Low-temperature corrosion affects the heating surfaces of tubular and regenerative air heaters, low-temperature economizers, as well as metal gas ducts and chimneys at metal temperatures below the dew point flue gases. The source of low-temperature corrosion is sulfuric anhydride SO 3 , which forms sulfuric acid vapor in flue gases, which condenses at flue gas dew point temperatures. A few thousandths of a percent of SO 3 in gases is enough to cause metal corrosion at a rate exceeding 1 mm/year. Low-temperature corrosion slows down when organizing a furnace process with small excesses of air, as well as when using fuel additives and increasing the corrosion resistance of the metal.
High-temperature corrosion is exposed to furnace screens of drum and once-through boilers when burning solid fuel, superheaters and their fastenings, as well as screens of the lower radiation part of supercritical pressure boilers when burning sulfurous fuel oil.
Corrosion of the inner surface of the pipes is a consequence of the interaction with the metal of the pipes of gases of oxygen and carbon dioxide) or salts (chlorides and sulfates) contained in the boiler water. AT modern boilers supercritical steam pressure, the content of gases and corrosive salts as a result of deep desalination of feed water and thermal deaeration is insignificant, and the main cause of corrosion is the interaction of metal with water and steam. Corrosion of the inner surface of pipes is manifested in the formation of pockmarks, pits, shells and cracks; the outer surface of damaged pipes may not differ from healthy ones.
Damage due to internal pipe corrosion also includes:
oxygen parking corrosion affecting any parts of the inner surface of pipes. The areas covered with water-soluble deposits are most intensively affected (pipes of superheaters and the transition zone of once-through boilers);
undersludge alkaline corrosion of boilers and screen pipes, arising under the action of concentrated alkali due to the evaporation of water under a layer of sludge;
corrosion fatigue, which manifests itself in the form of cracks in boiler and screen pipes as a result of simultaneous exposure to a corrosive environment and variable thermal stresses.
Scale is formed on pipes as a result of their overheating to temperatures significantly higher than the calculated ones. In connection with the increase in the productivity of boiler units, cases of failure of superheater pipes due to insufficient scale resistance to flue gases have recently become more frequent. Intensive scaling is most often observed during the combustion of fuel oil.
The wear of the pipe walls occurs as a result of the abrasive action of coal and shale dust and ash, as well as steam jets coming out of damaged neighboring pipes or blower nozzles. Sometimes the cause of wear and hardening of the pipe walls is the shot used to clean the heating surfaces. The places and degree of wear of pipes are determined by external inspection and measurement of their diameter. The actual wall thickness of the pipe is measured with an ultrasonic thickness gauge.
Warping of screen and boiler pipes, as well as individual pipes and sections wall panels The radiation part of once-through boilers occurs when pipes are installed with an uneven tightness, pipe fasteners are broken, water is lost, and due to the lack of freedom for their thermal movements. Warping of the coils and screens of the superheater occurs mainly due to the burning of hangers and fasteners, excessive and uneven tightness allowed during installation or replacement individual elements. Warping of the water economizer coils occurs due to burnout and displacement of supports and hangers.
Fistulas, bulges, cracks and ruptures can also appear as a result of: deposits in pipes of scale, corrosion products, technological scale, welding flash and other foreign objects that slow down the circulation of water and contribute to overheating of the pipe metal; shot hardening; non-compliance of steel grade with steam parameters and gas temperature; external mechanical damage; operational violations.
Identification of types of corrosion is difficult, and, therefore, errors are not uncommon in determining technologically and economically optimal measures to counteract corrosion. The main necessary measures are taken in accordance with the regulations, which set the limits of the main initiators of corrosion.
GOST 20995-75 “Stationary steam boilers with pressure up to 3.9 MPa. Quality indicators of feed water and steam” standardizes 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 alkali and acid corrosion and also foaming in the boiler drum; oxygen content - prevention of oxygen corrosion; nitrite content - prevention of nitrite corrosion; oil content - prevention of foaming in the boiler drum.
The values of the norms are determined by GOST depending on the pressure in the boiler (hence, on the temperature of the water), on the power of the local heat flow and on the technology of water treatment.
When investigating the causes of corrosion, first of all, it is necessary to inspect (where available) the 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.
On external examination, it is possible to suspect the following types corrosion.
