Corrosion in hot water boilers, heating systems, heating systems is much more common than in steam condensate systems. In most cases, this situation is explained by the fact that when designing a water heating system, this is given less attention, although the factors of 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 the hot water boilers with feed water cause different kinds corrosion - alkaline (intercrystalline), oxygen, chelate, 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 the water intended for make-up. It is necessary to ensure the binding of free carbon dioxide, oxygen, to bring the pH value to an acceptable level, to take measures to protect aluminum, bronze and copper elements of heating equipment and boilers, pipelines and heating equipment from corrosion.
Recently, special chemicals have been used for high-quality correctional heating networks, hot water boilers and other equipment.
Water at the same time is a universal solvent and an inexpensive coolant, it is beneficial to use it in heating systems. But insufficient preparation for it can lead to unpleasant consequences, one of which is boiler corrosion. Possible risks are primarily associated with the presence of a large amount of undesirable impurities in it. It is possible to prevent the formation and development of corrosion, but only if you clearly understand the causes of its occurrence, and also be familiar with modern technologies.
For hot water boilers, however, as for any heating systems that use water as a coolant, three types of problems are characteristic due to the presence of the following impurities:
- mechanical insoluble;
- precipitate-forming dissolved;
- corrosive.
Each of these types of impurities can cause corrosion and failure of a hot water boiler or other equipment. In addition, they contribute to a decrease in the efficiency and productivity of the boiler.
And if used for a long time in heating systems not passed special training water, this can lead to serious consequences - breakdown of circulation pumps, reduction in the diameter of the water supply and subsequent damage, failure of control and shutoff valves. The simplest mechanical impurities - clay, sand, ordinary dirt - are present almost everywhere, as in tap water, and in artesian sources. Also in coolants in large quantities there are corrosion products of heat transfer surfaces, pipelines and other metal elements of the system that are constantly in contact with water. Needless to say, their presence over time provokes very serious malfunctions in the functioning of hot water boilers and all heat and power equipment, which are mainly associated with boiler corrosion, formation lime deposits, entrainment of salts and foaming of boiler water.
The most common reason for which boiler corrosion, these are carbonate deposits that occur when using water of increased hardness, the removal of which is possible through. It should be noted that as a result of the presence of hardness salts, scale is formed even in a low-temperature heating equipment. But this is not the only cause of corrosion. For example, after heating water to a temperature of more than 130 degrees, the solubility of calcium sulfate is significantly reduced, resulting in a layer of dense scale. In this case, the development of corrosion of metal surfaces of hot water boilers is inevitable.
Corrosion phenomena in boilers most often appear on the internal heat-stressed surface and relatively less often on the outer one.
In the latter case, the destruction of the metal is due - in most cases - to the combined action of corrosion and erosion, which sometimes has a predominant significance.
An external sign of erosion destruction is a clean metal surface. Under corrosive action, corrosion products usually remain on its surface.
Internal (in the water environment) corrosion and scale processes can aggravate external corrosion (in the gas environment) due to the thermal resistance of the layer of scale and corrosion deposits, and, consequently, the temperature increase on the metal surface.
External metal corrosion (from the side of the boiler furnace) depends on various factors, but, above all, on the type and composition of the fuel being burned.
Corrosion of gas-oil boilers
Fuel oil contains organic compounds of vanadium and sodium. If molten deposits of slag containing vanadium (V) compounds accumulate on the wall of the pipe facing the furnace, then with a large excess of air and / or a metal surface temperature of 520-880 ° C, the following reactions occur:
4Fe + 3V2O5 = 2Fe2O3 + 3V2O3 (1)
V2O3 + O2 = V2O5 (2)
Fe2O3 + V2O5 = 2FeVO4 (3)
7Fe + 8FeVO4 = 5Fe3O4 + 4V2O3 (4)
(Sodium compounds) + O2 = Na2O (5)
Another corrosion mechanism involving vanadium (liquid eutectic mixture) is also possible:
2Na2O. V2O4 . 5V2O5 + O2 = 2Na2O. 6V2O5 (6)
Na2O. 6V2O5 + M = Na2O. V2O4 . 5V2O5 + MO (7)
(M - metal)
Compounds of vanadium and sodium during the combustion of fuel are oxidized to V2O5 and Na2O. In deposits adhering to the metal surface, Na2O is a binder. The liquid formed as a result of reactions (1)-(7) melts the protective film of magnetite (Fe3O4), which leads to the oxidation of the metal under the deposits (melting temperature of the deposits (slag) is 590-880 °C).
As a result of these processes, the walls screen pipes facing the firebox are evenly thinned.
An increase in the temperature of the metal, at which vanadium compounds become liquid, is facilitated by internal scale deposits in pipes. And thus, when the yield point temperature of the metal is reached, a pipe break occurs - a consequence of the combined action of external and internal deposits.
The fastening parts of the pipe screens, as well as the protrusions of the pipe welds, also corrode - the temperature rise on their surface accelerates: they are not cooled by the steam-water mixture, like pipes.
Fuel oil may contain sulfur (2.0-3.5%) in the form of organic compounds, elemental sulfur, sodium sulfate (Na2SO4), which enters the oil from formation waters. On the metal surface under such conditions, vanadium corrosion is accompanied by sulfide-oxide corrosion. Their combined effect is most pronounced when the deposits contain 87% V2O5 and 13% Na2SO4, which corresponds to the content of vanadium and sodium in the fuel oil in a ratio of 13/1.
In winter, when heating fuel oil with steam in tanks (to facilitate draining), additional water enters it in an amount of 0.5-5.0%. Consequence: the amount of deposits on the low-temperature surfaces of the boiler increases, and, obviously, the corrosion of fuel oil pipelines and fuel oil tanks increases.
In addition to the above-described scheme for the destruction of boiler screen tubes, corrosion of superheaters, festoon tubes, boiler bundles, economizers has some features due to increased - in some sections - gas velocities, especially those containing unburned fuel oil particles and exfoliated slag particles.
Corrosion identification
The outer surface of the pipes is covered with a dense enamel-like layer of gray and dark gray deposits. On the side facing the firebox, there is a thinning of the pipe: flat sections and shallow cracks in the form of “marks” are clearly visible if the surface is cleaned of deposits and oxide films.
