A number of boilers uses river and heat networks for feeding water water With low pH and low rigidity. Additional river water treatment on a tap station usually leads to a decrease in the PN, a decrease in alkalinity and an increase in aggressive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in the circuits of connection used for large heat supply systems with direct water treatment hot water (2000h3000 t / h). Water softening according to the Na-cation scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - stiffery salts.
With a poorly established deaeration of water and possible increasing concentrations of oxygen and carbon dioxide due to the lack of additional protective measures in the heat supply systems of the internal corrosion, Heatinglery equipment of the CHP.
During the examination of the feed tract by one of the CHP of Leningrad, the following data was obtained by corrosion velocity, g / (m2 · 4):
Corrosion Indicator Installation Place
In the pipeline of the feeding water after heaters of the heating system before the deaerators of the pipe with a thickness of 7 mm climbed over the year of operation in places up to 1 mm in some sections, through fistulas were formed.
The causes of ulcerative corrosion of water boilers are as follows:
insufficient removal of oxygen from feeding water;
low pH value due to the presence of aggressive carbon dioxide
(up to 10h15 mg / l);
the accumulation of products of oxygen corrosion of iron (Fe2O3;) on heat transfer surfaces.
Operation of equipment on network water with iron concentration Over 600 μg / l usually leads to the fact that several thousand hours of operation of hot water boilers are observed intensive (over 1000 g / m2) by iron-oxide deposits of their heating surfaces. At the same time, often emerging leaks in the pipes of the convective part are noted. In the composition of deposits, the content of iron oxides usually reaches 80ch90%.
Especially important for the operation of hot water boilers are starting periods. In the initial period of operation on one CHP, the removal of oxygen was not ensured to the norms installed by the PTE. The content of oxygen in the feed water exceeded these norms 10 times.
The concentration of iron in the feed water reached - 1000 μg / l, and in reverse water Heating networks - 3500 μg / l. After the first year of operation, cutting from pipelines of the network water were made, it turned out that the contamination of their surface with corrosion products was over 2000 g / m2.
It should be noted that on this CHP, before turning on the boiler, the internal surfaces of the on-screen pipes and the pipes of the convective beam were subjected to chemical cleaning. By the time the screening of samples of the on-screen pipes, the boiler worked 5300 hours. The sample of the on-screen pipe had an uneven layer of yellow-pointed sediments of black and brown color, firmly related to the metal; The height of the tubercles 10h12 mm; Specific contamination 2303 g / m2.
The composition of deposits,%
The surface of the metal under the layer of deposits was amazed by ulcers with a depth of 1 mm. A convective beam tubes from the inside were brought by deposits of iron oxide type of black and brown color with a height of tubercles up to 3h4 mm. The surface of the metal under deposits is covered with ulcers of various sizes with a depth of 0.3 h1.2 and a diameter of 0.35h0.5 mm. Separate tubes had through holes (fistulas).
When the water-heating boilers are installed in the old systems of centralized heat supply, in which a significant amount of iron oxides have accumulated, there are cases of depositing these oxides in the heated boiler pipes. Before turning on the boilers, it is necessary to make a thorough flushing of the entire system.
A number of researchers recognize an important role in the occurrence of submissive corrosion of the rusting process of water boilers under their downtime, when not taken proper measures to prevent parking corrosion. Corrosion foci arising from atmospheric air to the wet surfaces of the boilers continue to function when the boilers are working.
The system of iron - water steam is thermodynamically unstable. The interaction of these substances can proceed with the formation of the Magnetite Fe 3 O 4 or Vystit Feo:
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The analysis of reactions (2.1) - (2.3) indicates a peculiar decomposition of water vapor when interacting with a metal with the formation of molecular hydrogen, which is not a consequence of the actual thermal dissociation of water vapor. From equations (2.1) - (2.3) it follows that during corrosion of steels in an overheated pair in the absence of oxygen on the surface only Fe 3 O 4 or FEO may form.