Oxygen corrosion
: inlet pipe sections of steel economizers; supply pipelines when meeting with insufficiently deoxygenated (above normal) water - “breakthroughs” of oxygen in case of poor deaeration; feed water heaters; all wet areas of the boiler during its 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 it with water, for example vertical pipes 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) manifests itself in the steam separation devices of the boiler drums and on the wall of the drums at the water level boundary; in downpipes. In rising pipes, corrosion does not occur due to the deaerating effect of steam bubbles.
Type and nature of damage. Ulcers of various depths and diameters, 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 tubercles - a black liquid precipitate, probably magnetite (Fe 3 O 4) mixed with sulfates and chlorides. With damped corrosion, there is a void under the crust, and the bottom of the ulcer is covered with deposits of scale and sludge.
At 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.
At a water velocity of more than 2 m/s, the tubercles may take an oblong shape in the direction of the jet.
. The magnetite crusts are sufficiently 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 shutdowns and starts of the boiler, pulsating movement of the steam-water mixture, stratification of the steam-water mixture into separate steam and water plugs following one after another.
Corrosion intensifies with an increase in temperature (up to 350 °C) and an increase in the chloride content in the boiler water. Sometimes corrosion is enhanced by thermal decomposition products of certain organic matter feed water.
Rice. 1. The appearance of oxygen corrosion
Alkaline (in a narrower sense - intergranular) corrosion
Places of corrosion damage to the metal. Pipes in high power heat flow zones (burner area and opposite the elongated torch) - 300-400 kW / m 2 and where the metal temperature is 5-10 ° C higher than the boiling point of water at a given pressure; oblique and horizontal pipes where the water circulation is weak; places under thick deposits; zones near the backing rings and in the welds themselves, for example, in the places of welding of intra-drum steam separator devices; 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 recesses are covered with a hard crust. On the side of the pipes facing the furnace, the recesses can be connected, forming a so-called corrosion path 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 cracks: rivets, rolling joints, welding points of steam separation devices.
Causes of corrosion damage. At high temperatures- more than 200 ° C - and a high concentration of caustic soda (NaOH) - 10% or more - the protective film (crust) on the metal is destroyed:
4NaOH + Fe 3 O 4 \u003d 2NaFeO 2 + Na 2 FeO 2 + 2H 2 O (1)
The intermediate product NaFeO 2 undergoes hydrolysis:
4NаFeО 2 + 2Н 2 О = 4NаОН + 2Fe 2 О 3 + 2Н 2 (2)
That is, in this reaction (2), sodium hydroxide is reduced, in reactions (1), (2) it is not consumed, but acts as a catalyst.
When magnetite is removed, sodium hydroxide and water can react with iron directly to release atomic hydrogen:
2NaOH + Fe \u003d Na 2 FeO 2 + 2H (3)
4H 2 O + 3Fe \u003d 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 \u003d 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 undersludge corrosion); bubble boiling crisis, when a stable vapor film is formed over the metal - there the metal is almost not damaged, but caustic soda is concentrated along the edges of the film, where active evaporation takes place; 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 inside the metal (a type of intergranular corrosion is crevice corrosion).
Intergranular corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.
Rice. Fig. 3. Intergranular corrosion: a - metal microstructure before corrosion, b - microstructure at the stage of corrosion, formation of cracks along the metal grain boundary
Such a corrosive effect on the 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 (mentioned above), where deep evaporation of boiler water can occur and where the accumulated caustic soda dissolves the protective film of iron oxides (NaOH concentration is more than 10%, water temperature is above 200 ° C and - especially - closer to 300 ° C). If the boiler is operated with a pressure lower than the passport one (for example, 0.6-0.7 MPa instead of 1.4 MPa), then the probability of this type of corrosion decreases;
- an unfavorable combination of substances in boiler water, in which there are no necessary protective concentrations of inhibitors of this type of corrosion. Sodium salts can act as inhibitors: sulfates, carbonates, phosphates, nitrates, sulfite cellulose liquor.
Rice. 4. Appearance of intergranular corrosion
Corrosion cracks do not develop if the 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 - the content of sodium sulfate, sodium carbonate, sodium phosphate, sodium nitrate and sodium hydroxide, respectively, mg / kg.
Boilers currently manufactured do not have at least one of these corrosion conditions.
The presence of silicon compounds in boiler water can also enhance intergranular corrosion.
NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (Сl -) are corrosion accelerators, due to their high mobility and small size, they easily penetrate protective oxide films and form highly soluble salts with iron (FeCl 2, FeCl 3) instead of poorly soluble iron oxides.
In the water of boiler houses, the values of the total mineralization are traditionally controlled, and not the content of individual salts. Probably, for this reason, rationing was introduced not according to the indicated ratio (6), but according to the value of the relative alkalinity of boiler water:
SH kv rel = SH ov rel = SH ov 40 100/S ov ≤ 20, (7)
where U q 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 the treated (additional) water (including the content of chlorides), mg / l.
The total alkalinity of the treated (additional) water can be taken equal, mmol/l:
- after sodium cationization - total alkalinity of the source water;
- after hydrogen-sodium cationization parallel - (0.3-0.4), or sequential with "hungry" regeneration of the hydrogen-cationite 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 - (W about ref - D to), where W about ref - total alkalinity of the source water, mmol/l; D to - dose of coagulant, mmol/l;
- after soda lime at 30-40 °C - (1.0-1.5), and at 60-70 °C - (1.0-1.2).
The values of the relative alkalinity of boiler water according to the norms of Rostekhnadzor are accepted,%, not 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. The result of intergranular corrosion
According to the norms of Rostekhnadzor, U kv rel is one of the criteria safe work boilers. It is more correct to check the criterion of potential alkaline aggressiveness of boiler water, which does not take into account the content of chlorine ion:
K u = (S ov - [Сl - ]) / 40 u ov, (8)
where K u - criterion of potential alkaline aggressiveness of boiler water; S s - salinity of the treated (additional) water (including the content of chlorides), mg/l; Cl - - the content of chlorides in the treated (additional) water, mg/l; Shch ov - total alkalinity of treated (additional) water, mmol/l.
The value of K u can be taken:
- for boilers with riveted drums with a pressure of 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.
Subslurry corrosion
This name combines several different types of corrosion (alkaline, oxygen, etc.). The accumulation of loose and porous deposits and sludge in different zones of the boiler causes corrosion of the metal under the sludge. main reason: contamination of feed water with iron oxides.
Nitrite corrosion
. Screen and boiler pipes of the boiler on the side facing the furnace.
Type and nature of damage. Rare, sharply limited large ulcers.
. In the presence of nitrite ions (NO - 2) in the feed water of more than 20 μg / l, water temperature of more than 200 ° C, nitrites serve as cathodic depolarizers electrochemical corrosion, recovering to HNO 2, NO, N 2 (see above).
Steam-water corrosion
Places of corrosion damage to the metal. Superheater coil outlets, superheated steam lines, horizontal and slightly inclined steam generating pipes in areas of poor water circulation, sometimes upper generatrix outlet coils of boiling water economizers.
Type and nature of damage. Plaques of dense black oxides of iron (Fe 3 O 4), firmly bonded to the metal. With fluctuations in temperature, the continuity of the plaque (crust) is broken, the scales fall off. Uniform thinning of metal with bulges, longitudinal cracks, breaks.
It can be identified as under-slurry corrosion: in the form of deep pits with indistinctly demarcated edges, more often near welds protruding inside the pipe, where slurry accumulates.
Causes of corrosion damage:
- washing medium - steam in superheaters, steam pipelines, steam "pillows" under a layer of sludge;
- the temperature of the metal (steel 20) is more than 450 ° C, the heat flux to the metal section is 450 kW / m 2;
- violation of the combustion mode: slagging of burners, increased contamination of pipes inside and outside, unstable (vibration) combustion, elongation of the torch towards the pipes of the screens.
As a 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.
Places of metal damage. Pipes and containers to the boiler with water of the specified temperature.
Type and nature of damage. tubercles different sizes: diameter from a few millimeters to several centimeters, rarely - several tens of centimeters. The tubercles are covered with dense iron oxides - a waste product of aerobic bacteria. Inside - black powder and suspension (iron sulfide FeS) - a product of sulfate-reducing anaerobic bacteria, under the black formation - round ulcers.
Causes of damage. Iron sulfates, oxygen and various bacteria are always present in natural water.
In the presence of oxygen, iron bacteria 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 gives rise to the formation of sulfurous (very unstable) and sulfuric acids, and the metal corrodes.
This type of corrosion has an indirect effect on the corrosion of the boiler: the flow of water 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 can occur in the boiler itself, if during a long shutdown of the boiler in the reserve it is filled with water at a temperature of 50-60 ° C, and the temperature is maintained due to accidental steam breakthroughs from neighboring boilers.
