The conditions in which the elements of steam boilers are located during operation are extremely varied.
As numerous corrosion tests and industrial observations have shown, low-alloy and even austenitic steels can be subject to intense corrosion during boiler operation.
Corrosion of the metal heating surfaces of steam boilers causes premature wear and sometimes leads to serious problems and accidents.
Most emergency shutdowns of boilers occur due to through corrosion damage to the screen, grain economizer, steam superheating pipes and boiler drums. The appearance of even one corrosion fistula in a once-through boiler leads to the shutdown of the entire unit, which is associated with a lack of electricity production. Corrosion of drum boilers high and above high pressure became the main cause of failures in the operation of thermal power plants. 90% of operational failures due to corrosion damage occurred on drum boilers with a pressure of 15.5 MPa. Significant amount of corrosion damage screen pipes The salt compartments were in the zones of maximum thermal loads.
Inspections of 238 boilers (units with a capacity from 50 to 600 MW) conducted by US specialists revealed 1,719 unscheduled downtimes. About 2/3 of boiler downtime was caused by corrosion, of which 20% was due to corrosion of steam generating pipes. In the USA, internal corrosion was recognized as a serious problem in 1955 after the commissioning of a large number of drum boilers with a pressure of 12.5-17 MPa.
By the end of 1970, about 20% of the 610 such boilers were damaged by corrosion. Screen pipes were mostly susceptible to internal corrosion, while superheaters and economizers were less affected by it. With the improvement of the quality of feed water and the transition to a coordinated phosphating regime, with an increase in parameters on drum boilers of US power plants, instead of viscous, plastic corrosion damage, sudden brittle fractures of screen pipes occurred. “As of J970 t. for boilers with pressures of 12.5, 14.8 and 17 MPa, the destruction of pipes due to corrosion damage was 30, 33 and 65%, respectively.
According to the conditions of the corrosion process, a distinction is made between atmospheric corrosion, which occurs under the influence of atmospheric and also wet gases; gas, caused by the interaction of the metal with various gases - oxygen, chlorine, etc. - at high temperatures, and corrosion in electrolytes, in most cases occurring in aqueous solutions.
Due to the nature of corrosion processes, boiler metal can be subject to chemical and electrochemical corrosion, as well as their combined effects.
When operating the heating surfaces of steam boilers, high-temperature gas corrosion occurs in the oxidizing and reducing atmospheres of flue gases and low-temperature electrochemical corrosion of the tail heating surfaces.
Research has established that high-temperature corrosion of heating surfaces occurs most intensely only in the presence of excess free oxygen in the flue gases and in the presence of molten vanadium oxides.
High-temperature gas or sulfide corrosion in the oxidizing atmosphere of flue gases affects pipes of screen and convective superheaters, the first rows of boiler bundles, metal spacers between pipes, racks and suspensions.
High temperature gas corrosion in a reducing atmosphere was observed on the screen pipes of the combustion chambers of a number of high and supercritical pressure boilers.
Corrosion of heating surface pipes on the gas side is a complex physical and chemical process of interaction of flue gases and external deposits with oxide films and pipe metal. The development of this process is influenced by time-varying intense heat flows and high mechanical stresses arising from internal pressure and self-compensation.
On medium and low pressure boilers, the screen wall temperature, determined by the boiling point of water, is lower, and therefore this type of metal destruction is not observed.
Corrosion of heating surfaces from the side flue gases(external corrosion) is the process of metal destruction as a result of interaction with combustion products, aggressive gases, solutions and melts of mineral compounds.
Metal corrosion is understood as the gradual destruction of metal that occurs as a result of chemical or electrochemical exposure to the external environment.
\ The processes of metal destruction, which are a consequence of their direct chemical interaction with the environment, are classified as chemical corrosion.
Chemical corrosion occurs when metal comes into contact with superheated steam and dry gases. Chemical corrosion in dry gases is called gas corrosion.
In the furnace and flue ducts of the boiler, gas corrosion of the outer surface of the pipes and racks of superheaters occurs under the influence of oxygen, carbon dioxide, water vapor, sulfur dioxide and other gases; the inner surface of the pipes - as a result of interaction with steam or water.
Electrochemical corrosion, unlike chemical corrosion, is characterized by the fact that the reactions occurring during it are accompanied by the appearance of an electric current.
The carrier of electricity in solutions are the ions present in them due to the dissociation of molecules, and in metals - free electrons:
The internal boiler surface is mainly subject to electrochemical corrosion. According to modern concepts, its manifestation is due to two independent processes: anodic, in which metal ions pass into solution in the form of hydrated ions, and cathodic, in which excess electrons are assimilated by depolarizers. Depolarizers can be atoms, ions, molecules, which are reduced.
By external signs There are continuous (general) and local (local) forms of corrosion damage.
With general corrosion, the entire heating surface in contact with the aggressive environment is corroded, evenly thinning on the inside or outside. With local corrosion, destruction occurs in individual areas of the surface, the rest of the metal surface is not affected by damage.
Local corrosion includes spot corrosion, ulcer corrosion, pitting corrosion, intergranular corrosion, stress-corrosion cracking, and metal corrosion fatigue.
A typical example of destruction from electrochemical corrosion.
Destruction from the outer surface of NRCh 042X5 mm pipes made of steel 12Kh1MF of TPP-110 boilers occurred in a horizontal section in the lower part of the lifting and lowering loop in the area adjacent to the bottom screen. On the back side of the pipe, an opening occurred with a slight thinning of the edges at the point of destruction. The cause of the destruction was the thinning of the pipe wall by approximately 2 mm due to corrosion due to deslagging with a jet of water. After stopping the boiler with a steam production of 950 t/h, heated by anthracite pellet dust (liquid slag removal), a pressure of 25.5 MPa and a superheated steam temperature of 540 °C, wet slag and ash remained on the pipes, in which electrochemical corrosion proceeded intensively. The outside of the pipe was coated with a thick layer of brown iron hydroxide. The internal diameter of the pipes was within the tolerances for pipes of high- and ultra-high-pressure boilers. Outer diameter dimensions have deviations beyond the minus tolerance: minimum outer diameter. amounted to 39 mm with a minimum allowable of 41.7 mm. The wall thickness near the point of corrosion failure was only 3.1 mm with a nominal pipe thickness of 5 mm.
The microstructure of the metal is uniform along the length and circumference. On the inner surface of the pipe there is a decarbonized layer formed during oxidation of the pipe during heat treatment. There is no such layer on the outside.
