Formation of charges static electricity This is due to the fact that oil and petroleum products are dielectrics, and therefore, with intense friction of their particles against each other, as well as against the air, electrostatic induction occurs.
To ensure electrostatic intrinsic safety of tanks it is necessary:
- ground all their electrically conductive components and parts;
- eliminate the processes of splashing and dispersing oil (petroleum products), as well as the possibility of sparking when taking samples and measuring the liquid level in tanks;
- limit the rate of filling tanks, as well as the flow of oil (petroleum products) during erosion of bottom sediments.
Grounding devices used for protection against static electricity are combined with similar devices in electrical equipment or lightning shields. The resistance of these devices should not exceed 100 Ohms.
A reinforced concrete tank is considered electrostatically grounded if the resistance at any point on its internal and external surface relative to the ground loop does not exceed 10 7 Ohms. To avoid spark discharges, the presence of ungrounded electrically conductive floating objects (pontoons, floating roofs, level gauge floats, etc.) in tanks on the surface of oil (petroleum products) is not allowed. Their grounding is carried out by connecting to the tank body. Moreover, the pontoon or floating roof is connected to it by at least two flexible steel bridges.
The use of non-electrically conductive floating devices and items (in particular those intended to reduce the loss of oil and petroleum products from evaporation) is allowed only in agreement with a specialized organization involved in protection against static electricity.
Process pipelines and equipment located in the tank farm and on tanks must be continuous throughout their entire length. electrical circuit and are connected to the ground loop in at least two places.
To avoid splashing and atomization of oil (petroleum products), leading to the formation of static electricity charges, tanks are filled only to the level. If this is not possible (when filling tanks after flaw detection or repair), then the rate of injection of oil (petroleum products) into it should not exceed 1 m/s until the inlet-dispenser pipe in RVS type tanks is flooded and until the pontoon or floating roof floats up in tanks of types RVSP and RVSPK.
When manually sampling or measuring the level of oil (petroleum products) in the tank through the gauging hatch, these operations must be performed no earlier than 10 minutes after the pumping operation has been stopped.
- for liquids with a specific volumetric electrical resistance of no more than 10 5 Ohm m, the injection speed into the tank should be no more than 10 m/s;
- for liquids with a specific volumetric electrical resistance of no more than 10 9 Ohm m - up to 5 m/s;
- for liquids with a specific volumetric electrical resistance of more than 10 9 Ohm m, permissible transportation and outflow rates are established on the basis of special calculations.
To reduce the rate of flow of oils (petroleum products) with a specific volumetric electrical resistance above 10 9 Ohm m into tanks, it is recommended to use so-called relaxation tanks, which are a horizontal section of a pipeline with a length L e and an increased diameter D e , located directly at the inlet to the tank:
D e = D √2 W; L e = 2.2·10 -11 ·ε·ρ v ,
Where D is the diameter of the pipeline; W - fluid speed in it, m/s; ε—dielectric constant of oil (petroleum product); ρ v - specific volumetric electrical resistance of the liquid, Ohm m.
When two bodies that differ in phase state come into contact, an electric double layer is formed.
There are three reasons for the formation of an electrical double layer:
1) preferential movement of charge carriers from one body to another - diffusion;
2) absorption processes take place at the interface, when the charges of one of the phases preferentially settle on the surface of the other phase;
3) polarization of the molecules of at least one of the phases takes place. This leads to the polarization of molecules of another phase. Moreover, the polarization in the second phase can be blurred (diffuse).
The electrical double layer depends on the resistivity of the substance. The greater the resistance of the substance, the more diffuse in depth the second electrical layer is.
If we consider pumping oil, then the eroded second electrical layer can be carried away by the movement of oil and accumulate in the bunker. The higher the speed of oil movement, the greater the electrification of oil.
The magnitude of static electricity charges significantly depends on the conditions under which electrification occurs and, in particular, on the fact that the surfaces of contacting bodies may be “contaminated” with other substances. Therefore, the basis of quantitative analysis is experiment or, at best, computational and experimental research.
Technological process of oil transportation
Static charging of fuels began to manifest itself sharply around the 60s and 70s, when clean fuel began to be used to improve the operating efficiency and service life of engines. Figure 1 shows the technological chain of oil transportation.
Fig.1. Increase in charge density in oil as it passes through the path
An increase in charge density in oil occurs in technological devices where oil comes into contact with materials, leading to its charging, and where the speed of oil flow increases. A decrease in charge is observed when oil moves through grounded pipelines.
When oil moves along the technological path up to the receiving tank, there is practically no danger from the accumulation of a charge of static electricity, since there are no air gaps in the apparatus and there is no possibility of electrical breakdown in the gas. A different situation exists in the receiving tank, where there must be a gas space above the oil surface.