"Chelated" corrosion
Locations of corrosion damage. Equipment where steam is separated from water: boiler drum, steam separators in and out of the drum, also - rarely - in feed water piping 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 compounds of polyamines 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 “cheered up”: roughness, metal islands.
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 an overdose of "chelate", but even at a normal dose is possible, since "chelate" is concentrated in areas where there is an intensive evaporation of water: nucleate boiling is replaced by filmy. In steam separation devices, there are cases of especially destructive effect of "chelate" corrosion due to high turbulent velocities of water and steam-water mixture.
All described corrosion damage can have a synergistic effect, so that the total damage from the combined action of various corrosion factors can exceed the amount 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 excites thermal fatigue corrosion: the number of starts from a cold state is more than 100, total number launches - more than 200. Since these types of metal destruction are rare, cracks, pipe ruptures look identical to metal damage from various types of corrosion.
Usually, to identify the cause of metal destruction, additional metallographic studies are required: X-ray, ultrasound, color and magnetic-powder flaw detection.
Various researchers have proposed programs for diagnosing types of corrosion damage to boiler steels. The VTI program is known (A.F. Bogachev with employees) - mainly for power boilers high pressure, and developments of the Energochermet association - mainly for power boilers of low and medium pressure and waste heat boilers.
The iron-water vapor system is thermodynamically unstable. The interaction of these substances can proceed with the formation of magnetite Fe 3 O 4 or wustite FeO:
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;An analysis of reactions (2.1) - (2.3) indicates a peculiar decomposition of water vapor when interacting with a metal with the formation of molecular hydrogen, which is not a consequence of the actual thermal dissociation of water vapor. From equations (2.1) - (2.3) it follows that during the corrosion of steels in superheated steam in the absence of oxygen, only Fe 3 O 4 or FeO can form on the surface.
In the presence of oxygen in the superheated steam (for example, in neutral water regimes, with dosing of oxygen into the condensate), hematite Fe 2 O 3 may form in the superheated zone due to the additional oxidation of magnetite.
It is believed that corrosion in steam, starting from a temperature of 570 ° C, is chemical. At present, the limiting superheat temperature for all boilers has been reduced to 545 °C, and, consequently, electrochemical corrosion occurs in superheaters. The outlet sections of the primary superheaters are made of corrosion-resistant austenitic stainless steel, the outlet sections of the intermediate superheaters, which have the same final superheat temperature (545 °C), are made of pearlitic steels. Therefore, corrosion of intermediate superheaters usually manifests itself to a large extent.
As a result of the action of steam on steel, on its initially clean surface, gradually a so-called topotactic layer is formed, tightly bonded to the metal itself and therefore protecting it from corrosion. Over time, a second so-called epitactic layer grows on this layer. Both of these layers for steam temperatures up to 545 °C are magnetite, but their structure is not the same - the epitactic layer is coarse-grained and does not protect against corrosion.
Steam decomposition rate
mgN 2 /(cm 2 h)
Rice. 2.1. The dependence of the decomposition rate of superheated steam
on wall temperature
It is not possible to influence the corrosion of overheating surfaces by water regime methods. Therefore, the main task of the water-chemical regime of the superheaters proper is to systematically monitor the state of the metal of the superheaters in order to prevent the destruction of the topotactic layer. This can occur due to the ingress of individual impurities into the superheaters and the deposition in them, especially salts, which is possible, for example, as a result of a sharp increase in the level in the drum of high-pressure boilers. The salt deposits associated with this in the superheater can lead both to an increase in the wall temperature and to the destruction of the protective oxide topotactic film, which can be judged by a sharp increase in the rate of steam decomposition (Fig. 2.1).
3.3. Corrosion of the feed water path and condensate lines
A significant part of the corrosion damage to the equipment of thermal power plants falls on the feed water path, where the metal is in the most difficult conditions, the cause of which is the corrosive aggressiveness of the chemically treated water, condensate, distillate and their mixture in contact with it. At steam turbine power plants, the main source of feedwater contamination with copper compounds is ammonia corrosion of turbine condensers and low-pressure regenerative heaters, the pipe system of which is made of brass.