If the pipe is destroyed in an emergency, then a through longitudinal narrow crack is visible.
Corrosion of pulverized coal boilers
In corrosion formed by the action of coal combustion products, sulfur and its compounds are of decisive importance. In addition, the course of corrosion processes is affected by chlorides (mainly NaCl) and alkali metal compounds. Corrosion is most likely when the coal contains more than 3.5% sulfur and 0.25% chlorine.
Fly ash containing alkali compounds and sulfur oxides is deposited on the metal surface at a temperature of 560-730 °C. In this case, as a result of the ongoing reactions, alkali sulfates are formed, for example, K3Fe(SO4)3 and Na3Fe(SO4)3. This molten slag, in turn, destroys (melts) the protective oxide layer on the metal - magnetite (Fe3O4).
The corrosion rate is maximum at a metal temperature of 680-730 °C, with its increase, the rate decreases due to the thermal decomposition of corrosive substances.
The greatest corrosion is in the outlet pipes of the superheater, where the steam temperature is highest.
Corrosion identification
On screen pipes, flat areas can be observed on both sides of the pipe, which are subjected to corrosion destruction. These areas are located at an angle of 30-45 °C to each other and are covered with a layer of sediments. Between them is a relatively "clean" area, subjected to the "frontal" impact of the gas flow.
The deposits consist of three layers: the outer layer is porous fly ash, the intermediate layer is whitish water-soluble alkali sulfates, and the inner layer is shiny black iron oxides (Fe3O4) and sulfides (FeS).
On the low-temperature parts of the boilers - economizer, air heater, exhaust fan - the temperature of the metal drops below the "dew point" of sulfuric acid.
When burning solid fuel, the gas temperature decreases from 1650 °C in the flare to 120 °C or less in the chimney.
Due to the cooling of gases, sulfuric acid is formed in the vapor phase, and upon contact with a colder metal surface, the vapors condense to form liquid sulfuric acid. The “dew point” of sulfuric acid is 115-170 °C (maybe more - it depends on the content of water vapor and sulfur oxide (SO3) in the gas stream).
The process is described by the reactions:
S + O2 = SO2 (8)
SO3 + H2O = H2SO4 (9)
H2SO4 + Fe = FeSO4 + H2 (10)
In the presence of iron and vanadium oxides, catalytic oxidation of SO3 is possible:
2SO2 + O2 = 2SO3 (11)
In some cases, sulfuric acid corrosion when burning coal is less significant than when burning brown, shale, peat, and even natural gas - due to the relatively greater release of water vapor from them.
Corrosion identification
This type of corrosion causes uniform destruction of the metal. Usually the surface is rough, with a slight rust coating, and is similar to a surface without corrosive phenomena. With prolonged exposure, the metal may be covered with deposits of corrosion products, which must be carefully removed during examination.
Corrosion during service interruptions
This type of corrosion appears on the economizer and in those places of the boiler where the outer surfaces are covered with sulfur compounds. As the boiler cools down, the temperature of the metal drops below the “dew point” and, as described above, if there are sulfur deposits, sulfuric acid is formed. Perhaps an intermediate compound is sulfurous acid (H2SO3), but it is very unstable and immediately turns into sulfuric acid.
Corrosion identification
Metal surfaces are usually coated with coatings. If they are removed, then areas of metal destruction will be found, where there were sulfur deposits and areas of non-corroded metal. Such appearance distinguishes corrosion on a stopped boiler from the above-described corrosion of the economizer metal and other "cold" parts of the operating boiler.
When washing the boiler, corrosion phenomena are distributed more or less evenly over the metal surface due to the erosion of sulfur deposits and insufficient drying of the surfaces. With insufficient washing, corrosion is localized where there were sulfur compounds.
metal erosion
Erosive destruction of metal under certain conditions is subjected to different systems boiler both internally and outer side heated metal, and where turbulent flows occur at high speed.
Only turbine erosion is considered below.
Turbines are subject to erosion from the impact of solid particles and droplets of steam condensate. Solid particles (oxides) exfoliate from the inner surface of superheaters and steam pipelines, especially under conditions of transient thermal processes.
Droplets of steam condensate mainly destroy the surfaces of the blades of the last stage of the turbine and the drainage pipelines. Erosive and corrosive effects of steam condensate are possible if the condensate is "sour" - pH is below five units. Corrosion is also dangerous in the presence of chloride vapor (up to 12% by weight of deposits) and caustic soda in water droplets.
Erosion identification
The destruction of metal from impacts of condensate drops is most noticeable on the leading edges of turbine blades. The edges are covered with thin transverse teeth and grooves (grooves), there may be inclined conical protrusions directed towards the impacts. There are protrusions on the leading edges of the blades and are almost absent on their rear planes.
Damage from solid particles is in the form of gaps, microdents and notches on the leading edges of the blades. Grooves and inclined cones are absent.
Corrosion of steel in steam boilers, proceeding under the action of water vapor, is reduced mainly to the following reaction:
3Fe + 4H20 = Fe2O3 + 4H2
We can assume that the inner surface of the boiler is a thin film of magnetic iron oxide. During the operation of the boiler, the oxide film is continuously destroyed and re-formed, and hydrogen is released. Since the surface film of magnetic iron oxide is the main protection for steel, it should be maintained in a state of least water permeability.
For boilers, fittings, water and steam pipelines, mainly 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 proceed varies from the temperature of the room where the boiler is inactive to the boiling point of saturated solutions during boiler operation, sometimes reaching 700 °. The solution may have a temperature much higher than the critical temperature of pure 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 that explored at lower temperatures in less critical equipment. The difference is that the corrosion rate in boilers is much higher due to the high temperature and pressure. The high rate of heat transfer from the boiler walls to the medium, reaching 15 cal/cm2sec, also enhances corrosion.
PITTING CORROSION
The shape of corrosion pits and their distribution on the metal surface can vary over a wide range. Corrosion pits sometimes form inside pre-existing pits and are often so close together that the surface becomes extremely uneven.Recognition of pitting
Finding out 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 is cooled from high temperature and when the water is drained, sometimes masks the phenomena that occurred during operation. However, experience greatly helps to recognize pitting in boilers. For example, it has been observed that the presence of black magnetic iron oxide in a corrosive cavity or on the surface of a tubercle indicates that an active process was taking place in the boiler. Such observations are often used in the verification of measures taken to protect against corrosion.
