment, the use of antistatic substances, the application of electrically conductive films, a decrease in the speed of movement of charging materials, an increase in the purity of the treatment of rubbing surfaces, etc.
If it is impossible to use means of protection against static electricity, it is recommended to neutralize the charges by air ionization in the places of their occurrence or accumulation. For this, special devices are used - ionizers, which create positive and negative ions around an electrified object. Ions that have a charge opposite to that of the dielectric are attracted to the object and neutralize it. To remove static electricity from the human body, conductive floors or grounded areas, work platforms, stair handrails, instrument handles, etc. are provided; provide workers with conductive shoes with a sole resistance of not more than 108 ohms, as well as antistatic overalls.
According to the current rules, protection against static electricity discharges should be carried out in explosive and fire hazardous industries in the presence of zones of classes B-I, B-Ia, B-II, B-IIa, P-I and P-II, in which substances with specific volumetric electrical resistance Ohm∙m.
In other cases, protection is carried out only when static electricity poses a danger to operating personnel, adversely affects the technological process or product quality.
The main ways to eliminate the danger from static electricity are (slide):
1) grounding of equipment, communications, devices and vessels, as well as ensuring constant electrical contact with the grounding of the human body;
2) reduction of specific volumetric and surface electrical resistance by increasing air humidity or using antistatic impurities;
3) ionization of air or medium, in particular, inside the apparatus, vessel, etc.
In addition to these methods, they use: preventing the formation of explosive concentrations, limiting the speed of liquid movement, replacing flammable liquids with non-combustible solvents, etc. A practical way to eliminate the danger from static electricity is chosen taking into account efficiency and economic feasibility.
Let us dwell in more detail on the above methods for eliminating the danger from static electricity.
Grounding (18 min)- the most commonly used measure of protection against static electricity. Its purpose is to eliminate the risk of electrical discharges from conductive parts of the equipment. Therefore, all conductive parts of the equipment and conductive non-metallic objects are subject to mandatory grounding, regardless of whether other methods of protection against static electricity are used. It is necessary to ground not only those parts of the equipment that are involved in the generation of static electricity, but also all other properties mentioned above, since they can be charged according to the law of electrostatic induction.
In cases where the equipment is made of electrically conductive materials, grounding is the main and almost always sufficient method of protection.
If deposits of non-conductive substances (resins, films, sediments) form on the outer surface or inner walls of metal apparatuses, tanks and pipelines, grounding becomes ineffective. Grounding does not eliminate the danger when using devices with enamel and other non-conductive coatings.
Non-metallic equipment is considered to be electrostatically grounded if the resistance to the spread of current to the ground from any point on its outer and inner surface is ohms at relative humidity. Such resistance provides the necessary value of the relaxation time constant within a tenth of a second in a non-explosive and thousandths of a second in an explosive environment. The relaxation time constant is related to the resistance R grounding of the apparatus or equipment and its capacity C ratio τ = R∙ C.
Pipelines of outdoor installations (on overpasses or in channels), equipment and pipelines located in workshops must represent an electrical circuit throughout and be connected to grounding devices. It is believed that the electrical conductivity of flanged connections of pipelines and apparatuses, joints of covers with apparatus bodies, etc. high enough that no special parallel jumpers need to be installed.
Each system of apparatus and pipelines within the workshop must be grounded in at least two places. All tanks and containers with a capacity of more than 50 m 3 and a diameter of more than 2.5 m are grounded at least at two opposite points. There should be no floating objects on the surface of flammable liquids in tanks.
The loading risers of racks for filling railway tanks and the rails of the railway tracks within the loading and unloading front must be electrically connected to each other and reliably grounded. Tank trucks, bulk carriers, aircraft under loading (draining) of flammable liquids and liquefied gases must also be grounded. Contact devices (without means of explosion protection) for connecting grounding conductors must be installed outside the explosive zone (at least 5 m from the place of filling or draining, PUE). In this case, the conductors are first connected to the body of the grounding object, and then to the grounding device.
It should be noted that the grounding conductors used so far for grounding tankers do not provide the required level of fire and explosion safety for the technology of loading or unloading fuel and other flammable liquids. Therefore, at present, special grounding devices for tank trucks (UZA) of the UZA-2MI, UZA-2MK and UZA-2MK-03 types have been developed and are commercially produced, which meet the requirements of GOSTs and can be installed in explosive zones of class B-Ig.
If grounding is used for ESD protection of conductive non-metallic equipment with a conductive lining, then the same requirements apply as for the grounding of metallic equipment. For example, grounding of a pipeline made of a dielectric material, but with a conductive coating (paint, varnish), can be performed by connecting it to the ground loop using metal clamps and conductors every 20–30 m.