The feed water path of a steam turbine power plant can be divided into two main sections: before and after the thermal deaerator, and the flow conditions in their corrosion rates are sharply different. The elements of the first section of the feed water path, located before the deaerator, include pipelines, tanks, condensate pumps, condensate pipelines and other equipment. A characteristic feature of the corrosion of this part of the nutrient tract is the absence of the possibility of depletion of aggressive agents, i.e., carbonic acid and oxygen contained in the water. Due to the continuous inflow and movement of new portions of water along the tract, there is a constant replenishment of their loss. The continuous removal of part of the products of the reaction of iron with water and the influx of fresh portions of aggressive agents create favorable conditions for the intensive course of corrosion processes.
The source of oxygen in the turbine condensate is air suction in the tail section of the turbines and in the glands of the condensate pumps. Heating water containing O 2 and CO 2 in surface heaters located in the first section of the feed duct, up to 60–80 °С and above leads to serious corrosion damage to brass pipes. The latter become brittle, and often brass after several months of work acquires a spongy structure as a result of pronounced selective corrosion.
The elements of the second section of the feed water path - from the deaerator to the steam generator - include feed pumps and lines, regenerative heaters and economizers. The water temperature in this area as a result of sequential heating of water in regenerative heaters and water economizers approaches the boiler water temperature. The cause of corrosion of the equipment related to this part of the tract is mainly the effect on the metal of free carbon dioxide dissolved in the feed water, the source of which is additional chemically treated water. At an increased concentration of hydrogen ions (pH< 7,0), обусловленной наличием растворенной углекислоты и значительным подогревом воды, процесс коррозии на этом участке питательного тракта развивается преимущественно с выделением водорода. Коррозия имеет сравнительно равномерный характер.
In the presence of equipment made of brass (low pressure heaters, condensers), the enrichment of water with copper compounds through the steam condensate path proceeds in the presence of oxygen and free ammonia. The increase in the solubility of hydrated copper oxide occurs due to the formation of copper-ammonia complexes, such as Сu(NH 3) 4 (OH) 2 . These corrosion products of brass tube heaters low pressure begin to decompose in sections of the tract of high-pressure regenerative heaters (p.v.d.) with the formation of less soluble copper oxides, partially deposited on the surface of the p.v. tubes. e. Cuprous deposits on pipes a.e. contribute to their corrosion during operation and long-term parking of equipment without preservation.
With insufficiently deep thermal deaeration of feed water, pitting corrosion is observed mainly at the inlet sections of economizers, where oxygen is released due to a noticeable increase in feed water temperature, as well as in stagnant sections of the feed tract.
The heat-using equipment of steam consumers and pipelines, through which the production condensate is returned to the CHPP, are subject to corrosion under the action of oxygen and carbonic acid contained in it. The appearance of oxygen is explained by the contact of condensate with air in open tanks (at open circuit collection of condensate) and suction through leaks in the equipment.
The main measures to prevent corrosion of equipment located in the first section of the feed water path (from the water treatment plant to the thermal deaerator) are:
1) the use of protective anti-corrosion coatings on the surfaces of water treatment equipment and tank facilities, which are washed with solutions of acidic reagents or corrosive waters using rubber, epoxy resins, perchlorovinyl-based varnishes, liquid nayrite and silicone;
2) the use of acid-resistant pipes and fittings made of polymeric materials (polyethylene, polyisobutylene, polypropylene, etc.) or steel pipes and fittings lined inside with protective coatings applied by flame spraying;
3) the use of pipes of heat exchangers made of corrosion-resistant metals (red copper, stainless steel);
4) removal of free carbon dioxide from additional chemically treated water;
5) constant removal of non-condensable gases (oxygen and carbonic acid) from the steam chambers of low-pressure regenerative heaters, coolers and heaters of network water and rapid removal of the condensate formed in them;
6) careful sealing of glands of condensate pumps, fittings and flange connections of supply pipelines under vacuum;
7) ensuring sufficient tightness of turbine condensers from the side of cooling water and air and monitoring air suction with the help of recording oxygen meters;
8) equipping condensers with special degassing devices to remove oxygen from the condensate.
