This corrosion is often more significant and dangerous in size and intensity than the corrosion of boilers during operation.
When water is left in systems, depending on its temperature and air access, a wide variety of cases of standstill corrosion can occur. First of all, it should be noted that it is extremely undesirable to have water in the pipes of the units when they are in reserve.
If water for one reason or another remains in the system, then severe static corrosion can be observed in the steam and especially in the water space of the tank (mainly along the waterline) at a water temperature of 60-70°C. Therefore, in practice, stop-time corrosion of varying intensity is often observed, despite the same shutdown modes of the system and the quality of the water contained in them; devices with significant thermal accumulation are subject to more severe corrosion than devices with a firebox size and heating surface, since the boiler water in them cools faster; its temperature becomes below 60-70°C.
At water temperatures above 85-90°C (for example, during short-term shutdowns of apparatus), overall corrosion decreases, and the corrosion of the metal of the steam space, in which increased condensation of vapors is observed in this case, may exceed the corrosion of the metal of the water space. Standstill corrosion in the steam space is in all cases more uniform than in the water space of the boiler.
The development of standstill corrosion is greatly facilitated by sludge accumulating on the surfaces of the boiler, which usually retains moisture. In this regard, significant corrosion pits are often found in units and pipes along the lower generatrix and at their ends, i.e., in areas of greatest accumulation of sludge.
Methods for preserving equipment in reserve
The following methods can be used to preserve equipment:
a) drying - removing water and moisture from aggregates;
b) filling them with solutions of caustic soda, phosphate, silicate, sodium nitrite, hydrazine;
c) filling technological system nitrogen.
The preservation method should be selected depending on the nature and duration of downtime, as well as the type and design features equipment.
Equipment downtime can be divided into two groups based on duration: short-term—no more than 3 days and long-term—more than 3 days.
There are two types of short-term downtime:
a) planned, related to being put into reserve on weekends due to a drop in load or put into reserve at night;
b) forced - due to failure of pipes or damage to other equipment components, the elimination of which does not require a longer shutdown.
Depending on the purpose, long-term downtime can be divided into the following groups: a) putting equipment into reserve; b) current repairs; c) major repairs.
During short-term equipment downtime, it is necessary to use preservation by filling with deaerated water and maintaining overpressure or gas (nitrogen) method. If emergency shutdown is necessary, nitrogen preservation is the only acceptable method.
When putting the system into reserve or for a long period of downtime without performing repair work, it is advisable to preserve it by filling it with a solution of nitrite or sodium silicate. In these cases, nitrogen conservation can also be used, making sure to take measures to create system density in order to prevent excessive gas consumption and unproductive operation of the nitrogen plant, as well as create safe conditions when servicing equipment.
Preservation methods by creating excess pressure and filling with nitrogen can be used regardless of the design features of the heating surfaces of the equipment.
To prevent parking corrosion of metal during major and current repairs Only conservation methods are applicable that make it possible to create on the metal surface protective film, retaining its properties for at least 1-2 months after draining the preservative solution, since emptying and depressurization of the system is inevitable. The validity period of the protective film on the metal surface after treating it with sodium nitrite can reach 3 months.
Preservation methods using water and reagent solutions are practically unacceptable for protecting boiler intermediate superheaters from standstill corrosion due to the difficulties associated with filling them and subsequent cleaning.
Methods for preserving hot water and steam boilers low pressure, as well as other equipment of closed technological circuits of heat and water supply, differ in many respects from the currently used methods for preventing stop-time corrosion at thermal power plants. Below we describe the main ways to prevent corrosion during equipment idle mode of devices such as circulation systems taking into account the specifics of their work.
Simplified preservation methods
It is advisable to use these methods for small boilers. They consist of completely removing water from the boilers and placing desiccant in them: calcined calcium chloride, quicklime, silica gel at the rate of 1-2 kg per 1 m 3 of volume.
This preservation method is suitable at room temperatures below and above zero. In rooms heated in winter time, one of the contact preservation methods can be implemented. It comes down to filling the entire internal volume of the unit with an alkaline solution (NaOH, Na 3 P0 4, etc.), ensuring complete stability of the protective film on the metal surface even when the liquid is saturated with oxygen.
Typically, solutions containing from 1.5-2 to 10 kg/m 3 NaOH or 5-20 kg/m 3 Na 3 P0 4 are used, depending on the content of neutral salts in the source water. Lower values apply to condensate, higher values apply to water containing up to 3000 mg/l of neutral salts.
