Electrochemical protection of gas pipelines. Protection of gas pipelines against soil corrosion and stray currents

Corrosion protection of pipelines can be performed using a variety of technologies, the most effective of which is the electrochemical method, which includes cathodic protection. Often, anticorrosive cathodic protection is applied in combination with the treatment of the steel structure with insulating compounds.

In this article, the electrochemical protection of pipelines is considered and its cathodic subspecies are studied in particular in detail. You will learn what the essence of this method is, when it can be used and what equipment is used for cathodic protection of metals.

Content of the article

Varieties of cathodic protection

Cathodic corrosion protection for steel structures was invented in the 1820s. For the first time, the method was applied in shipbuilding - the copper hull of the ship was sheathed with protective anode protectors, which significantly reduced the rate of copper corrosion. The technique was adopted and began to actively develop, which made it one of the most effective methods of anti-corrosion protection today.

Cathodic protection of metals, according to the execution technology, is classified into two types:

• method No. 1 - an external current source is connected to the protected structure, in the presence of which the metal product itself acts as a cathode, while third-party inert electrodes act as anodes.
• method number 2 - " galvanic technology“: The structure to be protected is in contact with a protector plate made of a metal with a higher electronegative potential (such metals include zinc, aluminum, magnesium and their alloys). Both metals perform the function of the anode in this method, while the electrochemical dissolution of the metal of the protector plate ensures that the required minimum of the cathodic current flows through the protected structure. After the lapse of time, the tread plate is completely destroyed.

Method # 1 is the most common. It is an easy-to-implement anti-corrosion technology that effectively tackles many types of metal corrosion:

• intercrystalline corrosion of stainless steel;
• pitting corrosion;
• cracking of brass from increased stress;
• corrosion due to stray currents.

Unlike the first method, which is suitable for protecting large structures (used for underground and aboveground pipelines), electrochemical protection is intended for use with small-sized products.

The galvanic method is widespread in the United States, in Russia it is practically not used, since the technology for the construction of pipelines in our country does not provide for the treatment of highways with a special insulating coating, which is a prerequisite for galvanic electrochemical protection.

Note that without significantly increased corrosion of steel under the influence of groundwater, which is especially typical for the spring and autumn. In winter, after the water freezes, moisture corrosion slows down significantly.

The essence of technology

Cathodic anticorrosive protection is carried out through the use of direct current, which is supplied to the protected structure from an external source (most often rectifiers that convert alternating current into direct current are used) and makes its potential negative.

The object itself, connected to direct current, is a “minus” - the cathode, while the anode ground connected to it is a “plus”. A key condition for the effectiveness of cathodic protection is the presence of a well-conductive electrolytic medium, which serves as the ground for the protection of underground pipelines, while electronic contact is achieved through the use of highly conductive metal materials.

In the process of implementing the technology, the required difference in current potential is constantly maintained between the electrolytic medium (soil) and the object, the value of which is determined using a high-resistance voltmeter.

Features of cathodic protection of pipelines

Corrosion is the main cause of leakage in all types of pipelines. Rust damage to the metal creates tears, cavities and cracks on the metal, leading to the destruction of the steel structure. This problem is especially critical for underground pipelines, which are constantly in constant contact with groundwater.

Cathodic protection of gas pipelines against corrosion is performed by one of the above methods (by means of an external rectifier or by galvanic method). The technology in this case allows to reduce the rate of oxidation and dissolution of the metal from which the pipeline is made, which is achieved by shifting its natural corrosion potential to the negative side.

Through practical tests, it was found that the potential of cathodic polarization of metals, at which all corrosion processes slow down, is equal to -0.85V, while in underground pipelines in natural mode it is -0.55 V.

For anti-corrosion protection to be effective, it is necessary to reduce the cathodic potential of the metal from which the pipeline is made by -0.3 V by means of direct current. In this case, the corrosion rate of steel does not exceed 10 micrometers during the year.

Cathodic protection is the most effective method of protecting underground pipelines from stray currents. The concept of stray currents means an electric charge that enters the ground as a result of the operation of grounding points for power lines, lightning rods, or the movement of trains along railway lines. It is impossible to find out the exact time and place of the appearance of stray currents.

The corrosive effect of stray currents on the metal occurs if the metal structure has a positive potential with respect to the electrolyte (for underground pipelines, the electrolyte is the soil). Cathodic protection makes the potential of the metal of underground pipelines negative, which eliminates the risk of oxidation under the influence of stray currents.