Examination of the NRF pipes after the first rupture made it possible to find out the cause of the destruction. It was decided to replace the NRF and change the deslagging technology. IN in this case electrochemical corrosion occurred due to the presence of a thin film of electrolyte.
Pit corrosion occurs intensely on individual small areas surface, but often to a considerable depth. When the diameter of the ulcers is about 0.2-1 mm, it is called pinpoint.
In places where ulcers form, fistulas can form over time. The pits are often filled with corrosion products, as a result of which they cannot always be detected. An example is the destruction of steel economizer pipes due to poor deaeration of feed water and low speeds of water movement in the pipes.
Despite the fact that a significant part of the metal of the pipes is affected, due to through fistulas it is necessary to completely replace the economizer coils.
The metal of steam boilers is subject to the following dangerous types of corrosion: oxygen corrosion during operation of the boilers and when they are under repair; intercrystalline corrosion in places where boiler water evaporates; steam-water corrosion; corrosion cracking of boiler elements made of austenitic steels; sub-sludge - howling corrosion. a brief description of specified types corrosion of boiler metal is given in table. YUL.
During the operation of boilers, metal corrosion is distinguished - corrosion under load and standing corrosion.
Corrosion under load is most susceptible to heating. manufactured boiler elements in contact with a two-phase medium, i.e. screen and boiler pipes. The inner surface of economizers and superheaters is less affected by corrosion during boiler operation. Corrosion under load also occurs in an oxygen-free environment.
Parking corrosion occurs in undrained areas. elements of vertical superheater coils, sagging pipes of horizontal superheater coils
Corrosion of hot water boilers, heating systems, district heating systems are much more common than in steam and condensate systems. In most cases, this situation is explained by the fact that when designing a water heating system, less attention is paid to this, although the factors for the formation and subsequent development of corrosion in boilers remain exactly the same as for steam boilers and all other equipment. Dissolved oxygen, which is not removed by deaeration, hardness salts, carbon dioxide entering hot water boilers with feed water, cause various types of corrosion - alkaline (intercrystalline), oxygen, chelate, sub-sludge. It must be said that chelate corrosion in most cases is formed in the presence of certain chemical reagents, the so-called “complexons”.
In order to prevent the occurrence of corrosion in hot water boilers and its subsequent development, it is necessary to take seriously and responsibly the preparation of the characteristics of water intended for make-up. It is necessary to ensure the binding of free carbon dioxide and oxygen, bring the pH value to an acceptable level, and take measures to protect aluminum, bronze and copper elements of heating equipment and boilers, pipelines and heating equipment from corrosion.
IN Lately For high-quality correction heating networks, hot water boilers and other equipment, special chemical reagents are used.
Water is at the same time a universal solvent and an inexpensive coolant; it is beneficial to use in heating systems. But insufficient preparation can lead to unpleasant consequences, one of which is corrosion of hot water boilers. Possible risks are primarily associated with the presence of a large number of undesirable impurities in it. It is possible to prevent the formation and development of corrosion, but only if you clearly understand the reasons for its occurrence, and also be familiar with modern technologies.
Hot water boilers, as well as any heating systems that use water as a coolant, are characterized by three types of problems caused by the presence of the following impurities:
Each of the types of impurities listed can cause corrosion and failure of a hot water boiler or other equipment. In addition, they contribute to reducing the efficiency and performance of the boiler.
And if used for a long time heating systems water that has not undergone special preparation, this can lead to serious consequences - breakdown of circulation pumps, reduction in the diameter of the water supply system and subsequent damage, failure of control and shut-off valves. The simplest mechanical impurities - clay, sand, ordinary dirt - are present almost everywhere, as in tap water, and in artesian springs. Also, coolants contain large quantities of corrosion products of heat transfer surfaces, pipelines and other metal elements of the system that are constantly in contact with water. It goes without saying that their presence over time provokes very serious problems in the functioning of hot water boilers and all thermal power equipment, which are mainly associated with corrosion of boilers, the formation of lime deposits, the removal of salts and foaming of boiler water.
The most common reason that causes corrosion of hot water boilers, these are carbonate deposits that occur when using water of high hardness, the removal of which is possible through. It should be noted that as a result of the presence of hardness salts, scale forms even in low-temperature heating equipment. But this is far from the only cause of corrosion. For example, after heating water to a temperature of more than 130 degrees, the solubility of calcium sulfate decreases significantly, resulting in the formation of a layer of dense scale. In this case, the development of corrosion of the metal surfaces of hot water boilers is inevitable.
Corrosion of steel in steam boilers, occurring under the influence of water steam, comes down mainly to the following reaction:
3Fe + 4H20 = Fe2O3 + 4H2
We can assume that the inner surface of the boiler represents a thin film of magnetic iron oxide. During operation of the boiler, the oxide film is continuously destroyed and formed again, and hydrogen is released. Since the surface film of magnetic iron oxide represents the main protection for steel, it should be maintained in a state of least permeability to water.
For boilers, fittings, water and steam pipelines, predominantly simple carbon or low-alloy steels are used. The corrosive medium in all cases is water or water vapor of varying degrees of purity.
The temperature at which the corrosion process can occur ranges from the temperature of the room where the inactive boiler is located to the boiling point of saturated solutions when the boiler is operating, sometimes reaching 700°. The solution may have a temperature significantly higher than the critical temperature clean water(374°). However, high salt concentrations in boilers are rare.
The mechanism by which physical and chemical causes can lead to film failure in steam boilers is essentially different from the mechanism studied at lower temperatures in less critical equipment. The difference is that the corrosion rate in boilers is much greater due to the high temperature and pressure. The high rate of heat transfer from the boiler walls to the environment, reaching 15 cal/cm2sec, also increases corrosion.
Recognizing pitting corrosion
Determining the cause of the formation of corrosion damage of a certain type is often very difficult, since several causes can act simultaneously; in addition, a number of changes that occur when the boiler cools from high temperature and when water is drained sometimes masks the phenomena that took place during operation. However, experience greatly helps in recognizing pitting corrosion in boilers. For example, it was observed that the presence of black magnetic iron oxide in a corrosion shell or on the surface of a tubercle indicates that an active process was occurring in the boiler. Such observations are often used to check measures taken to protect against corrosion.
The iron oxide that forms in areas of active corrosion should not be mixed with black magnetic iron oxide, which is sometimes present as a suspension in boiler water. It must be remembered that neither the total amount of finely dispersed magnetic iron oxide, nor the amount of hydrogen released in the boiler can serve as a reliable indicator of the degree and extent of corrosion occurring. Ferrous hydrate entering the boiler from foreign sources, such as condensate tanks or boiler supply piping, may partly explain the presence of both iron oxide and hydrogen in the boiler. Ferrous hydroxide supplied with the feed water reacts in the boiler by reaction.
