Electrochemical protection of pipelines against corrosion. Big encyclopedia of oil and gas

Corrosion underground pipelines and protection from it

Corrosion of underground pipelines is one of the main reasons for their depressurization due to the formation of cavities, cracks and ruptures. Corrosion of metals, i.e. their oxidation is the transition of metal atoms from a free state to a chemically bound, ionic state. In this case, metal atoms lose their electrons, and oxidizing agents accept them. On an underground pipeline, due to the heterogeneity of the pipe metal and due to the heterogeneity of the soil (both in terms of physical properties and chemical composition), sections with different electrode potential appear, which causes the formation of galvanic corrosion. The most important types of corrosion are: surface (continuous over the entire surface), local in the form of shells, pitting, crevice and fatigue corrosion cracking. The last two types of corrosion are the most dangerous for underground pipelines. Surface corrosion is only rare cases causes damage, while pitting causes the most damage. The corrosion situation in which a metal pipeline is located in the ground depends on a large number of factors related to soil and climatic conditions, route features, and operating conditions. These factors include:

soil moisture,
chemical soil composition,
soil electrolyte acidity,
ground structure,
transported gas temperature

The strongest negative manifestation of stray currents in the ground, caused by electrified direct current rail transport, is the electrocorrosive destruction of pipelines. The intensity of stray currents and their impact on underground pipelines depends on factors such as:

contact resistance rail-to-ground;
longitudinal resistance of running rails;
distance between traction substations;
current consumption by electric trains;
number and section of suction lines;
specific electrical resistance of soil;
distance and location of the pipeline relative to the path;
transitional and longitudinal resistance of the pipeline.

It should be noted that stray currents in the cathodic zones have a protective effect on the structure, therefore, in such places, cathodic protection of the pipeline can be carried out without large capital costs.

Methods for protecting underground metal pipelines from corrosion are divided into passive and active.

The passive method of corrosion protection involves the creation of an impenetrable barrier between the metal of the pipeline and the surrounding soil. This is achieved by applying special protective coatings to the pipe (bitumen, coal tar pitch, polymer tapes, epoxy resins, etc.).

In practice, it is not possible to achieve complete continuity of the insulating coating. Different kinds coatings have different diffusion permeability and therefore provide different insulation of the pipe from the environment. During construction and operation, cracks, scuff marks, dents and other defects occur in the insulating coating. The most dangerous are penetrating damage. protective coating, where, in practice, ground corrosion occurs.

Since the passive method fails to provide complete protection of the pipeline against corrosion, active protection is simultaneously applied, associated with the control of electrochemical processes occurring at the interface between the pipe metal and ground electrolyte. This protection is called comprehensive protection.

The active method of corrosion protection is carried out by cathodic polarization and is based on a decrease in the rate of dissolution of the metal as its corrosion potential shifts to more negative values than the natural potential. Empirically established that the value of the potential cathodic protection steel is minus 0.85 volts relative to the copper sulfate reference electrode. Since the natural potential of steel in the soil is approximately equal to -0.55 ... -0.6 Volts, then for the implementation of cathodic protection it is necessary to shift the corrosion potential by 0.25 ... 0.30 Volts in the negative direction.

Applying an electric current between the metal surface of the pipe and the ground, it is necessary to achieve a decrease in the potential in defective places of the pipe insulation to a value below the protective potential criterion, equal to - 0.9 V. As a result, the corrosion rate is significantly reduced.

2. Cathodic protection installations
Cathodic protection of pipelines can be carried out in two ways:

the use of magnesium sacrificial anode protectors (galvanic method);
the use of external DC sources, the minus of which is connected to the pipe, and the plus to the anode ground (electrical method).

The galvanic method is based on the fact that different metals in the electrolyte have different electrode potentials. If you form a galvanic pair of two metals and place them in an electrolyte, then the metal with a more negative potential will become the anode and will be destroyed, thereby protecting the metal with a less negative potential. In practice, protectors made of magnesium, aluminum and zinc alloys are used as sacrificial galvanic anodes.

The use of cathodic protection using protectors is effective only in low-resistance soils (up to 50 Ohm-m). In high-resistivity soils, this method does not provide the necessary protection. Cathodic protection by external current sources is more complex and time-consuming, but it does not depend much on the resistivity of the soil and has an unlimited energy resource.

As a direct current source, as a rule, converters of various designs are used, powered by an alternating current network. Converters allow you to adjust the protective current over a wide range, ensuring the protection of the pipeline in any conditions.