But grounding does not solve the problem of protecting a tank filled with an electrified liquid from static electricity, it only excludes the accumulation of charge (leaking from the liquid volume) on its walls, but does not accelerate the process of charge dissipation in the liquid. This is explained by the fact that the rate of relaxation of static electricity charges in the volume of the dielectric liquid of petroleum products is determined by the relaxation time constant . Consequently, in the reservoir filled with electrified products, during the entire time of fluid injection and for a time approximately equal to after its completion, there is an electric field of charges, regardless of whether this reservoir is filled or not. It is during this period of time that there may be a danger of ignition of the vapor-air mixture of oil products in the tank by discharges of static electricity.
In view of the foregoing, there is a significant risk of taking samples from a tank immediately after it has been filled. But after a period of time approximately equal to , after filling the grounded tank, the charges of static electricity in it practically disappear and the sampling of the liquid becomes safe.
For light oil products with low electrical conductivity (at Ohm∙m), the required holding time after filling the tank, ensuring the safety of further operations, should be at least 10 minutes.
Grounding the tank and holding the required time after filling will not give the desired safety effect if there are isolated objects floating on the surface of the liquid in the tank, which can acquire a charge of static electricity when the tank is filled and keep it for a time significantly exceeding . In this case, if a floating object comes into contact with a grounded conductive body, dangerous sparking can occur.
Decrease in volume and surface electrical resistivity (8 min).
This increases the electrical conductivity and ensures the ability of the dielectric to divert charges of static electricity. Eliminating the danger of static electrification of dielectrics by this method is very effective and can be achieved by increasing the humidity of the air, chemical surface treatment, the use of electrically conductive coatings and antistatic substances (additives).
A. Increasing the relative humidity of the air.
Most fires from sparks of static electricity usually occur in winter, when the relative humidity of the air is high. At a relative humidity of 65÷70%, as studies and practice show, the number of flashes and fires becomes insignificant.
The acceleration of the drain of electrostatic charges from dielectrics at high humidity is explained by the fact that a thin film of moisture is adsorbed on the surface of hydrophilic dielectrics, usually containing a large amount of ions from impurities and a dissolved substance, due to which sufficient surface electrical conductivity of an electrolytic nature is ensured.
However, if the material is at a temperature higher than that at which the film can be held on the surface, said surface cannot become conductive even at very high air humidity. The effect will also not be achieved if the charged surface of the dielectric is hydrophobic (non-wettable: sulfur, paraffin, oils and other hydrocarbons) or the speed of its movement is greater than the rate of surface film formation.
An increase in humidity is achieved by spraying water vapor or water, circulating moist air, and sometimes free evaporation from the water surface or cooling the electrifying surface by 10 ° C below the ambient temperature.
B. Chemical surface treatment, electrically conductive coatings.
Reducing the specific surface resistance of polymeric materials can be achieved by chemical treatment of their surface with acids (for example, sulfuric or chlorosulfonic). As a result, the polymer surfaces (polystyrene, polyethylene and polyester films) are oxidized or sulfonated and the resistivity is reduced to 10 6 Ohm at a relative air humidity of 75%.
A positive effect is also achieved when processing products made of polystyrene and polyolefins by immersing samples in petroleum ether with simultaneous exposure to ultrasound. Chemical processing methods are effective, but require precise adherence to technological conditions.
Sometimes the desired effect is achieved by applying a surface conductive film to the dielectric, for example, a thin metal film, obtained by sputtering, spraying, evaporation in a vacuum, or gluing a metal foil. Carbon-based films are obtained by spraying carbon in a liquid medium or powder with particles smaller than 1 µm.
B. The use of antistatic agents.
Most combustible and flammable liquids are characterized by high electrical resistivity. Therefore, during some operations, for example, with petroleum products, static electricity charges accumulate, which prevents the intensification of technological operations, and also serves as a source of explosions and fires at oil refineries and petrochemical enterprises.
The movement of liquid hydrocarbons relative to a solid, liquid or gaseous medium can lead to the separation of electric charges on the contact surfaces. When a liquid moves through a pipe, a layer of charges located on the surface of the liquid is carried away by its flow, and charges of the opposite sign remain on the surface of the pipe in contact with the liquid and, if the metal pipe is grounded, drain into the ground. If the metal pipeline is insulated or made of dielectric materials, then it acquires a positive charge, and the liquid becomes negative.
The degree of electrization of petroleum products depends on the composition and concentration of active impurities contained in them, the physical and chemical composition of petroleum products, the condition of the inner surface of the pipeline or process apparatus (presence of corrosion, roughness, etc.), dielectric properties, viscosity and density of the liquid, as well as speed fluid movement, diameter and length of the pipeline. For example, the presence of 0.001% mechanical impurities transforms an inert hydrocarbon fuel into an electrified one to dangerous limits.