3Fe (OH)2 = Fe3O4 + 2H2O + H2.
Reasons influencing the development of pitting corrosion
Foreign impurities and stresses. Non-metallic inclusions in steel, as well as stress, can create anodic areas on the metal surface. Typically, corrosion pits come in different sizes and are scattered randomly across the surface. In the presence of stresses, the location of the shells obeys the direction of the applied stress. Typical examples include fin tubes where fins have cracked, as well as boiler tube flaring areas.
Dissolved oxygen.
It is possible that the most powerful activator of pitting corrosion is oxygen dissolved in water. 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 scale or contamination, where stagnant areas are created. The usual measure to combat this type of corrosion is deaeration.
Dissolved carbonic anhydride.
Since solutions of carbonic anhydride have a slightly acidic reaction, it accelerates corrosion in boilers. Alkaline boiler water reduces the aggressiveness of dissolved carbonic anhydride; however, the resulting benefit does not extend to steam-washed surfaces or condensate piping. Removal of carbonic anhydride along with dissolved oxygen by mechanical deaeration is common.
Recently, attempts have been made to use cyclohexylamine to eliminate corrosion in steam and condensate lines in heating systems.
Deposits on the walls of the boiler.
Very often, corrosion pits can be found along the outer surface (or under the surface) of deposits such as mill scale, boiler sludge, boiler scale, corrosion products, and oil films. Once started, pitting corrosion will continue to develop unless the corrosion products are removed. This type of local corrosion is enhanced by the cathodic (in relation to boiler steel) nature of the deposits or by the depletion of oxygen under the deposits.
Copper in boiler water.
Considering the large quantities of copper alloys used for auxiliary equipment (capacitors, pumps, etc.), it is not surprising that in most cases boiler deposits contain copper. It is usually present in a metallic state, sometimes in the form of an oxide. The amount of copper in deposits varies from fractions of a percent to almost pure copper.
The question of the significance of copper deposits in boiler corrosion cannot be considered resolved. Some argue that copper is only present during the corrosion process and does not affect it in any way; others, on the contrary, believe that copper, being a cathode in relation to steel, can contribute to pitting corrosion. None of these points of view has been confirmed by direct experiments.
In many cases, little (or even no) corrosion was observed despite the deposits throughout the boiler containing significant amounts of copper metal. There is also evidence that when copper comes into contact with low-carbon steel in alkaline boiler water at elevated temperatures, the copper is destroyed more quickly than the steel. Copper rings, crimping ends of flared pipes, copper rivets and screens of auxiliary equipment through which boiler water passes are almost completely destroyed even at relatively low temperatures. In view of this, it is believed that copper metal does not increase the corrosion of boiler steel. The deposited copper can be considered simply as the end product of the reduction of copper oxide by hydrogen at the time of its formation.
On the contrary, very strong corrosion pitting of boiler metal is often observed in the vicinity of deposits that are especially rich in copper. These observations led to the suggestion that copper, because it is cathodic to steel, promotes pitting corrosion.
The surface of boilers rarely presents exposed metallic iron. Most often, it has a protective layer consisting mainly of iron oxide. It is possible that where cracks form in this layer, a surface is exposed that is anodic to copper. In such places, the formation of corrosion pits increases. This can also explain, in some cases, accelerated corrosion in those places where a shell has formed, as well as severe pitting corrosion, sometimes observed after cleaning boilers with the use of acids.
Improper maintenance of idle boilers.
One of the most common causes of corrosion pits is the lack of proper care for idle boilers. An idle boiler must be kept 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 will later become centers around which the corrosion process will develop.
Common instructions for protecting idle boilers from corrosion are as follows:
1) draining water from a still hot boiler (about 90°); blowing the boiler with air until it is completely dry and kept dry;
2) filling the boiler with alkaline water (pH = 11), containing an excess of SO3 ions (about 0.01%), and storing under a water or steam seal;
3) filling the boiler with an alkaline solution containing chromic acid salts (0.02-0.03% CrO4").
When chemically 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 continuous layer and shells will appear on them, even in the absence of copper. Therefore, it is recommended immediately after chemical cleaning restore the iron oxide layer by treating with a boiling alkaline solution (similar to what is done for new boilers coming into operation).
Corrosion of economizers
The general provisions regarding boiler corrosion apply equally to economizers. However, the economizer, heating the feed water and located in front of the boiler, is especially sensitive to the formation of corrosion pits. It represents the first high-temperature surface that experiences the destructive action of oxygen dissolved in the feed water. In addition, the water passing through the economizer generally has a low pH value and does not contain chemical retardants.
The fight against corrosion of economizers involves deaerating the water and adding alkali and chemical retarders.
Sometimes boiler water is treated by passing part of it through an economizer. In this case, sludge deposits in the economizer should be avoided. The effect of such boiler water recirculation on steam quality must also be taken into account.
Stress required to cause alkali embrittlement
Practice shows a low probability of brittle fracture of conventional boiler steel if the stresses do not exceed the yield strength. Stresses created by steam pressure or a uniformly distributed load from the structure’s own weight cannot lead to the formation of cracks. However, the stresses created by rolling sheet material intended for the manufacture of boilers, deformation during riveting or any cold working associated with permanent deformation can cause the formation of cracks.
The presence of externally applied stresses is not necessary for the formation of cracks. A boiler steel sample previously held under constant bending stress and then released may crack in an alkaline solution whose concentration is equal to the increased alkali concentration in the boiler water.
Alkali concentration
The normal concentration of alkali in the boiler drum cannot cause cracks, because it does not exceed 0.1% NaOH, and the lowest concentration at which alkali brittleness is observed is approximately 100 times higher than normal.
Such high concentrations may result from extremely slow percolation of water through a rivet seam or some other gap. This explains the appearance of hard salts on the outside of most rivet joints in steam boilers. The most dangerous leak is one that is difficult to detect. It leaves a residue of solid material inside the rivet joint where there are high residual stresses. The combined action of stress and a concentrated solution can cause the appearance of alkali brittleness cracks.
Alkali embrittlement detection device
A special device for monitoring the composition of water reproduces the process of water evaporation with increasing alkali concentration on a stressed steel sample under the same conditions in which this occurs in the area of the rivet seam. Cracking of the control sample indicates that boiler water of this composition is capable of causing alkali embrittlement. Therefore, in this case, water treatment is necessary to eliminate its hazardous properties. However, cracking of the control sample does not mean that cracks have already appeared or will appear in the boiler. In rivet seams or other joints there is not necessarily both leakage (steaming), stress, and an increase in alkali concentration, as in the control sample.
