The conditions in which the elements of steam boilers are located during operation are extremely varied.

As numerous corrosion tests and industrial observations have shown, low-alloy and even austenitic steels can be subject to intense corrosion during boiler operation.

Corrosion of the metal heating surfaces of steam boilers causes premature wear and sometimes leads to serious problems and accidents.

Most emergency shutdowns of boilers occur due to through corrosion damage to the screen, grain economizer, steam superheating pipes and boiler drums. The appearance of even one corrosion fistula in a once-through boiler leads to the shutdown of the entire unit, which is associated with a lack of electricity production. Corrosion of high- and ultra-high-pressure drum boilers has become the main cause of failures in thermal power plants. 90% of operational failures 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 the salt compartments occurred in areas of maximum thermal loads.

Inspections of 238 boilers (units with a capacity from 50 to 600 MW) conducted by US specialists revealed 1,719 unscheduled downtimes. About 2/3 of boiler downtime was caused by corrosion, of which 20% was due to corrosion of steam generating pipes. In the USA, internal corrosion was recognized as a serious problem in 1955 after the commissioning of a large number of drum boilers with a pressure of 12.5-17 MPa.

By the end of 1970, about 20% of the 610 such boilers were damaged by corrosion. Screen pipes were mostly susceptible to internal corrosion, while superheaters and economizers were less affected by it. With improved quality feed water and the transition to the coordinated phosphating regime, with increasing parameters on drum boilers of US power plants, instead of viscous, plastic corrosion damage, sudden brittle fractures of screen pipes occurred. “As of J970 t. for boilers with pressures of 12.5, 14.8 and 17 MPa, the destruction of pipes due to corrosion damage was 30, 33 and 65%, respectively.

According to the conditions of the corrosion process, a distinction is made between atmospheric corrosion, which occurs under the influence of atmospheric and also wet gases; gas, caused by the interaction of the metal with various gases - oxygen, chlorine, etc. - at high temperatures, and corrosion in electrolytes, in most cases occurring in aqueous solutions.

Due to the nature of corrosion processes, boiler metal can be subject to chemical and electrochemical corrosion, as well as their combined effects.

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

Research has established that high-temperature corrosion of heating surfaces occurs most intensely only in the presence of excess free oxygen in the flue gases and in the presence of molten vanadium oxides.

High-temperature gas or sulfide corrosion in the oxidizing atmosphere of flue gases affects pipes of screen and convective superheaters, the first rows of boiler bundles, metal spacers between pipes, racks and suspensions.

High temperature gas corrosion in a reducing atmosphere was observed on the screen pipes of the combustion chambers of a number of high and supercritical pressure boilers.

Corrosion of heating surface pipes on the gas side is a complex physical and chemical process of interaction of flue gases and external deposits with oxide films and pipe metal. The development of this process is influenced by time-varying intense heat flows and high mechanical stresses arising from internal pressure and self-compensation.

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

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

Metal corrosion is understood as the gradual destruction of metal that occurs as a result of chemical or electrochemical exposure to the external environment.

\ Metal destruction processes resulting from their direct chemical interaction with the environment, refer to chemical corrosion.

Chemical corrosion occurs when metal comes into contact with superheated steam and dry gases. Chemical corrosion in dry gases is called gas corrosion.

In the furnace and gas ducts of the boiler, gas corrosion of the outer surface of the pipes and superheater racks occurs under the influence of oxygen, carbon dioxide, water vapor, sulfur dioxide and other gases; the inner surface of the pipes - as a result of interaction with steam or water.

Electrochemical corrosion, unlike chemical corrosion, is characterized by the fact that the reactions occurring during it are accompanied by the appearance of an electric current.

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

The internal boiler surface is mainly subject to electrochemical corrosion. According to modern ideas, its manifestation is due to two independent processes: anodic, in which metal ions pass into solution in the form of hydrated ions, and cathodic, in which excess electrons are assimilated by depolarizers. Depolarizers can be atoms, ions, molecules, which are reduced.

Based on external signs, continuous (general) and local (local) forms of corrosion damage are distinguished.

With general corrosion, the entire heating surface in contact with the aggressive environment is corroded, evenly thinning on the inside or outside. With local corrosion, destruction occurs in individual areas of the surface, the rest of the metal surface is not affected by damage.