The charge accumulated in the receiving tank can be determined from the condition of its increase due to the flow of charged oil into the tank, taking into account the relaxation (draining) of the charge onto the grounded structures of the tank:
dQ/dt | total = dQ/dt | input + dQ/dt | relaxation
Here, charge relaxation occurs according to an exponential dependence:
Q(t) = Q 0 e -t/τ
where τ = εε 0 /γ v is the relaxation time constant, and ε and γ are the relative dielectric constant and conductivity of the oil, respectively.
dQ/dt | relax = - Q 0 /τ ⋅ e -t/τ = -Q/τ
Let's rewrite the original equation, taking into account that dQ/dt | in = I in, where I in is the current of static electricity charges at the entrance to the tank.
dQ/dt | total = I input - Q/τ
The solution to the differential equation is:
Q = I input τ(1 - e -t/τ)
In Fig. Figure 2 shows the dependence of the change in density and total volumetric charge of oil in the receiving tank. 
Fig.2. Dependence of the total volumetric charge of oil in the receiving tank on filling time
From the dependences it is clear that the rate of charge growth decreases exponentially, and the total volume charge, increasing, exponentially tends to the limiting value determined by the product I in τ.
Therefore, there are two ways to reduce the charge accumulated in the receiving tank. The first is to reduce the relaxation time constant by adding special additives to the oil that increase its conductivity. This direction was chosen by the Dutch company Shell. The disadvantage of this method is the continuous monitoring of the amount of additive in the oil and its precise dosage, since when purifying oil with filters, the additive is simultaneously removed.
The second way is to directly reduce the charge in the receiving tank. For this purpose, special devices called static electricity neutralizers are used. The diagram of the static electricity neutralizer is shown in Fig. 3. 
Fig.3. Static Eliminator
Around the needle-shaped electrodes, as a result of ionization processes, areas with an increased content of ions are formed that have a charge of the opposite sign to the excess charge of oil (in our case, positive ions). As a result of the recombination of negative and positive ions, the excess charge of the oil is reduced.
To solve the problem of preventing the ignition of oil vapors due to static electricity discharges, it is necessary to determine the magnitude and distribution of charges in the receiving tank depending on the parameters of the transportation system, calculate the field distribution and determine the possibility of discharges and ignition of vapors depending on the minimum energy required for ignition . If the likelihood of ignition is high, then neutralizers should be used or restrictions on pumping modes should be introduced (for example, pumping speed restrictions). The risk of static electricity discharges depends on the size and shape of the containers used (Fig. 4). 
Fig.4. Types of tanks
a) rectangular; b) horizontal cylindrical; c) vertical
cylindrical; d) vertical cylindrical with a central post
Ignition of oil vapors
The charge of oil entering the reservoir is unevenly distributed throughout the volume. This is due to the relaxation of the charge onto the grounded walls of the structure. Therefore, the further the volume of oil in question is from the wall of the tank, the greater the charge in the volume. In addition, on the oil surface, the charge relaxes more slowly (especially when the level approaches the upper wall of the tank) due to the influence of the large capacitance between the oil surface and the upper wall.
This means that a large charge accumulates on the surface of the oil at the point furthest from the walls of the tank, which creates an electric field between this point on the surface of the oil and the grounded walls of the tank. As the charge accumulates, the electric field strength increases up to a value equal to the value at which the discharge begins. In the developing discharge, the energy accumulated in the oil is released. In order for oil vapor to ignite, a certain energy is required equal to the minimum ignition energy. It varies for different substances:
Minimum ignition energy of steam-air
and oxygen (in parentheses) mixtures (mJ)
The energy released during the breaking of a gas gap is determined by the formula:
where, respectively, U is the voltage across the gap and i is the current flowing through the gap.
Microdischarges of static electricity do not lead to any noticeable change in voltage due to the very short duration of the discharges themselves and their low energy. Then we can approximately assume that U ≈ const. Hence
those. the energy is proportional to the amount of charge flowing through the channel.
In Fig. Figure 5 shows the dependence of the magnitude of the charges leading to the ignition of petroleum product vapors on the diameter of the grounded ball for positive and negative charges of static electricity. 
Fig.5. The igniting abilities of discharges depending on
from the diameter of the grounded ball
The ignitability of static electricity discharges is usually determined by placing a grounded spherical electrode near the surface of the liquid. It can be seen that the ignition ability of the discharges sharply decreases if the diameter of the sphere becomes less than 20 mm. The smallest value of the igniting charge corresponds to an electrode with a diameter of 20-30 mm. With negative polarity of the charge of oil and petroleum product, the ignition energy is lower than with positive polarity. In table 1 shows the parameters of fuel groups according to flammability.