The control device is installed directly on the steam boiler and allows you to judge the quality of the boiler water.
The test lasts 30 days or more with constant circulation of water through the control device.
Alkali Brittleness Crack Recognition
Alkali brittleness cracks in conventional boiler steel are of a different nature than fatigue or high stress cracks. This is illustrated in Fig. I9, which shows the intergranular nature of such cracks, forming a fine network. The difference between intergranular alkali brittleness cracks and intragranular cracks caused by corrosion fatigue can be seen by comparison.
In alloy steels (for example, nickel or silicon-manganese), used for locomotive boilers, cracks are also arranged in a grid, but do not always pass between crystallites, as in the case of ordinary boiler steel.
Alkali brittleness theory
Atoms in the crystal lattice of a metal located at the boundaries of crystallites experience less symmetrical influence from their neighbors than atoms in the rest of the grain mass. Therefore, they leave the crystal lattice more easily. One might think that with careful selection of an aggressive environment it will be possible to achieve such selective removal of atoms from crystallite boundaries. Indeed, experiments show that in acidic, neutral (with the help of a weak electric current, creating conditions favorable for corrosion) and concentrated alkali solutions, intergranular cracking can be obtained. If a solution causing general corrosion is altered by the addition of any substance that forms protective film on the surface of crystallites, corrosion is concentrated at the boundaries between crystallites.
The aggressive solution in this case is caustic soda solution. The sodium silica salt can protect the surfaces of crystallites without affecting the boundaries between them. The result of a combined protective and aggressive action depends on many circumstances: concentration, temperature, stressed state of the metal and composition of the solution.
There are also the colloidal theory of alkali brittleness and the theory of the action of hydrogen dissolving in steel.
Ways to combat alkaline embrittlement
One way to combat alkali brittleness is to replace boiler riveting with welding, which eliminates the possibility of leakage. Brittleness can also be eliminated by using steel that is resistant to intergranular corrosion, or chemical treatment boiler water. In riveted boilers currently used, the latter method is the only acceptable one.
Preliminary tests using a control sample are the best way to determine the effectiveness of certain water protective additives. Sodium sulfide salt does not prevent cracking. Sodium nitrogen salt has been successfully used to protect against cracking at pressures up to 52.5 kg/cm2. Concentrated sodium nitrogen salt solutions boiling at atmospheric pressure can cause stress corrosion cracks in mild steel.
Currently, sodium nitrogen salt is widely used in stationary boilers. The concentration of sodium nitrogen salt corresponds to 20-30% of the alkali concentration.
Hydrogen as a measure of corrosion rate
The steam temperature in modern boilers approaches the temperatures used in the industrial production of hydrogen by direct reaction between steam and iron.
The rate of corrosion of pipes made of carbon and alloy steel under the influence of steam, at temperatures up to 650°, can be judged by the volume of hydrogen released. Hydrogen evolution is sometimes used as a measure of general corrosion.
Recently, three types of miniature units for removing gases and air have been used in US power plants. They ensure complete removal of gases, and the degassed condensate is suitable for determining salts carried away by steam from the boiler. An approximate value of the total corrosion of the superheater during boiler operation can be obtained by determining the difference in hydrogen concentrations in steam samples taken before and after its passage through the superheater.
Corrosion caused by impurities in steam
The saturated steam entering the superheater carries with it small but measurable amounts of gases and salts from the boiler water. The most commonly encountered gases are oxygen, ammonia and carbon dioxide. When steam passes through the superheater, no noticeable change in the concentration of these gases is observed. Only minor corrosion of the metal superheater can be attributed to the action of these gases. It has not yet been proven that salts dissolved in water, dry, or deposited on superheater elements can contribute to corrosion. However, caustic soda, being the main component of the salts carried away by the boiler water, can contribute to corrosion of a very hot tube, especially if the alkali adheres to the metal wall.
Increasing the purity of saturated steam is achieved by thoroughly removing gases from the feed water. Reducing the amount of salts entrained in the steam can be achieved by thorough cleaning in the upper header, the use of mechanical separators, flushing the saturated steam with feed water, or suitable chemical treatment of the water.
Determination of the concentration and nature of gases entrained by saturated steam is carried out using the above-mentioned devices and chemical analysis. Determination of salt concentration in saturated steam conveniently performed by measuring the electrical conductivity of water or the evaporation of large amounts of condensate.
An improved method for measuring electrical conductivity is proposed, and appropriate corrections for some dissolved gases are given. The condensate in the miniature degassing units mentioned above can also be used to measure electrical conductivity.
When the boiler is idle, the superheater is a refrigerator in which condensation accumulates; In this case, normal underwater pitting is possible if the steam contained oxygen or carbon dioxide.
Steam boiler accidents associated with violations water regime, corrosion and erosion of metal
Normal water regime is one of the most important conditions reliability and efficiency of boiler plant operation. The use of water with increased hardness to feed boilers entails the formation of scale, excessive fuel consumption and increased costs for repairs and cleaning of boilers. It is known that scale formation can lead to a steam boiler failure due to burnout of heating surfaces. Therefore, the correct water regime in the boiler room should be considered not only from the point of view of increasing the efficiency of the boiler installation, but also as the most important preventive measure to combat accidents.
Currently, boiler plants of industrial enterprises are equipped with water treatment devices, so their operating conditions have improved and the number of accidents caused by scale formation and corrosion has significantly decreased.
However, at some enterprises, the administration, having formally fulfilled the requirement of the Boiler Inspection Rules to equip boilers with water treatment units, does not provide normal operating conditions for these installations, does not control the quality of feed water and the condition of the boiler heating surfaces, allowing the boilers to become contaminated with scale and sludge. Here are some examples of boiler failures for these reasons.
1. In the boiler room of the prefabricated plant reinforced concrete structures Due to violations of the water regime in the boiler DKVR-6, 5-13, three screen pipes ruptured, some of the screen pipes were deformed, and dents formed on many pipes.
The boiler room has a two-stage sodium cation exchange water treatment and deaerator, but the normal operation of the water treatment equipment was not given due attention. Regeneration of cation exchange filters was not carried out in established by the instructions the timing and quality of feed and boiler water were rarely checked, and the timing of periodic boiler purging was not observed. The water in the deaerator was not heated to the required temperature and therefore deoxygenation of the water did not actually occur.