As power sources for cathodic protection installations are used air lines 0.4; 6; 10 kV. The protective current imposed on the pipeline from the converter and creating a potential difference "pipe-to-ground" is distributed unevenly along the length of the pipeline. Therefore, the maximum absolute value of this difference is at the point of connection of the current source (drainage point). As you move away from this point, the potential difference "pipe-to-ground" decreases. Excessive overestimation of the potential difference adversely affects the adhesion of the coating and can cause hydrogen saturation of the pipe metal, which can cause hydrogen cracking. Cathodic protection is one of the methods for combating metal corrosion in aggressive chemical environments. It is based on the transfer of the metal from the active state to the passive state and maintaining this state with the help of an external cathode current. To protect underground pipelines from corrosion along the route of their occurrence, cathodic protection stations (CPS) are being built. The structure of the SKZ includes a direct current source (protective installation), anode grounding, a control and measuring point, connecting wires and cables. Depending on the conditions, protective installations can be powered by AC 0.4; 6 or 10 kV or from independent sources. When protecting multi-line pipelines laid in one corridor, several installations can be installed and several anode groundings can be built. However, taking into account the fact that during breaks in the operation of the protection system, due to the difference in natural potentials of pipes connected by a dead bridge, powerful galvanic couples are formed, leading to intense corrosion, the pipes should be connected to the installation through special blocks joint protection. These blocks not only separate the pipes from each other, but also allow you to set the optimal potential on each pipe. As sources of direct current for cathodic protection at RMS, converters are mainly used, which are powered by a 220 V power frequency network. The output voltage of the converter is adjusted manually, by switching the transformer winding taps, or automatically, using controlled valves (thyristors). If cathodic protection installations operate under time-varying conditions, which may be due to the influence of stray currents, changes in soil resistivity or other factors, then it is advisable to provide converters with automatic output voltage regulation. Automatic regulation can be carried out according to the potential of the protected structure (potentiostat converters) or according to the protection current (galvanostat converters).

3. Drainage protection installations

Electrical drainage is the simplest type of active protection that does not require a power source, since the pipeline is electrically connected to the traction rails of the stray current source. The source of protective current is the potential difference between the pipeline and the rail, resulting from the operation of electrified railway transport and the presence of a stray current field. The flow of the drain current creates the required potential shift in the underground pipeline. As a rule, fuses are used as a protective device, however, automatic maximum load switches with a return are also used, that is, restoring the drainage circuit after a current that is dangerous for the installation elements has fallen. As a polarized element, valve blocks are used, assembled from several avalanche silicon diodes connected in parallel. The regulation of the current in the drainage circuit is carried out by changing the resistance in this circuit by switching active resistors. If the use of polarized electrical drains is ineffective, then reinforced (forced) electrical drains are used, which are a cathodic protection installation, the anode ground electrode of which is the rails of an electrified railway. The forced drainage current operating in the cathodic protection mode should not exceed 100A, and its use should not lead to the appearance of positive potentials of the rails relative to the ground in order to exclude corrosion of the rails and rail fasteners, as well as the structures attached to them.

It is allowed to connect electrical drainage protection to the rail network directly only to the middle points of the track choke-transformers through two to the third throttling point. More frequent connection is allowed if a special protective device. As such a device, a choke can be used, the input impedance of which to the signal current of the main signaling system railways frequency of 50 Hz is at least 5 ohms.

4. Installations of galvanic protection

Installations of galvanic protection (tread installations) are used for cathodic protection of underground metal structures in cases where the use of installations powered by external current sources is not economically feasible: lack of power lines, short length of the facility, etc.

Typically, cathodic installations are used for cathodic protection of the following underground structures:

tanks and pipelines that do not have electrical contacts with adjacent extended communications;
individual sections of pipelines that are not provided with a sufficient level of protection against converters;
sections of pipelines electrically cut off from the main by insulating joints;
steel protective covers(cartridges), underground tanks and tanks, steel supports and piles and other concentrated objects;
the linear part of the main pipelines under construction before the commissioning of permanent cathodic protection installations.

Sufficiently effective protection with tread installations can be carried out in soils with a specific electrical resistance of not more than 50 Ohm.

5. Installations with extended or distributed anodes.

As already noted, when using traditional scheme cathodic protection, the distribution of the protective potential along the pipeline is uneven. The uneven distribution of the protective potential leads to both excessive protection near the drainage point, i.e. to non-productive consumption of electricity, and to a decrease in the protective zone of the installation. This disadvantage can be avoided by using a scheme with extended or distributed anodes. The technological scheme of ECP with distributed anodes allows to increase the length of the protective zone in comparison with the scheme of cathodic protection with lumped anodes, and also provides a more uniform distribution of the protective potential. When applying the technological scheme of the ZKhZ with distributed anodes, various layouts of anode grounding can be used. The simplest is the scheme with anode grounds evenly installed along the gas pipeline. The protective potential is adjusted by changing the anode grounding current using an adjusting resistance or any other device that ensures the current changes within the required limits. In the case of grounding from several grounding switches, the protective current can be adjusted by changing the number of connected grounding switches. In general, the earth electrodes closest to the converter should have a higher contact resistance. Protective protection Electrochemical protection using protectors is based on the fact that due to the potential difference between the protector and the protected metal in an electrolyte medium, the metal is reduced and the protector body is dissolved. Since the bulk of metal structures in the world is made of iron, metals with a more negative electrode potential than iron can be used as a protector. There are three of them - zinc, aluminum and magnesium. The main difference between magnesium protectors is the greatest potential difference between magnesium and steel, which has a beneficial effect on the radius of protective action, which ranges from 10 to 200 m, which allows the use of a smaller number of magnesium protectors than zinc and aluminum. In addition, magnesium and magnesium alloys, unlike zinc and aluminum, do not have polarization accompanied by a decrease in current output. This feature determines the main application of magnesium protectors for the protection of underground pipelines in soils with high resistivity.