One of the most effective ways to eliminate the electrification of petroleum products is the introduction of special antistatic substances. Adding them in thousandths or ten thousandths of a percent makes it possible to reduce the specific resistance of oil products by several orders of magnitude and to secure operations with them. Such antistatic substances include: oleates and naphthenates of chromium and cobalt, chromium salts based on synthetic fatty acids, additive "Sigbal" and others. Thus, an additive based on oleic acid, chromium oleate reduces the ρ v of gasoline B-70 by 1.2 ∙ 10 4 times. The Ankor-1 and ASP-1 additives have found wide application in parts washing operations.
To obtain a "safe" electrical conductivity of petroleum products under any conditions, it is necessary to introduce 0.001 ÷ 0.005% additives. They usually do not affect the physicochemical properties of petroleum products.
To obtain conductive solutions of polymers (adhesives), antistatic additives soluble in them are also used, for example, salts of metals of variable valence of higher carboxylic and synthetic acids.
Positive results are achieved with the use of antistatic agents in synthetic fiber processing plants, since they have the ability to increase their ionic conductivity and thereby reduce the electrical resistance of the fibers and materials obtained from them.
For the preparation of antistatic substances that affect the electrical properties of the fibers, paraffin hydrocarbons, fats, oils, hygroscopic substances, surfactants are used.
Antistatic agents are used in the polymer industry, for example in the processing of polystyrene and polymethyl methacrylate. Treatment of polymers with antistatic additives is carried out both by surface application and by introduction into the molten mass. As such additives, for example, surfactants are used. With surface application of surfactants, ρ s of polymers decreases by 5–8 orders of magnitude, but the period of effective action is short
(up to one month). The introduction of surfactants inside is more promising. the antistatic properties of polymers are retained for several years, the polymers become less susceptible to solvents, abrasion, etc. For each dielectric, the optimal surfactant concentrations are different and vary from 0.05 to 3.0%.
At present, pipes made of semi-conductive polymer compositions with fillers are widely used: acetylene black, aluminum powder. graphite, zinc dust. The best filler is acetylene black, which reduces resistance by 10–11 orders of magnitude even at 20% by weight of the polymer. Its optimal mass concentration for creating an electrically conductive polymer is 25%.
To obtain electrically conductive or antistatic rubber, fillers are introduced into it: powdered graphite, various carbon blacks, and finely dispersed metals. The specific resistance ρ v of such rubber reaches 5 ∙10 2 Ohm∙m, and the usual one up to 10 6 Ohm∙m.
Antistatic rubber grades KR-388, KR-245 are used in explosive industries, they cover floors, work tables, equipment parts and wheels of intrashop transport. Such a coating quickly removes the emerging charges, reduces the electrification of people to a safe level.
Recently, oil and petrol resistant electrically conductive rubber has been developed using nitric butadiene and polychloroprene rubbers, which is widely used for the manufacture of pressure hoses and hoses for pumping flammable liquids. Such sleeves significantly reduce the risk of ignition during the discharge and filling of flammable liquids into road and rail tanks and other containers, exclude the use of special devices for grounding filling funnels and tips.
An effective reduction in the potential of belt drives and belt conveyors made of materials with ρ s =10 5 Ohm∙m is achieved by increasing the surface conductivity of the belt and the obligatory grounding of the installation. To increase the surface conductivity of the belt, its inner surface is coated with antistatic grease, renewed at least once a week.
Air ionization (9 min).
The essence of this method is to neutralize or compensate for surface electric charges with ions of different signs, which are created by special devices - neutralizers. Ions having a polarity opposite to the polarity of the charges of electrified materials, under the influence of an electric field created by the charges of such materials, settle on their surfaces and neutralize the charges.
The ionization of air by an electric field of high tension is carried out using neutralizers of two types: induction and high-voltage.
Induction neutralizers come with spikes (Fig. 2, a) and wire ones (Fig. 2, b). A neutralizer with spikes in a wooden or metal rod has grounded spikes, thin wires or foil. A wire neutralizer uses a thin steel wire stretched across a moving charged material. They work as follows. Under the action of a strong electric field of an electrified body near the tip or wire, impact ionization occurs, as a result of which ions of both signs are formed. To increase the effectiveness of the neutralizers, they tend to reduce the distance between the tips of the needles or wire and the surface to be neutralized to 5–20 mm. Such neutralizers have a high ionizing power, especially those with spikes.
Rice. 2. Diagram of an induction neutralizer (slide):
a - with points; b- wire; 1 - points; 1 "- wire; 2- charged surface.
