Corrosion of steam boilers. Corrosion of pipelines and hot water boilers Methods for preventing corrosion in hot water boilers

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 are more often exposed entrance areas economizers. 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. Important contains sodium hydroxide, 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 heating of the tube wall to critical temperature, which accelerates the oxidation of the metal with water. 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.

Corrosion of hot water boilers, heating systems, district heating systems are much more common than in steam and condensate systems. In most cases, this situation is explained by the fact that when designing hot water system less attention is paid to this, although the factors for the formation and subsequent development of corrosion in boilers remain exactly the same as for steam boilers and all other equipment. Dissolved oxygen, which is not removed by deaeration, hardness salts, and carbon dioxide entering hot water boilers with feed water cause different kinds corrosion - alkaline (intercrystalline), oxygen, chelate, sub-sludge. It must be said that chelate corrosion in most cases is formed in the presence of certain chemical reagents, the so-called “complexons”.

In order to prevent the occurrence of corrosion in hot water boilers and its subsequent development, it is necessary to take seriously and responsibly the preparation of the characteristics of water intended for make-up. It is necessary to ensure the binding of free carbon dioxide and oxygen, bring the pH value to an acceptable level, and take measures to protect aluminum, bronze and copper elements of heating equipment and boilers, pipelines and heating equipment from corrosion.

IN Lately For high-quality correction heating networks, hot water boilers and other equipment, special chemical reagents are used.

Water is at the same time a universal solvent and an inexpensive coolant; it is beneficial to use in heating systems. But insufficient preparation can lead to unpleasant consequences, one of which is corrosion of hot water boilers. Possible risks are primarily associated with the presence of a large number of undesirable impurities in it. It is possible to prevent the formation and development of corrosion, but only if you clearly understand the reasons for its occurrence, and also be familiar with modern technologies.

Hot water boilers, as well as any heating systems that use water as a coolant, are characterized by three types of problems caused by the presence of the following impurities:

  • mechanical insoluble;
  • sediment-forming dissolved;
  • corrosive.

Each of the types of impurities listed can cause corrosion and failure of a hot water boiler or other equipment. In addition, they contribute to reducing the efficiency and performance of the boiler.

And if you use water that has not undergone special preparation in heating systems for a long time, this can lead to serious consequences - breakdown of circulation pumps, reduction in the diameter of the water supply system and subsequent damage, failure of the regulating and shut-off valves. The simplest mechanical impurities - clay, sand, ordinary dirt - are present almost everywhere, as in tap water, and in artesian springs. Also, coolants contain large quantities of corrosion products of heat transfer surfaces, pipelines and other metal elements of the system that are constantly in contact with water. It goes without saying that their presence over time provokes very serious problems in the functioning of hot water boilers and all thermal power equipment, which are mainly associated with corrosion of boilers, the formation of lime deposits, the removal of salts and foaming of boiler water.

The most common reason that causes corrosion of hot water boilers, these are carbonate deposits that occur when using water of high hardness, the removal of which is possible through. It should be noted that as a result of the presence of hardness salts, scale forms even in low-temperature heating equipment. But this is far from the only cause of corrosion. For example, after heating water to a temperature of more than 130 degrees, the solubility of calcium sulfate decreases significantly, resulting in the formation of a layer of dense scale. In this case, the development of corrosion of the metal surfaces of hot water boilers is inevitable.

Identification of types of corrosion is difficult, and, therefore, errors are common in determining technologically and economically optimal measures to combat corrosion. The main necessary measures are taken in accordance with regulatory documents, which establish the limits of the main corrosion initiators.

GOST 20995-75 “Stationary steam boilers with pressure up to 3.9 MPa. Indicators of quality of feed water and steam" normalizes the indicators in feed water: transparency, that is, the amount of suspended impurities; general hardness, content of iron and copper compounds - prevention of scale formation and iron and copper oxide deposits; pH value - prevention of alkaline and acid corrosion and also foaming in the boiler drum; oxygen content - preventing oxygen corrosion; nitrite content - prevention of nitrite corrosion; content of petroleum products - preventing foam formation in the boiler drum.

The norm values ​​are determined by GOST depending on the pressure in the boiler (therefore, on the water temperature), on the power of the local heat flow and on the water treatment technology.

When investigating the causes of corrosion, first of all, it is necessary to inspect (where available) places of metal destruction, analyze the operating conditions of the boiler in the pre-accident period, analyze the quality of feed water, steam and deposits, analyze design features boiler

Upon external inspection, the following types of corrosion may be suspected.

