Sea Site Russia Not October 05, 2016 Created: October 05, 2016 Updated: October 05, 2016 Views: 5363

Types of corrosion. In the process of operation, elements of the steam boiler are exposed to aggressive medium - water, steam and flue gases. Corrosive chemical and electrochemical.

Chemical corrosioncaused by steam or water destroys the metal evenly over the entire surface. The speed of such corrosion in modern ship boilers is low. Local chemical corrosion caused by aggressive chemical compounds contained in the sediments of ash (sulfur, vanadium oxides, etc.).

The most common and dangerous is electrochemical corrosionflowing into an aqueous solutions of electrolytes in the occurrence of electric current caused by the difference in potentials between individual parts of the metal, characterized by chemical heterogeneity, temperature, or processing quality.
The role of the electrolyte is carried out by water (with internal corrosion) or condensible pairs of water in sediments (with external corrosion).

The occurrence of such microgalvanic pairs on the surface of the pipes leads to the fact that the metal ion-atoms are moving into water as positively charged, and the surface of the pipe in this place acquires a negative charge. If the difference in the potentials of such microgalvanic pairs is slightly, then on the border, metal-water is gradually created by a double electric layer, which slows down the further course of the process.

However, in most cases, the potentials of individual sections are different, which causes the occurrence of EMF, directed from greater potential (anode) to a smaller (cathode).

At the same time, ion-atoms of metal are transmitted from the anode to the water, and excess electrons are accumulated on the cathode. As a result, the EDC and, therefore, the intensity of the process of the metal destruction is sharply reduced.

This phenomenon is called polarization. If the anode potential decreases as a result of the formation of a protective oxide film or a growth of metal ion concentration in the anode area, and the potential of the cathode is practically not changed, the polarization is called anodic.

In cathode polarization in the solution at the cathode, the concentration of ions and molecules capable of removing excess electrons from the metal surface drops sharply. It follows from this that the main point of the control of electrochemical corrosion is to create such conditions when both types of polarization will be supported.
It is impossible to almost achieve this, since there are always depolarizers in boiler water - substances that cause violation of polarization processes.

Depolarizers include molecules about 2 and CO 2, ions H +, CL - and SO - 4, as well as iron and copper oxides. Dissolved in water CO 2, CL - and SO - 4 inhibit the formation on the anode dense protective oxide film and thereby contribute to the intensive flow of the anode processes. Hydrogen ions H + reduce the negative cathode charge.

The effect of oxygen on the corrosion rate began to appear in two opposite directions. On the one hand, oxygen increases the rate of the corrosion process, as it is a strong depolarizer of cathode areas, and the passivating effect on the surface has a passivating effect.
Typically, the parts of the boiler made of steel have a sufficiently strong initial oxide film that protects the material from the effects of oxygen until it is destroyed under the action of chemical or mechanical factors.

The speed of heterogeneous reactions (to which the corrosion refers) is regulated by the intensity of the following processes: a supply to the surface of the reagent material (primarily depolarizers); the destruction of the protective oxide film; removal of the reaction products from its place of leakage.

The intensity of these processes is largely determined by hydrodynamic, mechanical and thermal factors. Therefore, measures to reduce the concentration of aggressive chemical reagents at high intensity of two other processes, as the experience of operating boilers shows, usually ineffective.

It follows that the solution to the problem of preventing corrosion damage must be integrated when all factors affect the initial causes of the destruction of materials are taken into account.

Electrochemical corrosion

Depending on the location of the substances learning in reactions, the following types of electrochemical corrosion distinguish:

• oxygen (and its kind - parking),
• subwind (sometimes called "shell"),
• intercrystalline (alkaline fragility of boiler steels),
• skalvaya I.
• sulfur.

Oxygen corrosionit is observed in economizers, reinforcement, nutrient and lower pipes, steaming collectors and intraverter devices (shields, pipes, steam detergents, etc.). Especially strongly susceptible to oxygen corrosion of coils of the second contour of dual circuit boilers, utilization boilers and steam air heaters. Oxygen corrosion proceeds during the action of boilers and depends on the concentration of oxygen dissolved in boiler water.

The speed of oxygen corrosion in the main boilers is low, which is due to the effective work of deaerators and phosphate-nitrate water regime. In auxiliary water-tube boilers, it often reaches 0.5 - 1 mm / year, although on average lies in the range of 0.05 - 0.2 mm / year. The nature of damage to boiler steels is small sizes.

A more dangerous variety of oxygen corrosion is parking corrosionflowing during the inactivity of the boiler. Due to the specifics of the work, all ship boilers (and auxiliary especially) are susceptible to intensive parking corrosion. As a rule, parking corrosion does not lead to a boiler failure, but the metal that has undergone corrosion during stops, other things being equal intensively destroys the boiler.

The main cause of the occurrence of parking corrosion is the falling of oxygen into water, if the boiler is filled, or in the moisture film on the metal surface, if the boiler is died. In this case, chlorides and NaOH contained in water, and water-soluble sediments of salts play a major role.

In the presence of chlorides in the water, uniform corrosion of the metal is intensified, and if it contains a slight amount of alkalis (less than 100 mg / l), then corrosion is localized. To avoid parking corrosion at a temperature of 20-5 ° C in water should be contained up to 200 mg / l NaOH.

External signs of corrosion with oxygen participation: Local ulcers of small size (Fig. 1, a) filled with corrosion products of brown color, which form tubercles over ulcers.

Removal of oxygen from nutrient water is one of the important measures to reduce oxygen corrosion. Since 1986, oxygen content in nutrient water for ship auxiliary and utilization boilers is limited to 0.1 mg / l.

However, with such oxygen-containing nutritional water in operation, corrosion damage of the boiler elements are observed, which indicates the prevailing effect of the processes of destruction of the oxide film and washing the reaction products from corrosion foci. The most visual example illustrating the influence of these processes on corrosion damage is the destruction of coils of utilization boilers with forced circulation.

Fig. 1. Damage to oxygen corrosion

Corrosion damagewith oxygen corrosion, usually strictly localized: on the inner surface of the input sections (see Fig. 1, a), in the region of flexures (Fig. 1, b), at the weekend and in the knee of the coil (see Fig. 1, B), as well as in stealing collectors of utilization boilers (see Fig. 1, d). It is in these areas (2 - the area of \u200b\u200bthe wondl cavitation) the hydrodynamic features of the flow create conditions for the destruction of the oxide film and intensive flushing of corrosion products.
Indeed, any deformations of the flow of water and steaming mixture are accompanied by occurrence. cavitation in the wall layersexpanding flow 2, wheremed and immediately collapsed steam bubbles determine the destruction of the oxide film due to the energy of hydraulic microdes.
This also contributes to alternate stresses in the film caused by vibration of coils and fluctuations in temperature and pressures. Increased local turbulization of the flow in these areas causes an active washout of corrosion products.