The technology of using an external current source for cathodic protection of underground pipelines is preferable. Its advantages are unlimited energy resources, capable of overcoming soil resistivity.

Overhead power lines with a capacity of 6 and 10 kW are used as a current source for anti-corrosion protection, but if there are no power lines on the territory of power lines, mobile generators operating on gas and diesel fuel can be used.

Detailed review of cathodic corrosion protection technology (video)

Cathodic protection equipment

For anti-corrosion protection of underground pipelines, special equipment is used - cathodic protection stations(SKZ), consisting of the following nodes:

• grounding (anode);
• constant current source;
• control, control and measurement point;
• connecting cables and wires.

One SKZ connected to the power grid or to an autonomous generator can perform cathodic protection of several nearby underground pipelines at once. The current adjustment can be performed manually (by replacing the winding on the transformer) or in automatic mode (if the system is equipped with thyristors).

Among the cathodic protection stations used in the domestic industry, the most technologically advanced installation is considered to be Minerva-3000 (designed by engineers from France by order of Gazprom). The capacity of this RPS is sufficient to effectively protect 30 km of the underground pipeline.

Installation benefits include:

• increased power;
• overload recovery function (update takes 15 seconds);
• availability of digital control systems to control operating modes;
• complete tightness of critical units;
• the ability to connect equipment for remote control.

Also, ASKG-TM units are widely in demand in domestic construction, in comparison with Minerva-3000 they have a reduced power (1-5 kW), however, in the stock configuration, the system is equipped with a telemetry complex that automatically monitors the operation of the SCZ and has the ability to remotely control ...

Cathodic protection stations Minerva-3000 and ASKG-TM require power supply from a 220 V power supply. Remote control of equipment is performed by means of built-in GPRS modules. SKZ have rather larger dimensions - 50 * 40 * 90 cm and weight - 50 kg. The devices have a minimum lifespan of 20 years.

Corrosion of underground pipelines and protection against it

Corrosion of underground pipelines is one of the main reasons for their depressurization due to the formation of caverns, 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 one. In this case, the metal atoms lose their electrons, and the oxidizers accept them. On the underground pipeline, due to the heterogeneity of the pipe metal and due to the heterogeneity of the soil (both in physical properties and in chemical composition), areas 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 pits, pitting, crevice and fatigue corrosion cracking. The last two types of corrosion pose the greatest danger to underground pipelines. Surface corrosion only rarely results in damage, while pitting causes the greatest number of damage. The corrosive situation in which a metal pipeline is in the ground depends on a large number of factors related to soil and climatic conditions, the characteristics of the route, and operating conditions. These factors include:

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

The strongest negative manifestation of stray currents in the ground, caused by electrified DC rail transport, is the electro-corrosive destruction of pipelines. The intensity of stray currents and their effect on underground pipelines depends on factors such as:

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

It should be noted that stray currents in the 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 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 types of coatings have different diffusion permeabilities and therefore provide different insulation of the pipe from the environment. During construction and operation, cracks, galls, dents and other defects appear in the insulating coating. The most dangerous are through damage to the protective coating, where, in practice, soil corrosion occurs.

Since the passive method fails to fully protect 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 ground electrolyte. Such protection is called comprehensive protection.

An active method of corrosion protection is carried out by cathodic polarization and is based on a decrease in the rate of dissolution of a metal as its corrosion potential shifts to a region of more negative values ​​than the natural potential. It was experimentally found that the value of the cathodic protection potential of 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 soil, it is necessary to achieve a reduction in the potential in defective places of the pipe insulation to a value below the criterion of the protective potential equal to 0.9 V. As a result, the corrosion rate is significantly reduced.

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

• the use of magnesium sacrificial anodes-protectors (galvanic method);
• the use of external DC sources, the minus of which is connected to the pipe, and the plus to the 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 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 with protectors is effective only in low-resistance soils (up to 50 Ohm-m). In high-resistance soils, this method does not provide the necessary protection. Cathodic protection by external current sources is more complex and laborious, but it depends little on the specific resistance of the soil and has an unlimited energy resource.

