Cathodic protection. A.I. Kheifets, System of electrochemical protection of pipelines of heating networks
Corrosion has a detrimental effect on the technical condition of underground pipelines; under its influence, the integrity of the gas pipeline is compromised and cracks appear. To protect against such a process, electrochemical protection of the gas pipeline is used.
Corrosion of underground pipelines and means of protection against it
Per condition steel pipelines is influenced by soil moisture, its structure and chemical composition. The temperature of the gas conveyed through pipes, currents wandering in the ground caused by electrified transport and climatic conditions generally.
Types of corrosion:
- Superficial. Spreads in a continuous layer over the surface of the product. Represents the least danger to the gas pipeline.
- Local. Manifests itself in the form of ulcers, cracks, spots. The most dangerous type of corrosion.
- Fatigue corrosion failure. The process of gradual accumulation of damage.
Methods of electrochemical protection against corrosion:
- passive method;
- active method.
The essence of the passive method of electrochemical protection is to apply a special protective layer to the surface of the gas pipeline that prevents harmful effects environment. Such coverage could be: 
- bitumen;
- polymer tape;
- coal tar pitch;
- epoxy resins.
In practice, it is rarely possible to apply an electrochemical coating evenly to a gas pipeline. In places of gaps, the metal is still damaged over time.
The active method of electrochemical protection or the cathodic polarization method is to create a negative potential on the surface of the pipeline, preventing the leakage of electricity, thereby preventing the occurrence of corrosion.
Operating principle of electrochemical protection
To protect a gas pipeline from corrosion, it is necessary to create a cathodic reaction and eliminate the anodic reaction. To do this, a negative potential is forcibly created on the protected pipeline.
Anode electrodes are placed in the ground, and the negative pole of an external current source is connected directly to the cathode - the protected object. To complete the electrical circuit, the positive pole of the current source is connected to the anode - an additional electrode installed in general environment with a protected pipeline.
The anode in this electrical circuit performs the grounding function. Due to the fact that the anode has a more positive potential than the metal object, its anodic dissolution occurs.
The corrosion process is suppressed under the influence of the negatively charged field of the protected object. With cathodic protection against corrosion, the anode electrode will be directly subjected to deterioration.
To increase the service life of anodes, they are made from inert materials, resistant to dissolution and other influences of external factors.
An electrochemical protection station is a device that serves as a source of external current in a cathodic protection system. This installation is connected to the network, 220 W and produces electricity with set output values.
The station is installed on the ground next to the gas pipeline. It must have a degree of protection IP34 or higher, since it works outdoors.
Cathodic protection stations can have different technical specifications and functional features.
Types of cathodic protection stations:
- transformer;
- inverter
Transformer stations for electrochemical protection are gradually becoming a thing of the past. They are a structure consisting of a transformer operating at a frequency of 50 Hz and a thyristor rectifier. The disadvantage of such devices is the non-sinusoidal shape of the generated energy. As a result, a strong current pulsation occurs at the output and its power decreases.
An inverter electrochemical protection station has an advantage over a transformer one. Its principle is based on the operation of high-frequency pulse converters. A feature of inverter devices is the dependence of the size of the transformer unit on the frequency of current conversion. With a higher signal frequency, less cable is required and heat loss is reduced. In inverter stations, thanks to smoothing filters, the ripple level of the produced current has a smaller amplitude.
The electrical circuit that powers the cathodic protection station looks like this: anodic grounding - soil - insulation of the protected object.
When installing a corrosion protection station, the following parameters are taken into account:
- position of the anode grounding (anode-ground);
- soil resistance;
- electrical conductivity of the object's insulation.
Drainage protection installations for gas pipelines
With the drainage method of electrochemical protection, a current source is not required; the gas pipeline, using currents wandering in the ground, communicates with the traction rails of the railway transport. Electrical interconnection is achieved due to the potential difference between the railway rails and the gas pipeline.
By means of the drainage current, a displacement of the electric field of the gas pipeline located in the ground is created. The protective role in this design is played by fuses, as well as automatic maximum load switches with reset, which adjust the operation of the drainage circuit after a drop in the high voltage.
The polarized electric drainage system is carried out using valve block connections. Voltage regulation with this installation is carried out by switching active resistors. If the method fails, more powerful electrical drains are used in the form of electrochemical protection, where a railway rail serves as the anode grounding conductor.