On direct weekend sections of coils, the oxide film is destroyed due to the blows of the water droplets surface during turbulent pulsations of the steaming mixture, the dispersed-ring mode of which goes here to the dispersion at the flow rate to 20-25 m / s.
Under these conditions, even low oxygen-containing (~ 0.1 mg / l) determines the intensive destruction of the metal, which leads to the appearance of fistulas at the input sections of coils of utilization boilers of La MONT in 2-4 years of operation, and in the rest of the sections - after 6-12 years.

Fig. 2. Corrosion damage to co-crewing coils of recycling boilers Coup1500R ship "Indira Gandhi".

As an illustration, we will consider the reasons for damage to the process meters of economizers of two recycling boilers of the TP1500R type, installed on the Entra Gandhi Lighter Code (type "Alexey Kosygin"), which has been commissioned in October 1985. Almost in February 1987 due to damage Replaced economizers of both boilers. After 3 years and in these economizers there are damage to coils located in areas up to 1-1.5 m from the input manifold. The nature of the damage indicates (Fig. 2, A, b) on typical oxygen corrosion, followed by fatigue destruction (transverse cracks).

However, the nature of fatigue in some sections is different. The appearance of the crack (and earlier - cracking of the oxide film) in the weld area (see Fig. 2, a) is a consequence of the alternate stresses due to the vibration of the beam of pipes and the constructive feature of the coil coating node with a collector (to the curved decorator with a diameter with a diameter of 22x3 22x2).
The destruction of the oxide film and the formation of fatigue cracks on the inner surface of direct sections of coils remote from the entrance by 700-1000 mm (see Fig. 2, b) are due to the alternate thermal stresses arising during the input period of the boiler when it is on a hot surface Cold water serves. In this case, the effect of thermal stresses is enhanced by the fact that the fins of coils make it difficult to the free expansion of the pipe metal, creating additional stresses in the metal.

Podllam corrosionit is usually observed in the main water-tube boilers on the inner surfaces of the on-screen and vapor-forming pipes of triberate beams facing a torch. The character of submissive corrosion is an oval shape with a large axis size (parallel axis of the pipe) to 30-100 mm.
On ulcers there is a dense layer of oxides in the form of "seashells" 3 (Fig. 3). The submissible corrosion flows in the presence of solid depolarizers - iron and copper oxides 2, which are deposited on the most heat-stressed areas of pipes in places of active corrosion centers arising from the destruction of oxide films. .
From above, a loose layer of scale and corrosion products is formed from above 1. The formed "shells" from the corrosion products are firmly linked with the main metal and can be removed on the mechanical way. Under the "shells" deteriorates heat exchange, which leads to the overheating of the metal and the appearance of the metal.
For auxiliary boilers, this type of corrosion is not characteristic, but at high heat loads and the corresponding water treatment modes, the appearance of submissive corrosion is not excluded and in these boilers.

Corrosion of steel in steam boilers occurring under the action of water vapor is reduced, mainly to the next reaction:

ZFE + 4N20 \u003d Fe2O3 + 4H2

It can be assumed that the inner surface of the boiler is a thin film of the magnetic oxide of iron. During the operation of the boiler, the oxide film is continuously destroyed and is formed again, and hydrogen is distinguished. Since the surface film of the magnetic oxide of iron is the main protection for steel, it should be maintained in the state of the smallest permeability for water.
For boilers, reinforcements, water and steam lines, predominantly simple carbon or low-alloy steel are used. The corrosion medium in all cases are water or water vapor varying degrees of purity.
The temperature at which the corrosion process may flow, ranges from the temperature of the room where the inactive boiler is located, to the boiling point of saturated solutions when the boiler occurs, which is sometimes 700 °. The solution may have a temperature significantly higher than the critical temperature of clean water (374 °). However, high concentrations of salts in boilers are rare.
The mechanism by which physical and chemical causes can lead to the destruction of the film in steam boilers, essentially differs from the mechanism studied at lower temperatures on less responsible equipment. The difference lies in the fact that the corrosion rate in boilers is much larger due to high temperature and pressure. The high speed of heat transfer from the walls of the boiler to the medium reaches 15 feces / cm2sec, also enhances corrosion.

Pottle corrosion

The form of corrosion sinks and their distribution on the metal surface may vary widely. Corrosion sinks are sometimes formed inside the already existing shells and are often arranged so close to each other that the surface becomes extremely uneven.

Recognition of point corrosion

Finding out the causes of the formation of corrosion destruction of a certain type is often very difficult, since several reasons can act at the same time; In addition, a number of changes occurring during the cooling of the boiler from high temperature and during water descent, sometimes masks the phenomena that occurred during operation. However, experience significantly helps to recognize point corrosion in boilers. For example, it was observed that the presence in the corrosion sink or on the surface of the black magnetic oxide tuberculosis indicates that the active process was proceeded in the boiler. Such observations are often used when checking events adopted to protect against corrosion.
It should not be mixed by the oxide of iron, which is formed in places of active corrosion, with black magnetic oxide of iron, sometimes present in the form of suspension in boiler water. It must be remembered that neither the total amount of fine magnetic oxide of iron nor the amount of hydrogen released in the boiler can not serve as a reliable sign of the extent and the size of the corrosion of origin. Gyrat Zaksi Iron, entering the boiler from extraneous sources, for example, from tanks for condensate or from the pipeline boiler, can partially explain the presence in the boiler as iron and hydrogen oxide. Gyrat of iron zaisi coming with nutrient water interacts in the reaction boiler.

ZFE (OH) 2 \u003d FE3O4 + 2N2O + H2.