If there is an oxygen in a superheated pair (for example, in neutral aqueous modes, with dosing of oxygen into condensate), the formation of hematite Fe 2 O 3 is possible due to the milking milknetite.
It is believed that corrosion in a pair, starting at a temperature of 570 ° C, is a chemical. Currently, the limiting overheating temperature for all boilers is reduced to 545 ° C, and, therefore, electrochemical corrosion occurs in the steamers. The outlet sections of the primary steamers are performed from the corrosion-resistant austenitic stainless steel, the outlet sections of intermediate performances having the same finite overheating temperature (545 ° C), from pearlit steels. Therefore, corrosion of intermediate performances usually manifests itself to a strong extent.
As a result of the effects of steam on steel on its originally clean surface gradually a so-called topotactic layer is formed, tightly adhesive with the metal itself and therefore protecting it from corrosion. Over time, the second so-called epitactic layer is growing on this layer. Both of these layers for the temperature level of steam to 545 ° C are magnetite, but the structure is not the same - the rotary layer is coarse-grained and does not protect against corrosion.
Wheel decomposition rate
mGN. 2 /(cm 2 h)
Fig. 2.1. The dependence of the expansion speed of the superheated steam
from the temperature of the wall
Influence the corrosion of overheating surfaces does not manage to affect the water mode. Therefore, the main task of the water-chemical mode of the actually steamer is in systematic observation of the state of the metal of the steamers in order to prevent the destruction of the topotactic layer. This can occur due to falling into parirements and precipitation of individual impurities, especially salts, which is possible, for example, as a result of a sharp increase in the level of high pressure boilers. Associated with these sediments of salts in a steamer can lead both to an increase in the temperature of the wall and to the destruction of the protective oxide topotactic film, which can be judged by the sharp increase in the steam decomposition rate (Fig. 2.1).
A significant part of the corrosion damage to the equipment of thermal power plants is accounted for by the path of nutrient water, where the metal is under the most difficult conditions, the cause of which is the corrosive aggressiveness of chemically treated water, condensate, distillate and mixtures of them. On steam-turbine power plants, the main source of feed water pollution with copper compounds is ammonia corrosion of turbine condensers and low-pressure regenerative heaters, the pipe system of which is made of brass.
Tract nutrient water Parroid turbine power plants can be divided into two main areas: to the thermal deaerator and after it, and the conditions for the flow in these corrosion are sharply different. The elements of the first section of the feed water path, located to the deaerator, include pipelines, tanks, condensate pumps, condensate pipes and other equipment. A characteristic feature of corrosion of this part of the nutrient tract is the absence of the possibility of exhausting aggressive agents, that is, coalic acid and oxygen contained in water. Due to the continuous receipt and movement of new portions of water through the tract, there is a constant replenishment. Continuous removal of part of the iron reaction products with water and the influx of fresh portions of aggressive agents create favorable conditions for the intensive flow of corrosion processes.
The source of oxygen appearance in the condensate turbines are air supplies in the tail part of the turbines and in the condensate pumps. Heated water containing 2 and CO 2 in surface heaters located on the first section of the nutrient path, up to 60-80 ° C and above leads to serious corrosion damage to brass pipes. The latter become fragile, and often brass after several months of work acquires the spongy structure as a result of a pronounced electoral corrosion.
Elements of the second section of the path of nutrient water - from the deaerator to the steam generator - include nutritional pumps and highways, regenerative heaters and economizers. The water temperature in this area as a result of sequential heating of water in regenerative heaters and water economizers is approaching the temperature of the boiler water. The cause of corrosion of equipment belonging to this part of the path is mainly the impact on the metal dissolved in nutrient water of free carbon dioxide, the source of which is the added chemically treated water. With an elevated concentration of hydrogen ions (pH< 7,0), обусловленной наличием растворенной углекислоты и значительным подогревом воды, процесс коррозии на этом участке питательного тракта развивается преимущественно с выделением водорода. Коррозия имеет сравнительно равномерный характер.