Oxygen corrosion

: inlet sections of steel economizer pipes; supply pipelines when encountering insufficiently deoxygenated (above normal) water - “breakthroughs” of oxygen due to poor deaeration; feedwater heaters; all wet areas of the boiler during shutdown and failure to take measures to prevent air from entering the boiler, especially in stagnant areas, when draining water, from where it is difficult to remove steam condensate or completely fill with water, for example vertical pipes steam superheaters. During downtime, corrosion is enhanced (localized) in the presence of alkali (less than 100 mg/l).

Oxygen corrosion rarely (when the oxygen content in water is significantly higher than the norm - 0.3 mg/l) appears in the steam separation devices of boiler drums and on the drum wall at the water level boundary; in downpipes. Corrosion does not occur in riser pipes due to the deaerating effect of steam bubbles.

Type and nature of damage. Ulcers of varying depth and diameter, often covered with tubercles, the upper crust of which is reddish iron oxides (probably hematite Fe 2 O 3). Evidence of active corrosion: under the crust of the tubercles there is a black liquid sediment, probably magnetite (Fe 3 O 4) mixed with sulfates and chlorides. With extinct corrosion, there is a void under the crust, and the bottom of the ulcer is covered with deposits of scale and sludge.

At water pH > 8.5 - ulcers are rare, but larger and deeper, at pH< 8,5 - встречаются чаще, но меньших размеров. Только вскрытие бугорков помогает интерпретировать бугорки не как поверхностные отложения, а как следствие коррозии.

When the water speed is more than 2 m/s, the tubercles can take on an oblong shape in the direction of the jet movement.

. Magnetic crusts are quite dense and could serve as a reliable barrier to the penetration of oxygen into the tubercles. But they are often destroyed as a result of corrosion fatigue, when the temperature of water and metal changes cyclically: frequent stops and starts of the boiler, pulsating movement of the steam-water mixture, stratification of the steam-water mixture into separate plugs of steam and water, following each other.

Corrosion increases with increasing temperature (up to 350 °C) and increasing chloride content in boiler water. Sometimes corrosion is enhanced by thermal decomposition products of certain organic substances in the feedwater.

Rice. 1. Appearance oxygen corrosion

Alkaline (in a narrower sense - intergranular) corrosion

Places of metal corrosion damage. Pipes in areas of high power heat flow (burner area and opposite the elongated torch) - 300-400 kW/m2 and where the metal temperature is 5-10 °C higher than the boiling point of water at a given pressure; inclined and horizontal pipes where water circulation is poor; places under thick sediments; zones near the backing rings and in the welds themselves, for example, in places where intra-drum vapor separation devices are welded; places near the rivets.

Type and nature of damage. Hemispherical or elliptical depressions filled with corrosion products, often including shiny crystals of magnetite (Fe 3 O 4). Most of the depressions are covered with a hard crust. On the side of the pipes facing the firebox, the recesses can connect, forming a so-called corrosion track 20-40 mm wide and up to 2-3 m long.

If the crust is not sufficiently stable and dense, then corrosion can lead - under conditions of mechanical stress - to the appearance of cracks in the metal, especially near the cracks: rivets, rolling joints, welding points of vapor separation devices.

Causes of Corrosion Damage. At high temperatures ah - more than 200 °C - and a high concentration of caustic soda (NaOH) - 10% or more - the protective film (crust) on the metal is destroyed:

4NaOH + Fe 3 O 4 = 2NaFeO 2 + Na 2 FeO 2 + 2H 2 O (1)

The intermediate product NaFeO 2 undergoes hydrolysis:

4NaFeO 2 + 2H 2 O = 4NaOH + 2Fe 2 O 3 + 2H 2 (2)

That is, in this reaction (2) caustic soda is reduced, in reactions (1), (2) it is not consumed, but acts as a catalyst.

When the magnetite is removed, caustic soda and water can react with the iron directly to release atomic hydrogen:

2NaOH + Fe = Na 2 FeO 2 + 2H (3)

4H 2 O + 3Fe = Fe 3 O 4 + 8H (4)

The released hydrogen is able to diffuse into the metal and form methane (CH 4) with iron carbide:

4H + Fe 3 C = CH 4 + 3Fe (5)

It is also possible to combine atomic hydrogen into molecular hydrogen (H + H = H 2).

Methane and molecular hydrogen cannot penetrate into the metal; they accumulate at the grain boundaries and, in the presence of cracks, expand and deepen them. In addition, these gases prevent the formation and compaction of protective films.