Corrosion is a chemical and electrochemical reaction of a metal with its environment, causing damage to it. It flows at different speeds, which can be reduced. From a practical point of view, anticorrosive cathodic protection of metal structures in contact with the ground, water and transported media is of interest. The outer surfaces of pipes are especially damaged by the influence of soil and stray currents.

Inside corrosion depends on the properties of the medium. If it is a gas, it must be thoroughly cleaned from moisture and aggressive substances: hydrogen sulfide, oxygen, etc.

Principle of operation

Process objects electrochemical corrosion are the medium, the metal and the interface between them. The medium, which is usually moist soil or water, has good electrical conductivity. An electrochemical reaction takes place at the interface between it and the metal structure. If the current is positive (anode electrode), the iron ions pass into the surrounding solution, resulting in a mass loss of the metal. The reaction causes corrosion. With a negative current (cathode electrode), these losses are absent, since electrons pass into the solution. The method is used in electroplating for coating steel with non-ferrous metals.

Cathodic corrosion protection is achieved when a negative potential is applied to an iron object.

To do this, an anode electrode is placed in the ground and a positive potential is connected to it from a power source. The minus is applied to the protected object. Cathodic-anodic protection leads to active corrosion destruction of only the anode electrode. Therefore, it should be changed periodically.

Negative effect of electrochemical corrosion

Corrosion of structures can occur from the action of stray currents from other systems. They are useful for target objects, but cause significant damage to nearby structures. Stray currents can spread from the rails of electrified vehicles. They pass towards the substation and enter the pipelines. When leaving them, anode sections are formed, causing intense corrosion. For protection, electrical drainage is used - a special removal of currents from the pipeline to their source. It is also possible here. For this, it is necessary to know the magnitude of the stray currents, which is measured by special devices.

Based on the results of electrical measurements, a method of protecting the gas pipeline is selected. A universal remedy is a passive method of contact with the ground using insulating coatings. Cathodic protection of the gas pipeline refers to the active method.

Pipeline protection

Structures in the ground are protected from corrosion if the minus of a direct current source is connected to them, and the plus is connected to anode electrodes buried nearby in the ground. The current will go to the structure, protecting it from corrosion. In this way, cathodic protection of pipelines, tanks or pipelines located in the ground is carried out.

The anode electrode will degrade and should be replaced periodically. For a tank filled with water, the electrodes are placed inside. In this case, the liquid will be the electrolyte through which the current will flow from the anodes to the surface of the container. The electrodes are well controlled and easy to change. It is more difficult to do this in the ground.

Source of power

Near oil and gas pipelines, in heating and water supply networks that require cathodic protection, stations are installed from which voltage is supplied to objects. If they are placed outdoors, their degree of protection must be at least IP34. For dry rooms, any is suitable.

Cathodic protection stations for gas pipelines and other large structures have a capacity of 1 to 10 kW.

Their energy parameters primarily depend on the following factors:

resistance between soil and anode;
soil electrical conductivity;
length of the protective zone;
insulating effect of the coating.

Traditionally, a cathodic protection converter is a transformer installation. Now it is being replaced by an inverter one, which has smaller dimensions, better current stability and greater efficiency. In important areas, controllers are installed that have the functions of regulating current and voltage, equalizing protective potentials, etc.

The equipment is on the market in various options. For specific needs, the one providing the best operating conditions is used.

Current source parameters

For corrosion protection for iron, the protective potential is 0.44 V. In practice, it should be higher due to the influence of inclusions and the condition of the metal surface. The maximum value is 1 V. In the presence of coatings on the metal, the current between the electrodes is 0.05 mA/m 2 . If the insulation is broken, it rises to 10 mA/m 2 .

Cathodic protection is effective in combination with other methods, since less electricity is consumed. If the surface of the structure has paintwork, only places where it is broken are protected by the electrochemical method.

