Corrosion in boilers. Steam boiler metal corrosion
Steam boiler accidents associated with violation of the water regime, corrosion and metal erosion
The normal water regime is one of the most important conditions for the reliability and efficiency of the operation of a boiler plant. The use of water with increased hardness to feed boilers entails the formation of scale, excessive fuel consumption and an increase in the cost of repair and cleaning of boilers. It is known that scale formation can lead to an accident in a steam boiler due to burnout of heating surfaces. Therefore, the correct water regime in the boiler house should be considered not only from the point of view of increasing the efficiency of the boiler plant, but also as the most important preventive measure to combat accidents.
Currently, boiler plants of industrial enterprises are equipped with water treatment devices, so their operating conditions have improved and the number of accidents caused by scale formation and corrosion has significantly decreased.
However, at some enterprises, the administration, having formally fulfilled the requirement of the Rules for Boiler Supervision to equip boilers with water treatment plants, does not ensure normal operating conditions for these plants, does not control the quality of feed water and the condition of boiler heating surfaces, allowing boilers to be contaminated with scale and sludge. Here are a few examples of boiler failures for these reasons.
1. In the boiler house of the plant of prefabricated reinforced concrete structures, due to violations of the water regime in the boiler DKVR-6, 5-13, three screen pipes ruptured, some of the screen pipes were deformed, and bulges formed on many pipes.
The boiler house has a two-stage sodium cation exchanger and deaerator, but the normal operation of the water treatment equipment has not been given due attention. The regeneration of cationite filters was not carried out within the time limits set by the instructions, the quality of the feed and boiler water was rarely checked, and the timing of the periodic blowdown of the boiler was not observed. The water in the deaerator was not heated to the required temperature and, therefore, deoxygenation of the water did not actually occur.
It was also established that raw water was often supplied to the boiler, while not complying with the requirements of the “Rules for the Construction and safe operation steam and hot water boilers”, according to which the shut-off devices on the raw water line must be sealed in the closed position, and each supply raw water must be recorded in the water treatment log. From individual entries in the water treatment journal, it can be seen that the hardness of the feed water reached 2 mg-eq / kg or more, while 0.02 mg-eq / kg is permissible according to boiler supervision standards. Most often, such entries were made in the journal: “water is dirty, hard”, without indicating the results chemical analysis water.
When examining the boiler after stopping, deposits up to 5 mm thick were found on the inner surfaces of the screen pipes, individual pipes were almost completely clogged with scale and sludge. On the inner surface of the drum in the lower part, the thickness of the deposits reached 3 mm, the front part of the drum was littered with sludge by one third in height.
For 11 months Prior to this accident, similar damages (“cracks, bulges, deformations”) were found in 13 boiler screen tubes. The defective pipes were replaced, but the administration of the enterprise, in violation of the “Instructions for investigating accidents, but resulting in accidents at enterprises and facilities controlled by the USSR Gosgortekhnadzor”, did not investigate this case and did not take measures to improve the operating conditions of the boilers.
2. On the power train, raw water for feeding a single-drum water-tube shielded steam boiler with a capacity of 10 t/h and an operating pressure of 41 kgf/cm2 was treated by the cation exchange method. Due to the unsatisfactory operation of the cationic filter, the residual hardness of the softened water reached
0.7 meq/kg instead of 0.01 meq/kg envisaged by the project. The boiler was purged irregularly. When stopping for repairs, the boiler drum and screen collectors were not opened and examined. Due to scale deposits, a pipe ruptured, while the steam and burning fuel thrown out of the furnace burned the stoker.
The accident could not have happened if the furnace door of the boiler had been closed with a latch, as required by the rules for the safe operation of boilers.
3. At the cement plant, a newly installed single-drum water-tube boiler with a capacity of 35 t/h with an operating pressure of 43 kgf/cm2 was put into operation without chemical water treatment, the installation of which had not been completed by that time. During the month, the boiler was fed with untreated water. Water deaeration was not carried out for more than two months, since a steam pipeline was not connected to the deaerator.
Violations of the water regime were allowed even after preparatory equipment was included in the work. The boiler was often fed with raw water; purge mode was not observed; the chemical laboratory did not control the quality of the feed water, as it was not supplied with the necessary reagents.
Due to the unsatisfactory water regime, deposits on the inner surfaces of the screen pipes reached a thickness of 8 mm; as a result, bulges formed on 36 screen pipes, a significant part of the pipes was deformed, the walls of the drum were corroded from the inside.
4. At the factory of reinforced concrete products, the boiler of the Shukhov-Berlin system was fed with electromagnetically treated water. It is known that with this method of water treatment, timely effective removal of sludge from the boiler should be ensured.