Galvanic electrochemical protection installations
The use of protective installations for galvanic pipeline protection is justified if there is no voltage source near the facility - a power line, or the gas pipeline section is not large enough in size.
Galvanic equipment serves to protect against corrosion:
- underground metal structures not connected by an electrical circuit to external current sources;
- individual unprotected parts of gas pipelines;
- parts of gas pipelines that are isolated from the current source;
- pipelines under construction that are temporarily not connected to corrosion protection stations;
- other underground metal structures (piles, cartridges, tanks, supports, etc.).
Galvanic protection will work best in soils with electrical resistivity within 50 ohms.
Installations with extended or distributed anodes
When using a corrosion protection transformer station, the current is distributed along a sinusoid. This has an adverse effect on the protective electric field. Either excess voltage occurs at the protection point, which entails high energy consumption, or uncontrolled current leakage, which makes the electrochemical protection of the gas pipeline ineffective.
The practice of using extended or distributed anodes helps to circumvent the problem uneven distribution electricity. The inclusion of distributed anodes in the gas pipeline electrochemical protection scheme helps to increase the corrosion protection zone and smooth out the voltage line. With this scheme, anodes are placed in the ground along the entire gas pipeline.
An adjusting resistance or special equipment ensures that the current changes within the required limits, the anodic grounding voltage changes, and with this the protective potential of the object is regulated.
If several ground electrodes are used at once, the voltage of the protective object can be changed by changing the number of active anodes.
The ECP of a pipeline using protectors is based on the potential difference between the protector and the gas pipeline located in the ground. The soil in this case is an electrolyte; the metal is restored, and the protector body is destroyed.
Video: Protection against stray currents
A.I. Kheifets, Head of the Electrochemical Protection Service,
OJSC "Heating Network of St. Petersburg", St. Petersburg
Introduction
Protecting heating network pipelines from corrosion is a very important task, the solution of which largely determines the reliability of the entire centralized heat supply system. In St. Petersburg they prevail heating network underground laying, which are operated in corrosive conditions caused by both a dense network of long-distance underground communications and developed electrified transport, as well as the saturation of soils and soils with moisture and chemical reagents. There are two main ways to protect metals from corrosion: passive - by applying insulating coatings to their surface and active - by using electrochemical protection means.
A little theory
Metal structures operated in various environments (atmosphere, water, soil) are subject to the destructive effects of this environment. The destruction of a metal due to its interaction with the external environment is called corrosion. The essence of the corrosion process is the removal of atoms from the metal lattice, which can occur in two ways, which is why they distinguish between simply chemical and electrochemical corrosion.
Corrosion is chemical if, after breaking the metal bond, the metal atoms are directly connected chemical bond with those atoms or groups of atoms that are part of the oxidizing agents that take away the valence electrons of the metal. The process takes place without the participation of free electrons and is not accompanied by the appearance of electric current. An example is the formation of scale when iron-based materials interact with high temperature with oxygen.
Corrosion is electrochemical if a positively charged metal ion leaves the metal lattice, i.e. cation enters into contact not with the oxidizing agent, but with other components of the corrosive environment, and electrons are transferred to the oxidizing agent, released during the formation of the cation. In electrochemical corrosion, the removal of atoms from a metal lattice is carried out not as a result of one, as in chemical corrosion, but of two independent, but interconnected electrochemical processes: anodic (transition of “captured” metal cations into solution) and cathodic (binding of released electrons by an oxidizer). Oxidizing agents are hydrogen ions, which are found wherever water is present, and oxygen molecules. Electrochemical corrosion accompanied by the appearance of electric current.
Pipelines of heating networks are extended objects and their various sections are not in equal conditions from the point of view of the development of corrosion processes. Soils and soils absorb precipitation differently, melt water, have different air permeability. Specific electrical resistance soils are also different; it is its value (the lower, the more dangerous) that characterizes the corrosive aggressiveness of the environment. As a result, areas are formed along the surface of the pipelines where either anodic or cathodic reactions are predominantly carried out. The electrical conductivity of the metal is very high; electrons are almost instantly redistributed from the places where the anodic reaction occurs to the places where the cathodic reaction occurs (Fig. 1). In fact, something like galvanic cells or batteries arise, in which the soil plays the role of electrolyte, and the external circuit is an underground metal structure. The anodic zones are the positive electrode ("+"), and the cathodic zones are the negative electrode ("-"). When an electric current flows in the anode zones, atoms continuously escape from the metal lattice into external environment, i.e. metal dissolution.