A concentrated solution of caustic soda is formed in places of deep evaporation of boiler water: dense scale deposits of salts (a type of sub-sludge corrosion); a crisis of nucleate boiling, when a stable vapor film is formed above the metal - there the metal is almost not damaged, but at the edges of the film, where active evaporation occurs, caustic soda is concentrated; the presence of cracks where evaporation occurs, which is different from evaporation in the entire volume of water: caustic soda evaporates worse than water, is not washed away by water and accumulates. Acting on the metal, caustic soda forms cracks at the grain boundaries directed into the metal (a type of intergranular corrosion - crevice).

Intergranular corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.


Rice. 3. Intergranular corrosion: a - microstructure of the metal before corrosion, b - microstructure at the corrosion stage, formation of cracks along the grain boundaries of the metal

Such a corrosive effect on metal is possible only with the simultaneous presence of three factors:

  • local tensile mechanical stresses close to or slightly exceeding the yield strength, that is, 2.5 MN/mm 2 ;
  • loose joints of drum parts (indicated above), where deep evaporation of boiler water can occur and where accumulating caustic soda dissolves protective film iron oxides (NaOH concentration more than 10%, water temperature above 200 °C and - especially - closer to 300 °C). If the boiler is operated at a pressure lower than the rated pressure (for example, 0.6-0.7 MPa instead of 1.4 MPa), then the likelihood of this type of corrosion decreases;
  • an unfavorable combination of substances in boiler water, which lacks the necessary protective concentrations of inhibitors of this type of corrosion. Sodium salts can act as inhibitors: sulfates, carbonates, phosphates, nitrates, cellulose sulfite liquor.


Rice. 4. Appearance of intergranular corrosion

Corrosion cracks do not develop if the following ratio is observed:

(Na 2 SO 4 + Na 2 CO 3 + Na 3 PO 4 + NaNO 3)/(NaOH) ≥ 5.3 (6)

where Na 2 SO 4, Na 2 CO 3, Na 3 PO 4, NaNO 3, NaOH are the contents of sodium sulfate, sodium carbonate, sodium phosphate, sodium nitrate and sodium hydroxide, respectively, mg/kg.

In currently manufactured boilers, at least one of the specified conditions for the occurrence of corrosion is absent.

The presence of silicon compounds in boiler water can also increase intergranular corrosion.

NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (Cl -) are corrosion accelerators; due to their high mobility and small size, they easily penetrate protective oxide films and produce highly soluble salts with iron (FeCl 2, FeCl 3) instead of poorly soluble iron oxides.

In boiler water, the values ​​of total mineralization are traditionally monitored, rather than the content of individual salts. Probably for this reason, standardization was introduced not according to the indicated ratio (6), but according to the value of the relative alkalinity of the boiler water:

Sh q rel = Sh ov rel = Sh ov 40 100/S ov ≤ 20, (7)

where Shk rel - relative alkalinity of boiler water, %; Shch ov rel - relative alkalinity of treated (additional) water, %; Shch ov - total alkalinity of treated (additional) water, mmol/l; S ov - mineralization of treated (additional) water (including chloride content), mg/l.

The total alkalinity of the treated (additional) water can be taken equal, mmol/l:

  • after sodium cationization - the total alkalinity of the source water;
  • after hydrogen-sodium cationization parallel - (0.3-0.4), or sequential with “hungry” regeneration of the hydrogen-cation exchange filter - (0.5-0.7);
  • after sodium cationization with acidification and sodium chlorine ionization - (0.5-1.0);
  • after ammonium-sodium cationization - (0.5-0.7);
  • after liming at 30-40 °C - (0.35-1.0);
  • after coagulation - (Sh about ref - D k), where Sh about ref is the total alkalinity of the source water, mmol/l; D k - dose of coagulant, mmol/l;
  • after soda liming at 30-40 °C - (1.0-1.5), and at 60-70 °C - (1.0-1.2).

The values ​​of relative alkalinity of boiler water according to Rostechnadzor standards are accepted, %, no more than:

  • for boilers with riveted drums - 20;
  • for boilers with welded drums and pipes rolled into them - 50;
  • for boilers with welded drums and pipes welded to them - any value, not standardized.


Rice. 4. Result of intergranular corrosion

According to Rostekhnadzor standards, Shk kv rel is one of the criteria safe work boilers It is more correct to check the criterion for the potential alkaline aggressiveness of boiler water, which does not take into account the content of chlorine ion:

K sh = (S ov - [Cl - ])/40 Shch ov, (8)

where Ksh is a criterion for the potential alkaline aggressiveness of boiler water; S ov - mineralization of treated (additional) water (including chloride content), mg/l; Cl - - content of chlorides in treated (additional) water, mg/l; Shch ov - total alkalinity of treated (additional) water, mmol/l.

