Electrochemical protection of pipelines against corrosion. Great 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, the metal atoms lose their electrons, and the 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 physical properties and chemical composition), areas with different electrode potentials appear, which causes the formation of galvanic corrosion. The most important types of corrosion are: superficial (solid over the entire surface), local in the form of shells, pitting, crevice and fatigue corrosion cracking. The last two types of corrosion pose the greatest danger to underground pipelines. Surface corrosion only in in rare cases leads to damage, while pitting corrosion causes the greatest number of damage. The corrosion situation in which a metal pipeline is located in the ground depends on a large number of factors associated with soil and climatic conditions, route characteristics, and operating conditions. These factors include:

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

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

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

It should be noted that stray currents in cathode 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 creating 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, scuffs, dents and other defects appear in the insulating coating. The most dangerous are through damage protective coating, where, practically, ground corrosion occurs.

Since the passive method does not allow complete protection of the pipeline from corrosion, active protection is simultaneously applied, associated with the control of electrochemical processes occurring at the boundary of the pipe metal and the ground electrolyte. This type of protection is called comprehensive protection.

The active method of corrosion protection is carried out by cathodic polarization and is based on reducing the rate of dissolution of the metal as its corrosion potential shifts to an area of ​​more negative values ​​than the natural potential. It was experimentally established that the magnitude 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 ground is approximately -0.55...-0.6 Volts, to implement cathodic protection it is necessary to shift the corrosion potential by 0.25...0.30 Volts in the negative direction.

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

2. Cathodic protection installations
Cathodic protection of pipelines can be carried out using two methods:

• using magnesium sacrificial protector anodes (galvanic method);
• using external direct current sources, the minus of which is connected to the pipe, and the plus to anode grounding (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 couple from two metals and place them in an electrolyte, 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-resistivity 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 labor-intensive, but it depends little on the resistivity of the soil and has an unlimited energy resource.

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

The following are used as power sources for cathodic protection installations: air lines 0.4; 6; 10 kV. The protective current applied to the pipeline from the converter and creating a “pipe-ground” potential difference is distributed unevenly along the length of the pipeline. Therefore, the maximum absolute value of this difference is located at the point of connection of the current source (drainage point). As you move away from this point, the pipe-ground potential difference decreases. Excessively increasing the potential difference negatively affects the adhesion of the coating and can cause hydrogenation of the pipe metal, which can cause hydrogen cracking. Cathodic protection is one of the methods of combating metal corrosion in aggressive chemical environments. It is based on transferring a metal from an active state to a passive state and maintaining this state using an external cathode current. To protect underground pipelines from corrosion, cathodic protection stations (CPS) are built along their route. The VCS 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 from an alternating current network 0.4; 6 or 10 kV or from autonomous sources. When protecting multi-line pipelines laid in one corridor, several installations can be installed and several anode groundings can be constructed. However, taking into account that during interruptions in the operation of the protection system, due to the difference in natural potentials of the pipes connected by a blind jumper, powerful galvanic couples are formed, leading to intense corrosion, the connection of the pipes to the installation must be carried out through special blocks joint defense. These blocks not only disconnect the pipes from each other, but also allow you to set the optimal potential on each pipe. Converters powered by a 220 V industrial frequency network are mainly used as DC sources for cathodic protection in VSCs. The output voltage of the converter is adjusted manually, by switching the taps of the transformer winding, or automatically, using controlled valves (thyristors). If cathodic protection installations operate under time-varying conditions, which may be caused by the influence of stray currents, changes in soil resistivity or other factors, then it is advisable to provide converters with automatic control of the output voltage. 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

Electric drainage is the simplest type of active protection that does not require a current source, since the pipeline is electrically connected to the traction rails of the stray current source. The source of the protective current is the pipeline-rail potential difference, which arises as a result of the operation of electrified railway transport and the presence of a field of stray currents. The flow of drainage current creates the required potential shift in the underground pipeline. As a rule, fuses are used as a protective device, but maximum load circuit breakers with reset are also used, that is, they restore the drainage circuit after the current dangerous for the installation elements subsides. As a polarized element, valve blocks assembled from several avalanche silicon diodes connected in parallel are used. The current in the drainage circuit is regulated by changing the resistance in this circuit by switching active resistors. If the use of polarized electric drains is ineffective, then reinforced (forced) electric drains are used, which are a cathodic protection installation, the rails of an electrified railway are used as an anode grounding electrode. The current of forced drainage operating in cathodic protection mode should not exceed 100A, and its use should not lead to the appearance of positive rail potentials relative to the ground in order to prevent corrosion of rails and rail fastenings, as well as structures attached to them.

