Corrosion and erosion in the boilers of medium and low pressure from the furnace. Corrosion of pipelines and hot water boilers Boiler houses corrosion and damage to boilers
Corrosion of steel in steam boilers occurring under the action of water vapor is reduced, mainly to the next reaction:
ZFE + 4N20 \u003d Fe2O3 + 4H2
It can be assumed that the inner surface of the boiler is a thin film of the magnetic oxide of iron. During the operation of the boiler, the oxide film is continuously destroyed and is formed again, and hydrogen is distinguished. Since the surface film of the magnetic oxide of iron is the main protection for steel, it should be maintained in the state of the smallest permeability for water.
For boilers, reinforcements, water and steam lines, predominantly simple carbon or low-alloy steel are used. The corrosion medium in all cases are water or water vapor varying degrees of purity.
The temperature at which the corrosion process may flow, ranges from the temperature of the room where the inactive boiler is located, to the boiling point of saturated solutions when the boiler occurs, which is sometimes 700 °. The solution may have a temperature significantly higher than the critical temperature of clean water (374 °). However, high concentrations of salts in boilers are rare.
The mechanism by which physical and chemical causes can lead to the destruction of the film in steam boilers, essentially differs from the mechanism studied at lower temperatures on less responsible equipment. The difference lies in the fact that the corrosion rate in boilers is much larger due to high temperature and pressure. The high speed of heat transfer from the walls of the boiler to the medium reaches 15 feces / cm2sec, also enhances corrosion.
Pottle corrosion
The form of corrosion sinks and their distribution on the metal surface may vary widely. Corrosion sinks are sometimes formed inside the already existing shells and are often arranged so close to each other that the surface becomes extremely uneven.Recognition of point corrosion
Finding out the causes of the formation of corrosion destruction of a certain type is often very difficult, since several reasons can act at the same time; In addition, a number of changes occurring during the cooling of the boiler from high temperature and during water descent, sometimes masks the phenomena that occurred during operation. However, experience significantly helps to recognize point corrosion in boilers. For example, it was observed that the presence in the corrosion sink or on the surface of the black magnetic oxide tuberculosis indicates that the active process was proceeded in the boiler. Such observations are often used when checking events adopted to protect against corrosion.
It should not be mixed by the oxide of iron, which is formed in places of active corrosion, with black magnetic oxide of iron, sometimes present in the form of suspension in boiler water. It must be remembered that neither the total amount of fine magnetic oxide of iron nor the amount of hydrogen released in the boiler can not serve as a reliable sign of the extent and the size of the corrosion of origin. Gyrat Zaksi Iron, entering the boiler from extraneous sources, for example, from tanks for condensate or from the pipeline boiler, can partially explain the presence in the boiler as iron and hydrogen oxide. Gyrat of iron zaisi coming with nutrient water interacts in the reaction boiler.
ZFE (OH) 2 \u003d FE3O4 + 2N2O + H2.
Causes affecting the development of point corrosion
Foreign impurities and stresses. Non-metallic inclusions in steel, as well as voltages, are able to create anodic areas on a metal surface. Typically, corrosion sinks are of different sizes and scattered over the surface in disorder. In the presence of stresses, the location of the shell obeys the direction of the applied voltage. Typical examples can serve the fin tubes in places where the fins gave cracks, as well as the places of rolling of boiler tubes.
Dissolved oxygen.
It is possible that the strongest activator of point corrosion is dissolved in water oxygen. At all temperatures, even in an alkaline solution, oxygen serves as an active depolarizer. In addition, oxygen concentration elements can easily occur in boilers, especially under the scale or contaminants, where stagnation segments are created. The usual measure of the fight against this kind of corrosion is deaeration.
Dissolved coal anhydride.
Since the solutions of the coal anhydride have a weakly acidic reaction, it accelerates corrosion in boilers. Alkaline boiler water reduces the aggressiveness of the dissolved coal anhydride however, the benefit resulting from this does not apply to the surface is washed by steam, or on pipelines for condensate. Removal of coal anhydride together with dissolved oxygen by mechanical deaeration is a common event.
Attempts have recently been made to apply cyclohexilamine in order to eliminate corrosion in steam pipelines and pipelines for condensate heating systems.
Deposits on the walls of the boiler.
Very often, corrosive shells can be found along the outer surface (or below the surface) of such deposits such as rolling scale, boiler sludge, boiler room, corrosion products, oil films. When starting, the point corrosion will develop further, if not removing corrosion products. This type of local corrosion is enhanced by cathode (with respect to boiler steel) the character of precipitation or the depletion of oxygen under deposits.
Copper in boiler water.
If we take into account the large amounts of copper alloys used for the auxiliary equipment (capacitors, pumps, etc.), then there is nothing surprising in most cases in boiler sediments a copper contains copper. It is usually present in the metallic state, sometimes in the form of oxide. The amount of copper in sediments varies from the percentage of percent to almost pure copper.
The question of the value of copper sediments in the corrosion boiler house cannot be considered solved. Some argue that copper is only present in the corrosive process and does not affect it, others, on the contrary, believe that copper, being a cathode in relation to steel, can contribute to point corrosion. None of these points of view is confirmed by direct experiments.
In many cases, insignificant corrosion was observed (or even its complete absence), despite the fact that deposits throughout the boiler contained significant amounts of metallic copper. There is also information that when copper contact with low-carbon steel in alkaline boiler water, at elevated temperatures, copper is destroyed rather than steel. Copper rings, crimping ends of fragrant pipes, copper rivets and the screens of the auxiliary equipment through which boiler water passes, almost completely destroyed even at relatively low temperatures. In view of this, it is believed that metal copper does not enhance the corrosion of the boiler steel. The deposited copper can be considered simply as a final product of the reduction of oxide with hydrogen at its formation.
On the contrary, very strong corrosion ulceration of boiler metal is often observed next to sediments, especially rich copper. These observations led to the assumption that copper, since it is a cathode in relation to steel, contributes to point corrosion.
The surface of the boilers rarely represents nude metal iron. Most often, it has a protective layer consisting mainly of iron oxide. It is possible that the cracks are formed in this layer, the surface is the anode on copper. In such places, the formation of corrosion sinks is enhanced. This can explain in some cases the accelerated corrosion in those places where the sink was formed, as well as severe point corrosion, observed sometimes after cleaning the boilers using acids.