A particular danger for pipelines of heating networks is stray currents that arise due to leakage of part of the current from transport electrical circuits into the soil or aqueous solutions, where they fall on metal structures. Where current exits from these structures, anodic dissolution of the metal occurs again into the soil or water. Such zones are especially often observed in areas of ground electric transport. Corrosion due to stray currents is sometimes called electrical corrosion. Such currents can reach values of several amperes. To give an idea: a current of 1 A, in accordance with Faraday’s first law, causes the dissolution of iron in the amount of 9.1 kg over the course of a year. If the current is concentrated in an area of 1 m2, then this corresponds to a decrease in the thickness of the pipe wall by 1.17 mm per year, i.e. in 6 years it would decrease by 7 mm.
The principle of operation of electrochemical protection (ECP) of the outer surface of a metal against corrosion is based on the fact that by shifting the potential of the metal by passing an external electric current, it is possible to change the rate of its corrosion. The relationship between potential and corrosion rate is nonlinear and ambiguous.
ECP based on the application of cathodic current is called cathodic protection. In production conditions, it is implemented in two versions.
1. In the first option, the necessary potential shift is ensured by connecting the protected structure to an external voltage source as a cathode, and auxiliary electrodes are used as an anode (Fig. 2).
The source is an adjustable rectifier, which converts industrial frequency voltage into direct voltage, and the anode grounding electrodes are combined into a circuit, the composition and location of the electrodes of which are determined by calculation. During operation, the mass of the electrodes of the anode grounding circuit decreases monotonically.
Cathodic polarization of a bare metal structure to the minimum protective potential requires significant currents, so it is usually cathodic protection used in conjunction with insulating coatings applied to the outer surface of the protected structure. Surface coating reduces the required current by several orders of magnitude. With cathodic protection, it is also necessary to control the value of the maximum potential, because it's too much great importance may lead to peeling of the insulating coating from the pipeline wall. Regulatory documents (Standard instructions for the protection of heating network pipelines from external corrosion RD 153-34.0-20.518-2003) establish that the minimum protective potential for heating networks is 1.1 V, and the maximum 2.5 V in the negative direction with respect to the non-polarizing copper sulfate reference electrode. Such values must be ensured throughout the protected area, and this is achieved the more accurately the better the metal is isolated from the ground.
2. The second option for cathodic protection is galvanic (or sacrificial) protection (Fig. 3). The principle of its operation is based on the fact that different metals are characterized by different values of standard electrode potentials. Cathodic polarization of the protected structure is achieved due to its contact with a more electronegative metal. The latter acts as an anode, and its electrochemical dissolution ensures the flow of cathodic current through the protected metal. The anode itself, made of magnesium, zinc, aluminum and their alloys, gradually deteriorates. The advantage of tread protection is that it does not require an external voltage source, but this type of protection can only be used on relatively short sections of pipelines (up to 60 m), as well as on steel casings.
3. To protect pipelines of heating networks from external corrosion under the influence of stray currents, electrical drainage (drainage) is used - a metal conductor connecting the area from which these currents flow with a tram or tram rail railway tracks. At a large distance to the rail, when such drainage is difficult to implement, an additional cast iron anode is used, which is buried in the ground and connected to the protected area.
In places where the electrolytic effect of stray currents combines with the currents of galvanic couples, a sharp increase in the rate of corrosion processes can occur. In such cases, enhanced drainage installations are used (Fig. 4), which allow not only to remove stray currents from pipelines, but also to provide them with the necessary amount of protective potential. Reinforced drainage is a conventional cathode station, connected with the negative pole to the structure being protected, and with the positive pole not to the anodic grounding, but to the rails of electrified transport.
4. ECP installations of owners of adjacent underground utilities, such as gas pipelines, can have a strong corrosive effect on the pipelines of heating networks (Fig. 5a). If the pipelines are in the zone of action of the cathode current of a “foreign” installation, then the destruction at the places where this current exits the steel pipe into the ground will be the same as that caused by stray currents. For protection, it is necessary to connect the pipelines of heating networks with the negative pole of the voltage source (Fig. 5b).
It is possible to shift the potential of a metal to protect it from corrosion not only towards negative, but also towards positive values. In this case, some metals pass into a passive state, and the dissolution current of the metal drops tens of times. This type of protection is called anodic; its advantage is that low currents are required to maintain the passive state of the metal. However, if the electrolyte contains chlorine and sulfur ions, metal corrosion can increase sharply and the anodic-polarized equipment itself may fail. Anodic protection is not used for heating networks.