The value of K sch can be taken:

  • for boilers with riveted drums pressure more than 0.8 MPa ≥ 5;
  • for boilers with welded drums and pipes rolled into them with a pressure of more than 1.4 MPa ≥ 2;
  • for boilers with welded drums and pipes welded to them, as well as for boilers with welded drums and pipes rolled into them with a pressure of up to 1.4 MPa and boilers with riveted drums with a pressure of up to 0.8 MPa - do not standardize.

Sludge corrosion

This name combines several different types of corrosion (alkali, oxygen, etc.). The accumulation of loose and porous deposits and sludge in different areas of the boiler causes corrosion of the metal under the sludge. The main reason: contamination of feed water with iron oxides.

Nitrite corrosion

. Screen and boiler pipes of the boiler on the side facing the firebox.

Type and nature of damage. Rare, sharply limited large ulcers.

. If there are more than 20 μg/l of nitrite ions (NO - 2) in the feed water, and the water temperature is more than 200 ° C, nitrites serve as cathodic depolarizers of electrochemical corrosion, being reduced to HNO 2, NO, N 2 (see above).

Steam-water corrosion

Places corrosion damage metal. The outlet part of superheater coils, superheated steam steam pipelines, horizontal and slightly inclined steam generating pipes in areas of poor water circulation, sometimes along the upper form of the outlet coils of boiling water economizers.

Type and nature of damage. Plaques of dense black iron oxides (Fe 3 O 4), firmly adhered to the metal. When the temperature fluctuates, the continuity of the plaque (crust) is disrupted and the scales fall off. Uniform thinning of metal with bulges, longitudinal cracks, breaks.

It can be identified as sub-sludge corrosion: in the form of deep ulcers with vaguely demarcated edges, most often near welds protruding into the pipe, where sludge accumulates.

Causes of corrosion damage:

  • washing medium - steam in superheaters, steam pipelines, steam “pillows” under a layer of sludge;
  • metal temperature (steel 20) more than 450 °C, heat flow to the metal section - 450 kW/m2;
  • violation of the combustion regime: slagging of burners, increased contamination of pipes inside and outside, unstable (vibrating) combustion, elongation of the torch towards the screen pipes.

The result: direct chemical interaction of iron with water vapor (see above).

Microbiological corrosion

Caused by aerobic and anaerobic bacteria, appears at temperatures of 20-80 ° C.

Locations of metal damage. Pipes and containers to the boiler with water at the specified temperature.

Type and nature of damage. The tubercles are of different sizes: diameter from several millimeters to several centimeters, rarely - several tens of centimeters. The tubercles are covered with dense iron oxides - a waste product of aerobic bacteria. Inside there is a black powder and suspension (iron sulfide FeS) - a product of sulfate-reducing anaerobic bacteria; under the black formation there are round ulcers.

Causes of damage. Natural water always contains iron sulfates, oxygen and various bacteria.

Iron bacteria in the presence of oxygen form a film of iron oxides, under which anaerobic bacteria reduce sulfates to iron sulfide (FeS) and hydrogen sulfide (H 2 S). In turn, hydrogen sulfide starts the formation of sulfurous (very unstable) and sulfuric acids, and the metal corrodes.

This type has an indirect effect on boiler corrosion: a water flow at a speed of 2-3 m/s tears off the tubercles, carries their contents into the boiler, increasing the accumulation of sludge.

In rare cases, this corrosion may occur in the boiler itself if, during a long shutdown of the boiler, the reserve is filled with water at a temperature of 50-60 o C, and the temperature is maintained due to random breakthroughs of steam from neighboring boilers.

Chelate corrosion

Locations of corrosion damage. Equipment in which steam is separated from water: boiler drum, steam separation devices in and outside the drum, also - rarely - in feedwater pipelines and economizer.

Type and nature of damage. The surface of the metal is smooth, but if the medium moves at high speed, then the corroded surface is not smooth, has horseshoe-shaped depressions and “tails” oriented in the direction of movement. The surface is covered with a thin matte or black shiny film. There are no obvious deposits, and there are no corrosion products, because the “chelate” (organic polyamine compounds specially introduced into the boiler) has already reacted.

In the presence of oxygen, which rarely happens in a normally operating boiler, the corroded surface is “invigorated”: roughness, islands of metal.

Causes of Corrosion Damage. The mechanism of action of the “chelate” was described earlier (“Industrial and heating boiler houses and mini-CHP”, 1(6)΄ 2011, p. 40).

“Chelate” corrosion occurs when there is an overdose of “chelate,” but it is also possible with a normal dose, since the “chelate” is concentrated in areas where intense evaporation of water occurs: nucleate boiling is replaced by film boiling. In steam separation devices, there are cases of particularly destructive “chelate” corrosion due to high turbulent velocities of water and steam-water mixture.