Their disadvantages are that they act if the potential of an electrified body reaches several kV.
Their advantages: simple design, low cost, low operating costs, do not require a power source.
High-voltage neutralizers (Fig. 3) operate on alternating, direct and high frequency current. They consist of a transformer with a high output voltage and a spark gap. The DC converter also includes a high-voltage rectifier. Their principle of operation is based on the ionization of air by high voltage. The maximum distance between the discharge electrode and the neutralized material, while the neutralizer is still effective, for such neutralizers can reach 600 mm, but usually the working distance is taken equal to 200÷300 mm. The advantage of high-voltage neutralizers is a sufficient ionizing effect even at a low potential of an electrified dielectric material. Their disadvantage is the high energy of the resulting sparks, capable of igniting any explosive mixtures, therefore, for explosive areas, they can only be used in an explosion-proof version.
Fig. 3 Scheme of a high-voltage neutralizer (slide).
To protect operating personnel from high voltage, protective resistances are included in their high-voltage circuit, which limit the current to a value 50–100 times less than the life-threatening current.
Radioisotope neutralizers are very simple in design and do not require a power source. quite effective and safe when used in explosive environments. They are widely used in various industries. When using such neutralizers, it is necessary to provide for reliable protection of people, equipment and manufactured products from the harmful effects of radioactive radiation.
Radioisotope neutralizers are most often in the form of long plates or small disks. One side contains a radioactive substance that creates radioactive radiation that ionizes the air. In order not to contaminate the air, products and equipment, the radioactive substance is covered with a thin protective layer and a special enamel or foil. To protect against mechanical damage, the ionizer is placed in a metal casing, which simultaneously creates the desired direction of ionized air. Table 3 shows data on radioactive substances used in radioisotope neutralizers.
Data on radioactive substances of radioisotope neutralizers (slide).
Table 3
The most effective and safe radioactive substances with α-particles. The penetrating power of α-particles in air is up to 10 cm, and in denser media it is much less. For example, a sheet of ordinary clean paper completely absorbs it.
Neutralizers with such radiation are suitable for local ionization of air and neutralization of charges at the place of their formation. To neutralize electric charges in devices with a large volume, β-emitters are used.
Radioactive substance with γ-study is not used in neutralizers because of its high penetrating power and danger to people.
The main disadvantage of radioisotope neutralizers is the low ionization current compared to other neutralizers.
To neutralize electric charges, combined neutralizers, for example, radioactive induction, can be used. Similar neutralizers are produced by the industry and have improved performance. Performance characteristics express the dependence of the discharge ionization current on the magnitude of the potential of the charged body.
Additional ways to reduce the danger from static electricity (3 min, slide # 13).
The danger of static electrification of flammable liquids and flammable liquids can be significantly reduced or even eliminated by reducing the flow rate. v. Therefore, the following speed is recommended v dielectric liquids:
At p ≤ 10 5 Ohm∙m accept v≤ 10 m/s;
At ρ > 10 5 Ohm∙m accept v≤ 5 m/s.
For liquids with ρ > 10 9 Ohm∙m transport and flow rates are set separately for each liquid. Safe for such liquids is usually a speed of movement or expiration of 1.2 m/s.
For transporting liquids from ρ > 10 11 -10 12 Ohm∙m with speed v≥ 1.5 m/s, it is recommended to use relaxers (for example, horizontal pipe sections of increased diameter) directly at the entrance to the receiving tank. Required diameter D R, m of this section is determined by the formula
D R =1.4 D T ∙ . (7)
Relaxator length L p is determined by the formula
L p ≥ 2.2 ∙ 10 -11 ξρ, (8)
where ξ is the relative permittivity of the liquid;
ρ – specific volume resistance of the liquid Ohm∙m.
When filling the tank with liquid ρ >10 5 Ohm∙m until the loading pipe is flooded, it is recommended to supply liquids at a speed v ≤ 1 m/s and then at the specified speed v ≤ 5 m/s.
Sometimes it is required to increase the speed of liquids in the pipeline up to 4÷5 m/s.
The relaxator diameter calculated by formula (7) turns out to be unreasonably large in this case. Therefore, to increase the effectiveness of the relaxer, it is recommended to use them with strings or needles. In the first case, grounded strings are stretched inside the relaxer and along its axis, which reduces the electrization current by more than 50%, and in the second case, grounded needles are introduced into the liquid flow to remove charges from the liquid flow.
The maximum permissible and safe (with regard to the possibility of ignition of liquid vapor in an industrial tank) modes of transportation of petroleum products through long pipes with a diameter of 100 ÷ 250 mm can be estimated by the ratio
v T 2 D T ≤ 0.64 , (9)
where v T is the linear velocity of the liquid in the pipe, m/s, D T- pipe diameter, m.