The ECP at JSC Heating Network of St. Petersburg is operated and developed as a system, i.e. a set of interconnected components: stationary technical means, instrumental control and information database.
In accordance with the schedules, ECP service specialists routinely carry out corrosion measurements according to the established methodology in all sections of the main and distribution networks in places of access to underground pipelines (thermal chambers). After processing the measurement results, anode and cathode zones on pipelines, protection zones, and areas of dangerous influence of stray currents are determined. In addition, corrosion measurements are carried out during planned pitting and when eliminating defects in heating networks, where they are supplemented with the result chemical analysis soil. The measurement results are systematized and archived; they are valuable information for both proper organization operation of thermal mechanical equipment, and for planning the construction of additional ECP facilities.
More detailed and thorough corrosion inspections of heating main zones are carried out by a specialized contractor. These inspections are carried out in corrosion-hazardous areas, usually after reconstruction (relocation) of heating networks, because the use of modern types of insulation, structures and technologies ensures better galvanic isolation of metal from concrete and from the ground than before. This means, among other things, a possible change in the boundaries of the anode and cathode zones, areas of influence of stray currents. The results of the examinations are presented in the form of reports containing information about changes in the values of electrode potentials on different areas surfaces of pipelines under various operating modes (Fig. 6) not only of our own, but also of ECP equipment belonging to third-party organizations. Using mathematical modeling methods (Fig. 7), the type, quantity and location of the necessary additional ECP equipment for further design are calculated.
Currently, JSC Teploset St. Petersburg» owns 432 ECP installations, of which: cathodic protection installations - 204 pcs. (including cathodic protection installations belonging to the category of joint protection against external corrosion of pipelines of heating networks and gas pipelines laid nearby - 20 pcs.); enhanced drainage installations - 8 pcs.; tread protection installations - 220 pcs. Maintenance of cathodic joint protection installations is carried out by Antikor OJSC.
In accordance with the requirements of regulatory documents (Corrosion protection. Design of electrochemical protection of underground structures. STO Gazprom 2-3.5-047-2006), ECP installations should not have a negative impact on neighboring communications. OJSC Antikor, which is engaged in electrochemical protection of gas pipelines in St. Petersburg, during the reconstruction and new construction of its installations, promptly notifies OJSC Heating Network of St. Petersburg about technical feasibility connecting sections of heating networks to the ECP of gas pipelines, if provided for by the project.
During the operation of all, except drainage, ECP installations, the mass of their grounded electrodes is continuously lost, because this constitutes the physical essence of electrochemical protection. The moment of “death” of the anode grounding circuit or protector inevitably comes. It is possible and necessary to ensure a specified period of operation between major repairs of ECP installations using correct calculations
the required number and location of elements, choosing quality materials, strict adherence to installation technology. There may be cases of electrode failure due to local point damage. Since 2010, during reconstruction and new construction, we have been using ElZhK-1500 ferrosilide anode grounding conductors with contact unit protection instead of the previous EGT-1450. Over the course of a series recent years In ECP installations, only automatic converters of the UKZTA and PKZ-AR types are used (Fig. 8), which make it possible to continuously maintain the specified values of the anode current or protective potential on the pipeline.
The practice of equipping ECP installations with telemetric recorders has acquired particular importance (Fig. 9). These devices, manufactured in the form of built-in units, continuously remotely transmit information about the values of time-varying electrical quantities to a dedicated computer (Fig. 10). Archives are being created to analyze the operation of ECP installations. In addition, the telemetry system has an alarm function for unauthorized access of unauthorized persons to installations.
It is worth noting that before the start of construction and installation work, the contractor notifies the customer of the start date of work, design organization, an organization that carries out technical supervision of construction, and an organization to whose service the protective installations under construction will be transferred.
Our company has been engaged in electrochemical protection of heating networks from external corrosion since 1960, i.e. more than 50 years. Over the years, ECP specialists were part of various production divisions, and after the formation of OJSC Heating Network of St. Petersburg in 2010, a separate ECP service was created. Today it consists of 13 people who solve technical and organizational problems.
Technical tasks include: daily detours of two teams of electricians along given routes of ECP installations with Maintenance. At the same time, it is monitored whether third-party organizations are conducting correct design excavation in the area of our installations.