Corrosion can also be prevented by the overpressure method, in which the steam pressure in the stopped unit is constantly maintained at a level above atmospheric pressure, and the water temperature remains above 100°C, which prevents the access of the main corrosive agent - oxygen.
An important condition for the effectiveness and efficiency of any method of protection is the maximum possible tightness of the steam-water fittings in order to avoid too rapid a decrease in pressure, loss of protective solution (or gas) or moisture ingress. In addition, in many cases it is useful pre-cleaning surfaces from various deposits(salts, sludge, scale).
When implementing various methods of protection against parking corrosion, the following must be kept in mind.
1. For all types of preservation, it is necessary to first remove (rinse) deposits of easily soluble salts (see above) in order to avoid increased parking corrosion in certain areas of the protected unit. It is mandatory to carry out this measure during contact conservation, otherwise intense local corrosion is possible.
2. For similar reasons, it is desirable to remove all types of insoluble deposits (sludge, scale, iron oxides) before long-term preservation.
3. If the valves are unreliable, it is necessary to disconnect the backup equipment from the operating units using plugs.
Leakage of steam and water is less dangerous with contact conservation, but is unacceptable with dry and gas protection methods.
The choice of desiccant is determined by the relative availability of the reagent and the desirability of obtaining the highest possible specific moisture capacity. The best desiccant is granular calcium chloride. Quicklime is much worse than calcium chloride, not only due to its lower moisture capacity, but also due to the rapid loss of its activity. Lime absorbs not only moisture from the air, but also carbon dioxide, as a result of which it becomes covered with a layer of calcium carbonate, which prevents further absorption of moisture.
MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR
MAIN SCIENTIFIC AND TECHNICAL DIRECTORATE OF ENERGY AND ELECTRIFICATION
METHODOLOGICAL INSTRUCTIONS
BY WARNING
LOW TEMPERATURE
SURFACE CORROSION
HEATING AND GAS FLOW OF BOILERS
RD 34.26.105-84
SOYUZTEKHENERGO
Moscow 1986
DEVELOPED by the All-Union Twice Order of the Red Banner of Labor Thermal Engineering Research Institute named after F.E. Dzerzhinsky
PERFORMERS R.A. PETROSYAN, I.I. NADIROV
APPROVED BY THE CHIEF technical management on operation of power systems 04/22/84
Deputy Chief D.Ya. SHAMARAKOV
|
METHODOLOGICAL INSTRUCTIONS FOR PREVENTION OF LOW TEMPERATURE CORROSION OF HEATING SURFACES AND GAS FLUES OF BOILERS |
RD 34.26.105-84 |
Expiration date set
from 07/01/85
until 07/01/2005
These Guidelines apply to low-temperature heating surfaces of steam and hot water boilers (economizers, gas evaporators, air heaters of various types, etc.), as well as to the gas path behind the air heaters (gas ducts, ash collectors, smoke exhausters, chimneys) and establish methods for protecting surfaces heating from low temperature corrosion.
The guidelines are intended for thermal power plants operating on sulfur fuels and organizations designing boiler equipment.
1. Low-temperature corrosion is the corrosion of the tail heating surfaces, gas ducts and chimneys of boilers under the influence of condensing on them flue gases sulfuric acid vapor.
2. Condensation of sulfuric acid vapor, the volumetric content of which in flue gases when burning sulfurous fuels is only a few thousandths of a percent, occurs at temperatures significantly (50 - 100 °C) higher than the condensation temperature of water vapor.
4. To prevent corrosion of heating surfaces during operation, the temperature of their walls must exceed the dew point temperature of the flue gases at all boiler loads.
For heating surfaces cooled by a medium with a high heat transfer coefficient (economizers, gas evaporators, etc.), the temperature of the medium at their inlet should exceed the dew point temperature by approximately 10 °C.
5. For the heating surfaces of hot water boilers when operating on sulfur fuel oil, the conditions for completely eliminating low-temperature corrosion cannot be realized. To reduce it, it is necessary to ensure that the water temperature at the boiler inlet is 105 - 110 °C. When using water heating boilers as peak boilers, this mode can be ensured with full use of network water heaters. When using hot water boilers in the main mode, increasing the temperature of the water entering the boiler can be achieved by recirculating hot water.
In installations using the scheme for connecting hot water boilers to the heating network through water heat exchangers, the conditions for reducing low-temperature corrosion of heating surfaces are fully ensured.