Do not mix the iron oxide that forms in areas of active corrosion 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 the ongoing corrosion. Ferrous oxide hydrate entering the boiler from outside sources, such as condensate tanks or pipelines feeding the boiler, may partly explain the presence of both iron oxide and hydrogen in the boiler. Ferrous oxide hydrate, supplied with feed water, interacts in the boiler according to the reaction.
ZFe (OH) 2 \u003d Fe3O4 + 2H2O + H2.
Causes affecting the development of pitting corrosion
Foreign impurities and stresses. Non-metallic inclusions in steel, as well as stresses, are capable of creating anodic areas on a metal surface. Typically, corrosion cavities are different sizes and scattered over the surface in disorder. In the presence of stresses, the location of the shells obeys the direction of the applied stress. Typical examples fin tubes can serve in places where the fins have cracked, as well as places where the boiler tubes are flared.
dissolved oxygen.
It is possible that the most powerful pitting corrosion activator is oxygen dissolved in water. At all temperatures, even in alkaline solution, oxygen serves as an active depolarizer. In addition, oxygen concentration elements can easily form in boilers, especially under scale or contamination, where stagnant areas are created. The usual measure to combat this kind 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 corrosiveness of dissolved carbonic anhydride, but the resulting benefit does not extend to steam-flushed surfaces or condensate piping. Removal of carbonic anhydride along with dissolved oxygen by mechanical deaeration is a common practice.
Recently, attempts have been made to use cyclohexylamine to eliminate corrosion in steam and condensate pipes 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, oil films. Once started, pitting will continue to develop if corrosion products are not removed. This type of localized corrosion is exacerbated by the cathodic (relative to boiler steel) nature of precipitation or 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 most boiler deposits contain copper. It is usually present in the 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 in the corrosion process and does not affect it in any way, while others, on the contrary, believe that copper, being a cathode in relation to steel, can contribute to pitting. None of these points of view is confirmed by direct experiments.
In many cases, little or no corrosion was observed, despite the fact that deposits throughout the boiler contained significant amounts of metallic copper. There is also evidence that when copper comes into contact with mild steel in alkaline boiler water, at elevated temperatures, copper is destroyed faster than steel. Copper rings pressing the 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 metallic copper does not increase the corrosion of boiler steel. The deposited copper can be regarded simply as the end product of the reduction of copper oxide with 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 with respect to steel, promotes pitting.
The surface of the cauldrons rarely presents exposed metallic iron. Most often it has protective layer, consisting mainly of iron oxide. It is possible that where cracks form in this layer, a surface is exposed that is anodic with respect to copper. In such places, the formation of corrosion shells is enhanced. This may also explain the accelerated corrosion in some cases where the shell has formed, as well as the severe pitting sometimes observed after cleaning the boilers with acids.
Improper maintenance of inactive boilers.
One of the most common causes the formation of corrosion pits is the lack of proper care for idle boilers. The inactive boiler must be kept either completely dry or filled with water treated in such a way that corrosion is not possible.
The water remaining on the inner surface of the inactive boiler dissolves oxygen from the air, which leads to the formation of shells, which later become centers around which the corrosion process will develop.
The usual instructions for keeping inactive boilers from rusting are as follows:
1) draining water from the still hot boiler (about 90°); blowing the boiler with air until it is completely drained and kept in a dry state;
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 lock;
3) filling the boiler with an alkaline solution containing salts of chromic acid (0.02-0.03% CrO4").
During chemical cleaning of 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 to renew the iron oxide layer by treatment with a boiling alkaline solution (similar to how it is done for new boilers coming into operation).
Corrosion of economizers
The general provisions regarding boiler corrosion apply equally to economizers. However, the economizer, which heats the feed water and is located in front of the boiler, is especially sensitive to the formation of corrosion pits. It represents the first high temperature surface to be exposed to the damaging effects of oxygen dissolved in the feed water. In addition, the water passing through the economizer generally has a low pH and does not contain chemical retarders.
The fight against corrosion of economizers consists in deaeration of water and the addition of alkali and chemical retarders.
Sometimes the treatment of boiler water is carried out by passing part of it through an economizer. In this case, deposits of sludge in the economizer should be avoided. The effect of such boiler water recirculation on steam quality must also be taken into account.
BOILER WATER TREATMENT
When treating boiler water for corrosion protection, the formation and maintenance of a protective film on metal surfaces is paramount. The combination of substances added to the water depends on the operating conditions, especially on pressure, temperature, thermal stress of the quality of the feed water. However, in all cases, three rules must be observed: boiler water must be alkaline, must not contain dissolved oxygen and pollute the heating surface.Caustic soda provides protection best at pH = 11-12. In practice, with complex boiler water composition, the best results are obtained at pH = 11. For boilers operating at pressures below 17.5 kg/cm2, pH is usually maintained between 11.0 and 11.5. For higher pressures, due to the possibility of metal destruction due to improper circulation and local increase in the concentration of the alkali solution, pH is usually taken equal to 10.5 - 11.0.
To remove residual oxygen, chemical reducing agents are widely used: salts of sulfurous acid, ferrous oxide hydrate and organic reducing agents. Ferrous compounds are very good at removing oxygen but form sludge which has an undesirable effect on heat transfer. Organic reducing agents, due to their instability at high temperatures, are generally not recommended for boilers operating at pressures above 35 kg/cm2. There are data on the decomposition of sulphurous salts at elevated temperatures. However, their use in small concentrations in boilers operating under pressure up to 98 kg/cm2 is widely practiced. Many high pressure plants operate without any chemical deaeration at all.
The cost of special equipment for deaeration, despite its undoubted usefulness, is not always justified for small installations operating at relatively low pressures. At pressures below 14 kg/cm2, partial deaeration in the feed water heaters can bring the dissolved oxygen content to approximately 0.00007%. The addition of chemical reducing agents gives good results, especially when the pH of the water is above 11, and oxygen scavengers are added before the water enters the boiler, which ensures that oxygen is taken up outside the boiler.