Maintenance of ECP installations includes:
■ inspection of all elements of the installation in order to identify external defects, checking the tightness of contacts, serviceability of installation, absence of mechanical damage to individual elements, absence of burns and signs of overheating, absence of excavations on the route of drainage cables and anode groundings;
■ checking the serviceability of fuses (if any);
■ cleaning the housing of the drain and cathode converter, the joint protection unit outside and inside;
■ measurement of current and voltage at the output of the converter or between galvanic anodes (protectors) and pipes;
■ measuring the pipeline potential at the installation connection point;
■ making an entry in the installation log about the results of the work performed;
■ measuring potentials at permanently fixed measuring points.
Current repairs and performance monitoring of ECP equipment are periodically carried out. ECP service specialists conduct technical supervision of production overhaul, reconstruction and capital construction of ECP installations by contractors. The compliance of the construction and installation work performed with the project is monitored.
Current repairs include:
■ measuring the insulation resistance of power cables;
■ repair of power lines;
■ repair of the rectifier unit;
■ repair of drainage cable.
Monitoring the efficiency of an ECP installation involves measuring protective potentials at measuring points throughout the protection zone of a given ECP installation. Monitoring the effectiveness of ECP of heating network pipelines is carried out at least 2 times a year, as well as when operating parameters of ECP installations change and when corrosive conditions change associated with:
■ laying new underground structures;
■ in connection with the repair work on heating networks;
■ installation of ECP on adjacent underground utilities.
Specialists of the ECP service conduct technical supervision of the overhaul, reconstruction and capital construction of ECP installations by contractors. The compliance of the construction and installation work performed with the project is monitored.
Organizational tasks include, first of all, obtaining permission to supply power to ECP stations from the networks of JSC Lenenergo. This is a multi-step algorithm, accompanied by a large amount of documentation. In addition to power supply, the ECP service is engaged in the preparation of targeted programs for new construction and repairs, verification and approval of projects, and preparation of technical specifications.
ECP installations against external corrosion of metal structures have been used for 100 years. The physical and chemical principle of their operation remains unchanged, but to increase their service life, reduce capital and operating costs, it is necessary to search and find new ones technical solutions. The use of extended electrodes for anodic grounding seems promising. Elastomeric electrodes are laid horizontally in a trench along the heating network pipelines at a depth
1.5 m and are divided into several sections to increase maintainability. The cost of such installations is less than when using traditional anodic grounding loops. In 2011, two installations with horizontal electrodes were already built.
Equipping ECP installations with telemetry units will continue, and in the future, information about the operation of all installations will be remotely transmitted and archived.
In 2011, an automated electricity metering project was completed for 59 ECP installations, and its implementation is scheduled for 2012
Work has already begun on entering the database of ECP installations into the unified information and analytical system of OJSC Heating Network of St. Petersburg. In the future, this will make it possible to quickly and more accurately determine priorities when drawing up a program for the reconstruction of sections of heating networks, and correctly organize excavation work when eliminating defects.
The main purpose of the ECP of heating networks is to ensure the operation of pipelines without damage throughout the entire regulatory period(25 years). To achieve this goal, it is necessary to treat the ECP as a system, without neglecting any of its components specified in this article. A few general considerations may be helpful.
1. In corrosion-hazardous areas, it is necessary to commission the ECP as soon as possible after the construction or reconstruction of a section of the heating network, i.e. protect metal from scratch.
2. On a section of pipelines that are electrically poorly insulated from the ground (destruction of thermal insulation, contact of metal with concrete structures, etc.), the installation of ECP will be of little effectiveness, because the protective current created by it will not be distributed over hundreds of meters along the pipes, but will flow into the ground at the “short-circuit” point.
3. If the low efficiency of the existing ECP installation is revealed (small difference in the value of the metal potential when the installation is turned on and off), it is necessary to reconstruct it by changing the location of the anode grounding loop (AGC) in relation to the protected pipelines.
4. When reconstructing and new construction of ECP installations, it is advisable to use the most best brands electrodes for KAZ, because failure of the circuit means failure of the entire installation, and to restore the KAZ, expensive excavation work will have to be carried out.
5. Coordination of activities regarding ECP with other owners of underground communications will make it possible to take measures to protect pipelines of heating networks from the harmful influence of “foreign” ECP installations, as well as, in some cases, organize joint protection.
The operating experience of heating networks of JSC Heating Network of St. Petersburg convincingly proves that ECP has been and remains an important component in a set of measures to increase the reliability of heat supply in St. Petersburg.