Causes affecting the development of point corrosion

Foreign impurities and stresses. Non-metallic inclusions in steel, as well as voltages, are able to create anodic areas on a metal surface. Typically, corrosion sinks are of different sizes and scattered over the surface in disorder. In the presence of stresses, the location of the shell obeys the direction of the applied voltage. Typical examples can serve the fin tubes in places where the fins gave cracks, as well as the places of rolling of boiler tubes.
Dissolved oxygen.
It is possible that the strongest activator of point corrosion is dissolved in water oxygen. At all temperatures, even in an alkaline solution, oxygen serves as an active depolarizer. In addition, oxygen concentration elements can easily occur in boilers, especially under the scale or contaminants, where stagnation segments are created. The usual measure of the fight against this kind of corrosion is deaeration.
Dissolved coal anhydride.
Since the solutions of the coal anhydride have a weakly acidic reaction, it accelerates corrosion in boilers. Alkaline boiler water reduces the aggressiveness of the dissolved coal anhydride however, the benefit resulting from this does not apply to the surface is washed by steam, or on pipelines for condensate. Removal of coal anhydride together with dissolved oxygen by mechanical deaeration is a common event.
Attempts have recently been made to apply cyclohexilamine in order to eliminate corrosion in steam pipelines and pipelines for condensate heating systems.
Deposits on the walls of the boiler.
Very often, corrosive shells can be found along the outer surface (or below the surface) of such deposits such as rolling scale, boiler sludge, boiler room, corrosion products, oil films. When starting, the point corrosion will develop further, if not removing corrosion products. This type of local corrosion is enhanced by cathode (with respect to boiler steel) the character of precipitation or the depletion of oxygen under deposits.
Copper in boiler water.
If we take into account the large amounts of copper alloys used for the auxiliary equipment (capacitors, pumps, etc.), then there is nothing surprising in most cases in boiler sediments a copper contains copper. It is usually present in the metallic state, sometimes in the form of oxide. The amount of copper in sediments varies from the percentage of percent to almost pure copper.
The question of the value of copper sediments in the corrosion boiler house cannot be considered solved. Some argue that copper is only present in the corrosive process and does not affect it, others, on the contrary, believe that copper, being a cathode in relation to steel, can contribute to point corrosion. None of these points of view is confirmed by direct experiments.
In many cases, insignificant corrosion was observed (or even its complete absence), despite the fact that deposits throughout the boiler contained significant amounts of metallic copper. There is also information that when copper contact with low-carbon steel in alkaline boiler water, at elevated temperatures, copper is destroyed rather than steel. Copper rings, crimping ends of fragrant pipes, copper rivets and the screens of the auxiliary equipment through which boiler water passes, almost completely destroyed even at relatively low temperatures. In view of this, it is believed that metal copper does not enhance the corrosion of the boiler steel. The deposited copper can be considered simply as a final product of the reduction of oxide with hydrogen at its formation.
On the contrary, very strong corrosion ulceration of boiler metal is often observed next to sediments, especially rich copper. These observations led to the assumption that copper, since it is a cathode in relation to steel, contributes to point corrosion.
The surface of the boilers rarely represents nude metal iron. Most often, it has a protective layer consisting mainly of iron oxide. It is possible that the cracks are formed in this layer, the surface is the anode on copper. In such places, the formation of corrosion sinks is enhanced. This can explain in some cases the accelerated corrosion in those places where the sink was formed, as well as severe point corrosion, observed sometimes after cleaning the boilers using acids.
Wrong care for inactive boilers.
One of the most frequent causes of the formation of corrosion shells is the lack of proper care for inactive boilers. An inactive boiler must be contained either completely dry or filled with water treated in such a way that corrosion is impossible.
The water remaining on the inner surface of an inactive boiler dissolves oxygen from the air, which leads to the formation of shells, which in the future will be centered around which the corrosion process will develop.
The usual instructions for the protection of inactive boilers from corrosion are as follows:
1) the descent of water from another hot boiler (about 90 °); blowing the boiler by air to its complete drainage and content in a dry state;
2) filling the boiler with alkaline water (pH \u003d 11) comprising an excess of SO3 ions "(about 0.01%), and storage under water or steam shutter;
3) filling the boiler by an alkaline solution containing, chromic acid salts (0.02-0.03% SG4 ").
When chemical cleaning boilers, the protective layer of iron oxide will be removed in many places. Subsequently, these places may not be covered with a newly formed solid layer and on them, even in the absence of copper, sinks will appear. Therefore, it is recommended immediately after chemical cleaning to resume layer of iron oxide by treating a boiling alkaline solution (just as it is done for new boilers entering into operation).

Corrosion Economymen

General provisions relating to boiler houses are equally applicable to economizers. However, the economizer, heated nutrient water and located in front of the boiler, especially sensitive to the formation of corrosion shells. It represents the first surface with a high temperature experiencing the destructive effect of oxygen dissolved in nutrient water. In addition, water passing through an economizer has, as a rule, a low pH value and does not contain chemical moderators.
The fight against corrosion of economizers is to deaeration of water and the addition of alkali and chemical retarders.
Sometimes the processing of boiler water is carried out by passing part of it through an economizer. In this case, deposits of the sludge in the economizer should be avoided. It is also necessary to take into account the effect of such recycling of boiler water on the quality of steam.

Processing of boiler water

When processing boiler water in order to protect against corrosion, the primary task is the formation and preservation of the protective film on metal surfaces. The combination of substances added to the water depends on the working conditions, especially on the pressure, temperature, thermal tension of the nutrient water quality. However, for all cases, three rules must be observed: the boiler water must be alkaline, should not contain dissolved oxygen and contaminate the heating surface.
The caustic satter best provides protection at pH \u003d 11-12. In practice, with a complex composition of boiler water, the best results are obtained at pH \u003d 11. For boilers operating at pressures below 17.5 kg / cm2, pH is usually supported within, between 11.0 and 11.5. For higher pressures, due to the possibility of the destruction of the metal as a result of incorrect circulation and local increase in the concentration of alkali solution, pH is usually taken equal to 10.5 - 11.0.
Chemical reducing agents are widely used to remove residual oxygen: sulfuric acid salts, iron zaisi hydrate and organic reducing agents. The compounds of bivalent iron are very good to remove oxygen, but form sludge, which has an undesirable effect on heat transfer. Organic reducing agents, due to their instability at high temperatures, are usually not recommended for boilers operating at pressures above 35 kg / cm2. There are data on the decomposition of sulfium salts at elevated temperatures. However, the use of them in small concentrations in pressure boilers up to 98 kg / cm2 is widely practiced. Many high-pressure installations work at all without chemical deaeration.
The cost of special equipment for deaeration, despite its undoubted benefit, is not always justified for small installations operating at relatively low pressures. At pressures below 14 kg / cm2, partial deaeration in nutrient heaters can bring the content of dissolved oxygen to approximately 0.00007%. The addition of chemical reducing agents gives good results, especially when the pH of water is above 11, and the substances that bind oxygen are added to the water supply to the boiler, which ensures the absorption of oxygen outside the boiler.