Electrical drainage protection can be connected to the rail network directly only to the middle points of track choke transformers through two to a third choke points. More frequent connection is allowed if a special connection is included in the drainage circuit protective device. A choke can be used as such a device, the total input resistance of which to the signal current of the trunk signaling system railways frequency 50 Hz is at least 5 ohms.

4. Galvanic protection installations

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

Typically, protector 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 from converters;
• sections of pipelines electrically isolated from the main line by insulating connections;
• steel protective covers(cartridges), underground tanks and containers, 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 protective installations can be carried out in soils with a specific electrical resistivity of no more than 50 Ohms.

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 unproductive energy consumption and to a reduction in the protective zone of the installation. This disadvantage can be avoided by using a circuit with extended or distributed anodes. The ECP technological scheme with distributed anodes makes it possible to increase the length of the protective zone compared to the cathodic protection scheme with concentrated anodes, and also ensures a more uniform distribution of the protective potential. When using the ZHZ technological scheme with distributed anodes, various layouts of anode grounding can be used. The simplest is the scheme with anode groundings evenly installed along the gas pipeline. Adjustment of the protective potential is carried out by changing the anodic grounding current using an adjusting resistance or any other device that ensures a change in the current within the required limits. In the case of grounding from several grounding electrodes, the protective current can be adjusted by changing the number of connected grounding electrodes. In general, the ground 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 environment, the metal is restored and the protector body dissolves. Since the bulk of metal structures in the world are 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 largest 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 fewer 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 use 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 its damage. It occurs at different speeds, which can be reduced. From a practical point of view, anti-corrosion 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 environment. If it is a gas, it must be thoroughly cleaned of moisture and aggressive substances: hydrogen sulfide, oxygen, etc.

Principle of operation

Process objects electrochemical corrosion are the medium, the metal and the interfaces between them. The medium, which is usually moist soil or water, has good electrical conductivity. An electrochemical reaction occurs at the interface between it and the metal structure. If the current is positive (anode electrode), iron ions move into the surrounding solution, which leads to a loss of metal mass. The reaction causes corrosion. With a negative current (cathode electrode), these losses do not exist, since electrons pass into the solution. The method is used in electroplating for applying non-ferrous metal coatings to steel.

Cathodic corrosion protection occurs 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 from a power source is connected to it. The minus signal is applied to the protected object. Cathodic-anodic protection leads to active destruction of only the anode electrode from corrosion. Therefore, it should be changed periodically.

Negative effects of electrochemical corrosion

Corrosion of structures can occur from the action of stray currents coming 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 transport. They pass towards the substation and end up on the pipelines. When leaving them, anodic areas are formed, causing intense corrosion. For protection, electrical drainage is used - a special drainage of currents from the pipeline to their source. It is also possible here. To do this, you need to know the magnitude of stray currents, which is measured with special instruments.

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

Pipeline protection

Structures in the ground are protected from corrosion if you connect the minus of a DC source to them, and the plus to the anode electrodes buried nearby in the ground. The current will flow 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 deteriorate and should be replaced periodically. For a tank filled with water, the electrodes are placed inside. In this case, the liquid will be an electrolyte through which the current will flow from the anodes to the surface of the container. The electrodes are well controlled and easy to replace. It is more difficult to do this in the ground.

Power supply

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

Cathodic protection stations for gas pipelines and other large structures have a power 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, the cathodic protection converter is a transformer unit. Now it is being replaced by an inverter, 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 presented on the market in various options. For specific needs, it is used that provides the best operating conditions.

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 increases to 10 mA/m2.

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

Features of cathodic protection

1. Power sources are stations or mobile generators.
2. The location of the anode grounding electrodes depends on the specifics of the pipelines. The placement method can be distributed or concentrated, and also located at different depths.
3. The anode material is selected with low solubility so that it lasts for 15 years.
4. 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

• Valid throughout the entire service life of the protective equipment.
• Protection against atmospheric surges.
• Placement of the station in block boxes or in a stand-alone, vandal-proof design.
• Anodic grounding is selected in areas with minimal electrical resistance of the soil.
• The characteristics of the converter are selected taking into account the aging of the protective coating of the pipeline.