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

Overhead lines 0.4 are used as power sources for cathodic protection installations; 6; 10 kV. The protective current imposed on the pipeline from the converter and creating a potential difference "pipe-ground" 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 (point of drainage). With distance from this point, the potential difference "pipe-ground" decreases. Excessive overestimation of the potential difference negatively affects the adhesion of the coating and can cause hydrogen saturation of the pipe metal, which can lead to hydrogen cracking. Cathodic protection is one of the methods for combating corrosion of metals in aggressive chemical environments. It is based on transferring a metal from an active state to a passive state and maintaining this state with the help of an external cathodic current. To protect underground pipelines from corrosion, cathodic protection stations (CPS) are being built along the route of their occurrence. The RMS 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 0.4; 6 or 10 kV or from autonomous sources. When protecting multi-line pipelines laid in the same corridor, several installations can be installed and several anode groundings can be built. However, given that during interruptions in the operation of the protection system, due to the difference in natural potentials of pipes connected by a blind jumper, powerful galvanic couples are formed, leading to intense corrosion, the connection of pipes to the installation should be carried out through special blocks of 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 DC sources for cathodic protection at the RMS, converters are mainly used, which are powered from a 220 V industrial frequency network. Adjustment of the output voltage of the converter is carried out 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 can be caused by the influence of stray currents, changes in soil resistivity or other factors, then it is advisable to provide converters with automatic regulation of the output voltage. Automatic regulation can be carried out by the potential of the protected structure (converters potentiostats) or by the protection current (converters galvanostats).

3. Installations of drainage protection

Electrical 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 potential difference between the pipeline and the rail resulting from the operation of electrified railway transport and the presence of a field of stray currents. The flow of the drain current creates the required potential displacement in the buried pipeline. As a rule, fuses are used as a protective device, however, automatic circuit breakers of maximum load with a return are also used, that is, restoring the drainage circuit after the current that is dangerous for the elements of the installation has dropped. As a polarized element, valve blocks assembled from several parallel-connected avalanche silicon diodes are used. 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 electric drains is ineffective, then reinforced (forced) electric drains are used, which is a cathodic protection installation, the rails of an electrified railway are used as an anode ground electrode. 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.

Electric drainage protection is allowed to be connected to the rail network directly only to the middle points of the line choke transformers through two to the third choke point. More frequent connection is allowed if a special protective device is included in the drain circuit. As such a device, a choke can be used, the total input resistance of which to the signal current of the signaling system of the main railway signaling system with a frequency of 50 Hz is at least 5 ohms.

4. Installations of galvanic protection

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

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

• reservoirs and pipelines that do not have electrical contacts with adjacent long communications;
• separate sections of pipelines that are not provided with a sufficient level of protection from converters;
• sections of pipelines electrically isolated from the mainline by insulating joints;
• steel protective casings (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 tread installations can be carried out in soils with a specific electrical resistance of no more than 50 ohms.

5. Installations with extended or distributed anodes.

As already noted, when using the traditional scheme of 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 power consumption, 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 makes it possible 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 using the technological scheme of ZHZ with distributed anodes, various layouts of anode grounding can be used. The simplest is the scheme with anode grounding, evenly installed along the gas pipeline. The control of the protective potential is carried out by changing the anode grounding current using an adjusting resistance or any other device that provides a change in the current within the required limits. In the case of performing grounding from several grounding electrodes, the protective current can be adjusted by changing the number of included grounding electrodes. In general, the ground electrodes closest to the converter must 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 is made of iron, metals with an electrode potential that are more negative 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 is from 10 to 200 m, which allows using fewer magnesium protectors than zinc and aluminum protectors. In addition, in magnesium and magnesium alloys, in contrast to zinc and aluminum, there is no polarization, accompanied by a decrease in current transfer. This feature determines the main application of magnesium protectors for the protection of underground pipelines in soils with high resistivity.

A. G. Semenov, general director, Joint venture Elkon, G. Chisinau; L. P. Sysa, leading engineer on ECP, NPK "Vector", G. Moscow

Introduction

Cathodic protection stations (SKZ) are a necessary element of the system of electrochemical (or cathodic) protection (ECP) of underground pipelines from corrosion. When choosing a VHC, most often they proceed from the lowest cost, convenience of service and the qualifications of their service personnel. The quality of the purchased equipment is usually difficult to assess. The authors propose to consider the technical parameters of the SCZ indicated in the passports, 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 defining concepts. In the process of communicating with the staff of the ECP services, we realized that it is necessary for these people to help organize terms and, more importantly, to give them an idea of ​​what is happening both in the power grid and in the VHC itself.