CORROSION IN CONCENTRATED BOILER WATER
Low concentrations of caustic soda (of the order of 0.01%) contribute to the preservation of the oxide layer on the steel in a state that reliably provides protection against corrosion. A local increase in concentration causes severe corrosion.Areas of the boiler surface, where the concentration of alkali reaches a dangerous value, are usually characterized by excessive, in relation to the circulating water, heat supply. Alkali-enriched zones near the metal surface can occur in different places boiler. Corrosion pits are arranged in strips or elongated sections, sometimes smooth, and sometimes filled with hard and dense magnetic oxide.
Tubes located horizontally or slightly inclined and exposed to intense radiation from above are corroded inside, along the upper generatrix. Similar cases were observed in boilers high power, and were also reproduced in specially designed experiments.
Pipes in which the water circulation is uneven or broken when the boiler is heavily loaded may be subject to destruction along the lower generatrix. Sometimes corrosion is more pronounced along the variable water level on the side surfaces. Often one can observe abundant accumulations of magnetic iron oxide, sometimes loose, sometimes representing dense masses.
Overheating steel often increases the destruction. This can happen as a result of the formation of a layer of steam at the top of the inclined tube. The formation of a steam jacket is also possible in vertical tubes with an increased heat supply, as indicated by temperature measurements in various places of the tubes during the operation of the boiler. Characteristic data obtained during these measurements are shown in Figs. 7. Limited areas of superheat in vertical tubes having a normal temperature above and below the "hot spot", possibly the result of film boiling of water.
Every time a steam bubble forms on the surface of the boiler tube, the temperature of the metal underneath rises.
An increase in the concentration of alkali in water should occur at the interface: steam bubble - water - heating surface. On fig. it has been shown that even a slight increase in the temperature of the water film in contact with the metal and with the expanding vapor bubble leads to the concentration of caustic soda, already measured in percent and not in parts per million. The film of water enriched with alkali, formed as a result of the appearance of each vapor bubble, affects a small area of the metal and for a very short time. However, the total effect of steam on the heating surface can be likened to the continuous action of a concentrated alkali solution, despite the fact that the total mass of water contains only millionths of caustic soda. Several attempts have been made to find a solution to the problem associated with a local increase in the concentration of caustic soda on heating surfaces. So it was proposed to add neutral salts (for example, metal chlorides) to water in a higher concentration than caustic soda. However, it is best to completely exclude the addition of caustic soda and provide the required pH value by introducing hydrolyzable salts of phosphoric acid. The relationship between the pH of the solution and the concentration of sodium phosphorus salt is shown in fig. Although water containing sodium phosphorus has a high pH value, it can be evaporated without a significant increase in the concentration of hydroxyl ions.
However, it should be remembered that the exclusion of the action of caustic soda only means that one factor accelerating corrosion has been removed. If a steam jacket forms in the tubes, then even though the water does not contain alkali, corrosion is still possible, although to a lesser extent than in the presence of caustic soda. The solution of the problem should also be sought by changing the design, taking into account at the same time the tendency to constant increase energy intensity of heating surfaces, which, in turn, certainly enhances corrosion. If the temperature of a thin layer of water, directly at the heating surface of the tube, exceeds the average temperature of the water in the coarse, even by a small amount, the concentration of caustic soda can increase relatively strongly in such a layer. The curve approximately shows the equilibrium conditions in a solution containing only caustic soda. The exact data depends, to some extent, on the pressure in the boiler.
ALKALINE FRITABILITY OF STEEL
Alkali brittleness can be defined as the appearance of cracks in the area of rivet seams or in other joints where a concentrated alkali solution can accumulate and where there are high mechanical stresses.The most serious damage almost always occurs in the area of rivet seams. Sometimes they cause the boiler to explode; more often it is necessary to make expensive repairs even of relatively new boilers. One American Railway registered cracks in 40 locomotive boilers in a year, requiring about $60,000 worth of repairs. The appearance of brittleness was also found on tubes in the places of flaring, on connections, manifolds and in places of threaded connections.
Stress required for alkali embrittlement to occur
Practice shows a low probability of brittle fracture of conventional boiler steel if the stresses do not exceed the yield strength. voltage, created by pressure steam or a uniformly distributed load from the own weight of the structure, cannot lead to the formation of cracks. However, stresses generated by rolling of the sheet material intended for the manufacture of boilers, deformation during riveting, or any cold working involving permanent deformation, can cause cracking.
The presence of externally applied stresses is not necessary for the formation of cracks. A sample of boiler steel, previously held at a constant bending stress and then released, can crack in an alkaline solution, the concentration of which is equal to the increased concentration of alkali in the boiler water.
Alkali concentration
The normal concentration of alkali in the boiler drum cannot cause cracking, because it does not exceed 0.1% NaOH, and the lowest concentration at which alkali embrittlement is observed is approximately 100 times higher than normal.
Such high concentrations can result from the extremely slow infiltration of water through the 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 solid deposit inside the rivet joint where there are high residual stresses. The combined action of stress and concentrated solution can cause alkali brittle cracks to appear.
Alkaline embrittlement device
A special device for controlling the composition of water reproduces the process of evaporation of water with an increase in the concentration of alkali on a stressed steel sample under the same conditions in which this occurs in the area of the rivet seam. Cracking of the test sample indicates that boiler water of this composition is capable of causing alkaline embrittlement. Therefore, in this case, water treatment is necessary to eliminate it. dangerous properties. However, the cracking of the control sample does not mean that cracks have already appeared or will appear in the boiler. In rivet seams or in other joints, there is not necessarily both a leak (steaming), a stress, and an increase in the concentration of alkali, as in the control sample.
The control device is installed directly on the steam boiler and makes it possible to judge the quality of the boiler water.
The test lasts 30 or more days with constant circulation of water through the control device.
Recognition of alkali embrittlement cracks
Alkali brittle cracks in conventional boiler steel are of a different nature than fatigue cracks or cracks formed due to high stresses. This is illustrated in Fig. I9, which shows the intergranular nature of such cracks forming a fine network. The difference between intergranular alkali brittle 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 the crystallites, as in the case of ordinary boiler steel.