6. For air heaters of steam boilers, complete exclusion of low-temperature corrosion is ensured when the design temperature of the wall of the coldest section exceeds the dew point temperature at all boiler loads by 5 - 10 °C (the minimum value refers to the minimum load).
7. Calculation of the wall temperature of tubular (TVP) and regenerative (RVP) air heaters is carried out according to the recommendations of “Thermal calculation of boiler units. Normative method" (Moscow: Energy, 1973).
8. When using replaceable cold cubes or cubes made from pipes with an acid-resistant coating (enameled, etc.), as well as those made from corrosion-resistant materials, as the first (air) stroke in tubular air heaters, the following are checked for the conditions of complete exclusion of low-temperature corrosion (by air) metal cubes of the air heater. In this case, the choice of the wall temperature of cold metal cubes, replaceable, as well as corrosion-resistant cubes, should exclude intense contamination of the pipes, for which their minimum wall temperature when burning sulfur fuel oils should be below the dew point of the flue gases by no more than 30 - 40 ° C. When burning solid sulfur fuels minimum temperature In order to prevent intensive contamination of the pipe wall, the temperature must be at least 80 °C.
9. In RVP, under the conditions of complete exclusion of low-temperature corrosion, their hot part is calculated. The cold part of the RVP is corrosion-resistant (enamelled, ceramic, low-alloy steel, etc.) or replaceable from flat metal sheets 1.0 - 1.2 mm thick, made of low-carbon steel. The conditions for preventing intense contamination of the packing are met when the requirements of paragraphs of this document are met.
10. The enameled packing is made from metal sheets with a thickness of 0.6 mm. The service life of enamel packing manufactured in accordance with TU 34-38-10336-89 is 4 years.
Porcelain tubes can be used as ceramic filling, ceramic blocks, or porcelain plates with projections.
Considering the reduction in fuel oil consumption by thermal power plants, it is advisable to use packing made of low-alloy steel 10KhNDP or 10KhSND for the cold part of the RVP, the corrosion resistance of which is 2 - 2.5 times higher than that of low-carbon steel.
11. To protect air heaters from low-temperature corrosion during the startup period, the measures set out in the “Guidelines for the design and operation of energy heaters with wire fins” (M.: SPO Soyuztekhenergo, 1981) should be carried out.
Ignition of a boiler using sulfur fuel oil should be carried out with the air heating system previously turned on. The air temperature in front of the air heater during the initial period of kindling should be, as a rule, 90 °C.
11a. To protect air heaters from low-temperature (“parking”) corrosion when the boiler is stopped, the level of which is approximately twice the corrosion rate during operation, before stopping the boiler, the air heaters should be thoroughly cleaned of external deposits. In this case, before stopping the boiler, it is recommended to maintain the air temperature at the inlet to the air heater at the level of its value at the rated load of the boiler.
Cleaning of TVP is carried out with shot with a feed density of at least 0.4 kg/m.s (clause of this document).
For solid fuels, taking into account the significant risk of corrosion of ash collectors, the temperature of the flue gases should be selected above the dew point of the flue gases by 15 - 20 °C.
For sulfur fuel oils, the temperature of the flue gases should exceed the dew point temperature at the rated boiler load by approximately 10 °C.
Depending on the sulfur content in the fuel oil, the calculated value of the flue gas temperature at the rated boiler load, indicated below, should be taken:
Flue gas temperature, ºС...... 140 150 160 165
When burning sulfur fuel oil with extremely low excess air (α ≤ 1.02), the temperature of the flue gases can be taken lower, taking into account the results of dew point measurements. On average, the transition from small to extremely small excess air reduces the dew point temperature by 15 - 20 °C.
The conditions for ensuring reliable operation of the chimney and preventing moisture loss on its walls are affected not only by the temperature of the flue gases, but also by their flow rate. Operating a pipe under load conditions significantly lower than design increases the likelihood of low-temperature corrosion.
When burning natural gas, it is recommended that the flue gas temperature be at least 80 °C.
13. When reducing the boiler load in the range of 100 - 50% of the nominal one, one should strive to stabilize the flue gas temperature, not allowing it to decrease by more than 10 °C from the nominal one.
The most economical way to stabilize the flue gas temperature is to increase the air preheating temperature in the air heaters as the load decreases.
The minimum permissible values of air preheating temperatures before the RAH are adopted in accordance with clause 4.3.28 of the “Rules for the technical operation of power plants and networks” (M.: Energoatomizdat, 1989).