In the presence of equipment made of brass (low pressure heaters, condensers), the enrichment of water with copper compounds by a parokondensate tract flows in the presence of oxygen and free ammonia. An increase in solubility of hydrated copper oxide occurs due to the formation of copper-ammonia complexes, for example, Cu (NH 3) 4 (OH) 2. These corrosion products of brass tubes of heaters low pressure We begin to decompose on the parts of the path of the regenerative heaters of high pressure (paragraph. d.) to form less soluble copper oxides, partially precipitated on the surface of the tubes p. D. Medical deposits on pipes p. d. contribute to their corrosion during operation and long-term parking equipment without preservation.
With an insufficiently deep thermal deaeration of nutritious water, ulcerative corrosion is observed mainly at the input sections of economizers, where oxygen is released due to a noticeable increase in the temperature of the nutrient water, as well as in the congestion sections of the nutrient path.
The heat-fonding equipment of steam consumers and pipelines, which returns the production condensate on the CHP, is corrosion under the action of oxygen and coal acid contained in it. The appearance of oxygen is explained by the contact of condensate with air in open tanks (when open scheme Condensate collection) and suspension through looseness in equipment.
The main activities to prevent corrosion of equipment located on the first section of the path of nutritious water (from water preparatory installation to thermal deaerator) are:
1) the use of protective anticorrosive coating surfaces of water preparatory equipment and a tank farm, which are washed with solutions of acid reagents or corrosive-aggressive waters using rubber, epoxy resins, perchlorvinyl-based varnishes, liquid nairita and silicone;
2) the use of acid-resistant pipes and reinforcements made of polymeric materials (polyethylene, polyisobutylene, polypropylene, etc.) or steel pipes and fittings, lined with protective coatings, applied by gasflame spraying method;
3) the use of pipes of heat exchange apparatuses from corrosion-resistant metals (red copper, stainless steel);
4) removal of free carbon dioxide from the added chemically treated water;
5) the constant output of non-condensable gases (oxygen and coalic acid) from the steam chambers of the regenerative heaters of low pressure, coolers and network water heaters and the rapid removal of the condensate formed in them;
6) careful sealing of condensate pumps, reinforcement and flange compounds of nutritional pipelines under vacuum;
7) ensuring sufficient tightness of turbine capacitors from cooling water and air and control over air suits using registering oxygen systems;
8) Equipment of capacitors with special degassing devices in order to remove oxygen from condensate.
To successfully combat the corrosion of equipment and pipelines located on the second section of the path of nutritious water (from thermal deaerators to steam generators), the following activities are applied:
1) Equipment of TPP thermal deaerators issued with any modes of operation deaerated water with residual oxygen content and carbon dioxide not exceeding permissible norms;
2) the maximum output of non-condensable gases from the steam chambers of the regenerative heaters of high pressure;
3) the use of corrosion-resistant metals for the manufacture of feed pumps in contact with water;
4) anticorrosive protection of nutrient and drainage tanks by applying non-metallic coatings, resistant at temperatures up to 80-100 ° C, for example asbvinyl (varnish ethinol mixtures with asbestos) or paintwork materials based on epoxy resins;
5) the selection of corrosion-resistant structural metals suitable for the manufacture of high-pressure regenerative heaters;
6) constant treatment of nutrient water with alkaline reagents in order to maintain the specified optimal meaning PH of nutrient water, in which carbon dioxide corrosion is suppressed and sufficient strength of the protective film is ensured;
7) constant treatment of nutrient water hydrazine for the binding of residual oxygen after thermal deaerators and creating an inhibitory effect of braking the transition of iron connections from the surface of the equipment into nutrient water;
8) sealing of nutritious water tanks by organizing the so-called closed system to prevent nutritious water from entering economizers of steam generators;
9) Implementation of the reliable preservation of equipment of the path of nutrient water during its downtime in reserve.