During operations with bulk and finely dispersed materials, the risk reduction from static electrization can be achieved by the following measures: when pneumotransporting them, use pipes made of polyethylene or from the same material (or similar in composition to the transported substance); the relative humidity of the air at the outlet of the pneumatic conveyor must be at least 65% (if this is unacceptable, it is recommended to ionize the air or use an inert gas).
Dust-air combustible mixtures should be avoided, dust should not fall, swirl or swirl. It is necessary to clean the equipment and structures of the building from settled dust.
During operations with combustible gases, it is necessary to monitor their cleanliness, the absence of ungrounded parts of equipment or devices on the paths of their movement.
A good effect in terms of fire and explosion safety from sparks of static electricity and all other sources of ignition is achieved by replacing organic solvents and flammable liquids with non-combustible ones if such a replacement does not disrupt the process and is economically feasible.
On dielectric materials after their friction between themselves or against metal objects, the formation of electric charges of increased density occurs. Thus, static electricity arises, measures of protection against which are absolutely necessary. First of all, this is due to the slow disappearance of the charge due to the fact that dielectrics have an extremely low electrical conductivity.
The appearance and danger of static electricity
The cause of electrification can also be induction. On a metal surface, an electric charge appears with the opposite value, the density of which is uniform in all places. Conditions for the occurrence of this phenomenon can be very different. Often the cause is the pumped liquid moving through pipelines or in the form of a falling jet. The same effect is given by compressed or liquefied gases, the operation of belt drives, the grinding and processing of organic and polymeric materials.
The electrification of dielectric materials often reaches a potential difference with a high voltage. For example, in the process of pumping gasoline through a pipeline with an isolated section, electrical potentials can range from 1460 to 14600 volts.
A serious danger is the accumulation of static electricity. In such cases, a strong spark discharge is often manifested. The released spark energy with a value of 0.01 J is already capable of causing a fire and explosion. A voltage of 300 volts results in an air spark discharge. Taking timely special measures helps to prevent the consequences of electrical discharges.
Protective measures against static electricity
To equalize the potentials and prevent the occurrence of sparks, all pipelines located in parallel, at a distance of less than 100 mm, are connected by jumpers every 20-25 meters. Piping and equipment systems must be grounded at least in two places. Checking the presence of grounding is carried out using a tester or once every 6 months and after repair work.
During loading, pumping and transportation of petroleum products, the emerging electrostatic discharges are removed by metal connection between pumps, pipelines, tanks and other devices. In case of spilling dielectric liquids into vessels made of glass and other insulating materials, it is necessary to use funnels made of electrically conductive materials. They are grounded and connected with copper cables to the supply hoses. Each funnel should reach the bottom of the vessel. If this is not possible, then a grounded cable is passed through the funnel, reaching to the bottom, along which the liquid will flow.
It should be remembered that the maximum electrification occurs in pipes, the material of which is mild steel. In the presence of a rough surface, static electricity appears, the protection against which is to eliminate the turbulence of the liquid that occurs during movement. To increase the electrification, the most favorable conditions that arise in certain places are necessary. Areas with less suitable conditions contribute to the loss of charges of the electrified liquid or keeping them at the same level.
The loading pipe must reach the bottom of the container when filling. The filling opening must have a large cross-section so that the jet cannot come into contact with the walls and surface of the liquid being poured. If these conditions cannot be met, it is necessary to reduce the loading speed as much as possible, bringing it to 0.5-0.7 m/s. The measures taken will ensure that unpleasant consequences are avoided.
Everyday activity of any person is connected with his movement in space. At the same time, he not only walks, but also travels by transport.
During any movement, there is a redistribution of static charges that change the balance of internal equilibrium between the atoms and electrons of each substance. It is associated with the process of electrification, the formation of static electricity.
In solids, the distribution of charges occurs due to the movement of electrons, and in liquid and gaseous - both electrons and charged ions. All of them together create a potential difference.
Reasons for the formation of static electricity
The most common examples of the manifestation of static forces are explained at school in the first physics lessons, when they rub glass and ebonite rods on woolen fabric and demonstrate the attraction of small pieces of paper to them.
Also known is the experience of deflecting a thin jet of water under the action of static charges concentrated on an ebonite rod.
In everyday life, static electricity manifests itself most often:
when wearing woolen or synthetic clothing;
walking in shoes with rubber soles or woolen socks on carpets and linoleum;
using plastic items.
The situation is aggravated:
dry indoor air;
reinforced concrete walls from which multi-storey buildings are made.
How is static charge created?
Usually the physical body contains an equal number of positive and negative particles, due to which a balance is created in it, ensuring its neutral state. When it is violated, the body acquires an electric charge of a certain sign.