Corrosion in concentrated boiler water

Low concentrations of caustic soda (about 0.01%) contribute to the preservation of the oxide layer on steel in a state that securely providing corrosion protection. Local concentration increase causes severe corrosion.
Plots of the boiler surface, on which the alkali concentration reaches a hazardous amount, are usually characterized by redundant, with respect to circulating water, heat supply. The zone enriched near the metal surface can occur in different places of the boiler. Corrosion ulcenes are located in the form of strips or elongated areas, sometimes smooth, and sometimes filled with solid and dense magnetic oxide.
Tubes located horizontally or slightly obliquely and susceptible to the intensive action of radiation from above, are erupted inside, along the upper generator. Such cases were observed in high power boilers, and also reproduced with specially set experiments.
Tubes in which water circulation is uneven or disrupted with a large load of the boiler, can be destroyed along the lower generator. Sometimes corrosion is more dramatically expressed along a variable water level on the side surfaces. Often you can observe abundant clusters of magnetic oxide of iron-sometimes loose, sometimes representing dense masses.
Overheating steel often enhances destruction. This can occur as a result of the formation of a pair layer at the top of the inclined tube. The formation of a steam shirt is possible in vertical tubes with a strengthened heat supply, which indicates temperature measurement in various parts of the tubes during the boiler operation. Characteristic data obtained under these dimensions are presented in Fig. 7. Limited areas of overheating in vertical tubes having a normal temperature above and below the "hot place" may be the result of a film boiling water.
Whenever on the surface of the boiler tube, a steam bubble is formed, the metal temperature is rising under it.
Increasing the concentration of alkali in water should occur on the surface of the section: the vapor bubble is water - the surface of heating. In fig. It is shown that even a slight increase in the temperature of the water film coming into contact with the metal and with an expanding vapor bubble leads to a concentration of caustic soda, measured by percentages and not by millions. The water film enriched with the alkali formed as a result of the appearance of each steam bubble affects the small section of the metal and for a very short time. However, the total effect of steam on the surface of heating can be amplified by the continuous action of a concentrated alkali solution, despite the fact that the total mass of water contains only million robes of the caustic soda. A few attempts have been made to find the issue of the issue associated with the local increase in the concentration of caustic soda on the surfaces of heating. So it was proposed to add neutral salts to water (for example, chloride metals) in a greater concentration than caustic nat. However, it is best to exclude the addition of caustic natra at all and to ensure the necessary pH value by administering hydrolyzing salts of phosphoric acid. The dependence between the pH of the solution and the concentration of phosphornation salts is presented in Fig. Despite the fact that the water containing the phosphornation salt has a high pH value, it can be paralleled without a significant increase in the concentration of hydroxyl ions.
However, it should be remembered that the exclusion of the use of caustic soda means only that one factor is removed, accelerating corrosion. If a steam shirt is formed in the tubes, then at least water and did not contain alkali, the corrosion is still possible, albeit to a lesser extent than in the presence of caustic soda. The solution to the problem should also be seen by changing the structure, given at the same time a tendency to a constant increase in the energy tension of the heating surfaces, which, in turn, certainly enhances corrosion. If the temperature of the thin layer of water, directly at the heating surface of the tube, exceeds the average temperature of the water in the rude hug, would be at a low value, in such a layer it can relatively grow the concentration of caustic soda. The curve approximately shows the conditions of equilibrium in the solution containing only the caustic soda. The exact data depends to some extent, from the pressure in the boiler.

Alkaline fragility of steel

Alkali fragility can be defined as the appearance of cracks in the area of \u200b\u200brivet seams or in other places of compounds where the concentrated alkali solution is possible and where there are high mechanical stresses.
The most serious damage almost always occurs in the area of \u200b\u200brivet seams. Sometimes they lead to the blast of the boiler; More often to produce expensive repair even relatively new boilers. One American railway for the year registered the formation of cracks in 40 locomotive boilers, which demanded the repairs worth about $ 60,000. The appearance of fragility was also installed on the tubes in the commodity places, on the links, manifolds and in the places of threaded connections.

The voltage required for the occurrence of alkaline fragility

Practice shows a small probability of fragile destruction of conventional boiler steel, if the voltages do not exceed the yield strength. The voltage created by the pressure of steam or evenly distributed load from its own weight of the structure cannot lead to the formation of cracks. However, the stresses created by the rolling of the sheet material intended for the manufacture of boilers, deformation during a riveting or any cold processing associated with residual deformation, can cause fracture formation.
The presence of the exempted voltages is optional to form cracks. A sample of boiler steel, pre-sustained with constant bending voltage, and then freed, can give a crack in an alkaline solution, the concentration of which is equal to an increased alkali concentration in boiler water.

Alkali concentration

The normal alkali concentration in the boiler drum cannot cause cracks, because it does not exceed 0.1% Naon, and the smallest concentration at which alkaline fragility is observed, above is normal at about 100 times.
Such high concentrations can be obtained as a result of extremely slow seeping of water through a riveting seam or any other clearance. This explains the appearance of solid salts outside the majority of rivet seams in steam boilers. The most dangerous flow is such that it is difficult to detect it leaves the precipitate of a solid inside the rivet seam where there are high residual stresses. The joint effect of the voltage and the concentrated solution may cause the appearance of alkaline fragility cracks.

Device for detecting alkaline fragility

A special device for controlling the composition of water reproduces the process of evaporation of water with an increase in alkali concentration on a stressed steel sample under the same conditions in which this occurs in the area of \u200b\u200brivet seam. The cracking of the control sample indicates that the boiler water of this composition is able to cause alkaline fragility. Consequently, in this case, water treatment is necessary, eliminating its dangerous properties. However, the cracking of the control sample does not mean that there are already cracks in the boiler or will appear. In the rivet seams or in other places, the compounds are not necessarily available at the same time and flow (steaming), and voltage, and an increase in alkali concentrations, as in the control sample.
The control device is installed directly on the steam boiler and allows you to judge the quality of boiler water.
The test lasts 30 or more days when the water circulation is constant through the control device.

Crack recognition of alkaline fragility

Crackers of alkaline fragility in conventional boiler steel are different in nature than fatigue cracks or cracks formed due to high voltages. This is illustrated in Fig. I9, which shows the intercrystalline character of such cracks forming a thin mesh. The difference between intercrystalline cracks of alkaline fragility and intracrystalline cracks caused by corrosion fatigue can be seen when compared.
In alloyed steels (for example, nickel or silicarganogenic) used for locomotive boilers, cracks are also located grid, but do not always pass between crystallites, as in the case of ordinary boiler steel.

Theory of alkaline fragility

Atoms in the crystal lattice of the metal, located at the boundaries of crystallites, are experiencing a less symmetrical effect of their neighbors than atoms in the rest of the grain. Therefore, they are easier to leave the crystal lattice. It can be thought that with a thorough selection of an aggressive environment, it will be possible to carry out such selective removal of atoms from the boundaries of crystallites. Indeed, experiments show that in acidic, neutral (with a weak electric current, creating conditions, favorable for corrosion) and concentrated alkali solutions, an intercrystalline cracking can be obtained. If a solution causing general corrosion is changed by the addition of any substance forming a protective film on the surface of crystallites, corrosion focuses on the boundaries between crystallites.
Aggressive solution in the case under consideration is a solution of caustic soda. The silicenry salt can protect the surfaces of the crystallites without acting on the borders between them. The result of a joint protective and aggressive action depends on many circumstances: concentration, temperature, intense state of the metal and the composition of the solution.
There is also a colloid theory of alkaline fragility and the theory of hydrogen action dissolving in steel.