Tread protection

The method is a type of cathodic protection with the connection of electrodes from a more electronegative metal through an electrically conductive medium. The difference is the absence of an energy source. The protector takes on corrosion by dissolving in 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 salt mixture that dissolves corrosion products, efficiency increases.

The influence of the tread is limited. The range 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 unfavorable when used in acidic environments due to the high rate of dissolution of the anodes. Protectors are installed in water, in soil or in a neutral environment. Anodes are usually not made from pure metals. The dissolution of zinc occurs unevenly, magnesium corrodes too quickly, and a strong film of oxides forms on aluminum.

Protector materials

In order for 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 water with low salt concentration.

Improper use of some types of protectors leads to negative consequences. Magnesium anodes can cause equipment to crack due to hydrogen embrittlement.

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

The distribution of 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 an uncoated pipeline for only 30 m.

Protection of car bodies from corrosion

If the coating is damaged, the thickness of the car body can decrease to 1 mm over 5 years, i.e., rust through. Restoring the protective layer is important, but besides it there is a way to completely stop the corrosion process using cathodic protection. If you turn the body into a cathode, metal corrosion stops. Anodes can be any conductive surfaces located nearby: metal plates, a ground loop, a garage body, a wet road surface. Moreover, the effectiveness of protection increases with increasing area of ​​the anodes. If the anode is a road surface, a “tail” made of metallized rubber is used for contact with it. It is placed opposite the wheels to allow splashes to fall better. The "tail" is isolated from the body.

The plus of the battery is connected to the anode through a 1 kOhm resistor and an LED connected in series with it. When the circuit is closed through the anode, when the negative is connected to the body, in normal mode the LED glows barely noticeably. If it lights up brightly, there is a short circuit 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 the grounding anode. During movement, the connection occurs through the “tail”.

Conclusion

Cathodic protection is a way to improve the operational reliability of underground pipelines and other structures. In this case, one should take into account its negative impact on neighboring pipelines from the influence of stray currents.

Pipelines running underground are subject to the destructive effects of corrosion. Pipeline corrosion affects metal pipes when conditions arise where metal atoms can become ionic.

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.

This 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 electrolytic reactions

• There are several reasons for the formation of a potential difference (the magnitude of its value) in individual sections of the pipe:
• different soil compositions according to physical and chemical properties;
• metal heterogeneity;
• soil moisture;
• the value of the operating temperature of the transported substance;
• indicator of soil electrolyte acidity;

the passage of an electric transport line that creates stray currents.

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

• As a result, two types of corrosion damage can occur:
• superficial, which does not lead to destruction of the pipeline;

local, which results in the formation of shells, cracks, and cracking.

Types of corrosion protection

To protect pipes from destruction, pipeline corrosion protection is used.

• There are two main methods of protection: passive, in which a protective shell is created around the pipes, completely separating them from the ground. This is usually a bitumen coating, epoxy resin
• , polymer tape;

active, allowing you to control the 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 caused by stray currents. Such currents are diverted in the direction of the source that creates them or directly into the soil layer. Drainage can be earthen (grounding the anode zones of the pipeline), direct (disconnection from the negative pole of the stray current source). Polarized and enhanced drainage is used less frequently.

Methods for organizing cathodic protection

Cathodic protection of a 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 undergo destruction.

• Cathode is divided into two types:
• electric, which uses an external direct current 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 metals to have different potentials when they are used in the form of an electrode. If the electrolyte contains two metals with different meaning potential, then the one with the least value will be destroyed.

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

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

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

To increase the efficiency and effectiveness of protection, 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 anodic polarizability.

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

The effectiveness of tread protection against pipeline corrosion depends on the magnitude of soil resistivity. It works well if this indicator does not exceed 50 Ohm*m, with higher value 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 complex to organize, but the most effective is the electric one. To organize it, an external direct current source is constructed - a cathodic protection station. The power station is converted alternating current to permanent.

Cathodic protection elements:

• anodic grounding;
• DC connection line;
• protective grounding;
• DC source;
• cathode terminal.

The electrical method is an analogue of the electrolysis process.

Under the influence of the external field of the current source, valence electrons move away from the anode grounding towards the current source and the pipe. The grounded anode is gradually destroyed. And near a pipeline from a direct current source, an incoming excess of free electrons leads to depolarization (like a cathode during electrolysis).