Wrong care for inactive boilers.
One of the most frequent causes of the formation of corrosion shells is the lack of proper care for inactive boilers. An inactive boiler must be contained either completely dry or filled with water treated in such a way that corrosion is impossible.
The water remaining on the inner surface of an inactive boiler dissolves oxygen from the air, which leads to the formation of shells, which in the future will be centered around which the corrosion process will develop.
The usual instructions for the protection of inactive boilers from corrosion are as follows:
1) the descent of water from another hot boiler (about 90 °); blowing the boiler by air to its complete drainage and content in a dry state;
2) filling the boiler with alkaline water (pH \u003d 11) comprising an excess of SO3 ions "(about 0.01%), and storage under water or steam shutter;
3) filling the boiler by an alkaline solution containing, chromic acid salts (0.02-0.03% SG4 ").
When chemical cleaning boilers, the protective layer of iron oxide will be removed in many places. Subsequently, these places may not be covered with a newly formed solid layer and on them, even in the absence of copper, sinks will appear. Therefore, it is recommended immediately after chemical cleaning to resume layer of iron oxide by treating a boiling alkaline solution (just as it is done for new boilers entering into operation).
Corrosion Economymen
General provisions relating to boiler houses are equally applicable to economizers. However, the economizer, heated nutrient water and located in front of the boiler, especially sensitive to the formation of corrosion shells. It represents the first surface with a high temperature experiencing the destructive effect of oxygen dissolved in nutrient water. In addition, water passing through an economizer has, as a rule, a low pH value and does not contain chemical moderators.
The fight against corrosion of economizers is to deaeration of water and the addition of alkali and chemical retarders.
Sometimes the processing of boiler water is carried out by passing part of it through an economizer. In this case, deposits of the sludge in the economizer should be avoided. It is also necessary to take into account the effect of such recycling of boiler water on the quality of steam.
Processing of boiler water
When processing boiler water in order to protect against corrosion, the primary task is the formation and preservation of the protective film on metal surfaces. The combination of substances added to the water depends on the working conditions, especially on the pressure, temperature, thermal tension of the nutrient water quality. However, for all cases, three rules must be observed: the boiler water must be alkaline, should not contain dissolved oxygen and contaminate the heating surface.The caustic satter best provides protection at pH \u003d 11-12. In practice, with a complex composition of boiler water, the best results are obtained at pH \u003d 11. For boilers operating at pressures below 17.5 kg / cm2, pH is usually supported within, between 11.0 and 11.5. For higher pressures, due to the possibility of the destruction of the metal as a result of incorrect circulation and local increase in the concentration of alkali solution, pH is usually taken equal to 10.5 - 11.0.
Chemical reducing agents are widely used to remove residual oxygen: sulfuric acid salts, iron zaisi hydrate and organic reducing agents. The compounds of bivalent iron are very good to remove oxygen, but form sludge, which has an undesirable effect on heat transfer. Organic reducing agents, due to their instability at high temperatures, are usually not recommended for boilers operating at pressures above 35 kg / cm2. There are data on the decomposition of sulfium salts at elevated temperatures. However, the use of them in small concentrations in pressure boilers up to 98 kg / cm2 is widely practiced. Many high-pressure installations work at all without chemical deaeration.
The cost of special equipment for deaeration, despite its undoubted benefit, is not always justified for small installations operating at relatively low pressures. At pressures below 14 kg / cm2, partial deaeration in nutrient heaters can bring the content of dissolved oxygen to approximately 0.00007%. The addition of chemical reducing agents gives good results, especially when the pH of water is above 11, and the substances that bind oxygen are added to the water supply to the boiler, which ensures the absorption of oxygen outside the boiler.
Corrosion in concentrated boiler water
Low concentrations of caustic soda (about 0.01%) contribute to the preservation of the oxide layer on steel in a state that securely providing corrosion protection. Local concentration increase causes severe corrosion.Plots of the boiler surface, on which the alkali concentration reaches a hazardous amount, are usually characterized by redundant, with respect to circulating water, heat supply. The zone enriched near the metal surface can occur in different places of the boiler. Corrosion ulcenes are located in the form of strips or elongated areas, sometimes smooth, and sometimes filled with solid and dense magnetic oxide.
Tubes located horizontally or slightly obliquely and susceptible to the intensive action of radiation from above, are erupted inside, along the upper generator. Such cases were observed in high power boilers, and also reproduced with specially set experiments.
Tubes in which water circulation is uneven or disrupted with a large load of the boiler, can be destroyed along the lower generator. Sometimes corrosion is more dramatically expressed along a variable water level on the side surfaces. Often you can observe abundant clusters of magnetic oxide of iron-sometimes loose, sometimes representing dense masses.
Overheating steel often enhances destruction. This can occur as a result of the formation of a pair layer at the top of the inclined tube. The formation of a steam shirt is possible in vertical tubes with a strengthened heat supply, which indicates temperature measurement in various parts of the tubes during the boiler operation. Characteristic data obtained under these dimensions are presented in Fig. 7. Limited areas of overheating in vertical tubes having a normal temperature above and below the "hot place" may be the result of a film boiling water.
Whenever on the surface of the boiler tube, a steam bubble is formed, the metal temperature is rising under it.
Increasing the concentration of alkali in water should occur on the surface of the section: the vapor bubble is water - the surface of heating. In fig. It is shown that even a slight increase in the temperature of the water film coming into contact with the metal and with an expanding vapor bubble leads to a concentration of caustic soda, measured by percentages and not by millions. The water film enriched with the alkali formed as a result of the appearance of each steam bubble affects the small section of the metal and for a very short time. However, the total effect of steam on the surface of heating can be amplified by the continuous action of a concentrated alkali solution, despite the fact that the total mass of water contains only million robes of the caustic soda. A few attempts have been made to find the issue of the issue associated with the local increase in the concentration of caustic soda on the surfaces of heating. So it was proposed to add neutral salts to water (for example, chloride metals) in a greater concentration than caustic nat. However, it is best to exclude the addition of caustic natra at all and to ensure the necessary pH value by administering hydrolyzing salts of phosphoric acid. The dependence between the pH of the solution and the concentration of phosphornation salts is presented in Fig. Despite the fact that the water containing the phosphornation salt has a high pH value, it can be paralleled without a significant increase in the concentration of hydroxyl ions.