An effective method for reducing the concentration of corrosion products in condensate, returned to CEP consumers with consumers, is the introduction of turbines to select consumers, film-forming amines - octadecylamine or its substitutes. At the concentration of these substances in a pair, equal to 2-3 mg / dm 3 , you can reduce the content of iron oxides in the production condensate 10-15 times. The dosing of the aqueous emulsion of polyamines using a pump-dispenser does not depend on the concentration in the condensate of coalic acid, since they are not associated with the neutralizing properties, but is based on the ability of these amines to form on the surface of steel, brass and other metals insoluble and unsatisted films with water.
Low-temperature corrosion are subjected to surface heating of tubular and regenerative air heaters, low-temperature economizers, as well as metal gas supplies and chimneys At metal temperatures below the dew point flue gases. The source of low-temperature corrosion is SO 3 sulfuride, forming a seam-acid pair in flue gases, which is condensed at temperatures of the dew point of flue gases. Several thousandths of the percentage of SO 3 in gases are sufficient to cause metal corrosion at a speed greater than 1 mm / year. Low-temperature corrosion slows down when organizing a foil process with small excess airs, as well as when using additives to fuel and increasing the corrosion resistance of the metal.
High-temperature corrosion are subjected to ellow screens of drum and straight-flow boilers when burning solid fuel, steam steampers and their attachments, as well as the screens of the lower radiation part of the supercritical pressure boilers when burning sulfur fuel oil.
Corrosion of the inner surface of the pipes is a consequence of the interaction with the metal of oxygen gas and carbon dioxide gas) or salts (chlorides and sulfates) contained in boiler water. In modern boilers of supercritical pressure of steam, the content of gases and corrosionactive salts as a result of deep desalting of nutritious water and thermal deaeration is slight and the main cause of corrosion is the interaction of metal with water and steam. Corrosion of the inner surface of the pipes manifests itself in the formation of OSPIN, Yazvin, shells and cracks; The outer surface of damaged pipes may not differ from healthy.
Damage as a result of internal corrosion of pipes also include:
Oxygen parking corrosion affecting any sections of the inner surface of the pipes. The most intensively affected areas covered with water-soluble sediments (pipe steamers and the transition zone of the forwarding boilers);
submissive alkaline corrosion of boiling and on-screen pipes, occurring under the action of concentrated alkali due to evaporation of water under the layer of sludge;
Corrosion fatigue manifested in the form of cracks in boiling and screen pipes as a result of the simultaneous effect of the corrosion medium and variable thermal stresses.
Okalo is formed on pipes due to overheating them to temperatures significantly exceeding the calculated one. Due to the increase in the productivity of the bootaggers, there were increasing cases of the failure of pipeline pipes due to insufficient loan resistance to the fuel gases. Intensive scale is most often observed when combing fuel oil.
Wearing pipe walls occurs as a result of an abreasting action of coal and shale dust and ash, as well as jets of steam coming out of damaged adjacent pipes or sniffing vehicles. Sometimes the cause of wear and stagnation of the pipe walls is the fraction used to clean the heating surfaces. Places and degree of wear of pipes are determined by outer inspection and measurement of their diameter. The actual thickness of the pipe wall is measured by ultrasonic thickness gauge.
Warning of screen and boiling pipes, as well as individual pipes and sections of wall panels of the radiation part of the direct-flow boilers occurs when the installation of pipes with unevenly tension, the cliff of fastening of pipes, water lunch and due to lack of freedom for their thermal displacements. Change the coils and shirm of the steamer occurs mainly due to the burning of suspensions and fasteners, excessive and uneven tension allowed when installing or replacing individual elements. Change the coil of the water economizer is due to the brave and displacement of supports and suspension.
Fistulas, foiling, cracks and breaks may also appear as a result: deposits in the pipes of scale, corrosion products, technological scale, welding graph and other foreign objects that slow down the circulation of water and contributing to the overheating of the pipes; stagnant fraction; The inconsistencies of the brand became parameters of steam and the temperature of the gases; external mechanical damage; Violations of operating modes.
The conditions in which elements of steam boilers are located are extremely diverse.