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

Allows you to extend service life metal structure, as well as preserve its technical and physical properties during operation. Despite the variety of methods for ensuring 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, to preserve the metal structure of pipelines, their best properties demonstrates electrochemical corrosion protection based on cathode performance. Preventing the formation of rust on such communications, of course, is not the only area of ​​application of this technology, but based on the totality of its characteristics, this area can be considered as the most relevant for electrochemical protection.

General information about electrochemical protection

Protection of metals from rust through 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 potential range 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 structure.

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

Operating principle of cathodic protection

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

In this case, cathodic protection can also have different variants execution. The above-described technique of polarization from an external source is widely practiced, but the method of deaerating the electrolyte by reducing the rate of cathodic processes, as well as creating a protective barrier, also works effectively.

It has been noted more than once that the principle of cathodic protection is implemented through an external current source. Actually, its main function is its work. These tasks are performed by special stations, which, as a rule, are part of the general infrastructure Maintenance pipelines.

Anti-corrosion stations

The main function of the cathode station is to provide stable current to the target metal object in accordance with the cathode 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, and the most common cathodic protection device contains:

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

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

Transformer cathodic protection installations

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

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

Modern cathodic protection against corrosion of pipelines with such equipment allows you to adjust the parameters of the output current, voltage indicators, and also 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 flaw is not explained by the sinusoidal shape of the current.

The problem with pulsation can be solved to a certain extent by introducing a low-frequency choke into the system, but its dimensions correspond to the dimensions of the transformer itself, which does not always make such an addition possible.

Inverter cathodic protection station

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

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. Thus, inverter cathodic protection against pipeline corrosion quickly pays for itself and requires minimum investment into technical content. However, these qualities are clearly noticeable only when compared with transformer installations, but today more efficient new means of providing current for pipelines are appearing.

Designs 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 allows not only to obtain an optimal design for specific needs, but also to ensure the necessary operational parameters.

Rigorous calculation of the station’s characteristics makes it possible to further optimize the costs of its installation, transportation and storage. For example, for small objects, cathodic protection against corrosion of pipelines based on an inverter weighing 10-15 kg and a power of 1.2 kW is quite suitable. Equipment with such characteristics can be serviced by a passenger car, however, for large-scale projects, more massive and heavier stations that require the connection of trucks, a crane and installation teams can be used.

Protective functionality

When developing cathode stations, special attention is paid to protecting the equipment itself. For this purpose, systems are being integrated to protect stations from short circuits and load breaks. In the first case, special fuses are used to handle emergency operation modes of installations.

As for voltage surges and breaks, the cathodic protection station is unlikely to be seriously damaged by them, but there may be a danger of electric shock. For example, if in normal mode the equipment is operated at low voltage, then after a break the jump in the readings can reach 120 V.

Other types of electrochemical protection

In addition to cathodic protection, electrical drainage technologies, as well as protective methods for preventing corrosion, are also practiced. Most promising direction It is considered to be special protection against the formation of corrosion. In this case, active elements are also connected to the target object, ensuring the transformation of the surface with cathodes through 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 considered new and, especially, innovative. The effectiveness of using such techniques in the fight against rusting processes has been mastered for a long time. However, widespread This method is hampered by one serious drawback. The fact is that cathodic protection against corrosion of pipelines inevitably produces 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 neighboring pipes.

Cathodic protection stations (CPS) are a necessary element of the electrochemical (or cathodic) protection system (ECP) of underground pipelines against corrosion. When choosing VCS, they most often proceed from the lowest cost, ease of service and the qualifications of their operating personnel. The quality of purchased equipment is usually difficult to assess. The authors propose to consider those 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 strictly scientific language in defining concepts. In the process of communicating with the personnel of 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 VCP itself.

ECP task

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

· soil resistance between the pipeline and the 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 a consumable element, the dissolution of which ensures the very possibility of implementing ECP. Its resistance during operation steadily increases due to dissolution, reduction effective area working surface and oxide formation.

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

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

VCS task

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

So, if the insulation is new, and it has not yet received damage, then its resistance electric current high and a small current is needed to maintain the required potential. As insulation ages, its resistance decreases. Consequently, the required compensating current from the SCZ 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.

VCS operating modes

There can be four operating modes of the ECP:

· without stabilization of output current or voltage values;

· I output voltage stabilization;

· output current stabilization;

· I stabilization of protective potential.