However, it should be remembered that the exclusion of the use of caustic soda means only that one factor is removed, accelerating corrosion. If a steam shirt is formed in the tubes, then at least water and did not contain alkali, the corrosion is still possible, albeit to a lesser extent than in the presence of caustic soda. The solution to the problem should also be seen by changing the structure, given at the same time a tendency to a constant increase in the energy tension of the heating surfaces, which, in turn, certainly enhances corrosion. If the temperature of the thin layer of water, directly at the heating surface of the tube, exceeds the average temperature of the water in the rude hug, would be at a low value, in such a layer it can relatively grow the concentration of caustic soda. The curve approximately shows the conditions of equilibrium in the solution containing only the caustic soda. The exact data depends to some extent, from the pressure in the boiler.
Alkaline fragility of steel
Alkali fragility can be defined as the appearance of cracks in the area of \u200b\u200brivet seams or in other places of compounds where the concentrated alkali solution is possible and where there are high mechanical stresses.The most serious damage almost always occurs in the area of \u200b\u200brivet seams. Sometimes they lead to the blast of the boiler; More often to produce expensive repair even relatively new boilers. One American railway for the year registered the formation of cracks in 40 locomotive boilers, which demanded the repairs worth about $ 60,000. The appearance of fragility was also installed on the tubes in the commodity places, on the links, manifolds and in the places of threaded connections.
The voltage required for the occurrence of alkaline fragility
Practice shows a small probability of fragile destruction of conventional boiler steel, if the voltages do not exceed the yield strength. The voltage created by the pressure of steam or evenly distributed load from its own weight of the structure cannot lead to the formation of cracks. However, the stresses created by the rolling of the sheet material intended for the manufacture of boilers, deformation during a riveting or any cold processing associated with residual deformation, can cause fracture formation.
The presence of the exempted voltages is optional to form cracks. A sample of boiler steel, pre-sustained with constant bending voltage, and then freed, can give a crack in an alkaline solution, the concentration of which is equal to an increased alkali concentration in boiler water.
Alkali concentration
The normal alkali concentration in the boiler drum cannot cause cracks, because it does not exceed 0.1% Naon, and the smallest concentration at which alkaline fragility is observed, above is normal at about 100 times.
Such high concentrations can be obtained as a result of extremely slow seeping of water through a riveting seam or any other clearance. This explains the appearance of solid salts outside the majority of rivet seams in steam boilers. The most dangerous flow is such that it is difficult to detect it leaves the precipitate of a solid inside the rivet seam where there are high residual stresses. The joint effect of the voltage and the concentrated solution may cause the appearance of alkaline fragility cracks.
Device for detecting alkaline fragility
A special device for controlling the composition of water reproduces the process of evaporation of water with an increase in alkali concentration on a stressed steel sample under the same conditions in which this occurs in the area of \u200b\u200brivet seam. The cracking of the control sample indicates that the boiler water of this composition is able to cause alkaline fragility. Consequently, in this case, water treatment is necessary, eliminating its dangerous properties. However, the cracking of the control sample does not mean that there are already cracks in the boiler or will appear. In the rivet seams or in other places, the compounds are not necessarily available at the same time and flow (steaming), and voltage, and an increase in alkali concentrations, as in the control sample.
The control device is installed directly on the steam boiler and allows you to judge the quality of boiler water.
The test lasts 30 or more days when the water circulation is constant through the control device.
Crack recognition of alkaline fragility
Crackers of alkaline fragility in conventional boiler steel are different in nature than fatigue cracks or cracks formed due to high voltages. This is illustrated in Fig. I9, which shows the intercrystalline character of such cracks forming a thin mesh. The difference between intercrystalline cracks of alkaline fragility and intracrystalline cracks caused by corrosion fatigue can be seen when compared.
In alloyed steels (for example, nickel or silicarganogenic) used for locomotive boilers, cracks are also located grid, but do not always pass between crystallites, as in the case of ordinary boiler steel.
Theory of alkaline fragility
Atoms in the crystal lattice of the metal, located at the boundaries of crystallites, are experiencing a less symmetrical effect of their neighbors than atoms in the rest of the grain. Therefore, they are easier to leave the crystal lattice. It can be thought that with a thorough selection of an aggressive environment, it will be possible to carry out such selective removal of atoms from the boundaries of crystallites. Indeed, experiments show that in acidic, neutral (with a weak electric current, creating conditions, favorable for corrosion) and concentrated alkali solutions, an intercrystalline cracking can be obtained. If a solution causing general corrosion is changed by the addition of any substance forming a protective film on the surface of crystallites, corrosion focuses on the boundaries between crystallites.
Aggressive solution in the case under consideration is a solution of caustic soda. The silicenry salt can protect the surfaces of the crystallites without acting on the borders between them. The result of a joint protective and aggressive action depends on many circumstances: concentration, temperature, intense state of the metal and the composition of the solution.
There is also a colloid theory of alkaline fragility and the theory of hydrogen action dissolving in steel.
Ways to combat alkaline fragility
One way to combat alkaline fragility is to replace the riveting boilers with welding, which eliminates the possibility of treating leaks. Fragility can also be eliminated by the use of steel, resistant against intercrystalline corrosion, or chemical processing of boiler water. In the rivet boilers currently used, the last method is the only acceptable.
Preliminary tests with the use of the control sample are the best way to determine the effectiveness of certain protective additives to water. The truly salty salt warns cracking. Nitrate Salt is successfully used to protect against cracking at pressures up to 52.5 kg / cm2. The concentrated solutions of a nitratethrium salt, boiling at atmospheric pressure, can cause corrosive cracks at a soft steel voltage.
Currently, the nitratetomatic salt is widely used in stationary boilers. The concentration of a nitratetric salt corresponds to 20-30% of the alkali concentration.