Ways to combat alkaline fragility

One way to combat alkaline fragility is to replace the riveting boilers with welding, which eliminates the possibility of treating leaks. Fragility can also be eliminated by the use of steel, resistant against intercrystalline corrosion, or chemical processing of boiler water. In the rivet boilers currently used, the last method is the only acceptable.
Preliminary tests with the use of the control sample are the best way to determine the effectiveness of certain protective additives to water. The truly salty salt warns cracking. Nitrate Salt is successfully used to protect against cracking at pressures up to 52.5 kg / cm2. The concentrated solutions of a nitratethrium salt, boiling at atmospheric pressure, can cause corrosive cracks at a soft steel voltage.
Currently, the nitratetomatic salt is widely used in stationary boilers. The concentration of a nitratetric salt corresponds to 20-30% of the alkali concentration.

Corrosion of steps

Corrosion on the inner surfaces of the tubes of steamers is primarily due to the interaction between the metal and the ferry at high temperature and to a lesser extent - by the departure of the strains of boiler water by steam. In the latter case, films of solutions with a high concentration of caustic soda can be formed on metal walls, directly corrosive steel or giving deposits hitting on the wall of the tubes, which can lead to the formation of duun. In the inactive boilers and in cases of steam condensation in relatively cold steps, point corrosion may develop under the influence of oxygen and coal anhydride.

Hydrogen as a measure of corrosion speed

The temperature of the steam in modern boilers is approaching temperatures used in the industrial production of hydrogen by a direct response between the steam and iron.
On the corrosion rate of carbon and alloyed steel pipes under the action of steam, at temperatures up to 650 °, one can be judged by the volume of the hydrogen released. Sometimes it is used to release hydrogen as a measure of general corrosion.
Recently, three types of miniature plants for removing gases and air are used in US power stations. They provide complete removal of gases, and degassed condensate is suitable for determining in it salts that blame the steam from the boiler. The approximate value of the overall corrosion of the steamer during the boiler operation can be obtained by determining the difference in hydrogen concentrations in steam samples taken before and after passing it through the steamer.

Corrosion caused by impurities in a pair

A rich steam, which is part of the steam steamper, takes with them small, but measurable amounts of gases and strands from boiler water. The most common gases are oxygen, ammonia and carbon dioxide. When pair passes through a steam-controller, a tangible change in the concentration of these gases is not observed. Only minor corrosion of the metal superheater can be attributed due to the action of these gases. It has not yet been proven that salts dissolved in water, in a dry form or deposited on the elements of the steamer, can contribute to corrosion. However, the caustic soda, being the main component of the salts fascinated by boiler water, can contribute to corrosion of a strongly heated tube, especially if the alkali sticks to the metal wall.
Increasing the purity of the saturated pair is achieved by preliminary careful removal of gases from nutritious water. Reducing the number of salts involved in steam is achieved by careful cleaning in the upper collector, using mechanical separators, washing a saturated pair of nutrient water or suitable chemical treatment of water.
Determination of the concentration and nature of gases involved in a saturated ferry is carried out by using these devices and chemical analysis. Determining the concentration of salts in a saturated pair is conveniently produced by measuring the electrical conductivity of water or evaporation of a large amount of condensate.
An improved method of measuring electrical conductivity is proposed, appropriate corrections for some dissolved gases are given. Condensate in the above mentioned miniature gas removal can also be used to measure electrical conductivity.
When the boiler is inactive, the steamer is a refrigerator in which condensate accumulates; In this case, ordinary underwater point corrosion is possible if the steam contained oxygen or carbon dioxide.

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Most emergency stops of boilers fall on through corrosion damage to screen, economy - grain, steam heating pipes and borants of boilers. The appearance of even one corrosion fistula in the direct-flow boiler leads to the stop of the entire block, which is associated with the non-performance of electricity. Corrosion of high and ultra-high drum boilers has become the main cause of failures in the work of the CHP. 90% of failures in work due to corrosion damage occurred on drum boilers with a pressure of 15.5 MPa. A significant amount of corrosion damage to the screen pipes of salt compartments was in the "zones of maximum thermal loads.

Conducted by US specialists by surveys 238 boilers (blocks with a capacity of 50 to 600 MW), 1719 unscheduled downtime were recorded. About 2/3 downtime boilers were caused by corrosion, of which 20% accounted for corrosion of steam generating pipes. In the US, internal corrosion "in 1955 was recognized as a serious problem after commissioning a large number of drum boilers with a pressure of 12.5-17 MPa.

By the end of 1970, about 20% of 610 such boilers were amazed by corrosion. Mainly internal corrosion exposed on-screen pipes, and steamers and economizers were stronger than it. With the improvement of nutrient water quality and the transition to the coordinated phosphating mode, with an increase in the parameters on the drum boilers of US power stations instead of viscous, plastic corrosion damage occurred sudden fragile destruction of the on-screen pipes. "As of J970 tons. For Kotlree with a pressure of 12.5; 14.8 and 17 MPa, the destruction of pipes due to corrosion damage was 30, 33 and 65%, respectively.

In terms of the conditions of the corrosion process, the atmospheric corrosion flows under the action of atmospheric, as well as humid gases; Gas, due to the interaction of metal with various gases - oxygen, chlorine, etc. - at high temperatures, and corrosion in electrolytes, in most cases occurring in aqueous solutions.

By the nature of corrosion processes, the boiler metal may be subject to chemical and electrochemical corrosion, as well as their joint impact.

When the surfaces of the heating of steam boilers occurs, high-temperature gas corrosion in the oxidative and reducing atmospheres of flue gases and low-temperature electrochemical corrosion of the tail surfaces of heating.

Studies found that high-temperature corrosion of heating surfaces is most intensively proceeds only if there are excess free oxygen in the furnace gas and in the presence of molten vanadium oxides.

High-temperature gas or sulphide corrosion in the oxidative atmosphere of flue gases affects the tubes of shirm and convective superheater, the first rows of boiling beams, the metal of distortioning spacers between pipes, racks and suspension.

High-temperature gas corrosion in restoration of the atmosphere was observed on the on-screen pipes of the heat chambers of a series of high and supercritical pressure boilers.