Table 1. Fuel groups by flammability level
Fig. 6. Dependence of the permissible speed of pumping petroleum products on the accumulated specific charge and conductivity of petroleum products
Studies have shown that the process of filling a tank is safe if the potential on the surface of the liquid is no more than 25 kV for “-” charged fuel and no more than 54 kV for “+” charged fuel.
Based on the operating modes of oil product pumping systems and their conditions safe work, the permissible productivity is determined when a certain charge accumulates in petroleum products (Fig. 6).
ANOO "TsPPiPK "Kubansky""
Electrification of petroleum products.
Methodical manual.
Developed by: teacher A.S. Nesteruk
Krasnodar
Electrification of petroleum products. Causes and measures to protect against static electricity.
Oil and petroleum products are good dielectrics and are able to retain electrical charges for a long time. In anhydrous, pure petroleum products electrical conductivity completely insignificant. This property is widely used in practice. Thus, paraffins are used in the electromechanical industry as an insulator, and special petroleum oils are used for filling transformers, capacitors and other equipment in the electrical and radio industries.
The high dielectric properties of petroleum products contribute to the accumulation of static electricity charges on their surface. The formation of static electricity can occur from a number of different reasons.
Conducted research and a detailed study of the facts of explosions and fires from static electricity made it possible to establish a number of reasons for the formation of a charge of static electricity in the settlement:
Ø friction of liquid liquid on the solid surface of the pipeline, tank walls and filter;
Ø friction of particles between each other when fuel passes through a medium of other liquids, such as water;
Ø passage of drops of finely sprayed liquid through air or air-steam mixture;
Ø sedimentation of solid suspended particles from n/a;
Ø deposition of liquid suspended particles from n/a, for example drops of water or other chemicals, as well as when air bubbles, light hydrocarbon vapors, etc. pass through a layer of liquid liquid;
Ø passage of water droplets, snowflakes, etc. through the vapor-air space.
Experiments have established that the ability of a nanoparticle to undergo electrification during pumping depends on its electrical conductivity: the lower the electrical conductivity of a nanoparticle, the easier the charge of static electricity accumulates and the slower it dissipates. In addition, the rate of formation of static electricity is influenced by operational factors:
ü pumping speed,
ü the presence of mechanical impurities, water, air,
ü storage conditions, temperature, etc.
The higher the pumping speed, the more electrified the pump is. The longer you pump the pump, the more it becomes electrified. Great influence Mechanical impurities and air bubbles also affect the electrification of the item: the more of them, the more electrified the item is. Water dissolved or dispersed in n/a significantly increases the formation of static electricity. However, water located at the bottom of the container in the form of a separate layer either does not have any effect on the rate of formation of static electricity, or helps to reduce it.
If insulated metal containers or pipelines receive high potentials relative to the ground, then a spark discharge is possible between them and grounded objects, which can cause a fire or explosion of petroleum products and oils. To prevent the occurrence of dangerous spark discharges from the surface of oil and petroleum products, equipment, as well as from the human body, it is necessary to provide measures that reduce the amount of charge and ensure the drainage of the resulting charge of static electricity.
To reduce the intensity of the accumulation of electrical charges, petroleum products must be pumped into reservoirs, tanks, and containers without splashing, atomizing, or violent mixing. Petroleum products must be supplied to tanks below the level of the remaining petroleum product in it. Filling light petroleum products with a free-falling jet is not allowed. The distance from the end of the loading tube to the end of the receiving vessel should not exceed 200 mm, and if this is not possible, then the jet should be directed along the wall. The speed of movement of petroleum products through pipelines should not exceed the maximum permissible values, which depend on the type of operations performed, the properties of petroleum products, the content and size of insoluble impurities and the properties of the material of the pipeline walls. For petroleum products, movement and outflow speeds are allowed up to 5 m/s. When filling an empty tank, oil products must be fed into it at a speed of no more than 1 m/s until the end of the receiving and dispensing pipe is flooded.
To ensure the drainage of the resulting electrical charge, all metal parts of the equipment, pumps and pipeline communications are grounded and constant electrical contact of the human body is made with grounding. Automobile and railway tanks that are loaded and discharged with flammable petroleum products must be connected to grounding devices during the entire time of filling and emptying.
Static electricity is a set of phenomena associated with the emergence, conservation and relaxation of a free electric charge on the surface and volume of a dielectric or on insulated conductors.
4.4.1. To prevent the occurrence of spark discharges from the surface of equipment, oil and petroleum products, as well as from the human body, it is necessary to provide, taking into account the specifics of production, the following measures to ensure the drainage of the resulting charge of static electricity:
- reducing the intensity of generation of static electricity charge;
- grounding equipment for tanks and communications, as well as ensuring constant contact of the human body with grounding;
- reduction of specific volume and surface electrical resistance;
- use of radioisotope, induction and other neutralizers.