TaskECP

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

· Soil resistance between the pipeline and the anode; I resistance to spreading of the anode;

· 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 that consumable element, the dissolution of which provides the very possibility of ECP implementation. Its resistance grows steadily during operation due to dissolution, a decrease in the effective area of ​​the working surface and the formation of oxides.

Consider the metal pipeline itself, which is the protected ECP element. 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 formed cracks in the hydraulic and thermal insulation of the pipeline and the metal of the pipe comes into contact with the ground, thus forming a galvanic pair, which promotes the removal of metal from the pipe. The more cracks and their sizes, the more metal is carried away. Thus, galvanic corrosion occurs, in which a current of metal ions flows, i.e. electricity.

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

TaskVHC

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

So, if the insulation is new, and it did not have time to get damaged, then its resistance to electric current is high and a small current is needed to maintain the required potential. As the insulation ages, its resistance drops. 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 not required.

ModesworkVHC

There are four modes of operation of the ECP:

· Without stabilization of output values ​​of current or voltage;

· I stabilization of 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 ECP task is fully ensured only when the fourth mode is used. Which is accepted as the standard for the operating mode of the RMS.

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 very potential sensor. It must be installed 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 problems arise with the mode of operation with potential feedback, proceed as follows. When using the third mode, it is assumed that the state of insulation in the short term changes little and its resistance remains practically stable. Therefore, it is sufficient to ensure that a stable current flows through a stable insulation resistance, and a stable protective potential is obtained. In the medium and long term, a specially trained lineman can make the necessary adjustments. The first and second modes do not impose high requirements on the RMS. These stations turn out to be simple in execution and, as a consequence, cheap, both in manufacture and in operation. Apparently, this circumstance determines the use of such SCZ in ECP of objects located in conditions of low corrosive activity of the environment. If the external conditions (the state of insulation, temperature, humidity, stray currents) change to the limits when an unacceptable mode is formed at the protected object, these stations cannot fulfill their task. To adjust their mode, the frequent presence of maintenance personnel is necessary, otherwise the ECP task is partially fulfilled.

SpecificationsVHC

First of all, the VHC must be selected based on the requirements set out in the 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 noise generated by the RMS should be no higher than the values ​​specified by GOST 16842, and the level of harmonics at the output should correspond to GOST 9.602.

The passport of the SKZ 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. 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 of the VHCs are far from operating in a nominal mode. The power load factor ranges from 0.3 to 1.0. In this case, the real efficiency for most of the stations produced today will noticeably fall with a decrease in the output power. This is especially noticeable for transformer RMS with the use of thyristors as a regulating element. For transformerless (high-frequency) RMS, the drop in efficiency with decreasing output power is significantly less.

The general view of the change in efficiency for the RMS of 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 equal to 70%, then be prepared for the fact that you have wasted another 30% of the electricity received from the network. And this is at its best the rated output power.

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

Ohmic (heat) losses in the windings of transformers, chokes and in active elements of the circuit;

· Energy consumption for the operation of the station control scheme;

· Energy losses in the form of radio emission; energy losses of pulsations of the output current of the station at the load.

This energy is radiated into the soil 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. It is not enough that at high levels of ripple and radio emission, electricity losses increase, but in addition, this uselessly scattered 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 figure it out with this parameter. The SKZ takes energy from the power grid and does it in every unit of time with such an intensity that we 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 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 in time when the mains voltage crosses zero, no power can be taken from it. However, when the voltage sine wave reaches its maximum, then at this moment our ability to take energy from the network is maximum. At any other point in time, this opportunity is less. Thus, it turns out that at any moment of time the power of the network differs from its power at the neighboring time. These values ​​of power are called instantaneous power at a given moment in time and it is difficult to operate with such a concept. 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 constant voltage. When we calculated the value of this constant voltage for our power grids, 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 by 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 RMS itself is not fully used, because it contains 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 returned energy is 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 match our capabilities with the capabilities of the supply network, i.e. synchronously with the 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 unity.

The station must transmit active energy as efficiently as possible to create a protective potential. The efficiency with which the VMS does this is assessed by the efficiency factor. 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, let's just say that transformer and transformer thyristor RMS 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 SCZ, 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 margin for improvement.

Consumerproperties

The consumer properties of such a device as VMS include the following:

1. Dimensions (edit), 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 during repair.

2. Maintainability. It is very important to be able to quickly replace a station or assembly on site. With subsequent repair in the laboratory, i.e. modular principle of building VMS.