Local corrosion includes spot corrosion, ulcer corrosion, pitting corrosion, intergranular corrosion, stress-corrosion cracking, and metal corrosion fatigue.

Typical example destruction from electrochemical corrosion.

Destruction from the outer surface of NRCh 042X5 mm pipes made of steel 12Kh1MF of TPP-110 boilers occurred in a horizontal section in the lower part of the lifting and lowering loop in the area adjacent to the bottom screen. On the back side of the pipe, an opening occurred with a slight thinning of the edges at the point of destruction. The cause of the destruction was the thinning of the pipe wall by approximately 2 mm due to corrosion due to deslagging with a jet of water. After stopping the boiler with a steam production of 950 t/h, heated by anthracite pellet dust (liquid slag removal), a pressure of 25.5 MPa and a superheated steam temperature of 540 °C, wet slag and ash remained on the pipes, in which electrochemical corrosion proceeded intensively. The outside of the pipe was coated with a thick layer of brown iron hydroxide. The internal diameter of the pipes was within the tolerances for pipes of high- and ultra-high-pressure boilers. Outer diameter dimensions have deviations beyond the minus tolerance: minimum outer diameter. amounted to 39 mm with a minimum allowable of 41.7 mm. The wall thickness near the point of corrosion failure was only 3.1 mm with a nominal pipe thickness of 5 mm.

The microstructure of the metal is uniform along the length and circumference. On the inner surface of the pipe there is a decarbonized layer formed during the oxidation of the pipe during the process heat treatment. On outside there is no such layer.

Examination of the NRF pipes after the first rupture made it possible to find out the cause of the destruction. It was decided to replace the NRF and change the deslagging technology. In this case, electrochemical corrosion occurred due to the presence of a thin film of electrolyte.

Pit corrosion occurs intensely on individual small areas surface, but often to a considerable depth. When the diameter of the ulcers is about 0.2-1 mm, it is called pinpoint.

In places where ulcers form, fistulas can form over time. The pits are often filled with corrosion products, as a result of which they cannot always be detected. An example is the destruction of steel economizer pipes due to poor deaeration of feedwater and low speeds movement of water in pipes.

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

The metal of steam boilers is subject to the following dangerous types of corrosion: oxygen corrosion during operation of the boilers and when they are under repair; intercrystalline corrosion in places where boiler water evaporates; steam-water corrosion; corrosion cracking of boiler elements made of austenitic steels; sub-sludge - howling corrosion. a brief description of the indicated types of corrosion of boiler metal are given in table. YUL.

During the operation of boilers, metal corrosion is distinguished - corrosion under load and standing corrosion.

Corrosion under load is most susceptible to heating. manufactured boiler elements in contact with a two-phase medium, i.e. screen and boiler pipes. The inner surface of economizers and superheaters is less affected by corrosion during boiler operation. Corrosion under load also occurs in an oxygen-free environment.

Parking corrosion manifests itself in undrained. elements of vertical superheater coils, sagging pipes of horizontal superheater coils

Introduction

Corrosion (from Latin corrosio - corrosion) is the spontaneous destruction of metals as a result of chemical or physical-chemical interaction with environment. IN general case This is the destruction of any material - be it metal or ceramics, wood or polymer. The cause of corrosion is the thermodynamic instability of structural materials to the effects of substances in the environment in contact with them. Example - oxygen corrosion of iron in water:

4Fe + 2H 2 O + ZO 2 = 2 (Fe 2 O 3 H 2 O)

IN Everyday life For iron alloys (steels), the term “rusting” is more often used. Cases of corrosion of polymers are less known. In relation to them, there is the concept of “aging”, similar to the term “corrosion” for metals. For example, the aging of rubber due to interaction with atmospheric oxygen or the destruction of some plastics under the influence of precipitation, as well as biological corrosion. The rate of corrosion, like any other chemical reaction depends very much on temperature. An increase in temperature of 100 degrees can increase the corrosion rate by several orders of magnitude.

Corrosion processes are characterized by a wide distribution and variety of conditions and environments in which it occurs. Therefore, there is no single and comprehensive classification of corrosion cases encountered. The main classification is made according to the mechanism of the process. There are two types: chemical corrosion and electrochemical corrosion. This abstract examines chemical corrosion in detail using the example of small and large-capacity ship boiler plants.