4.4.2. Grounding devices for protection against static electricity should generally be combined with grounding devices for electrical equipment. Such grounding devices must be made in accordance with the requirements of PUE-85, GOST 21130-75 SN 102-76, Instructions for the installation of grounding networks. The resistance of a grounding device intended solely for protection against static electricity is allowed to be no higher than 100 ohms.
All metallic and electrically conductive nonmetallic parts of tank equipment must be grounded, regardless of whether other ESD protection measures are in place.
Paint coating applied to grounded metal equipment, internal and external walls of tanks is considered electrostatic grounding if the resistance of the outer surface of the coating relative to the grounded equipment does not exceed 10 ohms.
4.4.3 Tanks with a capacity of more than 50 m3 (except for vertical diameters up to 2.5 m) must be connected to grounding conductors using at least two grounding conductors at diametrically opposite points.
4.4.4. Petroleum products must be pumped into tanks without splashing, atomizing or violent mixing. Filling of petroleum products with a free-falling jet is not allowed.
The distance from the end of the loading pipe to the bottom of the tank should not exceed 200 mm, and if possible, the jet should be directed along the wall. In this case, the shape of the pipe end and the rate of supply of the petroleum product must be selected in such a way as to prevent splashing.
4.4.5. The speed of movement of petroleum products through pipelines must be limited in such a way that the charge brought into the tank with the flow of petroleum product cannot cause a spark discharge from its surface, the energy of which is sufficient to ignite environment. The permissible speeds of liquid movement through pipelines and their flow into tanks depend on the following conditions affecting the relaxation of charges: type of filling, properties of the petroleum product, content and size of insoluble impurities, properties of the material of the walls of the pipeline and tank.
4.4.6. For petroleum products with a specific volumetric electrical resistance of no more than 10 9 Ohms. m, movement and outflow speeds are allowed up to 5 m/s.
For petroleum products with a specific volumetric electrical resistance of more than 10 9 Ohm.m, permissible transportation and outflow rates are established for each petroleum product separately.
To reduce the charge density in a flow of liquid having a specific volumetric electrical resistance of more than 10 9 Ohm.m to a safe value, if it is necessary to transport them through pipelines at a speed exceeding the safe one, special charge removal devices should be used.
A device for removing charges from a liquid product must be installed on the loading pipeline directly at the entrance to the tank being filled so that, at the maximum transport speed used, the time the product moves through the loading pipe after exiting the device before flowing into the apparatus does not exceed 0.1 of the charge relaxation time constant in liquid.
If this condition cannot be fulfilled structurally, then the removal of the charge arising in the loading pipe must be ensured inside the tank being filled before the charged flow reaches the surface of the liquid in the tank.
Notes. Neutralizers with strings can be used as devices for removing charge from a liquid product, the rules for the selection, design, installation and operation of which are set out in RTM 6.28-008-78 Devices for removing charge from a liquid flow with extended discharge electrodes (neutralizers with strings).
Cages made of a grounded metal mesh can be used as charge removal devices inside the filled tank, covering a certain volume at the end of the loading pipe so that the charged flow from the pipe enters the cell. In this case, the volume of the cell must be at least V = Q τ /3600, where V is the volume of the cell, m 3 ; Q—petroleum product pumping speed, m 3 /h; τ is the charge relaxation time constant in the oil product, s.
4.4.7. Data on the electrical parameters of light petroleum products and nomograms for determining permissible pumping speeds are given in the Recommendations for the prevention of dangerous electrification of petroleum products when loading into vertical and horizontal tanks, road and rail tanks, approved on November 12, 1985 by the State Oil Product Committee of the RSFSR.
4.4.8. Petroleum products must enter the tank below the level of the remaining petroleum product in it.
When filling an empty tank, oil products must be fed into it at a speed of no more than 1 m/s until the end of the receiving and dispensing pipe is flooded.
For further filling, the speed should be selected taking into account the requirements of clause 4.4.6.
4.4.9. To prevent the risk of spark discharges, there should be no ungrounded electrically conductive floating objects on the surface of oil products.
4.4.10. Pontoons made of electrically conductive materials, designed to reduce losses of petroleum products from evaporation, must be grounded using at least two flexible grounding conductors with a cross-sectional area of at least 6 mm 2 connected to the pontoon at diametrically opposite points.
4.4.11. Pontoons made of non-electrically conductive materials must have electrostatic protection.
4.4.12. Manual sampling of petroleum products from tanks is allowed no earlier than 10 minutes after the movement of the petroleum product ceases.
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