Features of cathodic protection

Stations or mobile generators serve as power sources.
The location of the anode ground electrodes depends on the specifics of the pipelines. The placement method can be distributed or concentrated, as well as located at different depths.
The anode material is chosen with low solubility to last for 15 years.
The protective field potential for each pipeline is calculated. It is not regulated if there are no protective coatings on the structures.

Gazprom standard requirements for cathodic protection

Action during the entire period of operation of protective equipment.
Protection against atmospheric surges.
Placement of the station in block-boxes or separately standing in anti-vandal design.
Anode grounding is selected in areas with a minimum electrical resistance of the soil.
The characteristics of the transducer are selected taking into account the aging of the protective coating of the pipeline.

Protective protection

The method is a type of cathodic protection with the connection of electrodes made of a more electronegative metal through an electrically conductive medium. The difference lies in the absence of an energy source. The tread absorbs corrosion by dissolving into the electrically conductive environment.

After a few years, the anode should be replaced as it wears out.

The effect of the anode increases with a decrease in its contact resistance with the medium. Over time, it can become covered with a corrosive layer. This leads to a breakdown in electrical contact. If the anode is placed in a mixture of salts, which ensures the dissolution of corrosion products, the efficiency increases.

Protector influence is limited. The radius of action is determined by the electrical resistance of the medium and the potential difference between

Protective protection is used in the absence of energy sources or when their use is not economically feasible. It is also disadvantageous in acidic applications due to the high dissolution rate of the anodes. Protectors are installed in water, in soil or in a neutral environment. Anodes are usually not made of pure metals. The dissolution of zinc occurs unevenly, magnesium corrodes too quickly, and a strong film of oxides forms on aluminum.

Tread materials

In order for the protectors to have the necessary performance properties, they are made from alloys with the following alloying additives.

Zn + 0.025-0.15% Cd + 0.1-0.5% Al - protection of equipment located in sea water.
Al + 8% Zn +5% Mg + Cd, In, Gl, Hg, Tl, Mn, Si (fractions of a percent) - operation of structures in flowing sea water.
Mg + 5-7% Al + 2-5% Zn - protection of small structures in soil or in water with a low salt concentration.

Incorrect use of some types of protectors leads to negative consequences. Magnesium anodes can cause equipment cracking due to the development of hydrogen embrittlement.

Joint sacrificial cathodic protection with anti-corrosion coatings increases its effectiveness.

The distribution of the protective current is improved, and significantly fewer anodes are required. One magnesium anode protects a bitumen-coated pipeline for a length of 8 km, and without a coating - only 30 m.

Protection of car bodies from corrosion

In case of violation of the coating, the thickness of the car body can decrease in 5 years to 1 mm, i.e., rust through. Restoration of the protective layer is important, but in addition to it, there is a way to completely stop the corrosion process using cathodic-protective protection. If you turn the body into a cathode, the corrosion of the metal stops. Anodes can be any conductive surfaces located nearby: metal plates, ground loop, garage body, wet road surface. In this case, the protection efficiency increases with an increase in the area of the anodes. If the anode is a road surface, a "tail" of metallized rubber is used to contact it. It is placed opposite the wheels so that splashes get better. "Tail" is isolated from the body.

The battery plus is connected to the anode through a 1 kΩ resistor and an LED connected in series with it. When the circuit is closed through the anode, when the minus is connected to the body, in normal mode the LED barely noticeably glows. If it burns brightly, then a short circuit has occurred in the circuit. The cause must be found and eliminated.

For protection, a fuse must be installed in series in the circuit.

When the car is in the garage, it is connected to a grounding anode. During the movement, the connection occurs through the "tail".

Conclusion

Cathodic protection is a way to improve the operational reliability of underground pipelines and other structures. At the same time, its negative impact on adjacent pipelines from the influence of stray currents should be taken into account.

Underground pipelines are exposed to the destructive effect of corrosion. Corrosion of the pipeline affects metal pipes if conditions arise when metal atoms can go into the state of an ion.

In order for a neutral atom to become an ion, it is necessary to give up an electron, and this is possible if there is an anode that will accept it. Such a situation is possible when a potential difference occurs between individual sections of the pipe: one section is the anode, the other is the cathode.

Reasons for the occurrence of electrolytic reactions

There are several reasons for the formation of a potential difference (the value of its value) in certain sections of the pipe:

different soil compositions according to physical and chemical properties;
heterogeneity of the metal;
soil moisture;
the value of the operating temperature of the transported substance;
soil electrolyte acidity index;
the passage of an electric transport line that creates stray currents.

Important! The areas that require the establishment of protection are determined at the design stage of the facility. All necessary structures are being built in parallel with the laying of pipes.

As a result, two types of corrosion damage can occur:

surface, which does not lead to the destruction of the pipeline;
local, as a result of which shells, cracks, cracks are formed.

Types of corrosion protection

To protect pipes from destruction, corrosion protection of pipelines is used.