All of the described corrosion damage can have a synenergetic effect, so that the total damage from the combined action of different corrosion factors can exceed the sum of damage from individual types of corrosion.

As a rule, the action of corrosive agents enhances the unstable thermal regime of the boiler, which causes corrosion fatigue and initiates thermal fatigue corrosion: the number of starts from a cold state is more than 100, the total number of starts is more than 200. Since these types of metal damage occur rarely, cracks, rupture pipes have an appearance identical to metal damage from various types of corrosion.

Usually, to identify the cause of metal destruction, additional metallographic studies are required: radiography, ultrasound, color and magnetic particle flaw detection.

Various researchers have proposed programs for diagnosing types of corrosion damage to boiler steels. The VTI program is known (A.F. Bogachev and co-workers) - mainly for power boilers high pressure, and developments of the Energochermet association - mainly for low and medium pressure power boilers and waste heat boilers.



Owners of patent RU 2503747:

TECHNICAL FIELD

The invention relates to thermal power engineering and can be used for protection against scale heating pipes steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems residential buildings And industrial facilities during current operation.

BACKGROUND OF THE ART

The operation of steam boilers is associated with simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the boiler heating surfaces are separate phases complex system, which is formed upon their contact. The result of the interaction of these phases is surface processes that occur at their interface. As a result of this, corrosion and scale formation occur in the metal of the heating surfaces, which leads to a change in the structure and mechanical properties of the metal, and which contributes to the development of various damages. Since the thermal conductivity of scale is fifty times lower than that of iron heating pipes, there are losses of thermal energy during heat transfer - with a scale thickness of 1 mm from 7 to 12%, and with 3 mm - 25%. Severe scale formation in the steam boiler system continuous action often causing production to stop for several days a year to remove scale.

The quality of feed water and, therefore, boiler water is determined by the presence of impurities that can cause various types of corrosion of the metal of internal heating surfaces, the formation of primary scale on them, as well as sludge as a source of secondary scale formation. In addition, the quality of boiler water also depends on the properties of substances formed as a result of surface phenomena during water transportation and condensate through pipelines during water treatment processes. Removing impurities from feed water is one of the ways to prevent the formation of scale and corrosion and is carried out by methods of preliminary (pre-boiler) water treatment, which are aimed at maximizing the removal of impurities found in the source water. However, the methods used do not allow us to completely eliminate the content of impurities in water, which is associated not only with technical difficulties, but also with the economic feasibility of using pre-boiler water treatment methods. In addition, since water treatment is complex technical system, it is redundant for boilers of low and medium productivity.

Known methods for removing already formed deposits mainly use mechanical and chemical cleaning methods. The disadvantage of these methods is that they cannot be produced during the operation of the boilers. In addition, chemical cleaning methods often require the use of expensive chemicals.

There are also known methods to prevent the formation of scale and corrosion, carried out during the operation of boilers.

US patent 1877389 proposes a method for removing scale and preventing its formation in hot water and steam boilers. In this method, the surface of the boiler is the cathode, and the anode is placed inside the pipeline. The method consists of passing a constant or alternating current through the system. The authors note that the mechanism of action of the method is that under the influence electric current Gas bubbles form on the surface of the boiler, which lead to the detachment of existing scale and prevent the formation of a new one. The disadvantage of this method is the need to constantly maintain the flow of electric current in the system.

US Pat. No. 5,667,677 proposes a method for treating a liquid, particularly water, in a pipeline to slow down the formation of scale. This method is based on the creation of an electromagnetic field in pipes, which repels calcium and magnesium ions dissolved in water from the walls of pipes and equipment, preventing them from crystallizing in the form of scale, which allows the operation of boilers, boilers, heat exchangers, and cooling systems on hard water. The disadvantage of this method is the high cost and complexity of the equipment used.

Application WO 2004016833 proposes a method for reducing the formation of scale on a metal surface exposed to a supersaturated alkaline aqueous solution which is capable of forming scale after a period of exposure, comprising applying a cathodic potential to said surface.

This method can be used in various technological processes, in which the metal is in contact with an aqueous solution, in particular in heat exchangers. The disadvantage of this method is that it does not protect the metal surface from corrosion after removing the cathodic potential.

Thus, there is currently a need to develop an improved method for preventing scale formation of heating pipes, hot water boilers and steam boilers, which would be economical and highly effective and provide anti-corrosion protection to the surface for a long period of time after exposure.

In the present invention, this problem is solved using a method according to which a current-carrying electric potential is created on a metal surface, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide an improved method for preventing the formation of scale in heating pipes of hot water and steam boilers.