Corrosion processes are characterized by a wide distribution and variety of conditions and environments in which it occurs. Therefore, there is no single and comprehensive classification of corrosion cases encountered.

Type aggressive environments, in which the destruction process occurs, corrosion can be of the following types:

1) -Gas corrosion

2) - Corrosion in non-electrolytes

3) -Atmospheric corrosion

4) -Corrosion in electrolytes

5) -Underground corrosion

6) -Biocorrosion

7) - Corrosion by stray current.

According to the conditions of the corrosion process, the following types are distinguished:

1) - Contact corrosion

2) - Crevice corrosion

3) -Corrosion during partial immersion

4) -Corrosion during full immersion

5) -Corrosion during alternating immersion

6) -Friction corrosion

7) -Stress corrosion.

By nature of destruction:

Complete corrosion covering the entire surface:

1) - uniform;

2) - uneven;

3) -selective.

Local (local) corrosion covering individual areas:

1) - spots;

2) - ulcerative;

3) - spot (or pitting);

4) - through;

5) - intercrystalline.

1. Chemical corrosion

Let's imagine metal in the process of producing rolled metal at a metallurgical plant: a red-hot mass moves along the stands of a rolling mill. Fiery splashes fly out from her in all directions. This is when particles of scale break off from the surface of the metal - a product of chemical corrosion resulting from the interaction of the metal with atmospheric oxygen. This process of spontaneous destruction of a metal due to the direct interaction of oxidizer particles and the oxidized metal is called chemical corrosion.

Chemical corrosion is the interaction of a metal surface with a (corrosive) environment, not accompanied by the occurrence of electrochemical processes at the phase boundary. In this case, the interactions of metal oxidation and reduction of the oxidizing component of the corrosive environment occur in one act. For example, the formation of scale when iron-based materials react at high temperatures with oxygen:

4Fe + 3O 2 → 2Fe 2 O 3

During electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidizing component of the corrosive environment occur in more than one act and their rates depend on electrode potential metal (for example, rusting of steel in sea water).

In chemical corrosion, metal oxidation and reduction of the oxidizing component of the corrosive environment occur simultaneously. Such corrosion is observed when metals are exposed to dry gases (air, fuel combustion products) and liquid non-electrolytes (oil, gasoline, etc.) and is a heterogeneous chemical reaction.

The process of chemical corrosion occurs as follows. The oxidizing component of the external environment, taking away valence electrons from the metal, simultaneously enters into contact with it. chemical compound, forming a film (corrosion product) on the metal surface. Further formation of the film occurs due to mutual two-way diffusion through the film of the aggressive medium towards the metal and metal atoms towards external environment and their interactions. Moreover, if the resulting film has protective properties, that is, it prevents the diffusion of atoms, then corrosion proceeds with self-inhibition over time. Such a film is formed on copper at a heating temperature of 100 °C, on nickel at 650, on iron at 400 °C. Heat steel products above 600 °C leads to the formation of a loose film on their surface. With increasing temperature, the oxidation process accelerates.

The most common type of chemical corrosion is the corrosion of metals in gases at high temperatures - gas corrosion. Examples of such corrosion are oxidation of furnace fittings, parts of internal combustion engines, grate bars, parts of kerosene lamps and oxidation during high-temperature processing of metals (forging, rolling, stamping). Other corrosion products may also form on the surface of metal products. For example, when exposed to sulfur compounds, sulfur compounds are formed on iron; on silver, when exposed to iodine vapor, silver iodide is formed, etc. However, most often a layer of oxide compounds is formed on the surface of metals.

Temperature has a great influence on the rate of chemical corrosion. As temperature increases, the rate of gas corrosion increases. Compound gas environment has a specific effect on the corrosion rate various metals. Thus, nickel is stable in an environment of oxygen and carbon dioxide, but is highly corroded in the atmosphere sulfur dioxide. Copper is susceptible to corrosion in an oxygen atmosphere, but is stable in a sulfur dioxide atmosphere. Chromium is corrosion resistant in all three gas environments.

To protect against gas corrosion, heat-resistant alloying with chromium, aluminum and silicon is used, creating protective atmospheres and protective coatings aluminum, chromium, silicon and heat-resistant enamels.