It was also established that the boiler was often fed raw water, at the same time, they did not comply with the requirements of the “Rules for the Design and Safe Operation of Steam and Hot Water Boilers”, according to which the shut-off devices on the raw water line must be sealed in closed position, and each case of feeding with raw water should be recorded in the water treatment log. From individual entries in the water treatment log it is clear that the hardness of the feed water reached 2 mEq/kg or more, while the permissible value according to boiler inspection standards is 0.02 mEq/kg. Most often, the following entries were made in the log: “the water is dirty, hard,” without indicating the results chemical analysis water.
When inspecting the boiler after shutdown, deposits up to 5 mm thick were found on the internal surfaces of the screen pipes; individual pipes were almost completely clogged with scale and sludge. On the inner surface of the drum in the lower part, the thickness of deposits reached 3 mm, the front part of the drum is filled with sludge to one third of its height.
In 11 months Before this accident, similar damage (“cracks, dents, deformation) was detected in 13 boiler screen pipes. The defective pipes were replaced, but the administration of the enterprise, in violation of the “Instructions for the investigation of accidents that resulted in accidents at enterprises and facilities controlled by the USSR State Technical Supervision Committee,” did not investigate this case and did not take measures to improve the operating conditions of the boilers.
2. On the power train, raw water to feed a single-drum water-tube shielded steam boiler with a capacity of 10 t/h and an operating pressure of 41 kgf/cm2 was treated by the cation exchange method. Due to unsatisfactory performance of the cation and waste filter, the residual hardness of the softened water reached
0.7 mEq/kg instead of the 0.01 mEq/kg envisaged by the project. The boiler was not blown regularly. When stopping for repairs, the boiler drum and screen collectors were not opened or inspected. Due to scale deposits, a pipe ruptured, and a fireman was burned by steam and burning fuel ejected from the firebox.
The accident could not have happened if the boiler combustion door had been closed with a latch, as required by the rules for the safe operation of boilers.
3. At the cement plant, a newly installed single-drum water-tube boiler with a capacity of 35 t/h and a working pressure of 43 kgf/cm2 was put into operation without chemical water treatment, the installation of which had not been completed by that time. For a month, the boiler was fed with untreated water. The water was not deaerated for more than two months, since the steam line was not connected to the deaerator.
Violations of the water regime were allowed even after... pre-production equipment was put into operation. The boiler was often fed with raw water; the purge regime was not followed; the chemical laboratory did not control the quality of the feed water, since it was not equipped with the necessary reagents.
Due to unsatisfactory water conditions, deposits on the internal surfaces of the screen pipes reached a thickness of 8 mm; as a result, bulges formed on 36 screen pipes”, a significant part of the pipes was deformed, the walls of the drum with inside have been corroded.
4. Factory reinforced concrete products The boiler of the Shukhov-Berlin system was powered by electromagnetically treated water. It is known that with this method of water treatment, timely and effective removal of sludge from the boiler must be ensured.
However, during operation of the boiler this condition was not met. The boiler was not purged regularly, and the boiler shutdown schedule for flushing and cleaning was not followed.
As a result, a large amount of sludge accumulated inside the boiler. The back part of the pipes was clogged with sludge at 70-80% of the cross-section, the mud trap - at 70% of the volume, the thickness of scale on the heating surfaces reached 4 mm. This led to overheating and deformation of the boiling pipes, pipe pipes and heads of tubular sections.
When choosing an electromagnetic method for processing iodine in this case, the quality of the feed water and the design features of the boiler were not taken into account, and no measures were taken to organize a normal blowing regime, which led to the accumulation of sludge and significant scale deposits in the boiler.
5. The issues of organizing a rational water regime to ensure reliable and economical operation of boilers at thermal power plants have acquired exceptional importance.
The formation of deposits on the heating surfaces of boiler units occurs as a result of complex physicochemical processes in which not only scale formers, but also metal oxides and easily soluble compounds are involved. Dialysis of deposits shows that, along with scale-forming salts, they contain a significant amount of iron oxides, which are products of corrosion processes.
Over the past years, our country has achieved significant success in organizing the rational water regime of thermal power plant boilers and chemical control of water and steam, as well as in the introduction of corrosion-resistant metals and protective coatings.
The use of modern water treatment means has dramatically increased the reliability and cost-effectiveness of operating power equipment.
However, at some thermal power plants, violations of the water regime are still allowed.
In June 1976, for this reason, at the thermal power plant of the pulp and paper mill, an accident occurred on a steam boiler type BKZ-220-100 f with a steam capacity of 220 t/h with steam parameters of 100 kgf/cm2 and 540 ° C, manufactured at the Barnaul Boiler Plant in 1964 d. Single-drum boiler with natural circulation, made according to a U-shaped design. The prismatic combustion chamber is completely shielded by pipes with an outer diameter of 60 mm, the pitch of which is 64 mm. The lower part of the screen surface forms a so-called cold funnel, along the slopes of which particles of slag in solid form roll down into the slag chest. The evaporation scheme is two-stage, with steam flushed with feed water. The first evaporation stage is included directly in the boiler drum, the second stage is remote steam separation cyclones included in the circulation circuit of the middle side screen blocks.
The boiler is fed with a mixture of chemically purified water (60%) and condensate coming from turbines and production shops (40%). Water for feeding the boiler is processed according to the following scheme: limestone - coagulation - magnesium desiliconization in
Clarifiers - two-stage cationization.
The boiler runs on coal from the Inta deposit with a relatively low ash melting point. Fuel oil is used as starting fuel. Before the accident, the boiler operated for 73,300 hours.
On the day of the accident, the boiler was turned on at 00:45 and operated without deviation from normal mode until 14:00. The pressure in the drum during this period of operation was maintained within the range of 84-102 kgf/cm2, steam consumption was 145-180 t/h, temperature superheated steam -520-535° C.
At 14:10, 11 pipes of the front screen ruptured in the cold funnel area at 3.7 m with partial destruction
lining. It is believed that a water pipe or two ruptured first, followed by the rupture of other pipes. The water level dropped sharply and the boiler was stopped by automatic protection.
The inspection showed that the inclined sections of the pipes of the cold funnel outside the bends were destroyed, while two pipes were torn off from the first front lower collector, and nine from the second. The rupture is brittle; the edges at the rupture sites are blunt and not thinned. The length of the ruptured sections of pipes ranges from one to three meters. On the inner surface of damaged pipes, as well as samples cut from undamaged pipes, loose deposits up to 2.5 mm thick were found, as well as a large number of pits, up to 2 mm deep, located in a chain up to 10 mm wide along two generatrices along the heating boundary of the pipe. It was in places of corrosion damage that the metal was destroyed.