However, during the operation of the boiler, this condition was not met. The boiler was purged irregularly, the schedule for shutting down the boiler for flushing and cleaning was not observed.
As a result, a large amount of sludge accumulated inside the boiler. The rear part of the pipes was clogged with sludge by 70-80% of the section, the sump - by 70% of the volume, the scale thickness on the heating surfaces reached 4 mm. This led to overheating and deformation of the boiler tubes, pipe extensions and heads of tubular sections.
When choosing an electromagnetic method for processing iodine, in this case, the quality of the feed water was not taken into account and design features boiler, while measures were not taken to organize the normal blowdown mode, which led to the accumulation of sludge and significant scale deposits in the boiler.
5. The issues of organizing a rational water regime to ensure reliable and economical operation of boilers at thermal power plants have acquired exceptional importance.
The formation of deposits on the heating surfaces of boiler units occurs as a result of complex physical and chemical processes, in which not only scale formers are involved, but also metal oxides and easily soluble compounds. Dialysis of deposits shows that, along with scale-forming salts, they contain a significant amount of iron oxides, which are products of corrosion processes.
Over the past years, our country has achieved significant success in organizing a rational water regime for boilers of thermal power plants and chemical control of water and steam, as well as in the introduction of corrosion-resistant metals and protective coatings.
Application modern means water treatment has made it possible to dramatically increase the reliability and efficiency of operation of power equipment.
However, violations of the water regime are still allowed at individual thermal power plants.
In June 1976, for this reason, an accident occurred at the CHPP of the pulp and paper mill on a steam boiler of the BKZ-220-100 f type with a steam capacity of 220 t / h with steam parameters of 100 kgf / cm2 and 540 ° C, manufactured at the Barnaul boiler plant in 1964 d. Single-drum boiler with natural circulation, made according to the U-shaped scheme. The prismatic combustion chamber is completely shielded by pipes with an outer diameter of 60 mm, the pitch of which is 64 mm. The lower part of the screen surface forms a so-called cold funnel, along the slopes of which solid slag particles roll down into the slag chest. The scheme of evaporation is two-stage, washing the steam with feed water. The first stage of evaporation is included directly in the boiler drum, the second stage is provided by remote steam separation cyclones included in the circulation scheme of the middle side blocks of the screen.
The boiler is fed with a mixture of chemically purified water (60%) and condensate coming from turbines and production shops (40%). Boiler feed water is processed according to the following scheme: lime - coagulation - magnesia desiliconization in
Clarifiers - two-stage cationization.
The boiler operates on coal from the Inta deposit with a relatively low ash melting point. Oil is used as starting fuel. Before the accident, the boiler worked 73,300 hours.
On the day of the accident, the boiler was turned on at 00:45 and worked without deviation from the normal mode until 14:00. superheated steam -520-535 ° C.
At 2:10 p.m., 11 pipes of the front screen ruptured in the zone of the cold funnel at the level of 3.7 m with partial destruction
brickwork. It is assumed that at first there was a rupture of the water or two pipes, and then the rupture of the remaining pipes followed. The water level dropped sharply, and the boiler was stopped by automatic protection.
The inspection showed that the inclined sections of the pipes of the cold funnel outside the bends were destroyed, while two pipes were torn off from the first front lower collector, and nine pipes from the second. The rupture is brittle, the edges at the rupture points are blunt and do not have thinning. The length of the burst sections of pipes is from one to three meters. On the inner surface of damaged pipes, as well as samples cut from undamaged pipes, loose deposits up to 2.5 mm thick were found, as well as a large number of pits, up to 2 mm deep, located in a chain up to 10 mm wide along two generators along the heating boundary of the pipe. It was in the places of corrosion damage that the destruction of the metal occurred.
During the investigation of the accident, it turned out that earlier during the operation of the boiler there were already ruptures of screen pipes. So, for example, two months before the accident, a pipe of the front screen broke at the level of 6.0 m. After 3 days, the boiler was again stopped due to the rupture of two pipes of the front screen at the level of 7.0 m. And in these cases, the destruction of the pipes was the result of corrosion damage to the metal.
In accordance with the approved schedule, the boiler was to be shut down for overhaul in the third quarter of 1976. During the repair period, it was planned to replace the pipes of the front screen in the area of the cold funnel. However, the boiler was not stopped for repairs and the pipes were not replaced.