Corrosion of pipes for heating surfaces with a gas side represents a complex physico-chemical process of interaction of flue gases and external sediments with oxides - films and metal pipes. The development of this process is influenced by time-changing intensive heat flows and high mechanical stresses arising from internal pressure and self-compensation.

On the boilers of medium and low pressure "the temperature of the walls of the screens determined by the boiling point of water is lower, and therefore this type of metal destruction is not observed.

Corrosion of heating surfaces from flue gases (outer corrosion) is the process of the destruction of the metal as a result of interaction with combustion products, aggressive gases, solutions and melts of mineral compounds.

Under corrosion of metal understands the gradual destruction of the metal, which is due to the chemical or electrochemical effects of the external environment.

\\ Metal destruction processes resulting from their direct chemical interaction with the environment are chemical corrosion.

Chemical corrosion occurs when the metal with superheated ferry and dry gases. Chemical corrosion in dry gases is called gas corrosion.

In the firebox and gas strokes of the boiler, gas corrosion of the outer surface of the pipes and steaks of steam-heater occurs under the influence of oxygen, carbon dioxide, water vapor, sulfur and other gases; The inner surface of the pipes - as a result of interaction with steam or water.

Electrochemical corrosion in contrast to the chemical is characterized by the fact that the reaction occurring with it is accompanied by the occurrence of electric current.

The carriers of electricity in solutions are the ions present in them due to the dissociation of molecules, and in metals - free electrons:

The intracerene surface is mainly susceptible to electrochemical corrosion. According to modern ideas, its manifestation is due to two independent processes: an anode, in which the metal ions are transferred to the solution in the form of hydrated ions, and cathodic, in which the assimilation of excess electrons depolarizers occurs. Depolarizers may be atoms, ions, molecules that are restored.

According to external signs, the solid (general) and local (local) form of corrosion destruction is distinguished.

With a general corrosion, the entire spoofing surface of heating with an aggressive medium is subjected to corrosion, evenly drowned with the inner or outdoor side. With local corrosion, the destruction occurs in separate areas of the surface, the remaining surface of the metal is not affected by damage.

Local local local stains include corrosive, ulcerative, point, intercrystalline, corrosion cracking, corrosion metal fatigue.

A typical example of destruction from electrochemical corrosion.

Destruction from the outer surface of the HDC 042x5 mm pipes from steel 12x1mf TPP-110 boilers occurred on a horizontal section at the bottom of the lifting-hydraulic loop in the zone adjacent to the sub-screen screen. On the back of the pipe there was a disclosure with a small refinement of the edges at the destroyer. The cause of the destruction was the thinning of the pipe wall of about 2 mm in corrosion due to the weasure of the jet of water. After the stop of the boiler, the 850 t / h with the anthracite binary dust (liquid slag), 25.5 MPa and the temperature of the superheated steam 540 ° C on the pipes remained wet slag and ash in which electrochemical corrosion was intensively flowed. Outside the pipe was covered with a thick layer of buoy hydroxide of iron. The internal diameter of the pipes was within tolerances on the pipes of high and ultra-high pressure boilers. The dimensions of the outer diameter have deviations that go beyond the minus tolerance: the minimum outer diameter. was 39 mm with minimally permissible 41.7 mm. The wall thickness near the damage from corrosion was only 3.1 mm at a nominal pipe thickness of 5 mm.

Metal microstructure is homogeneous in length and circle. On the inner surface of the pipe there is a deductible layer formed during the oxidation of the pipe in the process of heat treatment. There is no such layer on the outside.

Surveys of PCC pipes after the first break made it possible to find out the cause of destruction. It was decided to replace the HPC and about changing the technology of divisions. In this case, electrochemical corrosion proceeded due to the presence of a thin electrolyte film.

Ulcerative corrosion proceeds intensively on certain small areas of the surface, but often to a significant depth. When the diameter of Yazvin is about 0.2-1 mm, it is called point.

In places where yazvins are formed, swearing can be formed. Yazvins are often filled with corrosion products, as a result of which they are not always able to detect them. An example is the destruction of the steel economyzer pipes with poor deaeration of nutritious water and low speeds of water in the pipes.

Despite the fact that a significant part of the pipe metal is amazed, due to through fistulas, it is necessary to completely replace the economizer coils.

The metal of steam boilers is subjected to the following dangerous types of corrosion: oxygen corrosion during the boilers and find them in repair; intercrystallite corrosion in the places of evaporation of boiler water; conducting corrosion; corrosion cracking of elements of boilers made of austenitic steels; After corrosion. The brief characteristic of the specified types of metal corrosion of the boilers is given in Table. Yul.

During the work of boilers, metal corrosion is distinguished by corrosion under load and parking corrosion.

Corrosion under load are most susceptible. Moveless boiler elements in contact with a two-phase medium, i.e., screen and boiling pipes. The inner surface of the economizers and superheater when the boilers are affected by corrosion less. Corrosion under load flows in an enlightening medium.

Parking corrosion is manifested in underestimated. Elements of vertical coins of superheater, conductive pipes of horizontal coins of superheater

A number of boilers use river and tap water with low pH and low rigidity to feed thermal networks. Additional river water treatment on a tap station usually leads to a decrease in the PN, a decrease in alkalinity and an increase in aggressive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in the connection schemes used for large heat supply systems with direct hot water waterborne (2000h3000 t / h). Water softening according to the Na-cation scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - stiffery salts.

With a poorly established deaeration of water and possible increasing concentrations of oxygen and carbon dioxide due to the lack of additional protective measures in the heat supply systems of the internal corrosion, Heatinglery equipment of the CHP.

During the examination of the feed tract by one of the CHP of Leningrad, the following data was obtained by corrosion velocity, g / (m2 · 4):

Corrosion Indicator Installation Place

In the pipeline of the feeding water after heaters of the heating system before the deaerators of the pipe with a thickness of 7 mm climbed over the year of operation in places up to 1 mm in some sections, through fistulas were formed.

The causes of ulcerative corrosion of water boilers are as follows:

insufficient removal of oxygen from feeding water;

low pH value due to the presence of aggressive carbon dioxide

(up to 10h15 mg / l);

the accumulation of products of oxygen corrosion of iron (Fe2O3;) on heat transfer surfaces.

Operation of equipment on network water with iron concentration Over 600 μg / l usually leads to the fact that several thousand hours of operation of hot water boilers are observed intensive (over 1000 g / m2) by iron-oxide deposits of their heating surfaces. At the same time, often emerging leaks in the pipes of the convective part are noted. In the composition of deposits, the content of iron oxides usually reaches 80ch90%.

Especially important for the operation of hot water boilers are starting periods. In the initial period of operation on one CHP, the removal of oxygen was not ensured to the norms installed by the PTE. The content of oxygen in the feed water exceeded these norms 10 times.