To successfully combat corrosion of equipment and pipelines located in the second section of the feedwater path (from thermal deaerators to steam generators), the following measures are taken:
1) equipping thermal power plants with thermal deaerators, which, under any operating conditions, produce deaerated water with a residual content of oxygen and carbon dioxide that does not exceed permissible standards;
2) maximum removal of non-condensable gases from the steam chambers of high-pressure regenerative heaters;
3) the use of corrosion-resistant metals for the manufacture of elements of feed pumps in contact with water;
4) anti-corrosion protection of nutrient and drainage tanks by applying non-metallic coatings that are resistant at temperatures up to 80–100 ° C, for example, asbovinyl (a mixture of lacquer ethinol with asbestos) or paints and varnishes based on epoxy resins;
5) selection of corrosion-resistant structural metals suitable for the manufacture of pipes for high-pressure regenerative heaters;
6) continuous treatment of feed water with alkaline reagents in order to maintain the specified optimal pH value of feed water, at which carbon dioxide corrosion is suppressed and sufficient strength of the protective film is ensured;
7) continuous treatment of feed water with hydrazine to bind residual oxygen after thermal deaerators and create an inhibitory effect of inhibition of the transfer of iron compounds from the equipment surface to feed water;
8) sealing the feed water tanks by organizing a so-called closed system to prevent oxygen from entering the economizers of the steam generators with the feed water;
9) implementation of reliable conservation of the equipment of the feedwater tract during its downtime in reserve.
An effective method for reducing the concentration of corrosion products in the condensate returned to the CHPP by steam consumers is the introduction of film-forming amines - octadecylamine or its substitutes into the selective steam of turbines sent to consumers. At a concentration of these substances in a vapor equal to 2–3 mg / dm 3 , it is possible to reduce the content of iron oxides in the production condensate by 10–15 times. The dosing of an aqueous emulsion of polyamines using a dosing pump does not depend on the concentration of carbonic acid in the condensate, since their action is not associated with neutralizing properties, but is based on the ability of these amines to form insoluble and water-resistant films on the surface of steel, brass and other metals.
A number of power plants use river and tap water with low pH and low hardness. Additional processing of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of corrosive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in acidification schemes used for large heat supply systems with direct hot water intake (2000–3000 t/h). Water softening according to the Na-cationization scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - hardness salts.
With poorly adjusted water deaeration and possible increases in oxygen and carbon dioxide concentrations, due to the lack of additional protective measures in heat supply systems, pipelines are subject to internal corrosion, heat exchangers, storage tanks and other equipment.
It is known that an increase in temperature contributes to the development of corrosion processes that occur both with the absorption of oxygen and with the release of hydrogen. With an increase in temperature above 40 ° C, oxygen and carbon dioxide forms of corrosion increase sharply.
A special type of under-sludge corrosion occurs under conditions of a low content of residual oxygen (when the PTE standards are met) and when the amount of iron oxides is more than 400 µg/dm 3 (in terms of Fe). This type of corrosion, previously known in the practice of operating steam boilers, was found under conditions of relatively weak heating and the absence of thermal loads. In this case, loose corrosion products, consisting mainly of hydrated trivalent iron oxides, are active depolarizers of the cathode process.
During the operation of heating equipment, crevice corrosion is often observed, i.e., selective, intense corrosion destruction of the metal in the crack (gap). A feature of the processes occurring in narrow gaps is the reduced oxygen concentration compared to the concentration in the bulk solution and the slow removal of corrosion reaction products. As a result of the accumulation of the latter and their hydrolysis, a decrease in the pH of the solution in the gap is possible.
With constant replenishment of the heating network with open water intake with deaerated water, the possibility of the formation of through holes in pipelines is completely excluded only in normal hydraulic mode, when excess pressure above atmospheric pressure is constantly maintained at all points of the heat supply system.
Causes of pitting corrosion of pipes of hot water boilers and other equipment are as follows: poor-quality deaeration of make-up water; low pH value due to the presence of aggressive carbon dioxide (up to 10–15 mg / dm 3); accumulation of oxygen corrosion products of iron (Fe 2 O 3) on heat transfer surfaces. The increased content of iron oxides in the network water contributes to the drift of the heating surfaces of the boiler with iron oxide deposits.
A number of researchers recognize an important role in the occurrence of under-sludge corrosion of the process of rusting of pipes of water-heating boilers during their downtime, when proper measures are not taken to prevent parking corrosion. The centers of corrosion that occur under the influence of atmospheric air on the wet surfaces of the boilers continue to function during the operation of the boilers.
MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR
MAIN SCIENTIFIC AND TECHNICAL DEPARTMENT OF ENERGY AND ELECTRIFICATION
METHODOLOGICAL INSTRUCTIONS
BY WARNING
LOW TEMPERATURE
SURFACE CORROSION
HEATING AND GAS FLUES 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. NADYROV
APPROVED by the Main Technical Directorate for the Operation of Power Systems on April 22, 1984.