As shown numerous corrosion tests and industrial observations, low-alloyed and even austenitic steel during the operation of boilers can be subjected to intensive corrosion.
The corrosion of the metal surfaces of the steam boilers causes its premature wear, and sometimes leads to serious malfunctions and accidents.
Most emergency stops of boilers fall on through corrosion damage to screen, economy - grain, steam heating pipes and borants of boilers. The appearance of even one corrosion fistula in the direct-flow boiler leads to the stop of the entire block, which is associated with the non-performance of electricity. Corrosion of high and ultra-high drum boilers has become the main cause of failures in the work of the CHP. 90% of failures in work due to corrosion damage occurred on drum boilers Pressure 15.5 MPa. A significant amount of corrosion damage to the screen pipes of salt compartments was in the "zones of maximum thermal loads.
Conducted by US specialists by surveys 238 boilers (blocks with a capacity of 50 to 600 MW), 1719 unscheduled downtime were recorded. About 2/3 downtime boilers were caused by corrosion, of which 20% accounted for corrosion of steam generating pipes. In the US, internal corrosion "in 1955 was recognized as a serious problem after commissioning a large number of drum boilers with a pressure of 12.5-17 MPa.
By the end of 1970, about 20% of 610 such boilers were amazed by corrosion. Mainly internal corrosion exposed on-screen pipes, and steamers and economizers were stronger than it. With the improvement of nutrient water quality and the transition to the coordinated phosphating mode, with an increase in the parameters on the drum boilers of US power stations instead of viscous, plastic corrosion damage occurred sudden fragile destruction of the on-screen pipes. "As of J970 tons. For Kotlree with a pressure of 12.5; 14.8 and 17 MPa, the destruction of pipes due to corrosion damage was 30, 33 and 65%, respectively.
In terms of the conditions of the corrosion process, the atmospheric corrosion flows under the action of atmospheric, as well as humid gases; Gas, due to the interaction of metal with various gases - oxygen, chlorine, etc. - at high temperatures, and corrosion in electrolytes, in most cases occurring in aqueous solutions.
By the nature of corrosion processes, the boiler metal may be subject to chemical and electrochemical corrosion, as well as their joint impact.
When the surfaces of the heating of steam boilers occurs, high-temperature gas corrosion in the oxidative and reducing atmospheres of flue gases and low-temperature electrochemical corrosion of the tail surfaces of heating.
Studies found that high-temperature corrosion of heating surfaces is most intensively proceeds only if there are excess free oxygen in the furnace gas and in the presence of molten vanadium oxides.
High-temperature gas or sulphide corrosion in the oxidative atmosphere of flue gases affects the tubes of shirm and convective superheater, the first rows of boiling beams, the metal of distortioning spacers between pipes, racks and suspension.
High-temperature gas corrosion in restoration of the atmosphere was observed on the on-screen pipes of the heat chambers of a series of high and supercritical pressure boilers.
Corrosion of pipes for heating surfaces with a gas side represents a complex physico-chemical process of interaction of flue gases and external sediments with oxides - films and metal pipes. The development of this process is influenced by the time-changing intensive heat flows and high mechanical stresses arising from internal pressure and self-compensation.
On the boilers of medium and low pressure "the temperature of the walls of the screens determined by the boiling point of water is lower, and therefore this type of metal destruction is not observed.
Corrosion of heating surfaces from flue gases (outer corrosion) is the process of the destruction of the metal as a result of interaction with combustion products, aggressive gases, solutions and melts of mineral compounds.
Under corrosion of metal understands the gradual destruction of the metal, which is due to the chemical or electrochemical effects of the external environment.
\\ Metal destruction processes resulting from their direct chemical interaction with the environment are chemical corrosion.
Chemical corrosion occurs when the metal with superheated ferry and dry gases. Chemical corrosion in dry gases is called gas corrosion.
In the firebox and gas strokes of the boiler, gas corrosion of the outer surface of the pipes and steaks of steam-heater occurs under the influence of oxygen, carbon dioxide, water vapor, sulfur and other gases; The inner surface of the pipes - as a result of interaction with steam or water.