The concentration of iron in the feeding water reached - 1000 μg / l, and in the reverse water of the heating network - 3500 μg / l. After the first year of operation, cutting from pipelines of the network water were made, it turned out that the contamination of their surface with corrosion products was over 2000 g / m2.

It should be noted that on this CHP, before turning on the boiler, the internal surfaces of the on-screen pipes and the pipes of the convective beam were subjected to chemical cleaning. By the time the screening of samples of the on-screen pipes, the boiler worked 5300 hours. The sample of the on-screen pipe had an uneven layer of yellow-pointed sediments of black and brown color, firmly related to the metal; The height of the tubercles 10h12 mm; Specific contamination 2303 g / m2.

The composition of deposits,%

The surface of the metal under the layer of deposits was amazed by ulcers with a depth of 1 mm. A convective beam tubes from the inside were brought by deposits of iron oxide type of black and brown color with a height of tubercles up to 3h4 mm. The surface of the metal under deposits is covered with ulcers of various sizes with a depth of 0.3 h1.2 and a diameter of 0.35h0.5 mm. Separate tubes had through holes (fistulas).

When the water-heating boilers are installed in the old systems of centralized heat supply, in which a significant amount of iron oxides have accumulated, there are cases of depositing these oxides in the heated boiler pipes. Before turning on the boilers, it is necessary to make a thorough flushing of the entire system.

A number of researchers recognize an important role in the occurrence of submissive corrosion of the rusting process of water boilers under their downtime, when not taken proper measures to prevent parking corrosion. Corrosion foci arising from atmospheric air to the wet surfaces of the boilers continue to function when the boilers are working.

Ministry of Energy and Electrification of the USSR

Main Science and Technology Energy and Electrification

Methodical instructions
For warning
Low-temperature
Corrosion surfaces
Heating and gas pipes boilers

RD 34.26.105-84

Soyucehenergo

Moscow 1986.

Developed by the All-Union Twice Order of the Labor Red Banner Teply Engineering Research Institute named after F.E. Dzerzhinsky

Artists R.A. Petrosyan, I.I. Nadyrov

Approved by the Main Technical Operation Manual Energy Systems 22.04.84

Deputy Head of D.Ya. Shamarakov

Methodical guidelines for the prevention of low-temperature corrosion of heat and gas supplies of boilers

RD 34.26.105-84

The validity period is set
from 01.07.85
until 01.07.2005

These guidelines are applied to low-temperature surfaces of the heating of steam and hot water boilers (economizers, gas evaporators, air heaters of various types, etc.), as well as the gas tract for air heaters (gas ducts, ashors, smokers, flue pipes) and set surface protection methods Heating from low-temperature corrosion.

Methodical instructions are designed for thermal power plants operating on sulfur fuels, and organizations that design boiler equipment.

1. Low-temperature corrosion is the corrosion of the tail surfaces of heating, gas ducts and chimneys of boilers under the action of sulfuric acid vapors condensing from chimneal gases.

2. Condensation of sulfuric acid vapors, the volumetric content of which in flue gases when burning sulfur fuels is only a few thousandths of the percentage, occurs at temperatures, significantly (by 50 - 100 ° C) exceeding the temperature of the condensation of water vapor.

4. To prevent corrosion of heating surfaces during operation, the temperature of their walls should exceed the temperature point of the flue gases at all loads of the boiler.

For the heating surfaces cooled with a high heat transfer coefficient (economizers, gas evaporators, etc.), the temperature of the medium at the inlet in them should exceed the temperature of the dew point by about 10 ° C.

5. For the surfaces of heating the water boilers when working on a sulfur fuel oil, the conditions for the complete exception of low-temperature corrosion cannot be implemented. To reduce it, it is necessary to ensure the temperature of the water at the inlet to the boiler, equal to 105 - 110 ° C. When using water boilers as peaks, such a mode can be provided with the full use of network water heaters. When using water boilers in the main mode, an increase in water temperature at the inlet to the boiler can be achieved by recycling hot water.

In the installations using the scheme for the inclusion of water heating boilers in the heat carrier through water heat exchangers, the conditions for the reduction of low-temperature corrosion of the heating surfaces are fully ensured.

6. For aircraft heaters of steam boilers, the complete elimination of low-temperature corrosion is provided at the calculated wall temperature of the coldest area greater than the temperature of the dew point at all loads of the boiler by 5 - 10 ° C (the minimum value refers to the minimum load).

7. Calculation of the temperature of the wall of tubular (TVP) and regenerative (RWP) air heater is carried out on the recommendations of the "thermal calculation of boiler aggregates. Regulatory method "(M.: Energy, 1973).

8. When used in tubular air heaters as the first (by air) the movement of the changeable cold cubes or cubes from pipes with an acidic coating (enamelled, etc.), as well as made of corrosion-resistant materials to the conditions of complete exception of low-temperature corrosion, the following are checked for them (by air) Metal cubes air heater. In this case, the selection of the temperature of the cold metal cubes changeable, as well as corrosion-resistant cubes, should exclude intensive contamination of pipes, for which their minimal temperature of the wall when burning sulfur fuel oils should be lower than the dew point of flue gases by no more than 30 to 40 ° C. When burning solid sulfur fuels, the minimum temperature of the pipe wall under the conditions of the warning of intensive contamination should be taken at least 80 ° C.

9. In RVP, on the conditions of complete exception of low-temperature corrosion, their hot part is calculated. The cold part of the RVP is performed by corrosion-resistant (enameled, ceramic, from low-alloyed steel, etc.) or replaced from flat metal sheets with a thickness of 1.0 - 1.2 mm made of small-carbon steel. The conditions for preventing intensive packing pollution are complied with the requirements of claim. Of this document.

10. As an enameled, a filling of metal sheets with a thickness of 0.6 mm is applied. The service life of the enameled package made in accordance with TU 34-38-10336-89 is 4 years.

Porcelain tubes, ceramic blocks, or porcelain plates with protrusions can be used as ceramic packing.

Given the reduction in the consumption of fuel oil with thermal power plants, it is advisable to apply for the cold part of the RWP, a package of low-alloyed steel 10Hord or 10xst, the corrosion resistance of which is 2- 2.5 times higher than that of small-carbon steel.

11. To protect air heaters from low-temperature corrosion in the starting period, measures set out in the "Guidelines for the design and operation of energy heating calorifications with wire fins" (M.: SPO Uniontehenergo, 1981).

The milling of the boiler on the sulfur fuel oil should be carried out with a pre-enabled air heating system. The air temperature in front of the air heater in the initial period of the extracts should usually be 90 ° C.