Another objective of the present invention is to provide the possibility of eliminating or significantly reducing the need for descaling during operation of hot water and steam boilers.

Another objective of the present invention is to eliminate the need to use consumable reagents to prevent the formation of scale and corrosion of heating pipes of water heating and steam boilers.

Another object of the present invention is to enable work to begin to prevent the formation of scale and corrosion of heating pipes of hot water and steam boilers on contaminated boiler pipes.

The present invention relates to a method for preventing the formation of scale and corrosion on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale is capable of forming. This method consists in applying a current-carrying conductor to the specified metal surface. electric potential, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.

According to some private embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some private embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the interior surface of the heating tubes of a hot water or steam boiler.

The method disclosed herein has the following advantages. One advantage of the method is reduced scale formation. Another advantage of the present invention is the ability to use a working electrophysical apparatus once purchased without the need to use consumable synthetic reagents. Another advantage is the possibility of starting work on dirty boiler tubes.

The technical result of the present invention, therefore, is to increase the operating efficiency of hot water and steam boilers, increase productivity, increase heat transfer efficiency, reduce fuel consumption for heating the boiler, save energy, etc.

Other technical results and advantages of the present invention include providing the possibility of layer-by-layer destruction and removal of already formed scale, as well as preventing its new formation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the distribution of deposits on the internal surfaces of the boiler as a result of applying the method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention involves applying to a metal surface subject to scale formation a current-carrying electrical potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and scale-forming ions to the metal surface.

The term "conducting electrical potential" as used in this application means an alternating potential that neutralizes the electrical double layer at the interface of the metal and the steam-water medium containing salts that lead to scale formation.

As is known to a person skilled in the art, the carriers of electric charge in a metal, slow compared to the main charge carriers - electrons, are dislocations of its crystal structure, which carry an electric charge and form dislocation currents. Coming to the surface of the heating pipes of the boiler, these currents become part of the double electrical layer during the formation of scale. The current-carrying, electrical, pulsating (i.e., alternating) potential initiates the removal of the electrical charge of dislocations from the metal surface to the ground. In this respect, it is a conductor of dislocation currents. As a result of the action of this current-carrying electrical potential, the double electrical layer is destroyed, and the scale gradually disintegrates and passes into the boiler water in the form of sludge, which is removed from the boiler during periodic purging.

Thus, the term “current-carrying potential” is understandable to a person skilled in the art and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).

As a device for creating a current-carrying electrical potential, for example, a device described in RU 2100492 C1 can be used, which includes a converter with a frequency converter and a pulsating potential regulator, as well as a pulse shape regulator. Detailed description of this device is given in RU 2100492 C1. Any other similar device may also be used, as will be appreciated by one skilled in the art.

The conductive electrical potential according to the present invention can be applied to any part of the metal surface remote from the base of the boiler. The place of application is determined by the convenience and/or effectiveness of using the claimed method. One skilled in the art, using the information disclosed herein and using standard testing techniques, will be able to determine the optimal location for application of the current-sinking electrical potential.

In some embodiments of the present invention, the current-sinking electrical potential is variable.

The current-sinking electric potential according to the present invention can be applied for various periods of time. The time of application of the potential is determined by the nature and degree of contamination of the metal surface, the composition of the water used, the temperature regime and the operating characteristics of the heating device and other factors known to specialists in this field of technology. One skilled in the art, using the information disclosed herein and using standard test procedures, will be able to determine the optimal time to apply the current-sinking electrical potential based on the objectives, conditions, and condition of the thermal device.

The magnitude of the current-carrying potential required to neutralize the electrostatic component of the adhesion force can be determined by a specialist in the field of colloid chemistry based on information known from the prior art, for example from the book B.V. Deryagin, N.V. Churaev, V.M. Muller. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the magnitude of the current-carrying electrical potential is in the range from 10 V to 200 V, more preferably from 60 V to 150 V, even more preferably from 61 V to 150 V. Values ​​of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the double electrical layer, which is the basis of the electrostatic component of the adhesion forces in scale and, as a consequence, destruction of scale. Values ​​of the current-carrying potential below 61 V are insufficient to destroy scale, and at values ​​of the current-carrying potential above 150 V, unwanted electrical erosion destruction of the metal of the heating tubes is likely to begin.

The metal surface to which the method according to the present invention can be applied can be part of the following thermal devices: heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems of residential buildings and industrial facilities during ongoing operation. This list is illustrative and does not limit the list of devices to which the method according to the present invention can be applied.