There are two main ways to protect:

passive, in which a protective shell is created around the pipes, completely separating them from the ground. Usually it is a bitumen coating, epoxy resin, polymer tape;
active, which allows you to control electrochemical processes that occur at the points of contact between the pipe and the ground electrolyte.

The active method is divided into three types of protection:

cathode;
tread;
drainage.

Drainage protects pipelines from corrosion produced by stray currents. Such currents are diverted towards the source that creates them or directly into the soil layer. Drainage can be earth (grounding of the anode zones of the pipeline), direct (disconnection from the negative pole of the stray current source). Less commonly used drainage is polarized and reinforced.

Methods for organizing cathodic protection

Cathodic protection of the pipeline against corrosion is formed if an external electric field is used to organize the cathodic polarization of the pipeline, and the damage is transferred to the external anode, which will be destroyed.

The cathode is divided into two types:

galvanic using anode protectors, for the manufacture of which alloys of magnesium, aluminum, zinc are used;
electric, which uses an external DC source with a connection diagram: minus to the pipe, plus to the grounded anode.

The basis of the galvanic method of cathodic protection: the use of the property of the metal to have different potentials when used as an electrode. If the electrolyte contains two metals with different meaning potential, then the one that is of lesser value will be destroyed.

The material for the tread is selected so that certain requirements are met:

negative potential with a large value in comparison with the potential of the pipeline;
significant efficiency;
high rate of specific current output;
low anodic polarizability, so that oxide films do not form.

Note! The highest efficiency for anodes made of an alloy of zinc and aluminum, the lowest - for magnesium.

To increase the efficiency and effectiveness of protection, the protectors are immersed in an activator, which reduces the protector's own corrosion and the amount of resistance to current spreading from the protector, and reduces the anodic polarizability.

The protector protective installation consists of a protector, an activator, a conductor connecting the protector and the pipeline, a point for monitoring and measuring electrical parameters.

The effectiveness of tread protection against corrosion of pipelines depends on the value of soil resistivity. It works well if this indicator does not exceed 50 Ohm * m, with greater meaning protection will be partial. To increase efficiency, tape protectors are used.

The limitation for the use of sacrificial protection is the electrical contact of the pipeline and adjacent extended communications.

Cathodic protection stations

More difficult to organize, but the most effective is electric. For its organization, an external source of direct current is being built - a cathodic protection station. In the power plant is converted alternating current into permanent.

Elements of cathodic protection:

anode grounding;
DC connection line;
protective grounding;
direct current source;
cathode terminal.

The electrical method is analogous to the electrolysis process.

Under the action of the external field of the current source, the valence electrons move away from the anode ground to the current source and the pipe. The grounded anode is gradually destroyed. And at the pipeline from a direct current source, the incoming excess of free electrons leads to depolarization (as at the cathode during electrolysis).

To prevent corrosive destruction of several pipes, several stations are constructed and an appropriate number of anodes are installed.

Allow to extend service life metal structure, as well as to preserve its technical and physical properties during operation. Despite the variety of methods for providing anti-corrosion action, it is possible to completely protect objects from rust damage only in rare cases.

The effectiveness of such protection depends not only on the quality of the tread technology, but also on the conditions of its application. In particular, in order to preserve the metal structure of pipelines, their best properties demonstrates electrochemical corrosion protection based on the operation of cathodes. The prevention of rust formation on such communications is, of course, not the only area of application of this technology, but in terms of the combination of characteristics, this direction can be considered as the most relevant for electrochemical protection.

General information about electrochemical protection

The protection of metals from rust by electrochemical action is based on the dependence of the size of the material on the rate of the corrosion process. Metal structures must be operated in the range of potentials where their anodic dissolution will be below the permissible limit. The latter, by the way, is determined by the technical documentation for the operation of the facility.

In practice, electrochemical corrosion protection involves connecting to finished product source from direct current. Electric field on the surface and in the structure of the protected object, it forms the polarization of the electrodes, due to which the process of corrosion damage is also controlled. In essence, the anode zones on the metal structure become cathodic, which allows negative processes to be displaced, ensuring the preservation of the structure of the target object.

How Cathodic Protection Works

There is cathodic and anode protection of the electrochemical type. However, the first concept, which is used to protect pipelines, has gained the greatest popularity. By general principle, when implementing this method the object is supplied with a current with a negative pole from an external source. In particular, a steel or copper pipe can be protected in this way, as a result of which the polarization of the cathode sections will occur with the transition of their potentials to the anode state. As a result, the corrosive activity of the protected structure will be reduced to almost zero.

At the same time, cathodic protection can have different variants execution. The above-described technique of polarization from an external source is widely practiced, but the electrolyte deaeration method with a decrease in the rate of cathodic processes, as well as the creation of a protective barrier, also works effectively.