During the investigation of the accident, it turned out that earlier during the operation of the boiler there had already been ruptures of the screen pipes. So, for example, two months before the accident, a front screen pipe ruptured at 6.0 m. After 3 days, the boiler was shut down again due to the rupture of two front screen pipes at 7.0 m. And in these cases, the destruction of the pipes was result of corrosion damage to the metal.
In accordance with the approved schedule, the boiler had to be stopped at major renovation in the third quarter of 1976. During the repair period, it was planned to replace the pipes of the front screen in the area of the cold funnel. However, the boiler was not stopped for repairs and the pipes were not replaced.
Corrosion damage to the metal was a consequence of violations of the water regime, which were allowed for a long time during the operation of the boilers of the thermal power plant. The boilers were fed with water with a high content of iron, copper and oxygen. The total salt content in the feed water significantly exceeded acceptable standards, as a result of which, even in the circuits of the first evaporation stage, the salt content reached 800 mg/kg. Industrial condensates with an iron content of 400-600 mg/kg used to feed boilers were not purified. For this reason, and also because there was not sufficient anti-corrosion protection of the water treatment equipment (protection was partially carried out), there were significant deposits on the internal surfaces of the pipes (up to 1000 g/m2), mainly consisting of iron compounds. Amination and hydrazination of feed water was introduced only shortly before the accident. Pre-startup and operational acid flushing of the boilers was not carried out.
Other violations of the Rules for the Technical Operation of Boilers also contributed to the accident. At thermal power plants, boilers are lit very often, and the largest number of kindlings occurred in the boiler with which the accident occurred. The boilers are equipped with devices for steam heating, but they were not used for kindling. During kindling, the movements of the screen collectors were not controlled.
To clarify the nature of the corrosion process and determine the reasons for the formation of pits mainly in the first two panels of the front screen and the location of these pits in the form of chains, the materials of the accident investigation were sent to the CKTI. When reviewing these materials, attention was drawn to the fact that
the boilers operated with sharply variable loads, and a significant reduction in steam output was allowed (up to 90 t/h), which could lead to local circulation disruption. The boilers were heated in the following way: at the beginning of the kindling, two nozzles located opposite (diagonally) were turned on. This method led to a slowdown in the process of natural circulation in the panels of the first and second front screens. It is in these screens that the main focus of ulcerative lesions is found. Nitrites occasionally appeared in the feed water, the concentration of which was not monitored.
An analysis of the accident materials, taking into account the listed shortcomings, gave reason to believe that the formation of chains of ulcers on the side generatrices of the internal surfaces of the front screen pipes on the slope of the cold funnel is the result of a long-term process of sub-sludge electrochemical corrosion. The depolarizers of this process were nitrites and oxygen dissolved in water.
The arrangement of pits in the form of chains is, apparently, the result of the boiler operating during kindling with an unsteady process of natural circulation. During the period of the beginning of circulation, pore bubbles periodically form on the upper generatrix of the inclined pipes of the cold funnel, causing the effect of local thermal pulsations in the metal through the occurrence of electrochemical processes in the region of temporary phase separation. It was these places that became the foci for the formation of chains of ulcers. The predominant formation of pitting in the first two panels of the front screen was a consequence of improper kindling conditions.
6. At the TIC WB, during the operation of the PK-YUSH-2 boiler with a steam production capacity of 230 t/h with steam parameters of 100 kgf/cm2 and 540° C, steaming was noticed at the outlet from the fresh steam collection manifold to the main safety valve. The outlet is connected by welding to a cast tee welded into the prefabricated manifold.
The boiler was emergency stopped. During the inspection, an annular crack was discovered in the lower part of the pipe (168X13 mm) of the horizontal section of the bend in the immediate vicinity of the place where the bend is connected to the cast tee. The length of the crack on the outer surface is 70 mm and on the inner surface is 110 mm. On the inner surface of the pipe at the site of its damage, a large number of corrosion pits and individual cracks located parallel to the main one were revealed.
Metallographic analysis established that the cracks begin from pits in the decarbonized metal layer and then develop transcrystalline in the direction perpendicular to the surface of the pipe. The microstructure of the pipe metal is ferrite grains and thin pearlite chains along the grain boundaries. According to the scale given as an appendix to MRTU 14-4-21-67, the microstructure can be assessed with a score of 8.
The chemical composition of the metal of the damaged pipe corresponds to steel 12Х1МФ. Mechanical properties meet the requirements of technical delivery conditions. The diameter of the pipe in the damaged area does not exceed the plus tolerance.
The horizontal outlet to the safety valve with an unregulated fastening system can be considered as a cantilever beam welded to a tee rigidly fixed in the manifold, with maximum bending stresses at the sealing point, i.e. in the area where the pipe has been damaged. With absence
drainage in the outlet and the presence of a counter slope, due to elastic bending in the area from the safety valve to the fresh steam collection manifold, in the lower part of the pipe in front of the tee there may be a constant accumulation of a small amount of condensate, enriched with oxygen during shutdowns, conservation and commissioning of the boiler from the air. Under these conditions, corrosive erosion of the metal occurred, and the combined effect of condensate and tensile stresses on the metal caused its corrosion cracking. During operation, fatigue-corrosion cracks can develop in places of corrosion pits and shallow cracks as a result of aggressive environmental influences and alternating stresses in the metal, which apparently happened in this case.
To prevent condensate from accumulating, reverse steam circulation was installed in the outlet. To do this, the outlet pipe directly in front of the main safety valve was connected by a heating line (pipes with a diameter of 10 mm) to the intermediate chamber of the superheater, through which steam is supplied at a temperature of 430 ° C. With a small difference in excess pressure (up to 4 kgf/cm2), continuous steam flow is ensured and the temperature of the medium in the outlet is maintained at least 400° C. Reconstruction of the outlet was carried out on all boilers of PK-YUSH-2 CHPP.
In order to prevent damage to the outlets to the main safety valves on PK-YUSH-2 boilers and similar ones, it is recommended:
Ultrasound check the lower semi-perimeters of the branch pipes at the points of welding to the tees;
Check whether the required slopes are observed and, if necessary, adjust the systems for attaching steam pipelines to the main safety valves, taking into account the actual condition of the steam pipelines (insulation weight, actual weight of pipes, previously carried out reconstructions);
Make reverse steam circulation in the outlets to the main safety valves; the design and internal diameter of the heating steam pipeline in each individual case must be agreed with the equipment manufacturer;
All dead-end bends to safety valves carefully insulate.