Corrosion damage to the metal was the result of violations of the water regime, which were allowed for a long time during the operation of the CHP boilers. The boilers were fed with water with a high content of iron, copper and oxygen. The total salt content in the feed water significantly exceeded allowable norms, as a result of which, even in the circuits of the first stage of evaporation, the salt content reached 800 mg/kg. Industrial condensates with an iron content of 400–600 mg/kg used to feed the boilers were not purified. For this reason, and also due to the fact that there was no sufficient anti-corrosion protection of water treatment equipment (protection was partially implemented), there were significant deposits (up to 1000 g/m2) on the inner surfaces of the pipes, mainly consisting of iron compounds. Amination and hydrazine treatment of feed water was introduced only shortly before the accident. Pre-start and operational acid washing of boilers was not carried out.
Other violations of the Rules contributed to the accident. technical operation boilers. Boilers are often kindled at CHPPs, and the largest number of kindlings was in the boiler with which the accident occurred. The boilers are equipped with devices for steam heating, but they were not used for kindling. During kindling, the displacements of screen collectors were not controlled.
To clarify the nature of the corrosion process and to determine the reasons for the formation of pits mainly in the first two panels of the front screen and the arrangement of these pits in the form of chains, the materials of the accident investigation were sent to the TsKTI. In reviewing these materials, attention was drawn to the fact that
the boilers operated with a sharply variable load, while a significant reduction in steam production (up to 90 t/h) was allowed, at which local circulation disturbance is possible. The boilers were kindled in the following way: at the beginning of the kindling, two nozzles located opposite (diagonally) were turned on. This method slowed down the process. natural circulation in the panels of the first and second front screens. It was in these screens that the main focus of ulcerative lesions was found. Nitrite episodically appeared in the feed water, the concentration of which was not controlled.
An analysis of the accident materials, taking into account the listed shortcomings, gave reason to believe that the formation of chains of pits on the side generatrix of the inner surfaces of the pipes of the front screen on the slope of the cold funnel is the result of a long process of electrochemical corrosion under sludge. The depolarizers of this process were nitrites and oxygen dissolved in water.
The arrangement of pits in the form of chains is, apparently, the result of the operation of the boiler during kindling with an unsteady process of natural circulation. During the beginning of circulation, pore bubbles periodically form on the upper generatrix of the inclined tubes of the cold funnel, causing the effect of local thermal pulsations in the metal by the occurrence of electrochemical processes in the region of the temporary phase separation. It was these places that were the centers of the formation of chains of pits. The predominant formation of pits in the first two panels of the front screen was the result of an incorrect kindling regime.
6. During the operation of the PK-YuSh-2 boiler with a steam capacity of 230 t/h and steam parameters of 100 kgf/cm2 and 540°C, steaming was noticed at the outlet from the fresh steam collection header to the main safety valve at the TYTs vb. The outlet is connected by welding to a cast tee welded into the prefabricated manifold.
The boiler has been shut down. During the inspection, an annular crack was found in the lower part of the pipe (168X13 mm) of the horizontal section of the branch in the immediate vicinity of the point of connection of the branch to the cast tee. The crack length on the outer surface is 70 mm and on the inner surface is 110 mm. On the inner surface of the pipe at the site of its damage, a large number of corrosion pits and individual cracks located parallel to the main one were revealed.
Metallographic analysis established that cracks start from pits in the decarburized metal layer and then develop transcrystalline in the direction perpendicular to the pipe surface. Pipe metal microstructure - ferrite grains and thin pearlite chains along the grain boundaries. According to the scale given as an appendix to MRTU 14-4-21-67, the microstructure can be assessed with a score of 8.
The chemical composition of the metal of the damaged pipe corresponds to steel 12Kh1MF. Mechanical properties meet the requirements specifications supplies. The diameter of the pipe in the damaged section does not go beyond the plus tolerance.
A horizontal branch to a safety valve with an unadjusted fastening system can be considered as a cantilever beam welded to a tee rigidly fixed in the manifold, with maximum bending stresses at the termination point, i.e., in the area where the pipe has been damaged. With absence
drainage in the outlet and the presence of a counter slope, due to the elastic bend in the section from the safety valve to the live steam collection manifold, in the lower part of the pipe in front of the tee, a small amount of condensate may constantly accumulate, enriched with oxygen during shutdowns, conservation and start-up of the boiler from the air. Under these conditions, corrosion attack of the metal occurred, and the combined effect of condensate and tensile stresses on the metal caused its corrosion cracking. During operation, in places of corrosion pits and shallow cracks, as a result of the aggressive action of the medium and variable stresses in the metal, fatigue-corrosion cracks can develop, which, apparently, happened in this case.