Deputy Head D.Ya. SHAMARAKOV
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METHODOLOGICAL INSTRUCTIONS FOR THE PREVENTION OF LOW-TEMPERATURE CORROSION OF HEATING SURFACES AND GAS DUTS OF BOILERS |
RD 34.26.105-84 |
Expiry date set
from 01.07.85
until 01.07.2005
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 sour fuels and organizations designing boiler equipment.
1. Low-temperature corrosion is the corrosion of tail heating surfaces, gas ducts and chimneys of boilers under the action of sulfuric acid vapors condensing on them from flue gases.
2. Condensation of sulfuric acid vapors, the volume content of which in flue gases during the combustion of sulfurous fuels is only a few thousandths of a percent, occurs at temperatures that are significantly (by 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 flue gas dew point temperature at all boiler loads.
For heating surfaces cooled by a medium with a high heat transfer coefficient (economizers, gas evaporators, etc.), the temperatures of the medium at their inlet must exceed the dew point temperature by about 10 °C.
5. For the heating surfaces of hot water boilers when they are operated on sulphurous fuel oil, the conditions for the complete exclusion of low-temperature corrosion cannot be realized. To reduce it, it is necessary to ensure the temperature of the water at the inlet to the boiler, equal to 105 - 110 °C. When using hot water boilers as peak ones, such a mode can be provided with full use of network water heaters. When using hot water boilers in the main mode, an increase in 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 provided.
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 (RAH) air heaters is carried out according to the recommendations " Thermal calculation boiler units. Normative method” (M.: Energy, 1973).
8. When used in tubular air heaters as the first (by air) pass of replaceable cold cubes or cubes made of pipes with an acid-resistant coating (enamelled, etc.), as well as those made of corrosion-resistant materials, the following are checked for conditions for the 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 of replaceable, as well as corrosion-resistant cubes, should exclude intensive contamination of pipes, for which their minimum wall temperature during the combustion of sulfurous fuel oils should be below the dew point of flue gases by no more than 30 - 40 ° C. When burning solid sulfur fuels, the minimum temperature of the pipe wall, according to the conditions for preventing its intensive pollution, should be taken at least 80 °C.
9. In RAH, under conditions of complete exclusion of low-temperature corrosion, their hot part is calculated. The cold part of the RAH is made corrosion-resistant (enamelled, ceramic, low-alloy steel, etc.) or replaceable from flat metal sheets with a thickness of 1.0 - 1.2 mm, made of low-carbon steel. The conditions for preventing intense contamination of the packing are observed when fulfilling the requirements of clause of this document.
10. As an enameled packing, metal sheets with a thickness of 0.6 mm are used. The service life of enamelled packing, manufactured in accordance with TU 34-38-10336-89, is 4 years.
Porcelain tubes, ceramic blocks, or porcelain plates with protrusions can be used as ceramic packing.
Given the reduction in fuel oil consumption by thermal power plants, it is advisable to use for the cold part of the RAH a packing made of low-alloy steel 10KhNDP or 10KhSND, 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 start-up period, it is necessary to carry out the measures set forth in the “Guidelines for the design and operation of power heaters with wire fins” (M.: SPO Soyuztekhenergo, 1981).
Kindling of the boiler on sulphurous fuel oil should be carried out with the air heating system turned on beforehand. The temperature of the air in front of the air heater in the initial period of kindling should, as a rule, be 90 °C.
11a. To protect the air heaters from low-temperature ("station") corrosion on a stopped boiler, the level of which is approximately twice as high as the corrosion rate during operation, thoroughly clean the air heaters from external deposits before stopping the boiler. At the same time, before shutting down 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 (p. 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 sulphurous fuel oils, the flue gas temperature must exceed the dew point temperature at the rated load of the boiler by about 10 °C.
Depending on the sulfur content in the fuel oil, the calculated flue gas temperature at nominal boiler load should be taken as follows:
Flue gas temperature, ºС...... 140 150 160 165
When burning sulphurous fuel oil with extremely small excesses of air (α ≤ 1.02), the flue gas temperature can be taken lower, taking into account the results of dew point measurements. On average, the transition from small excesses of air to extremely small ones reduces the dew point temperature by 15–20 °C.