11a. To protect air heaters from low-temperature ("parking") corrosion on a stopped boiler, the level of which is about twice the rate of corrosion during operation, before stopping the boiler, it should be thoroughly clean the air heater from outdoor sediments. In this case, before stopping the boiler, the air temperature at the inlet into the air heater is recommended to maintain at the level of its value at the rated load of the boiler.

The cleaning of the TVP is carried out by a fraction with the density of its supply of at least 0.4 kg / pp (paragraph. Of this document).

For solid fuels, taking into account the significant hazard of the corrosion of the aspores, the temperature of the outgoing gases should be chosen above the dew point of the flue gases at 15 - 20 ° C.

For sulfur fuel oil, the temperature of the outgoing gases should exceed the temperature of the dew point at the rated load of the boiler by about 10 ° C.

Depending on the sulfur content in the fuel oil, the calculated value of the outgoing gases should be taken at the rated load of the boiler, indicated below:

The temperature of the outgoing gases, ºС ...... 140 150 160 165

When burning sulfur fuel oil with extremely small excess air (α ≤ 1.02), the temperature of the outgoing gases can be accepted lower taking into account the results of the dew point measurements. On average, the transition from small excess air to the maximum low reduces the temperature of the dew point by 15 to 20 ° C.

The conditions for ensuring reliable operation of the chimney and the prevention of moisture falling on its wall affects not only the temperature of the outgoing gases, but also their consumption. The work of the pipe with load modes is significantly lower than the project increases the likelihood of low-temperature corrosion.

When burning natural gas, the temperature of the outgoing gases is recommended to have no lower than 80 ° C.

13. With a decrease in the loading of the boiler in the range of 100 - 50% of the nominal one should strive to stabilize the temperature of the outgoing gases, not allowing its decline to more than 10 ° C from the nominal.

The most economical way to stabilize the temperature of the outgoing gases is to increase the temperature of the preheating of air in the carriers as the load decreases.

The minimum allowable values \u200b\u200bof the temperature preheating temperatures before RVP are accepted in accordance with clause 4.3.28 "Rules for the technical operation of electric stations and networks" (M.: Energoatomizdat, 1989).

In cases where the optimal temperature of the outgoing gases cannot be provided due to the insufficient surface of the RVP heating, the values \u200b\u200bof the preheating temperatures should be taken, at which the temperature of the outgoing gases does not exceed the values \u200b\u200bshown in these methodical instructions.

16. Due to the lack of reliable acid-resistant coatings to protect against low-temperature corrosion of metal gas ducts, their reliable operation can be achieved by careful insulation, ensuring the temperature difference between the flue gases and the wall of no more than 5 ° C.

At present, insulation materials and designs are not sufficiently reliable in long-term operation, therefore it is necessary to conduct periodic, at least once a year, control over their condition and, if necessary, carry out repair and restoration work.

17. When used in an experimental order to protect the gas ducts from low-temperature corrosion of various coatings, it should be borne in mind that the latter must provide heat resistance and gas content at temperatures exceeding the temperature of the outgoing gases at least 10 ° C, resistance to sulfuric acid concentration 50 - 80% In the temperature range, respectively, 60 - 150 ° C and the possibility of their repair and recovery.

18. For low-temperature surfaces, structural elements of RVP and boiler gas supplies, it is advisable to use low-alloyed steels of 10HNDP and 10XD, which are 2 - 2.5 times in corrosion resistance.

The absolute corrosion resistance is only very deficient and expensive high-alloy steel (for example, EI943 steel, containing up to 25% chromium and up to 30% nickel).

application

1. Theoretically, the temperature of the flue gas dew point with a predetermined content of sulfuric acid and water can be determined as a boiling point of a solution of sulfuric acid of such a concentration, at which there is the same content of water vapor and sulfuric acid.

The measured temperature point of the dew point depending on the measurement methodology may not be coincided with theoretical. In these recommendations for the temperature of the dew point of flue gases tR The surface temperature of a standard glass sensor with apart at a distance of 7 mm is taken one from the other platinum electrodes with a length of 7 mm, at which the resistance of the dew film between the electrodes in the steady state is 107 ohms. In the measuring circuit of the electrodes, an alternating current of low voltage is used (6 - 12 V).

2. When burning sulfur fuel oils with excess air 3 - 5% temperature point of dew flue gases depends on the sulfur content in fuel Sp. (Fig.).

When burning sulfur fuel oils with extremely low air excess (α ≤ 1.02), the temperature of the flue gases dew should be taken according to the results of special measurements. The conditions for the transfer of boilers in the mode with α ≤ 1.02 are set forth in the "Guidelines for the transfer of boilers operating on sulfur fuels, into combustion mode with extremely small excess airs" (M.: SPO SoyuceCenergo, 1980).

3. When burning sulfur solid fuels in the dust-shaped state temperature of the dew point of flue gases tP. It may be calculated according to the sulfur and ash content in the fuel SRP, ARPR and condensation temperature of water vapor ton according to the formula

where aUN - Share of ash in charge (usually received 0.85).

Fig. 1. The dependence of the temperature of the dew point of flue gases from the sulfur content in the combustion fuel oil

The value of the first term of this formula aUN \u003d 0.85 can be determined in fig. .

Fig. 2. The difference in temperature points of the dew of flue gases and condensation of water vapor in them depending on the sulfur contents ( SRP) and ash ( ARPR) In the fuel

4. When burning gaseous sulfur fuels, the dew point of flue gases can be determined in fig. Provided that the sulfur content in the gas is calculated as the above, that is, in a percentage by weight by 4186.8 kJ / kg (1000 kcal / kg) heat combustion of gas.

For gas fuel, the size of the sulfur content in percentage by weight can be determined by the formula

where m. - the number of sulfur atoms in the sulfur component molecule;

q. - bulk percentage of sulfur (sulfur component);

QN - heat combustion of gas in KJ / M3 (kcal / nm3);

FROM - coefficient equal to 4,187, if QN expressed in KJ / M3 and 1.0, if in kcal / m3.

5. The corrosion rate of the replaced metal packing of air heater during the combustion of the fuel oil depends on the temperature of the metal and the degree of corrosion activity of flue gases.

When burning sulfur fuel oil with an excess of air 3 - 5% and blend the surface of the corrosion (from two sides in mm / year), the RVP packing can be estimated according to Table. .

Table 1

	Corrosion rate (mm / year) at the wall temperature, ºС
				0.5 More than 2 0.20
St. 0.11 to 0.4 incl.
St. 0.41 to 1.0 incl.

6. For coal with a high content of calcium oxide, the dew point temperature is lower than those calculated according to claims of these methodical instructions. For such fuels, it is recommended to use the results of direct measurements.