In order to prevent condensate from accumulating, a reverse circulation of steam was made in the outlet. To do this, the outlet pipe directly before the main safety valve was connected by a heating line (pipes with a diameter of 10 mm) to the intermediate chamber of the superheater, through which steam is supplied at a temperature of 430 ° C. With a small excess pressure drop (up to 4 kgf / cm2), continuous steam flow is ensured and the temperature of the medium in the outlet is maintained at least 400°C.
In order to prevent damage to the outlets to the main safety valves on boilers PK-YuSh-2 and similar, it is recommended:
Check with ultrasound the lower half-perimeters of the branch pipes at the points of welding to the tees;
Check whether the required slopes are observed and, if necessary, adjust the systems for fastening steam pipelines to the main safety valves, taking into account the actual condition of the steam pipelines (weight of insulation, actual weight of pipes, previous reconstructions);
Make reverse circulation of steam in the outlets to the main safety valves; the design and internal diameter of the heating steam line in each individual case must be agreed with the equipment manufacturer;
All dead ends on safety valves insulate carefully.
(From the express information of SCNTI ORGRES - 1975)
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, this mode can be provided with full use of network water heaters. When using hot water boilers in the main mode, an increase in the water temperature at the inlet to the boiler can be achieved using recirculation 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 provided in full.
6. For air heaters of steam boilers, the 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. The calculation of the wall temperature of tubular (TVP) and regenerative (RAH) air heaters is carried out according to the recommendations of the “Thermal calculation of 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 can be used as ceramic packing, ceramic blocks, or porcelain plates with ledges.
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 out 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 switched 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, before shutting down the boiler, it is necessary to thoroughly clean the air heaters from external deposits. 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 air preheating temperature in the heaters as the load decreases.
The minimum allowable temperatures for air preheating before RAH are taken 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 optimal temperatures flue gases cannot be provided due to insufficient RAH heating surface, the 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 for protection 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.
The currently used 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 using on a trial basis to protect gas ducts from low-temperature corrosion of various coatings, it should be taken into account that the latter must provide heat resistance and gas tightness at temperatures exceeding the flue gas temperature by at least 10 ° C, resistance to sulfuric acid concentrations 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, RAH structural elements and boiler flues, it is advisable to use low-alloy steels 10KhNDP and 10KhSND, which are 2–2.5 times superior to carbon steel in corrosion resistance.
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 solution of sulfuric acid 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 t p the surface temperature of a standard glass sensor with 7 mm long platinum electrodes soldered at a distance of 7 mm from one another, at which the resistance of the dew film between for electrodes in steady state is equal to 10 7 Ohm. The measuring circuit of the electrodes uses low voltage alternating current (6 - 12 V).
2. When burning sulfurous fuel oils with excess air of 3 - 5%, the dew point temperature of flue gases 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 of 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 S p pr, A r pr and water vapor condensation temperature t con according to the formula
Where a un- 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 a un= 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 ( S p pr) and ash ( A r pr) 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, i.e. 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);
Q n- heat of combustion of gas in kJ / m 3 (kcal / nm 3);
WITH- coefficient equal to 4.187 if Q n expressed in kJ/m 3 and 1.0 if in kcal/m 3 .
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 tentatively estimated from the data in Table. .
Table 1
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Table 2
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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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 water heating in regenerative heaters and water economizers approaches the boiler water temperature. The cause of corrosion of 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 the feed water, pitting corrosion is observed mainly at the inlet sections of the economizers, where oxygen is released due to a noticeable increase in the temperature of the feed water, 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 (with an open condensate collection scheme) and suction through leaks in the equipment.
The main measures to prevent corrosion of equipment located in the first section of the feedwater 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 feedwater tanks by organizing a so-called closed system to prevent oxygen from entering the economizers of the steam generators with the feedwater;
9) the 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 boiler houses use river and tap water with a low pH value and low hardness to feed heating networks. Additional processing 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 connection schemes used for large heat supply systems with direct hot water intake (2000 h 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 the heat supply systems, the thermal power equipment of the CHPP is susceptible to internal corrosion.
When examining the make-up duct of one of the CHPPs in Leningrad, the following data were obtained on the corrosion rate, g/(m2 4):
Place of installation of corrosion indicators
In the make-up water pipeline after the heating network heaters in front of the deaerators, pipes 7 mm thick thinned over the year of operation in places up to 1 mm in some sections through holes were formed.
The causes of pitting corrosion of pipes of hot water boilers are as follows:
insufficient removal of oxygen from make-up water;
low pH value due to the presence of aggressive carbon dioxide
(up to 10h15 mg/l);
accumulation of oxygen corrosion products of iron (Fe2O3;) on heat transfer surfaces.