Theory of alkali embrittlement
The atoms in the crystal lattice of the metal, located at the boundaries of the crystallites, experience a less symmetrical effect of their neighbors than the atoms in the rest of the grain mass. Therefore, they leave the crystal lattice more easily. One might think that with careful selection of the aggressive medium, such a selective removal of atoms from the boundaries of crystallites will be possible. Indeed, experiments show that in acidic, neutral (using a weak electric current that creates conditions favorable for corrosion) and concentrated alkali solutions, intergranular cracking can be obtained. If the general corrosion solution is changed by the addition of some substance that forms a protective film on the surface of the crystallites, the corrosion is concentrated at the boundaries between the crystallites.
Aggressive solution in this case is a solution of caustic soda. Silicon sodium salt can protect the surfaces of crystallites without affecting the boundaries between them. The result of a joint protective and aggressive action depends on many circumstances: concentration, temperature, stress state of the metal and composition of the solution.
There is also a colloidal theory of alkali embrittlement and a theory of the effect of hydrogen dissolving in steel.
Ways to combat alkali embrittlement
One of the ways to combat alkaline brittleness is to replace the riveting of the boilers with welding, which eliminates the possibility of leakage. Brittleness can also be eliminated by using steel resistant to intergranular corrosion, or chemical treatment boiler water. In the riveted boilers currently used, the latter method is the only acceptable one.
Preliminary tests using a control sample represent the best way determining the effectiveness of certain protective additives to water. Sodium sulfide salt does not prevent cracking. Nitrogen-sodium salt is successfully used to prevent 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.
At present, sodium nitrogen salt is widely used in stationary boilers. The concentration of sodium nitrogen salt corresponds to 20-30% of the alkali concentration.
CORROSION OF STEAM SUPERHEATERS
Corrosion on the inner surfaces of superheater tubes is primarily due to the interaction between metal and steam at high temperature and, to a lesser extent, to entrainment of boiler water salts by steam. In the latter case, films of solutions with a high concentration of caustic soda can form on the metal walls, directly corroding the steel or giving deposits that sinter on the tube wall, which can lead to the formation of bulges. In idle boilers and in cases of steam condensation in relatively cold superheaters, pitting can develop under the influence of oxygen and carbonic anhydride.Hydrogen as a measure of corrosion rate
Steam temperature in modern boilers approaching the temperatures used in industrial production hydrogen by a direct reaction between steam and iron.
The rate of corrosion of pipes made of carbon and alloy steels 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 gas and air removal units have been used in US power plants. They provide complete removal of gases, and the degassed condensate is suitable for the determination of salts in it carried away by steam from the boiler. An approximate value of the general corrosion of the superheater during the operation of the boiler 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
Saturated steam entering the superheater carries with it small but measurable quantities of gases and salts from the boiler water. The most common 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 these gases. So far, it has not been proven that salts dissolved in water, in dry form or deposited on superheater elements, can contribute to corrosion. However, caustic soda, being the main component of salts entrained in boiler water, can contribute to the corrosion of a highly heated tube, especially if the alkali sticks to the metal wall.
Increasing the purity of saturated steam is achieved by preliminary careful removal of gases from the feed water. Reducing the amount of salt entrained in steam is achieved by thorough cleaning in the upper collector, using mechanical separators, saturated steam flushing with feed water or suitable chemical water treatment.
Determination of the concentration and nature of gases entrained in saturated steam is carried out using the above devices and chemical analysis. It is convenient to determine the concentration of salts in saturated steam by measuring the electrical conductivity of water or by evaporating a large amount 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 degassers mentioned above can also be used to measure electrical conductivity.
When the boiler is idle, the superheater is a refrigerator in which condensate accumulates; in this case, normal underwater pitting is possible if the steam contained oxygen or carbon dioxide.
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The most active corrosion of screen pipes is manifested in places where coolant impurities are concentrated. This includes sections of wall tubes with high thermal loads, where deep evaporation of boiler water occurs (especially if there are porous low-heat-conducting deposits on the evaporation surface). Therefore, in relation to the prevention of damage to the screen pipes associated with internal metal corrosion, it is necessary to take into account the need for an integrated approach, i.e. impact on both water-chemical and furnace regimes.
Wall tube damage is mainly of a mixed nature, they can be conditionally divided into two groups:
1) Damage with signs of steel overheating (deformation and thinning of the pipe walls at the point of destruction; the presence of graphite grains, etc.).
2) Brittle fractures without characteristic signs of overheating of the metal.
Significant deposits of a two-layer character were noted on the inner surface of many pipes: the upper one is weakly bonded, the lower one is scaling, tightly bonded to the metal. The thickness of the lower scale layer is 0.4-0.75 mm. In the damage zone, the scale on the inner surface is destroyed. Near the sites of destruction and at some distance from them, the inner surface of the pipes is affected by corrosion pits and brittle microdamages.
The general appearance of damage indicates the thermal nature of the destruction. Structural changes on the front side of the tubes - deep spheridization and perlite decomposition, graphite formation (transition of carbon to graphite 45-85%) - indicate that not only the operating temperature of the screens was exceeded, but also the allowable temperature for steel of 20,500 °C. The presence of FeO also confirms the high level of metal temperatures during operation (above 845 oK - i.e. 572 oC).
Brittle damage caused by hydrogen typically occurs in areas with high heat flows, under thick layers of deposits, and inclined or horizontal pipes, as well as in heat transfer areas near weld backing rings or other devices that prevent the free movement of flows. .Experience has shown that hydrogen damage occurs in boilers operating at pressures below 1000 psi. inch (6.9 MPa).
Hydrogen damage usually results in ruptures with thick edges. Other mechanisms that contribute to the formation of cracks with thick edges are stress corrosion cracking, corrosion fatigue, stress fractures, and (in some rare cases) severe overheating. It may be difficult to visually distinguish damage caused by hydrogen damage from other types of damage, but some of their features can help here.
For example, hydrogen damage is almost always associated with the formation of holes in the metal (see the precautions given in Chapters 4 and 6). Other types of damage (with the possible exception of corrosion fatigue, which often begins in individual shells) are usually not associated with severe corrosion.