It has been noted more than once that the principle of cathodic protection is implemented by means of an external current source. Actually, the main function lies in its work. Special stations perform these tasks, which, as a rule, are part of the general infrastructure. Maintenance pipelines.

Stations against corrosion

The main function of the cathode station is to provide stable current to the target metal object in accordance with the cathodic polarization method. Such equipment is used in the infrastructure of underground gas and oil pipelines, in water supply pipes, heating networks, etc.

There are many varieties of such sources, while the most common cathodic protection device provides for the presence of:

current converter equipment;
wires for connecting to the protected object;
anode grounding.

At the same time, there is a division of stations into inverter and transformer ones. There are other classifications, but they are focused on the segmentation of installations, either by application, or by technical characteristics and input data parameters. Basic principles The works most clearly illustrate the designated two types of cathode stations.

Transformer plants for cathodic protection

It should immediately be noted that this species stations is obsolete. It is being replaced by inverter analogues, which have both pluses and minuses. One way or another, transformer models are used even at new points for providing electrochemical protection.

A 50 Hz low-frequency transformer is used as the basis for such objects, and the simplest devices are used for the thyristor control system, including phase-pulse power controllers. A more responsible approach to solving control problems involves the use of controllers with wide functionality.

Modern cathodic corrosion protection of pipelines with such equipment allows you to adjust the parameters of the output current, voltage indicators, as well as equalize the protective potentials. As for the shortcomings of transformer equipment, they boil down to high degree output current ripple at low power factor. This defect is explained not by the sinusoidal form of the current.

To a certain extent, the introduction of a low-frequency choke into the system allows solving the problem with ripple, but its dimensions correspond to the dimensions of the transformer itself, which does not always make such an addition possible.

Cathodic protection inverter station

Inverter-type installations are based on pulsed high-frequency converters. One of the main advantages of using stations of this type is high efficiency, reaching 95%. For comparison, for transformer installations, this figure reaches an average of 80%.

Sometimes other advantages come to the fore. For example, the small dimensions of inverter stations expand the possibilities for their use in difficult areas. There are also financial advantages, which are confirmed by the practice of using such equipment. So, inverter cathodic protection against corrosion of pipelines quickly pays off and requires minimum investment into technical content. However, these qualities are clearly visible only when compared with transformer installations, but today there are more effective new means of providing current for pipelines.

Structures of cathode stations

Such equipment is presented on the market in different cases, shapes and dimensions. Of course, the practice of individual design of such systems is also widespread, which makes it possible not only to obtain an optimal design for specific needs, but also to provide the necessary operational parameters.

A rigorous calculation of the characteristics of the station allows further optimization of the costs of its installation, transportation and storage. For example, cathodic protection against corrosion of pipelines based on an inverter with a mass of 10-15 kg and a power of 1.2 kW is quite suitable for small objects. Equipment with such characteristics can be serviced by a car, however, for large-scale projects, more massive and heavy stations can be used, requiring the connection of trucks, a crane and installation teams.

Protective functionality

Particular attention in the development of cathode stations is paid to the protection of the equipment itself. For this, systems are integrated that allow protecting stations from short circuits and load interruptions. In the first case, special fuses are used to handle emergency operation of the installations.

With regard to power surges and breaks, the cathodic protection station is unlikely to be seriously affected by them, but there may be a risk of electric shock. For example, if in normal mode the equipment is operated with a low voltage, then after a break, the jump in indicators can be brought up to 120 V.

Other types of electrochemical protection

In addition to cathodic protection, electrical drainage technologies are also practiced, as well as tread methods for preventing corrosion. Most promising direction It is considered to be a special protection against the formation of corrosion. In this case, active elements are also connected to the target object, which ensure the transformation of the surface with cathodes by means of current. For example, a steel pipe as part of a gas pipeline can be protected by zinc or aluminum cylinders.

Conclusion

Methods of electrochemical protection cannot be attributed to new and, moreover, innovative. The effectiveness of the use of such techniques in the fight against rusting processes has been mastered for a long time. But, widespread This method is hindered by one serious drawback. The fact is that cathodic corrosion protection of pipelines inevitably produces the so-called They are not dangerous for the target structure, but can have a negative impact on nearby objects. In particular, stray current contributes to the development of the same corrosion on metal surface adjacent pipes.

Cathodic protection stations (CPS) are a necessary element of the system of electrochemical (or cathodic) protection (ECP) of underground pipelines against corrosion. When choosing a VCS, they most often proceed from the lowest cost, ease of maintenance and qualifications of their service personnel. The quality of the purchased equipment is usually difficult to assess. The authors propose to consider the data indicated in the passports technical specifications RMS, which determine how well the main task of cathodic protection will be performed.

The authors did not pursue the goal of expressing themselves in a strictly scientific language in the definition of concepts. In the process of communicating with the personnel of the ECP services, we realized that it is necessary to help these people systematize the terms and, more importantly, give them an idea of what is happening both in the power grid and in the VCS itself.