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 the technical parameters of the SCZ specified 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 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 steadily increases during operation due to dissolution and a decrease in the effective area work surface and formation of oxides.
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 formed cracks 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. How 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 has not been damaged, then its resistance to electric current is 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 feedback according to potential, 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 output harmonics 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 SSCs using thyristors as a regulating element. For transformerless (high-frequency) RMS, the drop in efficiency with a decrease in output power is significantly less.
A general view of the change in efficiency for VMS 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 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 the best case of 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 VCS passport also indicates the necessary full power, let's try to figure out 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 at a given time and this concept is difficult to operate with. 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 was called re active 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 this 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 SKZ does this is assessed by the efficiency factor. 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
The consumer properties of such a device as SKZ 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 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, the ability to remotely control and monitor 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 overload protection 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 in use. For convenience, the table shows stations of the same power, although many manufacturers can offer a whole range of produced stations.
METAL STRUCTURES"
Theoretical basis
Cathodic protection of underground metal structures
Operating principle of cathodic protection
When metal comes into contact with soils related to electrolytic environments, a corrosion process occurs, accompanied by the formation of an electric current, and a certain electrode potential is established. The magnitude of the electrode potential of the pipeline can be determined by the potential difference between two electrodes: the pipeline and the non-polarizing copper sulfate element. Thus, the value of the pipeline potential is the difference between its electrode potential and the potential of the reference electrode with respect to the ground. On the surface of the pipeline, electrode processes occur in a certain direction and changes in time are stationary in nature.
Stationary potential is usually called natural potential, implying the absence of stray and other induced currents on the pipeline.
The interaction of a corroding metal with an electrolyte is divided into two processes: anodic and cathodic, which take place simultaneously at different areas of the metal-electrolyte interface.
When protecting against corrosion, territorial separation of the anodic and cathodic processes is used. A current source with an additional grounding electrode is connected to the pipeline, with the help of which an external direct current is applied to the pipeline. In this case, the anodic process occurs on an additional grounding electrode.
Cathodic polarization of underground pipelines is carried out by applying an electric field from an external direct current source. The negative pole of the direct current source is connected to the structure being protected, while the pipeline is the cathode in relation to the ground, and the artificially created grounding anode is the positive pole.
Schematic diagram cathodic protection is shown in Fig. 14.1. With cathodic protection, the negative pole of the current source 2 is connected to pipeline 1, and the positive pole is connected to an artificially created anode-grounding device 3. When the current source is turned on, the current source from its pole through the anodic grounding enters the ground and through damaged areas of insulation 6 to the pipe. Then, through the drainage point 4 along the connecting wire 5, the current returns again to the minus of the power source. In this case, the process of cathodic polarization begins in the exposed sections of the pipeline.
Rice. 14.1. Schematic diagram of pipeline cathodic protection:
1 - pipeline; 2 - external DC source; 3 - anodic grounding;
4 - drainage point; 5 - drainage cable; 6 - cathode terminal contact;
7 - cathode terminal; 8 - damage to pipeline insulation
Since the voltage of the external current applied between the grounding electrode and the pipeline significantly exceeds the potential difference between the electrodes of the corrosion macropairs of the pipeline, the stationary potential of the anodic grounding does not play a decisive role.
With the inclusion of electrochemical protection ( j 0a.add) the distribution of currents of corrosion macropairs is disrupted, the values of the potential difference “pipe - ground” of the cathode sections ( j 0k) with the potential difference of the anode sections ( j 0a), conditions for polarization are provided.
Cathodic protection is regulated by maintaining the required protective potential. If, by applying an external current, the pipeline is polarized to the equilibrium potential ( j 0k = j 0a) dissolution of the metal (Fig. 14.2 a), then the anodic current stops and corrosion stops. Further increase in the protective current is impractical. With more positive values potential, the phenomenon of incomplete protection occurs (Fig. 14.2 b). It can occur during cathodic protection of a pipeline located in an area of strong influence of stray currents or when using protectors that do not have a sufficiently negative electrode potential (zinc protectors).
The criteria for protecting metal from corrosion are protective current density and protective potential.