Corrosion of steps
Corrosion on the inner surfaces of the tubes of steamers is primarily due to the interaction between the metal and the ferry at high temperature and to a lesser extent - by the departure of the strains of boiler water by steam. In the latter case, films of solutions with a high concentration of caustic soda can be formed on metal walls, directly corrosive steel or giving deposits hitting on the wall of the tubes, which can lead to the formation of duun. In the inactive boilers and in cases of steam condensation in relatively cold steps, point corrosion may develop under the influence of oxygen and coal anhydride.Hydrogen as a measure of corrosion speed
The temperature of the steam in modern boilers is approaching temperatures used in the industrial production of hydrogen by a direct response between the steam and iron.
On the corrosion rate of carbon and alloyed steel pipes under the action of steam, at temperatures up to 650 °, one can be judged by the volume of the hydrogen released. Sometimes it is used to release hydrogen as a measure of general corrosion.
Recently, three types of miniature plants for removing gases and air are used in US power stations. They provide complete removal of gases, and degassed condensate is suitable for determining in it salts that blame the steam from the boiler. The approximate value of the overall corrosion of the steamer during the boiler operation can be obtained by determining the difference in hydrogen concentrations in steam samples taken before and after passing it through the steamer.
Corrosion caused by impurities in a pair
A rich steam, which is part of the steam steamper, takes with them small, but measurable amounts of gases and strands from boiler water. The most common gases are oxygen, ammonia and carbon dioxide. When pair passes through a steam-controller, a tangible change in the concentration of these gases is not observed. Only minor corrosion of the metal superheater can be attributed due to the action of these gases. It has not yet been proven that salts dissolved in water, in a dry form or deposited on the elements of the steamer, can contribute to corrosion. However, the caustic soda, being the main component of the salts fascinated by boiler water, can contribute to corrosion of a strongly heated tube, especially if the alkali sticks to the metal wall.
Increasing the purity of the saturated pair is achieved by preliminary careful removal of gases from nutritious water. Reducing the number of salts involved in steam is achieved by careful cleaning in the upper collector, using mechanical separators, washing a saturated pair of nutrient water or suitable chemical treatment of water.
Determination of the concentration and nature of gases involved in a saturated ferry is carried out by using these devices and chemical analysis. Determining the concentration of salts in a saturated pair is conveniently produced by measuring the electrical conductivity of water or evaporation of a large amount of condensate.
An improved method of measuring electrical conductivity is proposed, appropriate corrections for some dissolved gases are given. Condensate in the above mentioned miniature gas removal can also be used to measure electrical conductivity.
When the boiler is inactive, the steamer is a refrigerator in which condensate accumulates; In this case, ordinary underwater point corrosion is possible if the steam contained oxygen or carbon dioxide.
Popular articles
Ministry of Energy and Electrification of the USSR
Main Science and Technology Energy and Electrification
Methodical instructions
For warning
Low-temperature
Corrosion surfaces
Heating and gas pipes boilers
RD 34.26.105-84
Soyucehenergo
Moscow 1986.
Developed by the All-Union Twice Order of the Labor Red Banner Teply Engineering Research Institute named after F.E. Dzerzhinsky
Artists R.A. Petrosyan, I.I. Nadyrov
Approved by the Main Technical Operation Manual Energy Systems 22.04.84
Deputy Head of D.Ya. Shamarakov
|
Methodical guidelines for the prevention of low-temperature corrosion of heat and gas supplies of boilers |
RD 34.26.105-84 |
The validity period is set
from 01.07.85
until 01.07.2005
These guidelines are applied to low-temperature surfaces of the heating of steam and hot water boilers (economizers, gas evaporators, air heaters of various types, etc.), as well as the gas tract for air heaters (gas ducts, ashors, smokers, flue pipes) and set surface protection methods Heating from low-temperature corrosion.
Methodical instructions are designed for thermal power plants operating on sulfur fuels, and organizations that design boiler equipment.
1. Low-temperature corrosion is the corrosion of the tail surfaces of heating, gas ducts and chimneys of boilers under the action of sulfuric acid vapors condensing from chimneal gases.
2. Condensation of sulfuric acid vapors, the volumetric content of which in flue gases when burning sulfur fuels is only a few thousandths of the percentage, occurs at temperatures, significantly (by 50 - 100 ° C) exceeding the temperature of the condensation of water vapor.
4. To prevent corrosion of heating surfaces during operation, the temperature of their walls should exceed the temperature point of the flue gases at all loads of the boiler.
For the heating surfaces cooled with a high heat transfer coefficient (economizers, gas evaporators, etc.), the temperature of the medium at the inlet in them should exceed the temperature of the dew point by about 10 ° C.
5. For the surfaces of heating the water boilers when working on a sulfur fuel oil, the conditions for the complete exception of low-temperature corrosion cannot be implemented. To reduce it, it is necessary to ensure the temperature of the water at the inlet to the boiler, equal to 105 - 110 ° C. When using water boilers as peaks, such a mode can be provided with the full use of network water heaters. When using water boilers in the main mode, an increase in water temperature at the inlet to the boiler can be achieved by recycling hot water.
In the installations using the scheme for the inclusion of water heating boilers in the heat carrier through water heat exchangers, the conditions for the reduction of low-temperature corrosion of the heating surfaces are fully ensured.
6. For aircraft heaters of steam boilers, the complete elimination of low-temperature corrosion is provided at the calculated wall temperature of the coldest area greater than the temperature of the dew point at all loads of the boiler by 5 - 10 ° C (the minimum value refers to the minimum load).
7. Calculation of the temperature of the wall of tubular (TVP) and regenerative (RWP) air heater is carried out on the recommendations of the "thermal calculation of boiler aggregates. Regulatory method "(M.: Energy, 1973).
8. When used in tubular air heaters as the first (by air) the movement of the changeable cold cubes or cubes from pipes with an acidic coating (enamelled, etc.), as well as made of corrosion-resistant materials to the conditions of complete exception of low-temperature corrosion, the following are checked for them (by air) Metal cubes air heater. In this case, the selection of the temperature of the cold metal cubes changeable, as well as corrosion-resistant cubes, should exclude intensive contamination of pipes, for which their minimal temperature of the wall when burning sulfur fuel oils should be lower than the dew point of flue gases by no more than 30 to 40 ° C. When burning solid sulfur fuels, the minimum temperature of the pipe wall under the conditions of the warning of intensive contamination should be taken at least 80 ° C.