2. Chemical corrosion in ship steam boilers.

Types of corrosion. During operation, the elements of a steam boiler are exposed to aggressive media - water, steam and flue gases. There are chemical and electrochemical corrosion.

Parts and components of machines operating at high temperatures, - piston and turbine engines, rocket engines, etc. The chemical affinity of most metals for oxygen at high temperatures is almost unlimited, since the oxides of all technically important metals are capable of dissolving in metals and leaving the equilibrium system:

2Me(t) + O 2 (g) 2MeO(t); MeO(t) [MeO] (solution)

Under these conditions, oxidation is always possible, but along with the dissolution of the oxide, an oxide layer also appears on the surface of the metal, which can inhibit the oxidation process.

The rate of metal oxidation depends on the rate of the chemical reaction itself and the rate of diffusion of the oxidizing agent through the film, and therefore the protective effect of the film is higher, the better its continuity and the lower its diffusion ability. The continuity of the film formed on the surface of the metal can be assessed by the ratio of the volume of the formed oxide or some other compound to the volume of the metal spent on the formation of this oxide (Pilling-Badwords factor). The coefficient a (Pilling-Badwords factor) for different metals has different meanings. Metals that have a<1, не могут создавать сплошные оксидные слои, и через несплошности в слое (трещины) кислород свободно проникает к поверхности металла.

Continuous and stable oxide layers are formed at a = 1.2-1.6, but at large values ​​of a the films are not continuous, easily separated from the metal surface (iron scale) as a result of internal stresses.

The Pilling-Badwords factor gives a very approximate estimate, since the composition of the oxide layers has a wide range of homogeneity, which is also reflected in the density of the oxide. So, for example, for chromium a = 2.02 (for pure phases), but the oxide film formed on it is very resistant to environmental influences. The thickness of the oxide film on the metal surface varies depending on time.

Chemical corrosion, caused by steam or water, destroys the metal evenly over the entire surface. The rate of such corrosion in modern marine boilers is low. More dangerous is local chemical corrosion caused by aggressive chemical compounds contained in ash deposits (sulfur, vanadium oxides, etc.).

Electrochemical corrosion, as its name indicates, is associated not only with chemical processes, but also with the movement of electrons in interacting media, i.e. with the appearance of electric current. These processes occur when the metal interacts with electrolyte solutions, which takes place in a steam boiler in which boiler water circulates, which is a solution of salts and alkalis that have disintegrated into ions. Electrochemical corrosion also occurs when the metal comes into contact with air (at normal temperature), which always contains water vapor, which condenses on the surface of the metal in the form of a thin film of moisture, creating conditions for electrochemical corrosion to occur.

a) Oxygen corrosion

Most often, steel water economizers of boiler units suffer from oxygen corrosion, which, due to unsatisfactory deaeration of the feed water, fail 2-3 years after installation.

The immediate result of oxygen corrosion of steel economizers is the formation of fistulas in the tubes, through which a stream of water flows out at high speed. Such jets directed at the wall of an adjacent pipe can wear it down to the point of forming through holes. Since the economizer pipes are located quite compactly, the resulting corrosion fistula can cause massive damage to the pipes if the boiler unit remains in operation for a long time with the resulting fistula. Cast iron economizers are not damaged by oxygen corrosion.

Oxygen corrosion the inlet sections of economizers are more often exposed. However, with a significant concentration of oxygen in the feed water, it penetrates into the boiler unit. Here, mainly drums and standpipes are exposed to oxygen corrosion. The main form of oxygen corrosion is the formation of depressions (ulcers) in the metal, which, when they develop, lead to the formation of fistulas.

An increase in pressure intensifies oxygen corrosion. Therefore, for boiler units with a pressure of 40 atm and above, even oxygen “slips” in deaerators are dangerous. The composition of the water with which the metal comes into contact is essential. The presence of a small amount of alkali enhances the localization of corrosion, while the presence of chlorides disperses it over the surface.

b) Parking corrosion

Boiler units that are idle are affected by electrochemical corrosion, which is called standstill corrosion. Depending on operating conditions, boiler units are often taken out of operation and placed in reserve or stopped for a long time.