Pipe failures as a result of hydrogen damage to metal often manifest themselves as the formation of a rectangular "window" in the pipe wall, which is not typical for other types of destruction.
To assess the damageability of screen pipes, it should be taken into account that the metallurgical (initial) content of gaseous hydrogen in pearlitic steel (including st. 20) does not exceed 0.5–1 cm3/100 g. When the hydrogen content is higher than 4--5 cm3/100g, the mechanical properties of the steel deteriorate significantly. In this case, it is necessary to focus mainly on the local content of residual hydrogen, since in the case of brittle fractures of screen pipes, a sharp deterioration in the properties of the metal is observed only in a narrow zone along the pipe cross section with invariably satisfactory structure and mechanical properties of the adjacent metal at a distance of only 0.2-2 mm.
The obtained values of the average concentrations of hydrogen at the edge of destruction are 5-10 times higher than its initial content for station 20, which could not but have a significant effect on the damage of pipes.
The presented results indicate that hydrogen embrittlement turned out to be the decisive factor in the damage of the wall tubes of the KrCHPP boilers.
An additional study was required, which of the factors has a decisive influence on this process: a) thermal cycling due to destabilization of the normal boiling regime in areas of increased heat flows in the presence of deposits on the evaporation surface, and, as a result, damage to the protective oxide films covering it; b) the presence in the working medium of corrosive impurities, concentrating in deposits near the evaporation surface; c) the combined action of factors "a" and "b".
The question of the role of the furnace regime is of particular interest. The nature of the curves indicates the accumulation of hydrogen in a number of cases near the outer surface of the screen tubes. This is possible, first of all, if there is a dense layer of sulfides on the indicated surface, which are largely impermeable to hydrogen diffusing from the inner surface to the outer. The formation of sulfides is due to: high sulfur content of the burned fuel; throwing a torch on the screen panels. Another reason for metal hydrogenation at the outer surface is the occurrence of corrosion processes when the metal comes into contact with flue gases. As the analysis of the external deposits of the boiler pipes showed, usually both of these causes took place.
The role of the combustion mode is also manifested in the corrosion of screen pipes under the action of pure water, which is most often observed on high-pressure steam generators. The centers of corrosion are usually located in the zone of maximum local thermal loads and only on the heated surface of the pipe. This phenomenon leads to the formation of round or elliptical depressions with a diameter greater than 1 cm.
Overheating of the metal occurs most often in the presence of deposits due to the fact that the amount of perceived heat will be almost the same for both a clean pipe and a pipe containing scale, the temperature of the pipe will be different.
The owners of the patent RU 2503747:
FIELD OF TECHNOLOGY
SUBSTANCE: invention relates to thermal power engineering and can be used to protect heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems of residential buildings and industrial facilities from scale during current operation.
BACKGROUND OF THE INVENTION
The operation of steam boilers is associated with the simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the heating surfaces of the boiler are separate phases of a complex system that is formed when they come into contact. The result of the interaction of these phases are surface processes that occur at the interface between them. As a result, corrosion and scale formation occur in the metal of the heating surfaces, which leads to a change in the structure and mechanical properties of the metal, and which contributes to the development of various damages. Since the thermal conductivity of the scale is fifty times lower than that of the iron of the heating pipes, there are losses of thermal energy during heat transfer - with a scale thickness of 1 mm from 7 to 12%, and with 3 mm - 25%. Severe scale formation in the steam boiler system continuous action often leads to a stoppage of production for several days a year for descaling.
The quality of the feed and, therefore, boiler water is determined by the presence of impurities that can cause various types of corrosion of the metal of the internal heating surfaces, the formation of primary scale on them, as well as sludge, as a source of secondary scale formation. In addition, the quality of boiler water also depends on the properties of substances formed as a result of surface phenomena during the transportation of water, and condensate through pipelines, in water treatment processes. Removal of impurities from feed water is one of the ways to prevent the formation of scale and corrosion and is carried out by methods of preliminary (pre-boiler) water treatment, which are aimed at maximizing the removal of impurities present in the source water. However, the methods used do not allow completely eliminating the content of impurities in water, which is associated not only with technical difficulties, but also economic feasibility application of pre-boiler water treatment methods. In addition, since water treatment is a complex technical system, it is redundant for small and medium capacity boilers.
Known methods for removing deposits that have already formed use mainly mechanical and chemical methods cleaning. The disadvantage of these methods is that they cannot be carried out during the operation of the boilers. In addition, chemical cleaning methods often require the use of expensive chemicals.
There are also known ways to prevent the formation of scale and corrosion, carried out during the operation of the boilers.
US Pat. No. 1,877,389 proposes a method for removing scale and preventing its formation in hot water and steam boilers. In this method, the surface of the boiler is the cathode, and the anode is placed inside the pipeline. The method consists in passing direct or alternating current through the system. The authors note that the mechanism of the method is that under the action of an electric current, gas bubbles form on the surface of the boiler, which lead to the exfoliation of the existing scale and prevent the formation of a new one. The disadvantage of this method is the need to constantly maintain the flow of electric current in the system.
US Pat. No. 5,667,677 proposes a method for treating a liquid, in particular water, in a pipeline in order to slow down scale formation. This method is based on creating an electromagnetic field in pipes, which repels calcium and magnesium ions dissolved in water from the walls of pipes and equipment, preventing them from crystallizing in the form of scale, which makes it possible to operate boilers, boilers, heat exchangers, and cooling systems on hard water. The disadvantage of this method is the high cost and complexity of the equipment used.
WO 2004016833 proposes a method for reducing scale formation on a metal surface exposed to a supersaturated alkaline aqueous solution that is capable of scale formation after a period of exposure, comprising applying a cathodic potential to said surface.
This method can be used in various technological processes in which the metal is in contact with an aqueous solution, in particular, in heat exchangers. The disadvantage of this method is that it does not protect the metal surface from corrosion after removing the cathode potential.
Thus, there is currently a need to develop an improved method for preventing the formation of scale in heating pipes, hot water and steam boilers, which is economical and highly effective and provides anti-corrosion protection of the surface for a long period of time after exposure.
In the present invention, this problem is solved using a method according to which a current-carrying electrical potential is created on the metal surface, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.