9. In RVP, on the conditions of complete exception of low-temperature corrosion, their hot part is calculated. The cold part of the RVP is performed by corrosion-resistant (enameled, ceramic, from low-alloyed steel, etc.) or replaced from flat metal sheets with a thickness of 1.0 - 1.2 mm made of small-carbon steel. The conditions for preventing intensive packing pollution are complied with the requirements of claim. Of this document.
10. As an enameled, a filling of metal sheets with a thickness of 0.6 mm is applied. The service life of the enameled package made in accordance with TU 34-38-10336-89 is 4 years.
Porcelain tubes, ceramic blocks, or porcelain plates with protrusions can be used as ceramic packing.
Given the reduction in the consumption of fuel oil with thermal power plants, it is advisable to apply for the cold part of the RWP, a package of low-alloyed steel 10Hord or 10xst, the corrosion resistance of which is 2- 2.5 times higher than that of small-carbon steel.
11. To protect air heaters from low-temperature corrosion in the starting period, measures set out in the "Guidelines for the design and operation of energy heating calorifications with wire fins" (M.: SPO Uniontehenergo, 1981).
The milling of the boiler on the sulfur fuel oil should be carried out with a pre-enabled air heating system. The air temperature in front of the air heater in the initial period of the extracts should usually be 90 ° C.
11a. To protect air heaters from low-temperature ("parking") corrosion on a stopped boiler, the level of which is about twice the rate of corrosion during operation, before stopping the boiler, it should be thoroughly clean the air heater from outdoor sediments. In this case, before stopping the boiler, the air temperature at the inlet into the air heater is recommended to maintain at the level of its value at the rated load of the boiler.
The cleaning of the TVP is carried out by a fraction with the density of its supply of at least 0.4 kg / pp (paragraph. Of this document).
For solid fuels, taking into account the significant hazard of the corrosion of the aspores, the temperature of the outgoing gases should be chosen above the dew point of the flue gases at 15 - 20 ° C.
For sulfur fuel oil, the temperature of the outgoing gases should exceed the temperature of the dew point at the rated load of the boiler by about 10 ° C.
Depending on the sulfur content in the fuel oil, the calculated value of the outgoing gases should be taken at the rated load of the boiler, indicated below:
The temperature of the outgoing gases, ºС ...... 140 150 160 165
When burning sulfur fuel oil with extremely small excess air (α ≤ 1.02), the temperature of the outgoing gases can be accepted lower taking into account the results of the dew point measurements. On average, the transition from small excess air to the maximum low reduces the temperature of the dew point by 15 to 20 ° C.
The conditions for ensuring reliable operation of the chimney and the prevention of moisture falling on its wall affects not only the temperature of the outgoing gases, but also their consumption. The work of the pipe with load modes is significantly lower than the project increases the likelihood of low-temperature corrosion.
When burning natural gas, the temperature of the outgoing gases is recommended to have no lower than 80 ° C.
13. With a decrease in the loading of the boiler in the range of 100 - 50% of the nominal one should strive to stabilize the temperature of the outgoing gases, not allowing its decline to more than 10 ° C from the nominal.
The most economical way to stabilize the temperature of the outgoing gases is to increase the temperature of the preheating of air in the carriers as the load decreases.
The minimum allowable values \u200b\u200bof the temperature preheating temperatures before RVP are accepted in accordance with clause 4.3.28 "Rules for the technical operation of electric stations and networks" (M.: Energoatomizdat, 1989).
In cases where the optimal temperature of the outgoing gases cannot be provided due to the insufficient surface of the RVP heating, the values \u200b\u200bof the preheating temperatures should be taken, at which the temperature of the outgoing gases does not exceed the values \u200b\u200bshown in these methodical instructions.
16. Due to the lack of reliable acid-resistant coatings to protect against low-temperature corrosion of metal gas ducts, their reliable operation can be achieved by careful insulation, ensuring the temperature difference between the flue gases and the wall of no more than 5 ° C.
At present, insulation materials and designs are not sufficiently reliable in long-term operation, therefore it is necessary to conduct periodic, at least once a year, control over their condition and, if necessary, carry out repair and restoration work.
17. When used in an experimental order to protect the gas ducts from low-temperature corrosion of various coatings, it should be borne in mind that the latter must provide heat resistance and gas content at temperatures exceeding the temperature of the outgoing gases at least 10 ° C, resistance to sulfuric acid concentration 50 - 80% In the temperature range, respectively, 60 - 150 ° C and the possibility of their repair and recovery.
18. For low-temperature surfaces, structural elements of RVP and boiler gas supplies, it is advisable to use low-alloyed steels of 10HNDP and 10XD, which are 2 - 2.5 times in corrosion resistance.
The absolute corrosion resistance is only very deficient and expensive high-alloy steel (for example, EI943 steel, containing up to 25% chromium and up to 30% nickel).
application
1. Theoretically, the temperature of the flue gas dew point with a predetermined content of sulfuric acid and water can be determined as a boiling point of a solution of sulfuric acid of such a concentration, at which there is the same content of water vapor and sulfuric acid.
The measured temperature point of the dew point depending on the measurement methodology may not be coincided with theoretical. In these recommendations for the temperature of the dew point of flue gases tR The surface temperature of a standard glass sensor with apart at a distance of 7 mm is taken one from the other platinum electrodes with a length of 7 mm, at which the resistance of the dew film between the electrodes in the steady state is 107 ohms. In the measuring circuit of the electrodes, an alternating current of low voltage is used (6 - 12 V).
2. When burning sulfur fuel oils with excess air 3 - 5% temperature point of dew flue gases depends on the sulfur content in fuel Sp. (Fig.).
When burning sulfur fuel oils with extremely low air excess (α ≤ 1.02), the temperature of the flue gases dew should be taken according to the results of special measurements. The conditions for the transfer of boilers in the mode with α ≤ 1.02 are set forth in the "Guidelines for the transfer of boilers operating on sulfur fuels, into combustion mode with extremely small excess airs" (M.: SPO SoyuceCenergo, 1980).
3. When burning sulfur solid fuels in the dust-shaped state temperature of the dew point of flue gases tP. It may be calculated according to the sulfur and ash content in the fuel SRP, ARPR and condensation temperature of water vapor ton according to the formula
where aUN - Share of ash in charge (usually received 0.85).