When the boiler unit is stopped in reserve, the pressure in it begins to drop and a vacuum arises in the drum, causing air to penetrate and enrich the boiler water with oxygen. The latter creates conditions for the occurrence of oxygen corrosion. Even when water is completely removed from the boiler unit, its internal surface is not dry. Fluctuations in air temperature and humidity cause the phenomenon of moisture condensation from the atmosphere contained inside the boiler unit. The presence of a film on the metal surface, enriched with oxygen when exposed to air, creates favorable conditions for the development of electrochemical corrosion. If there are deposits on the inner surface of the boiler unit that can dissolve in a film of moisture, the intensity of corrosion increases significantly. Similar phenomena can be observed, for example, in steam superheaters, which often suffer from standing corrosion.

If there are deposits on the inner surface of the boiler unit that can dissolve in a film of moisture, the intensity of corrosion increases significantly. Similar phenomena can be observed, for example, in steam superheaters, which often suffer from standing corrosion.

Therefore, when taking the boiler unit out of operation for a long period of downtime, it is necessary to remove existing deposits by washing.

Parking corrosion can cause serious damage to boiler units unless special measures are taken to protect them. Its danger also lies in the fact that the corrosion centers created by it during idle periods continue to act during operation.

To protect boiler units from parking corrosion, they are preserved.

c) Intergranular corrosion

Intergranular corrosion occurs in rivet seams and rolling joints of steam boiler units, which are washed off with boiler water. It is characterized by the appearance of cracks in the metal, initially very thin, invisible to the eye, which, as they develop, turn into large visible cracks. They pass between the grains of the metal, which is why this corrosion is called intergranular. In this case, the destruction of the metal occurs without deformation, therefore these fractures are called brittle.

Experience has established that intergranular corrosion occurs only when 3 conditions are simultaneously present:

1) High tensile stresses in the metal, close to the yield point.
2) Leaks in rivet seams or rolling joints.
3) Aggressive properties of boiler water.

The absence of one of the listed conditions eliminates the occurrence of brittle fractures, which is used in practice to combat intergranular corrosion.

The aggressiveness of boiler water is determined by the composition of the salts dissolved in it. The content of caustic soda is important, which at high concentrations (5-10%) reacts with the metal. Such concentrations are achieved in leaks in rivet seams and rolling joints, in which boiler water evaporates. This is why the presence of leaks can lead to brittle fractures under appropriate conditions. In addition, an important indicator of the aggressiveness of boiler water is relative alkalinity - Schot.

d) Steam-water corrosion

Steam-water corrosion is the destruction of metal as a result of chemical interaction with water vapor: 3Fe + 4H20 = Fe304 + 4H2
Metal destruction becomes possible for carbon steels when the pipe wall temperature increases to 400°C.

Corrosion products are hydrogen gas and magnetite. Steam-water corrosion has both a uniform and local (local) character. In the first case, a layer of corrosion products forms on the metal surface. The local nature of corrosion takes the form of ulcers, grooves, and cracks.

The main cause of steam corrosion is the heating of the tube wall to a critical temperature, at which the oxidation of the metal with water accelerates. Therefore, the fight against steam-water corrosion is carried out by eliminating the causes that cause overheating of the metal.

Steam-water corrosion cannot be eliminated by any change or improvement in the water chemistry of the boiler unit, since the causes of this corrosion lie in the combustion and intra-boiler hydrodynamic processes, as well as operating conditions.

e) Sludge corrosion

This type of corrosion occurs under a layer of sludge formed on the inner surface of the boiler unit pipe as a result of the boiler being fed with insufficiently purified water.

Metal damage that occurs during sludge corrosion is local (ulcerative) in nature and is usually located on the semi-perimeter of the pipe facing the furnace. The resulting ulcers look like shells with a diameter of up to 20 mm or more, filled with iron oxides, creating a “bump” under the ulcer.

MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR

MAIN SCIENTIFIC AND TECHNICAL DIRECTORATE OF ENERGY AND ELECTRIFICATION

METHODOLOGICAL INSTRUCTIONS
BY WARNING
LOW TEMPERATURE
SURFACE CORROSION
HEATING AND GAS FLOW OF BOILERS

RD 34.26.105-84

SOYUZTEKHENERGO

Moscow 1986

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

PERFORMERS R.A. PETROSYAN, I.I. NADIROV

APPROVED by the Main Technical Directorate for the Operation of Power Systems on April 22, 1984.