BRIEF DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide an improved method for preventing scaling of heating pipes in hot water and steam boilers.
Another object of the present invention is to provide the possibility of eliminating or significantly reducing the need for descaling during operation of hot water and steam boilers.
Another objective of the present invention is to eliminate the need for the use of consumable reagents to prevent the formation of scale and corrosion of the heating pipes of hot water and steam boilers.
Yet another object of the present invention is to enable work to be started to prevent scaling and corrosion of hot water and steam boiler heating pipes on contaminated boiler pipes.
The present invention relates to a method for preventing scale formation and corrosion on a metal surface made of an iron-containing alloy in contact with a water-steam environment from which scale is capable of forming. This method consists in applying a current-carrying current to the specified metal surface. electrical potential, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.
According to some particular embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some particular embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the inner surface of the heating pipes of a hot water or steam boiler.
Revealed in this description the method has the following advantages. One advantage of the method is reduced scale formation. Another advantage of the present invention is the possibility of using once purchased a working electrophysical apparatus without the need for consumable synthetic reagents. Another advantage is the possibility of starting work on contaminated boiler tubes.
The technical result of the present invention, therefore, is to increase the efficiency of hot water and steam boilers, increase productivity, increase heat transfer efficiency, reduce fuel consumption for heating the boiler, save energy, etc.
Other technical results and advantages of the present invention include the possibility of layer-by-layer destruction and removal of already formed scale, as well as preventing its new formation.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the distribution of deposits on the internal surfaces of the boiler as a result of applying the method according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The method according to the present invention consists in applying to a metal surface subject to scale formation a conductive electrical potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and scale-forming ions to the metal surface.
The term "current-carrying electrical potential" in the sense in which it is used in this application means an alternating potential that neutralizes a double electrical layer at the interface between the metal and the steam-water medium containing salts that lead to the formation of scale.
As is known to a person skilled in the art, electric charge carriers in a metal, which are slow compared to the main charge carriers - electrons, are dislocations of its crystal structure, which carry an electric charge and form dislocation currents. Coming to the surface of the heating pipes of the boiler, these currents are part of the double electric layer during the formation of scale. The current-carrying, electric, pulsating (that is, alternating) potential initiates the removal of the electric charge of dislocations from the metal surface to the ground. In this regard, it is a current-carrying dislocation current. As a result of the action of this current-carrying electrical potential, the electrical double layer is destroyed, and the scale gradually disintegrates and passes into the boiler water in the form of sludge, which is removed from the boiler during periodic blowdowns.
Thus, the term "current-removing potential" is understandable to a specialist in this field of technology and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).
The device described in RU 2100492 C1, which includes a converter with a frequency converter and a pulsating potential controller, as well as a pulse shape controller, can be used as a device for creating a current-carrying electrical potential, for example. Detailed description this device is given in RU 2100492 C1. Any other similar device can also be used, as will be understood by a person skilled in the art.
The conductive electrical potential according to the present invention can be applied to any part of the metal surface remote from the base of the boiler. The place of application is determined by the convenience and/or efficiency of the application of the claimed method. One skilled in the art, using the information disclosed herein and using standard test procedures, will be able to determine the optimal location for applying the current-dissipating electrical potential.
In some embodiments of the present invention, the conductive electrical potential is variable.
The conductive electrical potential according to the present invention may be applied for various periods of time. The potential application time is determined by the nature and degree of contamination of the metal surface, the composition of the water used, temperature regime and features of the operation of the heat engineering device and other factors known to specialists in this field of technology. A person skilled in the art, using the information disclosed in the present description and using standard test methods, will be able to determine the optimal time to apply a current-conducting electrical potential, based on the goals, conditions and condition of the thermal device.
The value of the current-carrying potential required to neutralize the electrostatic component of the adhesion force can be determined by a specialist in the field of colloid chemistry on the basis of information known from the prior art, for example, from the book Deryagin B.V., Churaev N.V., Muller V.M. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the value of the current-carrying electrical potential is in the range from 10 V to 200 V, more preferably from 60 V to 150 V, even more preferably from 61 V to 150 V. The values of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the electrical double layer, which is the basis of the electrostatic component of the adhesion forces in the scale and, as a result, to the destruction of the scale. Current-removing potential values below 61 V are insufficient for scale destruction, and at current-removing potential values above 150 V, undesirable electroerosive destruction of the metal of the heating tubes is likely to begin.
The metal surface, to which the method according to the present invention can be applied, can be part of the following heat engineering devices: heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during current operation. This list is illustrative and does not limit the list of devices to which the method of the present invention may be applied.
In some embodiments, the iron-containing alloy from which the metal surface to which the method of the present invention can be applied may be steel or other iron-containing material such as cast iron, kovar, fechral, transformer steel, alsifer, magnico, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron alloys to which the method of the present invention may be applied. A person skilled in the art, on the basis of knowledge known from the prior art, will be able to such iron-containing alloys that can be used according to the present invention.
The aqueous environment from which scale is capable of forming, according to some embodiments of the present invention, is tap water. The aqueous medium may also be water containing dissolved metal compounds. The dissolved metal compounds may be iron and/or alkaline earth metal compounds. The aqueous medium may also be an aqueous suspension of colloidal particles of iron and/or alkaline earth metal compounds.
The method according to the present invention removes previously formed deposits and serves as a reagent-free means of cleaning the internal surfaces during the operation of a heat engineering device, further ensuring its scale-free operation. At the same time, the size of the zone within which the prevention of scale formation and corrosion is achieved significantly exceeds the size of the effective scale destruction zone.
The method according to the present invention has the following advantages:
Does not require the use of reagents, i.e. environmentally friendly;
Easy to implement, does not require special devices;
Allows you to increase the heat transfer coefficient and increase the efficiency of boilers, which significantly affects economic indicators his works;
It can be used as an addition to the applied methods of pre-boiler water treatment, or separately;
Allows you to abandon the processes of softening and deaeration of water, which greatly simplifies technological scheme boiler rooms and makes it possible to significantly reduce costs during construction and operation.
Possible objects of the method can be hot water boilers, waste heat boilers, closed heat supply systems, thermal desalination plants. sea water, steam generators, etc.