In some embodiments, the iron-containing alloy from which the metal surface is made to which the method of the present invention can be applied may be steel or other iron-containing material such as cast iron, kovar, fechral, ​​transformer steel, alsifer, magneto, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron-containing alloys to which the method according to the present invention can be applied. One skilled in the art, based on knowledge known in the art, will be able to identify such iron-containing alloys that can be used according to the present invention.

Water environment, from which scale is capable of forming, according to some embodiments of the present invention, is tap water. The aqueous medium may also be water containing dissolved metal compounds. The dissolved metal compounds may be iron and/or alkaline earth metal compounds. The aqueous medium may also be an aqueous suspension of colloidal particles of iron and/or alkaline earth metal compounds.

The method according to the present invention removes previously formed deposits and serves as a reagent-free means of cleaning internal surfaces during operation of a heating device, subsequently ensuring its scale-free operation. In this case, the size of the zone within which the prevention of scale and corrosion is achieved significantly exceeds the size of the zone of effective scale destruction.

The method according to the present invention has the following advantages:

Does not require the use of reagents, i.e. environmentally friendly;

Easy to implement, does not require special devices;

Allows you to increase the heat transfer coefficient and increase the efficiency of boilers, which significantly affects the economic indicators of its operation;

Can be used as an addition to the applied methods of pre-boiler water treatment, or separately;

Allows you to abandon the processes of water softening and deaeration, which greatly simplifies the technological scheme of boiler houses and makes it possible to significantly reduce costs during construction and operation.

Possible objects of the method may be hot water boilers, waste heat boilers, closed systems heat supply, installations for thermal desalination of sea water, steam conversion plants, etc.

The absence of corrosion damage and scale formation on internal surfaces opens up the possibility of developing fundamentally new design and layout solutions for low- and medium-power steam boilers. This will allow, due to the intensification of thermal processes, to achieve a significant reduction in the weight and dimensions of steam boilers. Ensure the specified temperature level of heating surfaces and, therefore, reduce fuel consumption, volume flue gases and reduce their emissions into the atmosphere.

EXAMPLE OF IMPLEMENTATION

The method claimed in the present invention was tested at the Admiralty Shipyards and Krasny Khimik boiler plants. The method according to the present invention has been shown to effectively clean the internal surfaces of boiler units from deposits. In the course of these works, fuel equivalent savings of 3-10% were obtained, while the variation in savings values ​​is associated with varying degrees of contamination of the internal surfaces of the boiler units. The purpose of the work was to evaluate the effectiveness of the claimed method for ensuring reagent-free, scale-free operation of medium-power steam boilers under conditions of high-quality water treatment, compliance with the water chemical regime and high professional level equipment operation.

The method claimed in the present invention was tested on steam boiler unit No. 3 DKVR 20/13 of the 4th Krasnoselskaya boiler house of the South-Western branch of the State Unitary Enterprise "TEK SPb". The operation of the boiler unit was carried out in strict accordance with the requirements of regulatory documents. Everything is installed on the boiler necessary funds control of its operating parameters (pressure and flow rate of generated steam, temperature and flow rate of feed water, pressure of blast air and fuel on the burners, vacuum in the main sections of the gas path of the boiler unit). The steam output of the boiler was maintained at 18 t/hour, the steam pressure in the boiler drum was 8.1…8.3 kg/cm 2 . The economizer operated in heating mode. City water supply water was used as the source water, which met the requirements of GOST 2874-82 “Drinking water”. It should be noted that the amount of iron compounds entering the specified boiler room, as a rule, exceeds regulatory requirements (0.3 mg/l) and amounts to 0.3-0.5 mg/l, which leads to intensive overgrowing of internal surfaces with ferrous compounds.

The effectiveness of the method was assessed based on the condition of the internal surfaces of the boiler unit.

Assessment of the influence of the method according to the present invention on the condition of the internal heating surfaces of the boiler unit.

Before the start of the tests, an internal inspection of the boiler unit was carried out and the initial condition of the internal surfaces was recorded. A preliminary inspection of the boiler was carried out at the beginning heating season, a month after its chemical cleaning. As a result of the inspection, it was revealed: on the surface of the drums there are continuous solid deposits of a dark brown color, possessing paramagnetic properties and presumably consisting of iron oxides. The thickness of the deposits was up to 0.4 mm visually. In the visible part of the boiling pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of pipe length with a size of 2 to 15 mm and a visual thickness of up to 0.5 mm).