Deputy Chief D.Ya. SHAMARAKOV

METHODOLOGICAL INSTRUCTIONS FOR PREVENTION OF LOW TEMPERATURE CORROSION OF HEATING SURFACES AND GAS FLUES OF BOILERS

RD 34.26.105-84

Expiration date set
from 07/01/85
until 07/01/2005

These Guidelines apply to low-temperature heating surfaces of steam and hot water boilers (economizers, gas evaporators, air heaters of various types, etc.), as well as to the gas path behind the air heaters (gas ducts, ash collectors, smoke exhausters, chimneys) and establish methods for protecting surfaces heating from low temperature corrosion.

The guidelines are intended for thermal power plants operating on sulfur fuels and organizations designing boiler equipment.

1. Low-temperature corrosion is the corrosion of tail heating surfaces, flues and chimneys of boilers under the influence of sulfuric acid vapors condensing on them from the flue gases.

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

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

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

5. For the heating surfaces of hot water boilers when operating on sulfur fuel oil, the conditions for completely eliminating low-temperature corrosion cannot be realized. To reduce it, it is necessary to ensure that the water temperature at the boiler inlet is 105 - 110 °C. When using water heating boilers as peak boilers, this mode can be ensured with full use of network water heaters. When using hot water boilers in the main mode, increasing the temperature of the water entering the boiler can be achieved by recirculating hot water.

In installations using the scheme for connecting hot water boilers to the heating network through water heat exchangers, the conditions for reducing low-temperature corrosion of heating surfaces are fully ensured.

6. For air heaters of steam boilers, complete exclusion of low-temperature corrosion is ensured when the design temperature of the wall of the coldest section exceeds the dew point temperature at all boiler loads by 5 - 10 °C (the minimum value refers to the minimum load).

7. Calculation of the wall temperature of tubular (TVP) and regenerative (RVP) air heaters is carried out according to the recommendations of “Thermal calculation of boiler units. Normative method" (Moscow: Energy, 1973).

8. When using replaceable cold cubes or cubes made from pipes with an acid-resistant coating (enameled, etc.), as well as those made from corrosion-resistant materials, as the first (air) stroke in tubular air heaters, the following are checked for the conditions of complete exclusion of low-temperature corrosion (by air) metal cubes of the air heater. In this case, the choice of the wall temperature of cold metal cubes, replaceable, as well as corrosion-resistant cubes, should exclude intense contamination of the pipes, for which their minimum wall temperature when burning sulfur fuel oils should be below the dew point of the flue gases by no more than 30 - 40 ° C. When burning solid sulfur fuels, the minimum temperature of the pipe wall, in order to prevent intensive pollution, should be taken to be at least 80 °C.

9. In RVP, under the conditions of complete exclusion of low-temperature corrosion, their hot part is calculated. The cold part of the RVP is corrosion-resistant (enamelled, ceramic, low-alloy steel, etc.) or replaceable from flat metal sheets 1.0 - 1.2 mm thick, made of low-carbon steel. The conditions for preventing intense contamination of the packing are met when the requirements of paragraphs of this document are met.

10. The enameled packing is made from metal sheets with a thickness of 0.6 mm. The service life of enamel packing manufactured in accordance with TU 34-38-10336-89 is 4 years.

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

Considering the reduction in fuel oil consumption by thermal power plants, it is advisable to use packing made of low-alloy steel 10KhNDP or 10KhSND for the cold part of the RVP, the corrosion resistance of which is 2 - 2.5 times higher than that of low-carbon steel.

11. To protect air heaters from low-temperature corrosion during the startup period, the measures set out in the “Guidelines for the design and operation of energy heaters with wire fins” (M.: SPO Soyuztekhenergo, 1981) should be carried out.

Ignition of a boiler using sulfur fuel oil should be carried out with the air heating system previously turned on. The air temperature in front of the air heater during the initial period of kindling should be, as a rule, 90 °C.