The conditions for ensuring reliable operation of the chimney and preventing moisture from falling on its walls are affected not only by the temperature of the flue gases, but also by their flow rate. The operation of the pipe with load conditions significantly lower than the design ones increases the likelihood of low-temperature corrosion.
When burning natural gas, the flue gas temperature is recommended to be at least 80 °C.
13. When the boiler load is reduced 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 temperature preheating air in the heaters as the load decreases.
Minimum allowed values air preheating temperature before the RAH is taken in accordance with clause 4.3.28 of the Rules for Technical Operation power stations and networks” (M.: Energoatomizdat, 1989).
In cases where optimal temperatures flue gases cannot be provided due to insufficient RAH heating surface, air preheating temperatures must be taken at which the flue gas temperature does not exceed the values given in clauses of these Guidelines.
16. Due to the lack of reliable acid-resistant coatings to protect against low-temperature corrosion of metal gas ducts, their reliable operation can be ensured by thorough insulation, ensuring the temperature difference between the flue gases and the wall is not more than 5 °C.
Currently applied insulating materials and structures are not sufficiently reliable in long-term operation, therefore it is necessary to periodically, at least once a year, monitor their condition and, if necessary, perform repair and restoration work.
17. When used on a trial basis to protect gas ducts from low-temperature corrosion various coatings it should be borne in mind that the latter should 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 with a concentration of 50 - 80% in the temperature range of 60 - 150 ° C, respectively, and the possibility of their repair and restoration .
18. For low-temperature surfaces, structural elements of the RAH and flues 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.
Absolute corrosion resistance is possessed only by very scarce and expensive high-alloy steels (for example, steel EI943, 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 vapor and water can be defined as the boiling point of a sulfuric acid solution of such a concentration at which the same content of water vapor and sulfuric acid is present above the solution.
The measured dew point temperature may differ from the theoretical value depending on the measurement technique. In these recommendations for flue gas dew point temperature tr The surface temperature 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 the steady state is 107 Ohm. The measuring circuit of the electrodes uses low voltage alternating current (6 - 12 V).
2. When burning sulphurous fuel oils with excess air of 3 - 5%, the flue gas dew point temperature depends on the sulfur content in the fuel Sp(rice.).
When burning sulphurous fuel oils with extremely low air excesses (α ≤ 1.02), the flue gas dew point temperature should be taken from the results special measurements. The conditions for transferring boilers to the mode with α ≤ 1.02 are set out in the “Guidelines for the transfer of boilers operating on sulfurous fuels to the combustion mode with extremely small excess air” (M.: SPO Soyuztekhenergo, 1980).
3. When burning sulphurous solid fuels in a pulverized state, the dew point temperature of flue gases tp can be calculated from the reduced content of sulfur and ash in the fuel Sppr, Arpr and water vapor condensation temperature tkon according to the formula
where aun- the proportion of ash in the fly away (usually taken 0.85).
Rice. 1. Dependence of flue gas dew point temperature on sulfur content in combusted fuel oil
The value of the first term of this formula at aun= 0.85 can be determined from Fig. .
Rice. 2. Differences in temperatures of the dew point of flue gases and condensation of water vapor in them, depending on the reduced sulfur content ( Sppr) and ash ( Arpr) in fuel
4. When burning gaseous sulphurous fuels, the flue gas dew point can be determined from fig. provided that the sulfur content in the gas is calculated as reduced, that is, as a percentage by mass per 4186.8 kJ / kg (1000 kcal / kg) of the calorific value of the gas.
For gaseous fuels, the reduced mass percent sulfur content can be determined from the formula
where m- the number of sulfur atoms in the molecule of the sulfur-containing component;
q- volume percentage of sulfur (sulphur-containing component);
Qn- calorific value of gas in kJ/m3 (kcal/nm3);
FROM- coefficient equal to 4.187 if Qn expressed in kJ/m3 and 1.0 if in kcal/m3.
5. The corrosion rate of the replaceable metal packing of air heaters during fuel oil combustion depends on the temperature of the metal and the degree of corrosivity of flue gases.
When burning sulphurous 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 RAH packing can be roughly estimated from the data in Table. .
Table 1
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Corrosion rate (mm/year) at wall temperature, ºС |
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0.5More than 2 0.20 |
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St. 0.11 to 0.4 incl. |
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Over 0.41 to 1.0 incl. |
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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.