Cathodic polarization of a bare metal structure to the protective potential requires significant currents. The most probable values of current densities required for the polarization of steel in various environments to the minimum protective potential (-0.85 V) in relation to the copper-sulfate reference electrode are given in Table. 14.1
Rice. 14.2. Corrosion diagram for the case of complete polarization (a) and
incomplete polarization (b)
Typically, cathodic protection is used in conjunction with insulating coatings applied to the outer surface of the pipeline. Surface coating reduces the required current by several orders of magnitude. Thus, for cathodic protection of steel with a good coating in soil, only 0.01 ... 0.2 mA/m 2 is required.
Table 14.1
Current density required for cathodic protection
bare steel surface in various environments
The protective current density for insulated main pipelines cannot become a reliable protection criterion due to the unknown distribution of damaged pipeline insulation, which determines the actual contact area of the metal with the ground. Even for an uninsulated pipe (a cartridge at an underground passage through railways and highways), the protective current density is determined by the geometric dimensions of the structure and is fictitious, since the proportion of the surface of the cartridge remains unknown, covered with constantly present passive protective layers (scale, etc.) and not participating during the process of depolarization. Therefore, protective current density as a protection criterion is used in some laboratory studies performed on metal samples.
Ensuring the protection of pipes from corrosive effects is carried out using various technologies. One of the most effective techniques electrochemical treatment is considered, including cathodic protection. In most cases, this option is used in combination, along with the treatment of metal structures with insulating compounds.
Main types of cathodic protection
Cathodic protection of pipelines against corrosion was developed back in the nineteenth century. This technology is the first were used in the shipbuilding industry and - the hull of the floating vessel was sheathed with anodic protectors, which minimized the corrosion processes of the copper alloy. A little later, this technology began to be actively used in other areas. In addition, the cathodic technique is currently considered the most effective technology anti-corrosion protection.
There are two types of cathodic protection for metal alloys:
The first option is considered the most common today, as it is faster and simpler. Using this technology you can cope with different types of corrosion:
- intercrystal;
- cracking of brass due to excessive stress;
- corrosion caused by the influence of stray electric currents;
- pitting corrosion, etc.
It should be noted that the first technique allows the processing of large-sized metal structures, and galvanic chemical electrical protection is intended only for small products.
Galvanic technology is very popular in the United States, but in our country it is almost never used, since the technology for constructing pipelines in the Russian Federation does not imply special insulation treatment, which is necessary for galvanic protection.
Without such a coating, steel corrosion increases under the influence of groundwater, which is extremely important for autumn and spring. In winter, after the water freezes, the corrosion process is significantly slowed down.
Description of technology
Cathodic corrosion protection is carried out using a direct electric current applied to the workpiece, and makes the workpiece potential negative. Rectifiers are often used for this purpose.
The object that is connected to the source of electric current is considered the “minus”, that is, the cathode, and the connected ground is the anode, that is, the “plus”. The main condition is the presence of a good electrically conductive environment. For underground pipes, this is soil.
When implementing this technology, a difference in electric potential must be maintained between the soil (electrically conductive medium) and the object being processed. The value of this indicator can be determined using a high-resistance voltmeter.
Features of effective work
Corrosion is often the culprit of pipeline depressurization. Due to damage to the metal structure, cracks, cavities and ruptures form on the structure. This problem is extremely relevant for underground pipelines, because they are constantly in contact with groundwater.
In this situation, the cathodic technique makes it possible to minimize the process of dissolution and oxidation of the metal alloy by changing the initial corrosion potential.
Practical test results suggest that the polarization potential of metal alloys using cathodic techniques slows down corrosion.
In order to achieve effective protection, you need to use a direct electric current to reduce the cathode potential of the material that was used to create the pipeline. In this situation, the rate of metal corrosion will not exceed ten micrometers per year.
In addition, cathodic protection is the most The best decision to protect the pipeline underground from the influence of stray electric currents. Stray currents are an electric charge that penetrates the soil during the operation of a lightning rod, the movement of electric trains, etc.
To provide anti-corrosion protection, power lines or portable generators operating on diesel fuel or gas can be used.
Special equipment
For protection purposes, special stations are used. This equipment includes several units:
- source of electric current;
- anode (grounding);
- measurement, control and management point;
- connecting wires and cords.
Station anodic protection allows you to provide protection to several pipelines that are located next to each other at once. Adjustment of the supplied electric current can be automatic or manual.
In our country, the Minerva-3000 installation is especially popular. The power indicators of this SCP are sufficient to protect approximately 40 kilometers of underground pipeline from corrosion.
The advantages of the installation include:
Remote control of the equipment is carried out using GPRS modules, which are built into the design.