11a. To protect air heaters from low-temperature (“parking”) corrosion when the boiler is stopped, the level of which is approximately twice the corrosion rate during operation, before stopping the boiler, the air heaters should be thoroughly cleaned of external deposits. In this case, before stopping the boiler, it is recommended to maintain the air temperature at the inlet to the air heater at the level of its value at the rated load of the boiler.

Cleaning of TVP is carried out with shot with a feed density of at least 0.4 kg/m.s (clause of this document).

For solid fuels, taking into account the significant risk of corrosion of ash collectors, the temperature of the flue gases should be selected above the dew point of the flue gases by 15 - 20 °C.

For sulfur fuel oils, the temperature of the flue gases should exceed the dew point temperature at the rated boiler load by approximately 10 °C.

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

Flue gas temperature, ºС...... 140 150 160 165

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

The conditions for ensuring reliable operation of the chimney and preventing moisture loss on its walls are affected not only by the temperature of the flue gases, but also by their flow rate. Operating a pipe under load conditions significantly lower than design increases the likelihood of low-temperature corrosion.

When burning natural gas, it is recommended that the flue gas temperature be at least 80 °C.

13. When reducing the boiler load in the range of 100 - 50% of the nominal one, one should strive to stabilize the flue gas temperature, not allowing it to decrease by more than 10 °C from the nominal one.

The most economical way to stabilize the flue gas temperature is to increase the air preheating temperature in the air heaters as the load decreases.

The minimum permissible values ​​of air preheating temperatures before the RAH are adopted in accordance with clause 4.3.28 of the “Rules for the technical operation of power plants and networks” (M.: Energoatomizdat, 1989).

In cases where the optimal flue gas temperatures cannot be ensured due to insufficient heating surface of the RAH, air preheating temperatures should be adopted at which the temperature of the flue gases will not exceed the values ​​​​given in paragraph of these Guidelines.

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

The insulating materials and structures currently used are not reliable enough for long-term operation, so it is necessary to periodically, at least once a year, monitor their condition and, if necessary, carry out repair and restoration work.

17. When using various coatings on a trial basis to protect gas ducts from low-temperature corrosion, it should be taken into account that the latter must provide heat resistance and gas tightness at temperatures exceeding the temperature of flue gases by at least 10 ° C, resistance to sulfuric acid concentrations of 50 - 80% in the temperature range, respectively, 60 - 150 ° C and the possibility of their repair and restoration.

18. For low-temperature surfaces, structural elements of RVP and gas ducts of boilers, it is advisable to use low-alloy steels 10KhNDP and 10KhSND, which are 2 - 2.5 times superior in corrosion resistance to carbon steel.

Only very scarce and expensive high-alloy steels have absolute corrosion resistance (for example, EI943 steel, containing up to 25% chromium and up to 30% nickel).

Application

1. Theoretically, the dew point temperature of flue gases with a given content of sulfuric acid and water vapor can be defined as the boiling point of a solution of sulfuric acid of such a concentration at which the same content of water vapor and sulfuric acid exists above the solution.

The measured value of the dew point temperature, depending on the measurement technique, may not coincide with the theoretical one. In these recommendations for the flue gas dew point temperature tr The temperature of the surface of a standard glass sensor with 7 mm long platinum electrodes soldered at a distance of 7 mm from one another is assumed, at which the resistance of the dew film between the electrodes in a steady state is 107 Ohms. The electrode measuring circuit uses low voltage alternating current (6 - 12 V).

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

When burning sulfur fuel oils with extremely low excess air (α ≤ 1.02), the flue gas dew point temperature should be taken based on the results of special measurements. The conditions for transferring boilers to a mode with α ≤ 1.02 are set out in the “Guidelines for transferring boilers operating on sulfur fuels to a combustion mode with extremely low excess air” (M.: SPO Soyuztekhenergo, 1980).

3. When burning sulfurous solid fuels in a dusty state, the dew point temperature of the flue gases tp can be calculated based on the given content of sulfur and ash in the fuel Sppr, Arpr and water vapor condensation temperature tcon according to the formula

Where aun- the proportion of ash in the carryover (usually taken to be 0.85).

Rice. 1. Dependence of flue gas dew point temperature on sulfur content in burned fuel oil

The value of the first term of this formula at aun= 0.85 can be determined from Fig. .