Static refers to the state of rest, when the body is not moving. Inside its substance, polarization can occur - the movement of charges from one part to another or their transfer from a nearby object.
The electrification of substances occurs due to the acquisition, removal or separation of charges when:
interaction of materials due to forces of friction or rotation;
a sharp temperature drop;
irradiation in various ways;
separation or cutting of physical bodies.
They are distributed over the surface of an object or at a distance from it at several interatomic distances. For ungrounded bodies, they spread over the area of the contact layer, and for those connected to the earth contour, they flow onto it.
The acquisition of static charges by the body and their flow occurs simultaneously. Electrification is provided when the body receives more energy potential than it expends in the external environment.
A practical conclusion follows from this provision: in order to protect the body from static electricity, it is necessary to divert the acquired charges from it to the earth circuit.
Methods for assessing static electricity
According to the ability to form electric charges of different signs when interacting with other bodies by friction, physical substances are characterized according to the scale of the triboelectric effect. Some of them are shown in the picture.
The following facts can be cited as an example of their interaction:
walking in woolen socks or shoes with rubber soles on a dry carpet can charge the human body up to 5÷-6 kV;
the body of a car driving on a dry road acquires a potential of up to 10 kV;
the drive belt rotating the pulley is charged up to 25 kV.
As you can see, the potential of static electricity reaches very large values even in domestic conditions. But it does not cause us much harm because it does not have high power, and its discharge passes through the high resistance of the contact pads and is measured in fractions of a milliamp or a little more.
In addition, it significantly reduces the humidity of the air. Its effect on the amount of body stress in contact with various materials is shown in the graph.
From his analysis, the conclusion follows: in a humid environment, static electricity appears less. Therefore, various humidifiers are used to combat it.
In nature, static electricity can reach enormous levels. When clouds move over long distances, significant potentials accumulate between them, which are manifested by lightning, the energy of which is enough to split a century-old tree along the trunk or burn a residential building.
When static electricity is discharged in everyday life, we feel “pinching” of fingers, we see sparks emanating from woolen things, we feel a decrease in vigor and efficiency. The current that our body is exposed to in everyday life adversely affects the well-being, condition nervous system, but it does not cause obvious, visible damage.
Manufacturers of measuring industrial equipment produce devices that allow you to accurately determine the magnitude of the voltage of accumulated static charges both on the equipment cases and on the human body.
How to protect yourself from static electricity in your home
Each of us must understand the processes that form static discharges that pose a threat to our body. They should be known and limited. To this end, various educational events are held, including popular TV shows for the population.
They show with accessible means ways to create static voltage, the principles of its measurement and methods for implementing preventive measures.
For example, given the triboelectric effect, it is best to comb your hair with natural wood combs, rather than metal or plastic, as most people do. Wood has neutral properties and does not form charges when rubbed against hair.
To remove static potential from the body of the car when it is moving on a dry road, special tapes with antistatic agent are used, which are attached to the bottom. Their various types are widely presented on sale.
If there is no such protection on the car, then the voltage potential can be removed by briefly grounding the case through a metal object, for example, a car ignition key. It is especially important to perform this procedure before refueling.
When a static charge accumulates on clothes made of synthetic materials, it can be removed by treating vapors from a special spray can with the Antistatic composition. In general, it is better to use less such fabrics and wear natural materials made of linen or cotton.
Shoes with rubberized soles also contribute to the accumulation of charges. It is enough to put antistatic insoles made of natural materials into it, as the harmful effects on the body will be reduced.
The influence of dry air, typical for urban apartments in winter time, has already been discussed. Special humidifiers or even small pieces of moistened cloth placed on the bytarii improve the situation and reduce the formation of static electricity. But the regular performance of wet cleaning in the premises allows you to timely remove electrified particles and dust. This is one of the best defenses.
Household electrical appliances also accumulate static charges on the case during operation. The potential equalization system connected to the common ground loop of the building is designed to reduce their impact. Even a simple acrylic bathtub or an old cast iron structure with the same insert is subject to static and needs to be protected in a similar way.
How to protect against static electricity in production
Factors that reduce the performance of electronic equipment
Discharges that occur during the manufacture of semiconductor materials can cause great harm, disrupt the electrical characteristics of devices, or even disable them.
Under production conditions, the discharge can be random and depend on a number of different factors:
the values of the formed capacity;
energy potential;
electrical resistance of contacts;
type of transient processes;
other accidents.
In this case, at the initial moment of about ten nanoseconds, the discharge current increases to a maximum, and then it decreases within 100–300 ns.
The nature of the occurrence of a static discharge on a semiconductor device through the body of the operator is shown in the picture.