Fig. 1. The dependence of the temperature of the dew point of flue gases from the sulfur content in the combustion fuel oil
The value of the first term of this formula aUN \u003d 0.85 can be determined in fig. .
Fig. 2. The difference in temperature points of the dew of flue gases and condensation of water vapor in them depending on the sulfur contents ( SRP) and ash ( ARPR) In the fuel
4. When burning gaseous sulfur fuels, the dew point of flue gases can be determined in fig. Provided that the sulfur content in the gas is calculated as the above, that is, in a percentage by weight by 4186.8 kJ / kg (1000 kcal / kg) heat combustion of gas.
For gas fuel, the size of the sulfur content in percentage by weight can be determined by the formula
where m. - the number of sulfur atoms in the sulfur component molecule;
q. - bulk percentage of sulfur (sulfur component);
QN - heat combustion of gas in KJ / M3 (kcal / nm3);
FROM - coefficient equal to 4,187, if QN expressed in KJ / M3 and 1.0, if in kcal / m3.
5. The corrosion rate of the replaced metal packing of air heater during the combustion of the fuel oil depends on the temperature of the metal and the degree of corrosion activity of flue gases.
When burning sulfur fuel oil with an excess of air 3 - 5% and blend the surface of the corrosion (from two sides in mm / year), the RVP packing can be estimated according to Table. .
Table 1
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Corrosion rate (mm / year) at the wall temperature, ºС |
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0.5 More than 2 0.20 |
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St. 0.11 to 0.4 incl. |
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St. 0.41 to 1.0 incl. |
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6. For coal with a high content of calcium oxide, the dew point temperature is lower than those calculated according to claims of these methodical instructions. For such fuels, it is recommended to use the results of direct measurements.
Identification of corrosion types is difficult, and, therefore, there are no errors in determining technologically and economically optimal measures to counter corrosion. The main necessary measures are being taken in accordance with the regulatory documents, which establishes the limits of the main corrosion initiators.
GOST 20995-75 "Boilers steampody with pressure up to 3.9 MPa. Indicators of nutrient water and steam quality rates indicators in nutrient water: transparency, that is, the amount of suspended impurities; The overall rigidity, the content of iron and copper compounds - preventing scale formation and iron and copper-oxidic sediments; The pH is the prevention of alkaline and acid corrosion and also foaming in the boiler drum; oxygen content - prevention of oxygen corrosion; The content of nitrite is to prevent nitrite corrosion; The content of petroleum products is to prevent foaming in the boiler drum.
The values \u200b\u200bof the norms are determined by GOS, depending on the pressure in the boiler (consequently, on the temperature of the water), from the power of the local heat flux and the water treatment technology.
When studying the reasons for corrosion, first of all, it is necessary to inspect (where it is available) of the metal destruction sites, the analysis of the working conditions of the boiler in the destroyer period, the analysis of the quality of nutritious water, steam and deposits, analysis of the design features of the boiler.
With external inspection, the following types of corrosion can be suspected.
Oxygen corrosion
: Entrance sections of pipes of steel economizers; nutrient pipelines at a meeting with insufficiently enclosed (above normal) water - "breakthroughs" of oxygen with poor deaeration; native water heaters; All wet sections of the boiler during its stop and failure to prevent air flow to the boiler, especially in congestion, during the drainage of water, whence it is difficult to remove the condensate of the steam or completely pour water, for example, vertical pipes of steamers. During downtime, corrosion is amplified (localized) in the presence of alkali (less than 100 mg / l).
Oxygen corrosion rarely (with an oxygen content in water, a significant exceeding rate, - 0.3 mg / l) manifests itself in steaming devices of boiler drums and on the wall of the drums at the boundary of the water level; in lowered pipes. In lifting pipes, corrosion is not manifested due to the deaeeric action of steam bubbles.
Type and nature of damage. The ulcers of various depths and diameters, often covered with tubercles whose upper crust is reddish iron oxides (probably hematite Fe 2 O 3). Certificate of active corrosion: under the crust of tubercles - a black liquid precipitate, probably magnetite (Fe 3 o 4) in a mixture with sulfates and chlorides. When the corrosion is fucked under crust, the emptiness, and the bottom of the ulcers is covered with screaming and sludge.
With pH of water\u003e 8.5 - ulcers are rare, but larger and deep, with pH< 8,5 - встречаются чаще, но меньших размеров. Только вскрытие бугорков помогает интерпретировать бугорки не как поверхностные отложения, а как следствие коррозии.
With water speed, more than 2 m / s tubercles can take an oblong shape in the direction of the jet movement.
. Magnetic crusts are sufficiently dense and could serve as a reliable obstacle to oxygen penetration inside the tubercles. But they are often destroyed as a result of corrosion fatigue, when the temperature of the water and metal is cyclically change: frequent stops and boiler lands, pulsating the volatile mixture, bundle of a steam mixture into separate tubes of steam and water, following each other.
Corrosion is enhanced with increasing temperature (up to 350 ° C) and an increase in the content of chlorides in boiler water. Sometimes corrosion enhance the products of the thermal decay of some organic substances of the feed water.
Fig. 1. Appearance of oxygen corrosion
Alkaline (in a narrower sense - intercrystalline) corrosion
Metal corrosion damage. Pipes in the heat flux zones (burner area and opposite the elongated torch) - 300-400 kW / m 2 and where the temperature of the metal is 5-10 ° C above the boiling point of water at a given pressure; inclined and horizontal pipes where weak water circulation; places under fat sediments; zones near the lined rings and in the welds themselves, for example, in places of welding of internal pairs of steaming devices; Places near rivets.
Type and nature of damage. Hemispherical or elliptical recesses filled with corrosion products often include brilliant magnetite crystals (Fe 3 O 4). Most of the recesses are covered with solid crust. On the side of the pipes addressed to the furnace, the recesses can be connected, forming a so-called corrosion track with a width of 20-40 mm and up to 2-3 m long.
If the crust is not sufficiently stable and dense, then corrosion can lead - under the conditions of mechanical stress - to the appearance of cracks in the metal, especially about the cracks: rivets, rolling compounds, welding places for steaming devices.