Rice. 2. Temperature differences between the dew point of flue gases and the condensation of water vapor in them, depending on the given sulfur content ( Sppr) and ash ( Arpr) in fuel

4. When burning gaseous sulfur fuels, the dew point of the flue gases can be determined from Fig. provided that the sulfur content in the gas is calculated as given, that is, as a percentage by weight per 4186.8 kJ/kg (1000 kcal/kg) of the calorific value of the gas.

For gas fuel, the given sulfur content as a percentage by mass can be determined by the formula

Where m- the number of sulfur atoms in the molecule of the sulfur-containing component;

q- volume percentage of sulfur (sulfur-containing component);

Qn- heat of combustion of gas in kJ/m3 (kcal/Nm3);

WITH- coefficient equal to 4.187, if Qn expressed in kJ/m3 and 1.0 if in kcal/m3.

5. The rate of corrosion of the replaceable metal packing of air heaters when burning fuel oil depends on the temperature of the metal and the degree of corrosiveness of the flue gases.

When burning sulfur fuel oil with an excess of air of 3 - 5% and blowing the surface with steam, the corrosion rate (on both sides in mm/year) of the RVP packing can be approximately estimated from the data in Table. .

Table 1

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

6. For coals with a high content of calcium oxide in the ash, the dew point temperatures are lower than those calculated according to paragraphs of these Guidelines. For such fuels it is recommended to use the results of direct measurements.

A number of boiler houses use river and tap water with a low pH value and low hardness to feed heating networks. Additional treatment of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of aggressive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in connection schemes used for large heat supply systems with direct hot water supply (2000-3000 t/h). Softening water according to the Na-cationization scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - hardness salts.

With poorly established water deaeration and possible increases in oxygen and carbon dioxide concentrations, due to the lack of additional protective measures in heat supply systems, thermal power equipment of thermal power plants is susceptible to internal corrosion.

When examining the make-up tract of one of the thermal power plants in Leningrad, the following data were obtained on the corrosion rate, g/(m2 4):

Installation location of corrosion indicators

In the make-up water pipeline after the heaters of the heating network in front of the deaerators, the 7 mm thick pipes thinned over the year of operation, in some places up to 1 mm, and through fistulas formed in some areas.

The causes of pitting corrosion of hot water boiler pipes are as follows:

insufficient removal of oxygen from make-up water;

low pH value due to the presence of aggressive carbon dioxide

(up to 10h15 mg/l);

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

Operating equipment on network water with an iron concentration of over 600 µg/l usually results in intensive (over 1000 g/m2) contamination of their heating surfaces with iron oxide deposits for several thousand hours of operation of hot water boilers. In this case, frequent leaks are noted in the pipes of the convective part. The content of iron oxides in sediments usually reaches 80–90%.

Start-up periods are especially important for the operation of hot water boilers. During the initial period of operation at one thermal power plant, oxygen removal was not ensured to the standards established by the PTE. The oxygen content in the make-up water exceeded these standards by 10 times.

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

It should be noted that at this thermal power plant, before putting the boiler into operation, the internal surfaces of the screen pipes and convective beam pipes were subjected to chemical cleaning. By the time the samples of screen pipes were cut out, the boiler had worked for 5300 hours. The sample of the screen pipe had an uneven layer of black-brown iron oxide deposits, firmly bound to the metal; height of tubercles 10x12 mm; specific pollution 2303 g/m2.

Sediment composition, %

The metal surface under the layer of deposits was affected by ulcers up to 1 mm deep. The tubes of the convective beam were covered on the inside with deposits of iron oxide type, black-brown in color, with tubercles up to 3-4 mm high. The surface of the metal under the deposits is covered with ulcers of various sizes with a depth of 0.3x1.2 and a diameter of 0.35x0.5 mm. Some tubes had through holes (fistulas).

When hot water boilers are installed in old district heating systems in which significant amounts of iron oxides have accumulated, cases of deposits of these oxides in the heated boiler pipes are observed. Before turning on the boilers, it is necessary to thoroughly flush the entire system.

A number of researchers recognize the important role in the occurrence of subsludge corrosion of the process of rusting pipes of hot water boilers during their downtime, when proper measures have not been taken to prevent standstill corrosion. Foci of corrosion that arise under the influence of atmospheric air on the wet surfaces of boilers continue to function during operation of the boilers.