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

availability of all necessary indicators and measuring instruments, availability of the possibility of remote control and monitoring of the operating mode of the RMS.

conclusions

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 should have:

· High efficiency in the entire range of loads;

· Power factor (cos I) not lower than 0.75 in the entire load range;

· Coefficient of output voltage ripple no more than 2%;

· Range of current and voltage regulation from 0 to 100%;

· Lightweight, durable and small-sized body;

· Modular construction principle, i.e. have high maintainability;

· I energy efficiency.

Other requirements for cathodic protection stations, such as overload and short circuit protection; automatic maintenance of a given load current - and other requirements are generally accepted and mandatory for all RMS.

In conclusion, we offer consumers a table comparing the parameters of the main produced 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 produced stations.

Protection of gas pipelines against corrosion is subdivided into passive and active.

Passive protection. This type of protection provides for the insulation of the gas pipeline. At the same time, a coating based on bitumen-polymer, bitumen-mineral, polymer, ethylene and bitumen-rubber mastics is used. The anti-corrosion coating must have sufficient mechanical strength, ductility, good adhesion to the pipe metal, possess dielectric properties, and it must not deteriorate from biological effects and contain components that cause corrosion of the pipe metal.

One of the widely used methods of passive protection is insulation with adhesive polymer tapes in widths of 400, 450, 500 mm or on request. According to GOST 20477-86, depending on the thickness of the tape, its base can be grades A or B.

Active protection. Active protection methods (cathodic, protective, electric drainage) are basically reduced to creating such an electrical regime for a gas pipeline, in which pipeline corrosion stops.

Rice. 1. Scheme of cathodic protection:

/ - drainage cable; 2 - constant current source; 3 - connection cable; 4 - ground electrode (anode); 5 - gas pipeline; b - drainage point

Cathodic protection. With cathodic protection (Fig. 1), an external power source is used to create a galvanic pair 2. In this case, the cathode is the gas pipeline 5, connected to the drainage point 6 by means of a drain cable to the negative electrode of the power supply; the anode is a metal rod 4, buried in the ground below its freezing zone.

One cathode station provides protection for a gas pipeline with a length of up to 1,000 m.

Protective (electrode) protection. With the protector protection, the gas pipeline section turns into a cathode not due to the power source, but due to the use of the protector. The latter is connected by a conductor to the gas pipeline and forms a galvanic pair with it, in which the gas pipeline is the cathode, and the protector is the anode. A metal with a more negative potential than iron is used as a protector.

The principle of operation of the tread protection is shown in Fig. 2. Current from the protector 3 through the ground enters the gas pipeline 6, and then along the insulated connecting cable to the protector. The protector, when current flows from it, will collapse, protecting the gas line.

The coverage area of ​​the protector is approximately 70 m. The main purpose of the protector is in addition to drainage or cathodic protection on remote gas pipelines for the complete removal of positive potentials.

Rice. 2. Scheme of protector (electrode) protection:

/ - check Point; 2 - connecting cables; 3 - protector (electrode);

4 - filler (salt + clay + water); 5 — paths of movement of protective current in the ground; 6 - gas pipeline

Electric drainage protection. With electric drainage protection, the current is diverted from the anode zone of the gas pipeline to the source (rail or negative bus of the traction substation). The protection zone is about 5 km.

Three types of drainage are used: straight (simple), polarized and reinforced.

Direct drainage is characterized by two-way conductance (Fig. 3). The drain cable only connects to the minus bus. The main disadvantage is the appearance of a positive potential on the gas pipeline when the butt joints of the rails are broken, therefore, despite the simplicity, these installations are not used in urban gas pipelines.

Polarized drainage has one-way conductance from the pipeline to the source. When a positive potential appears on the rails, the drain cable is automatically disconnected, so it can be connected to the rails.

Rice. 3. Diagram of direct (simple) drainage:

/ - protected gas pipeline; 2 — adjusting rheostat; 3 - ammeter; 4 — fuse; 5 — negative bus (suction cable)

Reinforced drainage is used when a positive or alternating potential with respect to the ground remains on the gas pipeline, and the rail potential at the point of current drainage is higher than the gas pipeline potential. In reinforced drainage, an EMF source is additionally included in the circuit, which makes it possible to increase the drainage current. In this case, the ground is the rails.