The device for creating a current-carrying potential, described in RU 2100492 C1, was connected at point (1) to the hatch (2) of the upper drum on the back side of the boiler (see Fig. 1). The current-carrying electric potential was equal to 100 V. The current-carrying electric potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler unit, an almost complete absence of deposits (no more than 0.1 mm visually) was established on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the drum hatches (device connection points to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from the hatches, deposits (6) were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig. 1). Further, as you move towards the front, (at a distance of 3.0-3.5 m from the hatches) continuous deposits begin (7) up to 0.4 mm visually, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not evident. The current-carrying electric potential was equal to 100 V. The current-carrying electric potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler unit, an almost complete absence of deposits (no more than 0.1 mm visually) was established on the surface of the upper and lower drums within 2-2.5 meters from the drum hatches (attachment points of the device for creating current-carrying potential). At a distance of 2.5-3.0 m from the hatches, the deposits were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig. 1). Further, as you move towards the front (at a distance of 3.0-3.5 m from the hatches), continuous deposits of up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not evident.

In the visible part of the boiling pipes, within 3.5-4.0 m from the drum hatches, an almost complete absence of deposits was observed. Further, as we move towards the front, non-continuous solid deposits are found (up to five spots per 100 linear mm with a size ranging from 2 to 15 mm and a visual thickness of up to 0.5 mm).

As a result of this stage of testing, it was concluded that the method according to the present invention, without the use of any reagents, can effectively destroy previously formed deposits and ensure scale-free operation of the boiler unit.

On next stage During testing, the device for creating a current-carrying potential was connected at point “B” and the tests continued for another 30-45 days.

The next opening of the boiler unit was carried out after 3.5 months of continuous operation of the device.

An inspection of the boiler unit showed that the previously remaining deposits were completely destroyed and only a small amount remained in the lower sections of the boiler pipes.

This allowed us to draw the following conclusions:

The size of the zone within which scale-free operation of the boiler unit is ensured significantly exceeds the size of the zone of effective destruction of deposits, which allows subsequent transfer of the point of connection of the current-carrying potential to clean the entire internal surface of the boiler unit and further maintain its scale-free operation mode;

The destruction of previously formed deposits and the prevention of the formation of new ones is ensured by processes of different nature.

Based on the inspection results, it was decided to continue testing to the end heating season for the purpose of final cleaning of drums and boiling pipes and determining the reliability of ensuring scale-free operation of the boiler. The next opening of the boiler unit was carried out after 210 days.

The results of the internal inspection of the boiler showed that the process of cleaning the internal surfaces of the boiler within the upper and lower drums and boiling pipes resulted in almost complete removal of deposits. A thin, dense coating formed on the entire surface of the metal, black in color with a blue tarnish, the thickness of which, even in a moistened state (almost immediately after opening the boiler), did not visually exceed 0.1 mm.

At the same time, the reliability of ensuring scale-free operation of the boiler unit when using the method of the present invention was confirmed.

The protective effect of the magnetite film lasted up to 2 months after disconnecting the device, which is quite enough to ensure the preservation of the boiler unit using the dry method when transferring it to reserve or for repairs.

Although the present invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it may be practiced within the scope of the following claims.

1. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale can form, including applying a current-carrying electric potential to said metal surface in the range from 61 V to 150 V to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and ions forming scale.

The invention relates to heat power engineering and can be used to protect against scale and corrosion heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems of residential buildings and industrial facilities during operation. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale is capable of forming involves applying to said metal surface a current-carrying electric potential in the range from 61 V to 150 V to neutralize the electrostatic component of the adhesion force between the specified metal surface and colloidal particles and ions forming scale. The technical result is increasing the efficiency and productivity of hot water and steam boilers, increasing the efficiency of heat transfer, ensuring layer-by-layer destruction and removal of formed scale, as well as preventing its new formation. 2 salary f-ly, 1 ave., 1 ill.

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 various types etc.), as well as on the gas path behind the air heaters (gas ducts, ash collectors, smoke exhausters, chimneys) and establish methods for protecting heating surfaces 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 “ Thermal calculation 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 (“standby”) 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.

To ensure reliable operation chimney and preventing moisture loss, its walls are affected not only by the temperature of the exhaust 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 temperatures of the flue gases cannot be ensured due to insufficient heating surface of the RAH, the values ​​of the air preheating temperatures should be adopted at which the temperature of the flue gases will not exceed the values ​​​​given in paragraphs 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 long-term operation Therefore, it is necessary to conduct periodic, at least once a year, monitoring of their condition and, if necessary, carry out repair and restoration work.

17. When used on a trial basis to protect gas ducts from low-temperature corrosion various coatings it should be taken into account that the latter must provide heat resistance and gas tightness at temperatures exceeding the temperature of the flue gases by at least 10 ° C, resistance to the effects of sulfuric acid of a concentration 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.

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