ECP task

Cathodic protection is carried out when an electric current flows from the RMS through a closed electrical circuit formed by three resistors connected in series:

· soil resistance between pipeline and anode; I anode spreading resistance;

pipeline insulation resistance.

The soil resistance between the pipe and the anode can vary widely depending on the composition and external conditions.

The anode is an important part of the ECP system, and serves as the consumable element, the dissolution of which provides the very possibility of ECP implementation. Its resistance during operation steadily grows due to dissolution, reduction effective area working surface and the formation of oxides.

Consider the metal pipeline itself, which is the protected element of the ECP. The metal pipe is covered with insulation on the outside, in which cracks form during operation due to mechanical vibrations, seasonal and daily temperature changes, etc. Moisture penetrates through the cracks in the hydro- and thermal insulation of the pipeline and the metal of the pipe contacts the ground, thus forming a galvanic couple that contributes to the removal of metal from the pipe. The more cracks and their sizes, the more metal is carried out. Thus, galvanic corrosion occurs, in which a current of metal ions flows, i.e. electricity.

Since the current is flowing, then a wonderful idea arose to take an external current source and turn it on to meet this very current, due to which the removal of metal and corrosion occurs. But the question arises: what is the magnitude of this most man-made current to give? It seems to be such that plus to minus gives zero metal removal current. And how to measure this same current? The analysis showed that the tension between metal pipe and soil, i.e. on both sides of the insulation, must be between -0.5 and -3.5 V (this voltage is called the protective potential).

The task of the VHC

The task of the SKZ is not only to provide current in the ECP circuit, but also to maintain it in such a way that the protective potential does not go beyond the accepted limits.

So, if the insulation is new, and it has not had time to get damaged, then its resistance electric current high and need a small current to maintain the desired potential. As the insulation ages, its resistance decreases. Consequently, the required compensating current from the RMS increases. It will increase even more if cracks appear in the insulation. The station must be able to measure the protective potential and change its output current accordingly. And nothing more, from the point of view of the ECP task, is required.

SKZ operating modes

There are four modes of operation of the ECP:

without stabilization of output values of current or voltage;

I stabilize the output voltage;

stabilization of the output current;

· I stabilization of the protective potential.

Let's say right away that in the accepted range of changes of all influencing factors, the fulfillment of the ECP task is fully ensured only when using the fourth mode. Which is accepted as the standard for the operating mode of the SKZ.

The potential sensor gives the station information about the potential level. The station changes its current in the right direction. Problems begin from the moment when it is necessary to put this very potential sensor. You need to put it in a certain calculated place, you need to dig a trench for the connecting cable between the station and the sensor. Anyone who laid any communications in the city knows what a hassle it is. Plus, the sensor requires periodic maintenance.

In conditions where there are problems with the potential feedback mode, proceed as follows. When using the third mode, it is assumed that the state of the insulation changes little in the short term and its resistance remains practically stable. Therefore, it is enough to ensure the flow of a stable current through a stable insulation resistance, and we get a stable protective potential. In the medium and long term, the necessary adjustments can be made by a specially trained lineman. The first and second regimes do not impose high requirements on the SKZ. These stations are simple in execution and, as a result, cheap, both in manufacture and in operation. Apparently, this circumstance determines the use of such SCs in the ECP of objects located in conditions of low corrosive activity of the environment. If the external conditions (insulation state, temperature, humidity, stray currents) change to the limits when an unacceptable mode is formed on the protected object, these stations cannot perform their task. To adjust their mode, the frequent presence of maintenance personnel is necessary, otherwise the ECP task is partially performed.

Characteristics of SKZ

First of all, the VHC must be selected based on the requirements set out in normative documents. And, probably, the most important thing in this case will be GOST R 51164-98. Appendix "I" of this document states that the efficiency of the station must be at least 70%. The level of industrial noise generated by RMS should not exceed the values specified by GOST 16842, and the level of harmonics at the output should comply with GOST 9.602.

The SKZ passport usually indicates: I rated output power;

Efficiency at rated output power.

Rated output power - the power that the station can deliver at rated load. Typically this load is 1 ohm. The efficiency is defined as the ratio of the rated output power to the active power consumed by the station in the rated mode. And in this mode, the efficiency is the highest for any station. However, most VCSs operate far from the nominal mode. The power load factor ranges from 0.3 to 1.0. In this case, the real efficiency for most stations manufactured today will drop noticeably with a decrease in output power. This is especially noticeable for transformer SKZ using thyristors as a regulating element. For transformerless (high-frequency) RMS, the drop in efficiency with a decrease in output power is much less.

General view of the change in efficiency for RMS different execution can be seen in the figure.