The operation of equipment on network water with an iron concentration of more than 600 μg / l usually leads to the fact that for several thousand hours of operation of hot water boilers there is an intensive (over 1000 g / m2) drift of iron oxide deposits on their heating surfaces. At the same time, frequent leaks in the pipes of the convective part are noted. In the composition of deposits, the content of iron oxides usually reaches 80–90%.
Especially important for the operation of hot water boilers are start-up periods. During the initial period of operation, one CHPP did not ensure the removal of oxygen to the standards established by the PTE. The oxygen content in the make-up water exceeded these norms by 10 times.
The concentration of iron in the make-up water reached - 1000 µg/l, and in return water heating systems - 3500 mcg / l. After the first year of operation, cuttings were made from the network water pipelines, it turned out that the contamination of their surface with corrosion products was more than 2000 g/m2.
It should be noted that at this CHPP, before the boiler was put into operation, the inner surfaces of the screen tubes and tubes of the convective bundle were subjected to chemical cleaning. By the time of cutting out the wall tube samples, the boiler had operated for 5300 hours. The wall tube sample had an uneven layer of black-brown iron oxide deposits firmly bound to the metal; tubercles height 10x12 mm; specific contamination 2303 g/m2.
Deposit composition, %
The surface of the metal under the layer of deposits was affected by ulcers up to 1 mm deep. The tubes of the convective bundle from the inside were filled with deposits of the iron oxide type of black-brown color with a height of tubercles up to 3x4 mm. The surface of the metal under the deposits is covered with pits of various sizes with a depth of 0.3x1.2 and a diameter of 0.35x0.5 mm. Separate tubes had through holes (fistulas).
When hot water boilers are installed in old district heating systems in which a significant amount of iron oxides have accumulated, there are cases of deposits of these oxides in the heated pipes of the boiler. Before turning on the boilers, it is necessary to thoroughly flush the entire system.
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.
Corrosion in hot water boilers, heating systems, heating systems is much more common than in steam condensate systems. In most cases, this situation is explained by the fact that less attention is paid to this when designing a water heating system, 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, carbon dioxide entering boilers with feed water, cause different kinds corrosion - alkaline (intercrystalline), oxygen, chelate, sludge. It must be said that chelate corrosion in most cases is formed in the presence of certain chemical reagents, the so-called "complexons".
In order to prevent the occurrence of corrosion in hot water boilers and its subsequent development, it is necessary to take seriously and responsibly the preparation of the characteristics of the make-up water. It is necessary to ensure the binding of free carbon dioxide, oxygen, to bring the pH value to an acceptable level, to take measures to protect aluminum, bronze and copper elements of heating equipment and boilers, pipelines and heating equipment from corrosion.
IN Lately for high-quality correctional heating networks, hot water boilers and other equipment, special chemical reagents are used.
Water is at the same time universal solvent and an inexpensive coolant, it is advantageous to use it in heating systems. But insufficient preparation can lead to backfire, one of which is boiler corrosion. Possible risks are primarily associated with the presence of a large amount of undesirable impurities in it. It is possible to prevent the formation and development of corrosion, but only if you clearly understand the causes of its occurrence, and also be familiar with modern technologies.
For hot water boilers, however, as for any heating systems that use water as a coolant, three types of problems are characteristic due to the presence of the following impurities:
- mechanical insoluble;
- precipitate-forming dissolved;
- corrosive.
Each of these types of impurities can cause corrosion and failure of a hot water boiler or other equipment. In addition, they contribute to a decrease in the efficiency and productivity of the boiler.
And if you use water that has not undergone special preparation in heating systems for a long time, this can lead to serious consequences - a breakdown circulation pumps, reducing the diameter of the water supply and subsequent damage, failure of control and shutoff valves. The simplest mechanical impurities - clay, sand, ordinary dirt - are present almost everywhere, as in tap water, and in artesian sources. Also in coolants in large quantities there are corrosion products of heat transfer surfaces, pipelines and other metal elements of the system that are constantly in contact with water. Needless to say, their presence over time provokes very serious malfunctions in the functioning of hot water boilers and all heat and power equipment, which are mainly associated with corrosion of boilers, the formation of lime deposits, salt entrainment and foaming of boiler water.
Most common cause, which gives rise to boiler corrosion, these are carbonate deposits that occur when using water of increased hardness, the removal of which is possible through. It should be noted that as a result of the presence of hardness salts, scale is formed even in a low-temperature heating equipment. But this is not the only cause of corrosion. For example, after heating water to a temperature of more than 130 degrees, the solubility of calcium sulfate is significantly reduced, resulting in a layer of dense scale. In this case, the development of corrosion of metal surfaces of hot water boilers is inevitable.