(From express information from STSNTI ORGRES - 1975)
GOST 20995-75 “Stationary steam boilers with pressure up to 3.9 MPa. Indicators of quality of feed water and steam" normalizes the indicators in feed water: transparency, that is, the amount of suspended impurities; general hardness, content of iron and copper compounds - prevention of scale formation and iron and copper oxide deposits; pH value - prevention of alkaline and acid corrosion and also foaming in the boiler drum; oxygen content - preventing oxygen corrosion; nitrite content - prevention of nitrite corrosion; content of petroleum products - preventing foam formation in the boiler drum.
The norm values are determined by GOST depending on the pressure in the boiler (therefore, on the water temperature), on the power of the local heat flow and from water treatment technology.
When investigating the causes of corrosion, first of all, it is necessary to inspect (where available) places of metal destruction, analyze the operating conditions of the boiler in the pre-accident period, analyze the quality of feed water, steam and deposits, and analyze the design features of the boiler.
Upon external inspection, the following types of corrosion may be suspected.
: inlet sections of steel economizer pipes; supply pipelines when encountering insufficiently deoxygenated (above normal) water - “breakthroughs” of oxygen due to poor deaeration; feed water heaters; all wet areas of the boiler during shutdown and failure to take measures to prevent air from entering the boiler, especially in stagnant areas, when draining water, from where it is difficult to remove steam condensate or completely fill with water, for example vertical pipes steam superheaters. During downtime, corrosion is enhanced (localized) in the presence of alkali (less than 100 mg/l).
Oxygen corrosion rarely (when the oxygen content in water is significantly higher than the norm - 0.3 mg/l) appears in the steam separation devices of boiler drums and on the drum wall at the water level boundary; in downpipes. Corrosion does not occur in riser pipes due to the deaerating effect of steam bubbles.
Type and nature of damage. Ulcers of varying depth and diameter, often covered with tubercles, the upper crust of which is reddish iron oxides (probably hematite Fe 2 O 3). Evidence of active corrosion: under the crust of the tubercles there is a black liquid sediment, probably magnetite (Fe 3 O 4) mixed with sulfates and chlorides. With extinct corrosion, there is a void under the crust, and the bottom of the ulcer is covered with deposits of scale and sludge.
At water pH > 8.5 - ulcers are rare, but larger and deeper, at pH< 8,5 - встречаются чаще, но меньших размеров. Только вскрытие бугорков помогает интерпретировать бугорки не как поверхностные отложения, а как следствие коррозии.
When the water speed is more than 2 m/s, the tubercles can take on an oblong shape in the direction of the jet movement.
. Magnetic crusts are quite dense and could serve as a reliable barrier to the penetration of oxygen into the tubercles. But they are often destroyed as a result of corrosion fatigue, when the temperature of water and metal changes cyclically: frequent stops and starts of the boiler, pulsating movement of the steam-water mixture, stratification of the steam-water mixture into separate plugs of steam and water, following each other.
Corrosion increases with increasing temperature (up to 350 °C) and increasing chloride content in boiler water. Sometimes corrosion is enhanced by thermal decomposition products of certain organic substances in the feedwater.
Rice. 1. Appearance of oxygen corrosion
Places of metal corrosion damage. Pipes in areas of high power heat flow (burner area and opposite the elongated torch) - 300-400 kW/m2 and where the metal temperature is 5-10 °C higher than the boiling point of water at a given pressure; inclined and horizontal pipes where water circulation is poor; places under thick sediments; zones near the backing rings and in the welds themselves, for example, in places where intra-drum vapor separation devices are welded; places near the rivets.
Type and nature of damage. Hemispherical or elliptical depressions filled with corrosion products, often including shiny crystals of magnetite (Fe 3 O 4). Most of the depressions are covered with a hard crust. On the side of the pipes facing the firebox, the recesses can connect, forming a so-called corrosion track 20-40 mm wide and up to 2-3 m long.
If the crust is not sufficiently stable and dense, then corrosion can lead - under conditions of mechanical stress - to the appearance of cracks in the metal, especially near the cracks: rivets, rolling joints, welding points of vapor separation devices.
Causes of Corrosion Damage. At high temperatures - more than 200 ° C - and a high concentration of caustic soda (NaOH) - 10% or more - the protective film (crust) on the metal is destroyed:
4NaOH + Fe 3 O 4 = 2NaFeO 2 + Na 2 FeO 2 + 2H 2 O (1)
The intermediate product NaFeO 2 undergoes hydrolysis:
4NaFeO 2 + 2H 2 O = 4NaOH + 2Fe 2 O 3 + 2H 2 (2)
That is, in this reaction (2) caustic soda is reduced, in reactions (1), (2) it is not consumed, but acts as a catalyst.
When the magnetite is removed, caustic soda and water can react with the iron directly to release atomic hydrogen:
2NaOH + Fe = Na 2 FeO 2 + 2H (3)
4H 2 O + 3Fe = Fe 3 O 4 + 8H (4)
The released hydrogen is able to diffuse into the metal and form methane (CH 4) with iron carbide:
4H + Fe 3 C = CH 4 + 3Fe (5)
It is also possible to combine atomic hydrogen into molecular hydrogen (H + H = H 2).
Methane and molecular hydrogen cannot penetrate into 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 compaction of protective films.
A concentrated solution of caustic soda is formed in places of deep evaporation of boiler water: dense scale deposits of salts (a type of sub-sludge corrosion); a crisis of nucleate boiling, when a stable vapor film is formed above the metal - there the metal is almost not damaged, but at the edges of the film, where active evaporation occurs, caustic soda is concentrated; the presence of cracks where evaporation occurs, which is different from evaporation in the entire volume of water: caustic soda evaporates worse than water, is not washed away by water and accumulates. Acting on the metal, caustic soda forms cracks at the grain boundaries directed into the metal (a type of intergranular corrosion - crevice).
Intergranular corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.
Rice. 3. Intergranular corrosion: a - microstructure of the metal before corrosion, b - microstructure at the corrosion stage, formation of cracks along the grain boundaries of the metal
Such a corrosive effect on metal is possible only with the simultaneous presence of three factors:
Rice. 4. Appearance of intergranular corrosion
Corrosion cracks do not develop if the following ratio 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 are the contents of sodium sulfate, sodium carbonate, sodium phosphate, sodium nitrate and sodium hydroxide, respectively, mg/kg.