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

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

In conditions where problems arise with the operating mode with potential feedback, proceed as follows. When using the third mode, it is assumed that the state of the insulation in the short term changes little and its resistance remains practically stable. Therefore, it is enough to ensure the flow of stable current through a stable insulation resistance, and we obtain a stable protective potential. In the medium to long term, the necessary adjustments can be made by a specially trained lineman. The first and second modes do not impose high demands on VCS. These stations are simple in design and, as a result, cheap, both to manufacture and to operate. Apparently this circumstance determines the use of such SCZ in ECP of objects located in conditions of low corrosive activity of the environment. If external conditions (insulation state, temperature, humidity, stray currents) change to the extent that an unacceptable mode is formed at 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 completed.

Characteristics of VCS

First of all, VCS must be selected based on the requirements set out in regulatory 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 interference created by the RMS must not exceed the values ​​specified by GOST 16842, and the level of harmonics at the output must comply with GOST 9.602.

The SPS passport usually indicates: I rated output power;

Efficiency at rated output power.

Rated output power is the power that a station can deliver at rated load. Typically this load is 1 ohm. Efficiency is defined as the ratio of the rated output power to the active power consumed by the station in rated mode. And in this mode, the efficiency is the highest for any station. However, most VCSs do not operate in nominal mode. The power load factor ranges from 0.3 to 1.0. In this case, the real efficiency for most stations produced today will drop noticeably as the output power decreases. This is especially noticeable for transformer SPS using thyristors as a regulating element. For transformerless (high-frequency) RMS, the drop in efficiency with a decrease in output power is significantly less.

General view of the change in efficiency for VCS different designs can be seen in the figure.

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

With an output power of 0.7 of the rated value, you should be prepared for the fact that your electricity losses will be equal to the useful energy expended. Where is so much energy lost?

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

· energy costs for operation of the station control circuit;

· energy losses in the form of radio emission; loss of pulsation energy of the station output current on 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 pulsation coefficient, otherwise expensive energy is wasted. Not only do electricity losses increase at high levels of pulsation and radio emission, but in addition, this uselessly dissipated energy interferes with the normal operation of a large number of electronic equipment located in the surrounding area. The SKZ passport also indicates the required total power, let's try to understand this parameter. The SKZ takes energy from the power grid and does this in each unit of time with the same intensity that we allowed it to do with the adjustment knob on the station control panel. Naturally, you can take energy from the network with a power not exceeding the power of this very network. 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 network voltage passes through zero, no power can be taken from it. However, when the voltage sinusoid reaches its maximum, then at that moment our ability to take energy from the network is maximum. At any other time this opportunity is less. Thus, it turns out that at any moment in time the power of the network differs from its power at the next moment in 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 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, it turned out to be 220 V - it was called the effective voltage. And the maximum value of the voltage sinusoid was called the amplitude voltage, and it is equal to 320 V. By analogy with voltage, the concept of effective current value 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 VCS itself is not fully used, because it contains various reactive elements that do not waste energy, but use it as if 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 returned energy is called reactive energy. The energy that is transferred to the load is active energy. The parameter that indicates the relationship between the active energy that must be transferred to the load and the total energy supplied to the VMS 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 the sinusoidal change in the network voltage, we take power from it, then such a case is called ideal and the power factor of the VMS operating with the network in this way will be equal to unity.

The station must transfer 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 transmission and the operating mode. Without going into this extensive field for discussion, we will only say that transformer and transformer-thyristor SSCs have reached their limit of improvement. They don't have the resources to improve the quality of their work. The future belongs to high-frequency VMS, which are becoming more reliable and easier to maintain every year. 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 an SCS can include the following:

1. Dimensions, weight and strength. There is probably no need to say that the smaller and lighter the station, the lower the costs for its transportation and installation, both during installation and repair.

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

3. Ease of maintenance. Ease of maintenance, in addition to ease of transportation and repair, is determined, in our opinion, by the following:

presence of all necessary indicators and measuring instruments, availability of opportunity remote control and monitoring the operating mode of the VCS.

Based on the above, several conclusions and recommendations can be made:

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 over the entire load range;

· power factor (cos I) not lower than 0.75 over the entire load range;

· output voltage ripple factor no more than 2%;

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

· lightweight, durable and compact body;

· modular construction principle, 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 VCS.

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