Causes of corrosion damage. At high temperatures - more than 200 ° C - and a large concentration of caustic soda (NAON) - 10% or more - protective film (crust) on metal collapses:
4None + F 3 O 4 \u003d 2NFEO 2 + NA 2 FEO 2 + 2N 2 O (1)
The intermediate product NAFEO 2 is subjected to hydrolysis:
4NFEO 2 + 2N 2 O \u003d 4None + 2Fe 2 O 3 + 2N 2 (2)
That is, in this reaction (2), the caustic soda is restored, in reactions (1), (2) is not consumed, and acts as a catalyst.
When the magnetite is removed, then the caustic natter and water can react with iron directly with the release of atomic hydrogen:
2None + Fe \u003d NA 2 FEO 2 + 2N (3)
4N 2 O + 3FE \u003d Fe 3 O 4 + 8H (4)
The released hydrogen is capable of diffing inside the metal and to form methane carbide (CH 4):
4N + F 3 C \u003d CH 4 + 3F (5)
It is also possible to combine atomic hydrogen into molecular (H + H \u003d H 2).
Methane and molecular hydrogen can not penetrate the inside of the metal, they accumulate at the grain boundaries and in the presence of cracks expand and deepen them. In addition, these gases prevent the formation and sealing of protective films.
The concentrated solution of the caustic soda is formed in places of deep evaporation of the boiler water: dense scale deposits of salts (view of submissive corrosion); The crisis of bubble boiling, when a steady steam film is formed over the metal - there metal is almost not damaged, but at the edges of the film where active evaporation is underway, the caustic natra is concentrated; The presence of slots, where evaporation is essential from evaporation throughout the volume of water: the caustic natter evaporates worse than water, does not blur and accumulates. Acting for metal, the caustic soda forms the grains on the borders, directed into the metal (type of intercrystalline corrosion - slit).
Intercrystalline corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.
Fig. 3. Intercrystalline corrosion: A - Metal microstructure to corrosion, B - microstructure at the corrosion stage, the formation of cracks on the border of metal grains
Such a corrosion impact on metal is possible only with the simultaneous presence of three factors:
- local tensile mechanical stresses, close or somewhat higher than the yield strength, i.e. 2.5 mM / mm 2;
- the loose articulation of the drum details (indicated above), where a deep evaporation of the boiler water may occur and where the accumulating caustic Natro dissolves the protective film of iron oxides (NAO concentration of more than 10%, the water temperature is above 200 ° C and - especially - closer to 300 ° C). If the boiler is operated with a pressure less than a passport (for example, 0.6-0.7 MPa instead of 1.4 MPa), then the probability of this type of corrosion decreases;
- an unfavorable combination of substances in boiler water, in which there are no necessary protective concentrations of inhibitors of this type of corrosion. Sodium salts can act as inhibitors: sulfates, carbonates, phosphates, nitrates, sulfitecellulosic liquors.
Fig. 4. Appearance of intercrystalline corrosion
Corrosion cracks do not develop if the attitude is observed:
(NA 2 SO 4 + NA 2 CO 3 + NA 3 PO 4 + NAno 3) / (NaOH) ≥ 5, 3 (6)
where Na 2 So 4, Na 2 CO 3, NA 3 PO 4, NAnO 3, NaOH is the content of sodium sulfate, sodium of carbonate, sodium phosphate, nitrate sodium and sodium hydroxide, mg / kg.
In the currently manufactured boilers, at least one of these conditions for the occurrence of corrosion is absent.
The presence of silicon compounds in boiler water can also increase intercrystalline corrosion.
NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (CL -) - corrosion accelerators, due to high mobility and small sizes, they easily penetrate through protective oxide films and are provided with iron well soluble salts (FESL 2, FESL 3) instead of low-soluble iron oxides.
In water boilers, traditionally control the values \u200b\u200bof general mineralization, and not the content of individual salts. Probably, for this reason, normalization was introduced at the indicated relation (6), but by the value of the relative alkalinity of boiler water:
Uk kv \u003d u ov ov \u003d u s 40 100 / s ≤ 20, (7)
where u kvd is the relative alkalinity of boiler water,%; Р р ров - relative alkalinity of treated (added) water,%; Ov - the total alkalinity of the treated (additive) water, mmol / l; S os - mineralization of the treated (added) water (including - the chloride content), mg / l.
The total alkalinity of the treated (added) water can be taken equal to, mmol / l:
- after sodium-cationing - the total alkalicity of the original water;
- after hydrogen-sodium-cation of parallel - (0.3-0.4), or sequential with the "hungry" regeneration of hydrogen-cationic filter - (0.5-0.7);
- after sodium-cation with acidification and sodium-chlorine-ionics - (0.5-1.0);
- after ammonium sodium-cation - (0.5-0.7);
- after lime at 30-40 ° C - (0.35-1.0);
- after coagulation - (u about an est - d), where u is ch - the overall alkalicity of the original water, mmol / l; D K - the dose of coagulant, mmol / l;
- after co-operating at 30-40 ° C - (1.0-1.5), and at 60-70 ° C - (1.0-1.2).
The values \u200b\u200bof the relative alkalinity of boiler water according to Rostechnadzor standards are accepted,%, not more than:
- for boilers with riveted drums - 20;
- for boilers with welded drums and vvalted pipes - 50;
- for boilers with welded drums and tailored pipes - any value, not rationed.
Fig. 4. The result of intercrystalline corrosion
According to Rostekhnadzor ROS, one of the criteria for the safe work of the boilers. It is more correct to check the criterion of potential alkaline aggressiveness of boiler water, which does not take into account the content of the chlorine ion:
K sh \u003d (s ov - [Sl -]) / 40 u s, (8)
where kch is the criterion of potential alkaline aggressiveness of boiler water; S ov - mineralization of treated (added) water (including - chloride content), mg / l; CL - - the content of chlorides in the treated (added) water, mg / l; U ov - the total alkalinity of the treated (additive) water, mmol / l.
The value of ki can be taken:
- for boilers with riveted drums with a pressure of more than 0.8 MPa ≥ 5;
- for boilers with welded drums and vvalted pipes with a pressure of more than 1.4 MPa ≥ 2;
- for boilers with welded drums and wood-welded pipes, as well as for boilers with welded drums and vvalted pipes with pressure up to 1.4 MPa and boilers with riveted pressure drums up to 0.8 MPa - not to normalize.