Insulating flange connections and inserts. They are used in addition to electrochemical protection devices and allow you to break the gas pipeline into separate sections, reducing the conductivity and the strength of the current flowing through the gas pipeline. Electro-insulating joints (EIS) - gaskets between flanges made of rubber or ebonite. Polyethylene pipe inserts are used to cut off various underground structures from each other. The installation of the EIS leads to a reduction in electricity costs by eliminating the loss of overflow current to adjacent communications. EIS is installed at the inputs to consumers, underground and above-water crossings of gas pipelines through obstacles, as well as at the inputs of gas pipelines to the GDS, Hydraulic fracturing and the GRU.

Electrical jumpers. Electrical jumpers are installed on adjacent metal structures in the case when there are positive potentials on one structure (anode zone), and negative potentials on the other (cathode zone), while negative potentials are established on both structures. Jumpers are used when laying gas pipelines of various pressures along the same street.

Electrochemical corrosion protection consists of cathodic and drainage protection. Cathodic protection of pipelines is carried out by two main methods: the use of metal anodes-protectors (galvanic protective method) and the use of external DC sources, the minus of which is connected to the pipe, and the plus to the anode grounding (electrical method).

Rice. 1. Principle of operation of cathodic protection

Galvanic protective corrosion protection

The most obvious way to implement electrochemical protection of a metal structure in direct contact with an electrolytic medium is the method of galvanic protection, which is based on the fact that different metals in the electrolyte have different electrode potentials. Thus, 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 a protector anode and will be destroyed, protecting the metal with a less negative potential. Protectors essentially serve as portable power sources.

Magnesium, aluminum and zinc are used as the main materials for the manufacture of protectors. From a comparison of the properties of magnesium, aluminum and zinc, it can be seen that of the elements under consideration, magnesium has the greatest electromotive force. At the same time, one of the most important practical characteristics of treads is the efficiency, which shows the fraction of the tread mass used to generate useful electrical energy in the circuit. K.P.D. protectors made of magnesium and magnesium alloys rarely exceed 50% in, in contrast to protectors based on Zn and Al with K.P.D. 90% or more.

Rice. 2. Examples of magnesium protectors

Typically, protector installations are used for cathodic protection of pipelines that do not have electrical contacts with adjacent long-distance communications, individual sections of pipelines, as well as tanks, steel protective casings (cartridges), underground tanks and containers, steel supports and piles, and other concentrated objects.

At the same time, tread mounts are very sensitive to errors in their placement and configuration. The wrong choice or placement of protector installations leads to a sharp decrease in their effectiveness.

Cathodic corrosion protection

The most common method of electrochemical corrosion protection of underground metal structures is cathodic protection, which is carried out by cathodic polarization of the protected metal surface. In practice, this is done by connecting the protected pipeline to the negative pole of an external DC source called a cathodic protection station. The positive pole of the source is connected by a cable to an external auxiliary electrode made of metal, graphite or conductive rubber. This external electrode is placed in the same corrosive environment as the object to be protected, in the case of underground field pipelines, in the soil. Thus, a closed electrical circuit is formed: an additional external electrode - soil electrolyte - a pipeline - a cathode cable - a direct current source - an anode cable. As part of this electrical circuit, the pipeline is the cathode, and an additional external electrode connected to the positive pole of the DC source becomes the anode. This electrode is called anode ground. The negatively charged pole of the current source connected to the pipeline, in the presence of an external anode grounding, cathode polarizes the pipeline, while the potential of the anode and cathode sections is practically equalized.

Thus, the cathodic protection system consists of a protected structure, a direct current source (cathodic protection station), anode grounding, connecting anode and cathodic lines, the surrounding conductive medium (soil), as well as elements of the monitoring system - control and measuring points.

Drainage protection against corrosion

Drainage protection of pipelines against corrosion by stray currents is carried out by directional diversion of these currents to the source or to the ground. Installation of drainage protection can be of several types: earthen, direct, polarized and reinforced drainage.

Rice. 3. Drainage protection station

Earthen drainage is carried out by grounding pipelines with additional electrodes in the places of their anode zones, direct drainage - by creating an electrical bridge between the pipeline and the negative pole of the source of stray currents, for example, the rail network of an electrified railway. Polarized drainage, unlike direct drainage, has only one-sided conductivity, therefore, when a positive potential appears on the rails, the drainage is automatically turned off. In reinforced drainage, a current converter is additionally included in the circuit, which makes it possible to increase the drainage current.