The absence of corrosion damage, scale formation on the internal surfaces opens up the possibility for the development of fundamentally new design and layout solutions for steam boilers of small and medium power. This will allow, due to the intensification of thermal processes, to achieve a significant reduction in the mass and dimensions of steam boilers. To ensure the specified temperature level of heating surfaces and, consequently, to reduce fuel consumption, the volume of flue gases and reduce their emissions into the atmosphere.
IMPLEMENTATION EXAMPLE
The method claimed in the present invention was tested at the boiler plants "Admiralty Shipyards" and "Red Chemist". It has been shown that the method according to the present invention effectively cleans the internal surfaces of boilers from deposits. In the course of these works, an equivalent fuel saving of 3-10% was obtained, while the spread of savings values is associated with varying degrees of contamination of the internal surfaces of the boilers. The aim of the work was to evaluate the effectiveness of the proposed method to ensure a reagent-free, scale-free operation of medium-sized steam boilers under conditions of high-quality water treatment, compliance with water chemistry and high professional level operation of the equipment.
The test of the method claimed in the present invention was carried out on the steam boiler unit No. 3 DKVr 20/13 of the 4th Krasnoselskaya boiler house of the South-Western branch of the State Unitary Enterprise "TEK SPb". The operation of the boiler unit was carried out in strict accordance with the requirements of regulatory documents. The boiler is equipped with all the necessary means of monitoring the parameters of its operation (pressure and flow rate of generated steam, temperature and flow rate of feed water, pressure of blast air and fuel on burners, vacuum in the main sections of the gas path of the boiler unit). The steam capacity of the boiler was maintained at 18 t/h, the steam pressure in the boiler drum was 8.1...8.3 kg/cm 2 . The economizer worked in the heating mode. The source water was city water supply, which met the requirements of GOST 2874-82 "Drinking water". It should be noted that the amount of iron compounds at the input to the specified boiler room, as a rule, exceeds regulatory requirements(0.3 mg / l) and is 0.3-0.5 mg / l, which leads to intensive overgrowing of the internal surfaces with glandular compounds.
Evaluation of the effectiveness of the method was carried out according to the state of the internal surfaces of the boiler.
Evaluation of the influence of the method according to the present invention on the state of the internal heating surfaces of the boiler unit.
Prior to the start of the tests, an internal inspection of the boiler unit was carried out and the initial state of the internal surfaces was recorded. The preliminary inspection of the boiler was carried out at the beginning heating season, one month after its chemical cleaning. As a result of the inspection, it was revealed: on the surface of the drums there are solid dark brown deposits with paramagnetic properties and, presumably, consisting of iron oxides. The thickness of deposits was up to 0.4 mm visually. In the visible part of the boiler pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of the pipe length with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).
The device for creating a current-removing potential, described in EN 2100492 C1, was attached at point (1) to the hatch (2) of the upper drum from the back of the boiler (see Fig.1). The current-carrying electrical potential was equal to 100 V. The current-carrying electrical potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the hatches of the drums (connection points of the device to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from hatches deposits (6) are preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig.1). Further, as you move towards the front, (at a distance of 3.0-3.5 m from the hatches), continuous deposits (7) up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not manifested. The current-carrying electrical potential was equal to 100 V. The current-carrying electrical potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface of the upper and lower drums within 2-2.5 meters from the hatches of the drums (the connection point of the device for creating a current-discharging potential). At a distance of 2.5-3.0 m from the hatches, the deposits were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig.1). Further, as you move towards the front (at a distance of 3.0-3.5 m from the hatches), continuous deposits up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not manifested.
In the visible part of the boiler pipes, within 3.5-4.0 m from the hatches of the drums, there was an almost complete absence of deposits. Further, as we move towards the front, non-continuous solid deposits were found (up to five spots per 100 linear mm with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).
As a result of this stage of testing, it was concluded that the method according to the present invention, without the use of any reagents, effectively destroys previously formed deposits and provides a scale-free operation of the boiler.
At the next stage of testing, a device for creating a current-carrying potential was connected at point "B" and the tests continued for another 30-45 days.
The next opening of the boiler unit was made after 3.5 months of continuous operation of the device.
Inspection of the boiler unit showed that the previously remaining deposits were completely destroyed and only a small amount remained on the lower sections of the boiler pipes.
This led to the following conclusions:
The size of the zone within which the scale-free operation of the boiler unit is ensured significantly exceeds the size of the zone of effective destruction of deposits, which allows subsequent transfer of the connection point of the current-removing potential to clean the entire internal surface of the boiler unit and further maintain its scale-free mode of operation;
The destruction of previously formed deposits and the prevention of the formation of new ones is provided by processes of various nature.
Based on the results of the inspection, it was decided to continue testing until the end of the heating period in order to finally clean the drums and boiler pipes and determine the reliability of ensuring the boiler's scale-free operation. The next opening of the boiler unit was carried out after 210 days.
The results of the internal inspection of the boiler showed that the process of cleaning the internal surfaces of the boiler within the upper and lower drums and boiler pipes was almost completed. complete removal deposits. On the entire surface of the metal, a thin dense coating was formed, which had a black color with a blue tint, the thickness of which even in a wet state (almost immediately after opening the boiler) did not exceed 0.1 mm visually.
At the same time, the reliability of ensuring the scale-free operation of the boiler unit was confirmed when using the method of the present invention.
The protective effect of the magnetite film persisted for up to 2 months after the device was disconnected, which is quite enough to ensure the dry conservation of the boiler unit when transferring it to reserve or for repair.
Although the present invention has been described in relation to various specific examples and embodiments of the invention, it should be understood that this invention is not limited to them and that it can be practiced within the scope of the following claims.
1. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water medium from which scale can form, including applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and scale-forming ions.
The invention relates to thermal power engineering and can be used to protect against scale and corrosion of heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during operation. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water medium from which scale is capable of forming includes applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the adhesion force between specified metal surface and colloidal particles and scale-forming ions. EFFECT: increased efficiency and productivity of hot water and steam boilers, increased heat transfer efficiency, layer-by-layer destruction and removal of the formed scale, as well as prevention of its new formation. 2 w.p. f-ly, 1 pr., 1 ill.