Podllam corrosion
Under this title, several different types of corrosion are combined (alkaline, oxygen, etc.). The accumulation in different zones of the boiler loose and porous sediments, the sludge causes the metal corrosion under the sludge. The main reason: Pollution of nutrient water by iron oxides.
Nitrite corrosion
. Screen and boiler boiler pipes on the side facing the furnace.
Type and nature of damage. Rare, sharply limited large ulcers.
. If there are nitrite ions (NO - 2) in nutrient water, more than 20 μg / l, water temperature of more than 200 ° C, nitrites serve as cathode depolarisators of electrochemical corrosion, restoring to NNO 2, NO, N 2 (see above).
Carriage corrosion
Metal corrosion damage. Output part of steamers coils, superheated steam steamings, horizontal and slightly narcone steam generating pipes in areas of poor water circulation, sometimes along the upper generating weekend coils of boiling water economizers.
Type and nature of damage. The raids of dense ferrous iron oxides (Fe 3 O 4), firmly linked with the metal. With fluctuations in temperature, the inclusion of the plaque (crusts) is broken, the flakes fall off. Uniform thinning of metal with deductions, longitudinal cracks, breaks.
It can be identified as submissive corrosion: in the form of deep ulcers with fuzzy-degraded edges, often near the protruding pipes of the welds, where the sludge accumulates.
Causes of corrosion damage:
- washing medium - steam in steam steampers, steam pipes, steam "pillows" under the layer of sludge;
- metal temperature (steel 20) more than 450 ° C, heat flux to metal section - 450 kW / m 2;
- fiberglass Disrupting: Holding burners, increased contamination of pipes inside and outside, unstable (vibrating) burning, lengthening of the torch towards the pipes of screens.
As a result: the immediate chemical interaction of iron with water vapor (see above).
Microbiological corrosion
Caused by aerobic and anaerobic bacteria, appears at temperatures of 20-80 ° C.
Metal damage location. Pipes and capacities to boiler with water of the specified temperature.
Type and nature of damage. Bigrucks of different sizes: a diameter from a few millimeters to several centimeters, rarely - several tens of centimeters. The tubercles are covered with dense iron oxides - product of vital activity of aerobic bacteria. Inside - powder and black suspension (iron sulphide FES) - the product of sulfate-building anaerobic bacteria, under black education - round ulcers.
Causes of damage. In natural water, iron sulfates, oxygen and different bacteria are always present.
Jamming in the presence of oxygen form a film of iron oxides, the anaerobic bacteria under it is reduced to sulfide to iron sulfide (FES) and hydrogen sulfide (H 2 S). In turn, the hydrogen sulfide gives the formation of sulfur (very unstable) and sulfuric acids, and the metal corrodes.
At the corrosion of the boiler, this species has an indirect effect: the flow of water at a speed of 2-3 m / s breaks off the tubercles, takes their contents to the boiler, increasing the accumulation of the sludge.
In rare cases, it is possible to flow in this corrosion in the bolet itself, if during a long stop of the boiler in the reserve it is filled with water with a temperature of 50-60 o C, and the temperature is maintained at the expense of random steam breakthroughs from adjacent boilers.
"Chelate" corrosion
Corrosion damage locations. The equipment in which the pairs are separated from the water: the boiler drum, steaming devices in the drum and outside it, is also rarely in the nutrient water pipelines and the economizer.
Type and nature of damage. The surface of the metal is smooth, but if the medium moves at high speed, the corrosion surface is non-deployed, has horseshoe-shaped recesses and "tails" oriented in the direction of movement. The surface is covered with a thin matte or black shiny film. There are no obvious sediments, no corrosion products, because the "chelate" (specially injected in the boiler organic compounds of polyamines) has already reacted.
In the presence of oxygen, it rarely happens in a normally working boiler, a corrosion surface is "boiled": roughness, metal islands.
Causes of corrosion damage. The mechanism of action of the Helata is described earlier ("industrial and heating boilers and mini-CHP", 1 (6) 2011, p.40).
The "chelate" corrosion occurs in the overdose of "chelate", but also at a normal dose it is possible, since the "chelate" is concentrated in areas where there is an intensive evaporation of water: bubble boil is replaced by a film. In steaming devices there are cases of particularly destructive effects of "chelate" corrosion due to large turbulent water velocities and a steam mixture.
All described corrosion damage may have a syneergetic effect, so that the total damage from the joint action of different corrosion factors may exceed the amount of damage from certain types of corrosion.
As a rule, the effect of corrosion agents enhances the unstable heat mode of the boiler, which causes corrosion fatigue and excites heat-saline corrosion: the number of starts from a cold state is more than 100, the total number of launches is more than 200. Since these types of metal destruction are rarely manifested, then cracks, breaking The pipes are identical to metal lesions from different types of corrosion.
Usually, additional metallographic studies are required to identify the cause of metal destruction: radiography, ultrasound, color and magneto-powder flaw detection.
Different researchers proposed programs for the diagnosis of types of corrosion damage of boiler steels. The WTI Program (A.F. Bogachev with Employees) is mainly for high-pressure energy boilers, and the development of the Enerkoermet union is mainly for low and medium-sized energy boilers and waste disposal boilers.
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).
3.3. Corrosion of the path of feed water and condensate pipelines
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.
The path of nutrient water of a steam turbine power plant 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 the brass tubes of low-pressure heaters begin to decompose in the parts of the path of the regenerative heaters of high pressure (paragraphs. D.) To form less soluble copper oxides, partially precipitated on the surface of the tubes. 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 (with an open condensate collection scheme) and subcoases through looseness in the 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) anti-corrosion protection of nutrient and drainage tanks by applying non-metallic coatings, resistant at temperatures up to 80-100 ° C, for example asbobovinyl (lacquer mixtures of ethinol with asbestos) or paint 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 by alkaline reagents in order to maintain a given optimal pH value of nutrient water at which carbon dioxide corrosion is suppressed and a 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 is subjected to surface heating of tubular and regenerative air heaters, low-temperature economizers, as well as metal gas ducts and chimneys at metal temperatures below the flue gas dew point. 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 is subjected to eaves of drum and direct-flow boilers when burning solid fuels, steamers and their attachments, as well as the screens of the lower radiation part of the supercritical pressure boilers when combing 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.
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