The magnitude of the current is influenced by: the capacity of the charge accumulated by a person, the resistance of his body and contact pads.
In the production of electrical equipment, a static discharge can be created without the participation of the operator due to the formation of contacts through grounded surfaces.
In this case, the discharge current is affected by the charge capacity accumulated by the device case and the resistance of the contact pads formed. In this case, the semiconductor at the initial moment is simultaneously affected by the induced high voltage potential and the discharge current.
Due to such a complex impact, damage can be:
1. explicit, when the performance of the elements is reduced to such an extent that they become unusable;
2. hidden - by reducing the output parameters, sometimes even falling within the established factory specifications.
The second type of malfunctions is difficult to detect: they most often affect the loss of performance during operation.
An example of such damage from the action of high static voltage is shown by the deviation graphs of the current-voltage characteristics in relation to the KD522D diode and the integrated circuit BIS KR1005VI1.
The brown line under number 1 shows the parameters of semiconductor devices before testing with increased voltage, and curves with numbers 2 and 3 show their decrease under the action of an increased induced potential. In case #3, it has more impact.
Damage can be caused by:
overestimated induced voltage, which breaks through the dielectric layer of semiconductor devices or violates the structure of the crystal;
high flowing current density, causing a high temperature, leading to the melting of materials and burning of the oxide layer;
tests, electrical thermal training.
Hidden damage may not affect performance immediately, but after several months or even years of operation.
Methods for performing protection against static electricity in production
Depending on the type of industrial equipment, one of the following methods of maintaining operability or a combination of them is used:
1. exclusion of the formation of electrostatic charges;
2. blocking their entry into the workplace;
3. Increasing the resistance of devices and accessories to the action of discharges.
Methods No. 1 and No. 2 allow you to protect a large group of various devices in a complex, and No. 3 is used for individual devices.
The high efficiency of maintaining the operability of the equipment is achieved by placing it inside the Faraday cage - a space fenced on all sides with a fine-mesh metal mesh connected to the ground loop. External electric fields do not penetrate inside it, and static magnetic fields are present.
Shielded cables work according to this principle.
Static protection is classified according to the principles of execution into:
physical and mechanical;
chemical;
structural and technological.
The first two methods allow you to prevent or reduce the formation of static charges and increase the rate of their flow. The third technique protects devices from the effects of charges, but it does not affect their drain.
You can improve the stacking of discharges by:
creating a coronation;
increasing the conductivity of materials on which charges accumulate.
Solve these questions:
air ionization;
increase in working surfaces;
selection of materials with the best bulk conductivity.
Due to their implementation, pre-prepared lines are created for draining static charges to the ground loop, preventing them from getting on the working elements of the devices. At the same time, it is taken into account that the total electrical resistance of the created path should not exceed 10 ohms.
If the materials have high resistance, then protection is performed in other ways. Otherwise, charges begin to accumulate on the surface, which can be discharged upon contact with the ground.
An example of a comprehensive electrostatic protection of the workplace for an operator involved in the maintenance and adjustment of electronic devices is shown in the picture.
The surface of the table is connected to the ground loop through a connecting conductor and a conductive mat using special terminals. The operator works in special clothes, wears shoes with conductive soles and sits on a chair with a special seat. All these measures allow high-quality removal of accumulated charges to the ground.
Working air ionizers regulate humidity, reduce the potential of static electricity. When using them, it is taken into account that the increased content of water vapor in the air adversely affects human health. Therefore, they try to maintain it at a level of about 40%.
Also, an effective way can be regular ventilation of the room or the use of a ventilation system in it, when the air passes through the filters, ionizes and mixes, thus ensuring the neutralization of the resulting charges.
To reduce the potential accumulated by the human body, bracelets can be used to complement the set of antistatic clothing and shoes. They consist of a conductive strip that is attached to the arm with a buckle. The latter is connected to the ground wire.
With this method, the current flowing through the human body is limited. Its value should not exceed one milliamp. Larger values can cause pain and electrical shock.
During the charge draining to the ground, it is important to ensure the rate of its departure in one second. For this purpose, floor coverings with low electrical resistance are used.
When working with semiconductor boards and electronic components, protection against damage by static electricity is also provided:
forced shunting of the outputs of electronic boards and blocks during checks;
using tools and soldering irons with grounded working heads.
Containers with flammable liquids located on vehicles are grounded using a metal circuit. Even the fuselage of the aircraft is supplied with metal cables, which, during landing, work as protection against static electricity.
You must have come across in school physics lessons with such a definition as static electricity. Next, we will briefly analyze what exactly this definition is about, and also share our knowledge about why it arises and how to deal with this phenomenon in everyday life and at work. So, to your attention the causes of static electricity and measures to combat it.