Electrochemical corrosion in contrast to the chemical is characterized by the fact that the reaction occurring with it is accompanied by the occurrence of electric current.
The carriers of electricity in solutions are the ions present in them due to the dissociation of molecules, and in metals - free electrons:
The intracerene surface is mainly susceptible to electrochemical corrosion. According to modern ideas, its manifestation is due to two independent processes: an anode, in which the metal ions are transferred to the solution in the form of hydrated ions, and cathodic, in which the assimilation of excess electrons depolarizers occurs. Depolarizers may be atoms, ions, molecules that are restored.
According to external signs, the solid (general) and local (local) form of corrosion destruction is distinguished.
With a general corrosion, the entire spoofing surface of heating with an aggressive medium is subjected to corrosion, evenly drowned with the inner or outdoor side. With local corrosion, the destruction occurs in separate areas of the surface, the remaining surface of the metal is not affected by damage.
Local local local stains include corrosive, ulcerative, point, intercrystalline, corrosion cracking, corrosion metal fatigue.
A typical example of destruction from electrochemical corrosion.
Destruction from the outer surface of the HDC 042x5 mm pipes from steel 12x1mf TPP-110 boilers occurred on a horizontal section at the bottom of the lifting-hydraulic loop in the zone adjacent to the sub-screen screen. On the back of the pipe there was a disclosure with a small refinement of the edges at the destroyer. The cause of the destruction was the thinning of the pipe wall of about 2 mm in corrosion due to the weasure of the jet of water. After the stop of the boiler, the 850 t / h with the anthracite binary dust (liquid slag), 25.5 MPa and the temperature of the superheated steam 540 ° C on the pipes remained wet slag and ash in which electrochemical corrosion was intensively flowed. Outside the pipe was covered with a thick layer of buoy hydroxide of iron. The internal diameter of the pipes was within tolerances on the pipes of high and ultra-high pressure boilers. The dimensions of the outer diameter have deviations that go beyond the minus tolerance: the minimum outer diameter. was 39 mm with minimally permissible 41.7 mm. The wall thickness near the damage from corrosion was only 3.1 mm at a nominal pipe thickness of 5 mm.
Metal microstructure is homogeneous in length and circle. On the inner surface of the pipe there is a deductible layer formed during the oxidation of the pipe in the process of heat treatment. There is no such layer on the outside.
Surveys of PCC pipes after the first break made it possible to find out the cause of destruction. It was decided to replace the HPC and about changing the technology of divisions. In this case, electrochemical corrosion proceeded due to the presence of a thin electrolyte film.
Ulcerative corrosion proceeds intensively on individual small sites Surfaces, but often for considerable depth. When the diameter of Yazvin is about 0.2-1 mm, it is called point.
In places where yazvins are formed, swearing can be formed. Yazvins are often filled with corrosion products, as a result of which they are not always able to detect them. An example is the destruction of the steel economyzer pipes with poor deaeration of nutritious water and low speeds of water in the pipes.
Despite the fact that a significant part of the pipe metal is amazed, due to through fistulas, it is necessary to completely replace the economizer coils.
The metal of steam boilers is subjected to the following dangerous types of corrosion: oxygen corrosion during the boilers and find them in repair; intercrystallite corrosion in the places of evaporation of boiler water; conducting corrosion; corrosion cracking of elements of boilers made of austenitic steels; After corrosion. a brief description of The specified types of corrosion of metal boilers are given in Table. Yul.
During the work of boilers, metal corrosion is distinguished by corrosion under load and parking corrosion.
Corrosion under load are most susceptible. Moveless boiler elements in contact with a two-phase medium, i.e., screen and boiling pipes. The inner surface of the economizers and superheater when the boilers are affected by corrosion less. Corrosion under load flows in an enlightening medium.
Parking corrosion is manifested in underestimated. Elements of vertical coins of superheater, conductive pipes of horizontal coins of superheater
;