In currently manufactured boilers, at least one of the specified conditions for the occurrence of corrosion is absent.
The presence of silicon compounds in boiler water can also increase intergranular corrosion.
NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (Cl -) are corrosion accelerators; due to their high mobility and small size, they easily penetrate protective oxide films and produce highly soluble salts with iron (FeCl 2, FeCl 3) instead of poorly soluble iron oxides.
In boiler water, the values of total mineralization are traditionally monitored, rather than the content of individual salts. Probably for this reason, standardization was introduced not according to the indicated ratio (6), but according to the value of the relative alkalinity of the boiler water:
Sh q rel = Sh ov rel = Sh ov 40 100/S ov ≤ 20, (7)
where Shk rel - relative alkalinity of boiler water, %; Shch ov rel - relative alkalinity of treated (additional) water, %; Shch ov - total alkalinity of treated (additional) water, mmol/l; S ov - mineralization of treated (additional) water (including chloride content), mg/l.
The total alkalinity of the treated (additional) water can be taken equal, mmol/l:
The values of relative alkalinity of boiler water according to Rostechnadzor standards are accepted, %, no more than:
Rice. 4. Result of intergranular corrosion
According to Rostechnadzor standards, Shch kv rel is one of the criteria for the safe operation of boilers. It is more correct to check the criterion for the potential alkaline aggressiveness of boiler water, which does not take into account the content of chlorine ion:
K sh = (S ov - [Cl - ])/40 Shch ov, (8)
where Ksh is a criterion for the potential alkaline aggressiveness of boiler water; S ov - mineralization of treated (additional) water (including chloride content), mg/l; Cl - - content of chlorides in treated (additional) water, mg/l; Shch ov - total alkalinity of treated (additional) water, mmol/l.
The value of K sch can be taken:
Sludge corrosion
This name combines several different types of corrosion (alkali, oxygen, etc.). The accumulation of loose and porous deposits and sludge in different areas of the boiler causes corrosion of the metal under the sludge. main reason: contamination of feed water with iron oxides.
. Screen and boiler pipes of the boiler on the side facing the firebox.
Type and nature of damage. Rare, sharply limited large ulcers.
. If there are more than 20 μg/l of nitrite ions (NO - 2) in the feed water, and the water temperature is more than 200 ° C, nitrites serve as cathodic depolarizers of electrochemical corrosion, being reduced to HNO 2, NO, N 2 (see above).
Locations of metal corrosion damage. The outlet part of superheater coils, superheated steam steam pipelines, horizontal and slightly inclined steam generating pipes in areas of poor water circulation, sometimes along the upper form of the outlet coils of boiling water economizers.
Type and nature of damage. Plaques of dense black iron oxides (Fe 3 O 4), firmly adhered to the metal. When the temperature fluctuates, the continuity of the plaque (crust) is disrupted and the scales fall off. Uniform thinning of metal with bulges, longitudinal cracks, breaks.
It can be identified as sub-sludge corrosion: in the form of deep ulcers with vaguely demarcated edges, most often near welds protruding into the pipe, where sludge accumulates.
Causes of corrosion damage:
The result: direct chemical interaction of iron with water vapor (see above).
Caused by aerobic and anaerobic bacteria, appears at temperatures of 20-80 ° C.
Locations of metal damage. Pipes and containers to the boiler with water at the specified temperature.
Type and nature of damage. The tubercles are of different sizes: diameter from several millimeters to several centimeters, rarely - several tens of centimeters. The tubercles are covered with dense iron oxides - a waste product of aerobic bacteria. Inside there is a black powder and suspension (iron sulfide FeS) - a product of sulfate-reducing anaerobic bacteria; under the black formation there are round ulcers.
Causes of damage. IN natural water Iron sulfates, oxygen and various bacteria are always present.
Iron bacteria in the presence of oxygen form a film of iron oxides, under which anaerobic bacteria reduce sulfates to iron sulfide (FeS) and hydrogen sulfide (H 2 S). In turn, hydrogen sulfide starts the formation of sulfurous (very unstable) and sulfuric acids, and the metal corrodes.
This type has an indirect effect on boiler corrosion: a water flow at a speed of 2-3 m/s tears off the tubercles, carries their contents into the boiler, increasing the accumulation of sludge.
In rare cases, this corrosion may occur in the boiler itself if, during a long shutdown of the boiler, the reserve is filled with water at a temperature of 50-60 o C, and the temperature is maintained due to random breakthroughs of steam from neighboring boilers.
Locations of corrosion damage. Equipment in which steam is separated from water: boiler drum, steam separation devices in and outside the drum, also - rarely - in feedwater pipelines and economizer.
Type and nature of damage. The surface of the metal is smooth, but if the medium moves at high speed, then the corroded surface is not smooth, has horseshoe-shaped depressions and “tails” oriented in the direction of movement. The surface is covered with a thin matte or black shiny film. There are no obvious deposits, and there are no corrosion products, because the “chelate” (specially introduced into the boiler organic compounds polyamines) has already reacted.
In the presence of oxygen, which rarely happens in a normally operating boiler, the corroded surface is “invigorated”: roughness, islands of metal.
Causes of Corrosion Damage. The mechanism of action of the “chelate” was described earlier (“Industrial and heating boiler houses and mini-CHP”, 1(6)΄ 2011, p. 40).
“Chelate” corrosion occurs when there is an overdose of “chelate,” but it is also possible with a normal dose, since the “chelate” is concentrated in areas where intense evaporation of water occurs: nucleate boiling is replaced by film boiling. In steam separation devices, there are cases of particularly destructive “chelate” corrosion due to high turbulent velocities of water and steam-water mixture.
All of the described corrosion damage can have a synenergetic effect, so that the total damage from the combined action of different corrosion factors can exceed the sum of damage from individual types of corrosion.
As a rule, the action of corrosive agents enhances the unstable thermal regime of the boiler, which causes corrosion fatigue and initiates thermal fatigue corrosion: the number of starts from a cold state is more than 100, the total number of starts is more than 200. Since these types of metal damage occur rarely, cracks, rupture pipes have an appearance identical to metal damage from various types of corrosion.
Usually, to identify the cause of metal destruction, additional metallographic studies are required: radiography, ultrasound, color and magnetic particle flaw detection.
Various researchers have proposed programs for diagnosing types of corrosion damage to boiler steels. The VTI program (A.F. Bogachev and his colleagues) is known - mainly for high-pressure energy boilers, and the developments of the Energochermet association - mainly for low- and medium-pressure energy boilers and waste heat boilers.