From fig. it can be seen that if you use the station, for example, with a nominal efficiency of 70%, then be prepared for the fact that you have spent another 30% of the electricity received from the network uselessly. And this is in best case rated output power.

With an output power of 0.7 of the nominal, you should already be prepared for the fact that your energy losses will be equal to the useful energy spent. Where is so much energy being wasted?

ohmic (thermal) losses in the windings of transformers, chokes and active elements of the circuit;

· energy costs for the operation of the station control circuit;

Loss of energy in the form of radio emission; energy losses of the output current ripple of the station at the load.

This energy is radiated into the ground from the anode and does not produce useful work. Therefore, it is so necessary to use stations with a low ripple coefficient, otherwise expensive energy is wasted. Not only that, at high levels of ripples and radio emission, the loss of electricity increases, but besides this, this uselessly dissipated energy interferes with the normal operation of a large number of electronic equipment located in the vicinity. The required total power is also indicated in the SKZ passport, let's try to deal with this parameter. The SKZ takes energy from the power grid and does it in every unit of time with such intensity as we have allowed it to do with the adjustment knob on the station control panel. Naturally, it is possible to take energy from the network with a power not exceeding the power of this network itself. And if the voltage in the network changes sinusoidally, then our ability to take energy from the network changes sinusoidally 50 times per second. For example, at the moment when the mains voltage passes through zero, no power can be taken from it. However, when the voltage sinusoid reaches its maximum, then at this moment our ability to take energy from the network is maximum. At any other time, this possibility is less. Thus, it turns out that at any time the power of the network differs from its power at a neighboring time. These power values are called instantaneous power in this moment time and such a concept is difficult to operate. Therefore, we agreed on the concept of the so-called effective power, which is determined from an imaginary process in which a network with a sinusoidal voltage change is replaced by a network with a constant voltage. When we calculated the value of this constant voltage for our electrical networks, we got 220 V - it was called the effective voltage. And the maximum value of the sinusoid of the voltage was called the amplitude voltage, and it is equal to 320 V. By analogy with the voltage, the concept of the effective value of the current was introduced. The product of the effective voltage value and the effective current value is called the total power consumption, and its value is indicated in the RMS passport.

And the full power in the SKZ itself is not fully used, because. it has various reactive elements that do not waste energy, but use it, as it were, to create conditions for the rest of the energy to pass into the load, and then return this tuning energy back to the network. This energy returned back was called reactive energy. The energy that is transferred to the load is active energy. The parameter that indicates the ratio between the active energy that must be transferred to the load and the total energy supplied to the RMS is called the power factor and is indicated in the station passport. And if we coordinate our capabilities with the capabilities of the supply network, i.e. synchronously with a sinusoidal change in the voltage of the network, we take power from it, then such a case is called ideal and the power factor of the RMS operating with the network in this way will be equal to one.

The station must transmit active energy as efficiently as possible to create a protective potential. The efficiency with which the VHC does this is assessed by the coefficient useful action. How much energy it spends depends on the method of energy transfer and on the mode of operation. Without going into this vast field for discussion, we will only say that transformer and transformer-thyristor SKZs have reached their limit of improvement. They do not have the resources to improve the quality of their work. The future belongs to high-frequency VMS, which every year become more reliable and easier to maintain. In terms of efficiency and quality of their work, they already surpass their predecessors and have a large reserve for improvement.

Consumer properties

TO consumer properties such a device as SKZ include the following:

1. Dimensions, weight and strength. Probably, it is not necessary to say that the smaller and lighter the station, the lower the cost of its transportation and installation, both during installation and repair.

2. Maintainability. The ability to quickly replace a station or node on site is very important. With subsequent repairs in the laboratory, i.e. modular principle of construction of SKZ.

3. Ease of maintenance. Ease of maintenance, in addition to ease of transportation and repair, is determined, in our opinion, as follows:

availability of all necessary indicators and measuring instruments, possibility remote control and monitoring the operation of the SKZ.

Based on the foregoing, several conclusions and recommendations can be drawn:

1. Transformer and thyristor-transformer stations are hopelessly outdated in all respects and do not meet modern requirements, especially in the field of energy saving.

2. A modern station must have:

· high efficiency in all range of loadings;

power factor (cos I) not less than 0.75 in the entire load range;

output voltage ripple factor no more than 2%;

· current and voltage regulation range from 0 to 100%;

lightweight, durable and small-sized body;

· modular principle of construction, i.e. have high maintainability;

· I energy efficiency.

Other requirements for gas pipeline cathodic protection stations, such as protection against overloads and short circuits; automatic maintenance of a given load current - and other requirements are generally accepted and mandatory for all SKZ.

In conclusion, we offer consumers a table comparing the parameters of the main manufactured and currently used cathodic protection stations. For convenience, the table shows stations of the same power, although many manufacturers can offer a whole range of manufactured stations.