What it is?
The reasons for the occurrence of this natural phenomenon are quite interesting. With an incorrect balance inside an atom or inside a molecule, and as a result of the loss (acquisition) of a new electron, static electricity occurs. Normally, each atom should be in "balance" because of the equal number of protons and neutrons in it. Well, in turn, electrons, moving from atom to atom, can form negative ions or positive ions. And in the absence of balance, this natural phenomenon is obtained.
To learn more about what an electrostatic charge is and how to use it to your advantage, you can find out in this video:
What is the danger of the phenomenon?
The most important danger of static electricity is the risk of electric shock (which we will discuss below), but there is also the risk of fire. It is believed that not every production is at risk of fire, but for enterprises such as the polygraph itself, this is very dangerous, since they use solvents that are highly flammable in production.
- Energy, type and power of static discharge.
- The need for the presence of a medium that ignites easily.
The danger of this phenomenon and the rules for dealing with it are clearly demonstrated in the video example:
By the way, you should know that the negative impact of static electricity on the human body is not only in getting injured, but also in violation of the nervous system!
Causes and sources of occurrence
To date, we are confident that static electricity occurs due to several reasons, namely:
- Due to the presence of any contact between the surfaces of 2 materials with their subsequent separation from each other (for example, the friction of a rubber ball on a woolen sweater or in production when winding materials).
- The presence of ultraviolet, radiation, etc.
- With rapid temperature changes.
Most often, static electricity manifests itself in the first cause. This procedure is not completely clear, but it is the most accurate explanation of all.
It's no secret that both in production and in everyday life this phenomenon occurs more often and in order to control it, it is necessary to accurately identify the site of the problem area and take measures to protect it. Interesting fact: This phenomenon can cause a "spark" around an object that has the ability to accumulate a charge of electricity. And you ask, what is the danger of this? And the fact that with the accumulation of a large charge there is the possibility of defeating the working personnel in production. To date, only 2 main causes of static electricity shock are known.
The first reason is induced charge. If a person is in an electric field and if he holds a charged object with his hands, then the body of this person can be charged.
If this person is wearing protective boots with insulating soles, then the charge of electricity will remain in him. Could the charge be gone? Of course, the reason for this will be the moment when he touches a grounded object. It is at this moment that the worker will be defeated. electric shock(at the moment of charge leakage to the ground). The described method of receiving an electric shock is obtained by having electrically insulating shoes on his feet. After all, when you touch a charged object, because of the shoes, the charge remains in the human body, and when he touches the object designed to protect against him (grounded equipment), the charge quickly passes through the human body and “shocks” with current. The occurrence of this process is possible both in everyday life and in production, we can say that no one is protected from it. When exposed to synthetic carpets and shoes during the movement of a person, a charge of static electricity appears. Measures to combat this dangerous phenomenon in everyday life are demonstrated in the video:
Have you ever been hit by a discharge of electricity when leaving the car and still do not know what to do in this case? This occurs when your hand is exposed to a metal door due to the fact that, during the exit from the car, there is a "provocation" of charge between your clothes and the seat. Unfortunately, as mentioned earlier, the only way to get rid of this dilemma is to touch the car door so that the current through the car “goes down” to the ground through it. There is no other easier way to remove static electricity from yourself.
The second reason for the defeat of static electricity in the workplace - charging of the equipment. This type of electric shock happens quite rarely, unlike the above example.
So for your protection and so that you know how to get rid of this trouble, we will consider this whole process. Let's imagine that a certain object has an impressive charge of static electricity, it happens that your fingers have accumulated a charge in such an amount that a "breakdown" occurs and as a result of this - a discharge. So here's a little tip for you: for your protection in the workplace, you need to wear rubber gloves (just in case). We have covered everything in the corresponding article!
Measures and remedies
At a time when the question of “how to remove” the danger of static electricity and organize protection against it is on the production floor, many oilmen turn to the decision of the Gosgortekhnadzor. It is known that absolutely all equipment that is grounded can be considered protected, even if the equipment has a painted metal case.
To be honest, we have already discussed the protection of equipment from static electricity damage. How to deal with this phenomenon in the house and apartment is described in the video provided above. It is important to note that humidifiers are really good at removing static electricity. About that, we talked about in the corresponding article.
Another example of protection are windshields for cars. Strictly speaking, the stacker is just a “piece” of rubber attached to the car so that one side of it touches the car and the other the ground, a kind of “mobile ground electrode”. As a precaution, it is recommended to install drains on the car, as shown in the photo below. This will remove the electrostatic charge that could harm you.
That's all I wanted to tell you about what are the causes of static electricity and what methods of dealing with this phenomenon exist today. We hope the information was useful and interesting for you!