In cases where optimal temperatures flue gases cannot be provided due to the insufficient heating surface of the RAH, air preheating temperatures must be adopted at which the temperature of the flue gases will not exceed the values given in paragraph of these Guidelines.
16. Due to the lack of reliable acid-resistant coatings to protect metal flues from low-temperature corrosion, their reliable operation can be ensured by careful insulation, ensuring a temperature difference between the flue gases and the wall of no more than 5 °C.
The insulating materials and structures currently used are not reliable enough for long-term operation, so it is necessary to periodically, at least once a year, monitor their condition and, if necessary, carry out repair and restoration work.
17. When using various coatings on a trial basis to protect gas ducts from low-temperature corrosion, it should be taken into account that the latter must provide heat resistance and gas tightness at temperatures exceeding the temperature of flue gases by at least 10 ° C, resistance to sulfuric acid concentrations of 50 - 80% in the temperature range, respectively, 60 - 150 ° C and the possibility of their repair and restoration.
18. For low-temperature surfaces, structural elements of RVP and gas ducts of boilers, it is advisable to use low-alloy steels 10KhNDP and 10KhSND, which are 2 - 2.5 times superior in corrosion resistance to carbon steel.
Only very scarce and expensive high-alloy steels have absolute corrosion resistance (for example, EI943 steel, containing up to 25% chromium and up to 30% nickel).
Application
1. Theoretically, the dew point temperature of flue gases with a given content of sulfuric acid and water vapor can be defined as the boiling point of a solution of sulfuric acid of such a concentration at which the same content of water vapor and sulfuric acid exists above the solution.
The measured value of the dew point temperature, depending on the measurement technique, may not coincide with the theoretical one. In these recommendations for the flue gas dew point temperature t r The temperature of the surface of a standard glass sensor with platinum electrodes 7 mm long, soldered at a distance of 7 mm from one another, at which the resistance of the dew film between y electrodes in steady state is equal to 10 7 Ohm. The electrode measuring circuit uses low voltage alternating current (6 - 12 V).
2. When burning sulfur fuel oils with excess air of 3 - 5%, the dew point temperature of the flue gases depends on the sulfur content in the fuel S p(rice.).
When burning sulfur fuel oils with extremely low excess air (α ≤ 1.02), the flue gas dew point temperature should be taken based on the results of special measurements. The conditions for transferring boilers to a mode with α ≤ 1.02 are set out in the “Guidelines for transferring boilers operating on sulfur fuels to a combustion mode with extremely low excess air” (M.: SPO Soyuztekhenergo, 1980).
3. When burning sulfurous solid fuels in a dusty state, the dew point temperature of the flue gases t p can be calculated based on the given content of sulfur and ash in the fuel S r pr, A r pr and water vapor condensation temperature t con according to the formula
Where a un- the proportion of ash in the carryover (usually taken to be 0.85).
Rice. 1. Dependence of flue gas dew point temperature on sulfur content in burned fuel oil
The value of the first term of this formula at a un= 0.85 can be determined from Fig. .
Rice. 2. Temperature differences between the dew point of flue gases and the condensation of water vapor in them, depending on the given sulfur content ( S r pr) and ash ( A r pr) in fuel
4. When burning gaseous sulfur fuels, the dew point of the flue gases can be determined from Fig. provided that the sulfur content in the gas is calculated as given, that is, as a percentage by weight per 4186.8 kJ/kg (1000 kcal/kg) of the calorific value of the gas.
For gas fuel the given sulfur content as a percentage by mass can be determined by the formula
Where m- the number of sulfur atoms in the molecule of the sulfur-containing component;
q- volume percentage of sulfur (sulfur-containing component);
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 rate of corrosion of the replaceable metal packing of air heaters when burning fuel oil depends on the temperature of the metal and the degree of corrosiveness of the flue gases.
When burning sulfur fuel oil with an excess of air of 3 - 5% and blowing the surface with steam, the corrosion rate (on both sides in mm/year) of the RVP packing can be approximately estimated from the data in Table. .
Table 1
|
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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Corrosion phenomena in boilers most often appear on the internal heat-stressed surface and relatively less often on the external surface.
In the latter case, the destruction of the metal is caused - in most cases - by the combined action of corrosion and erosion, which sometimes has a predominant significance.
An external sign of erosion destruction is a clean metal surface. When exposed to corrosion, corrosion products usually remain on its surface.
Internal (in an aquatic environment) corrosion and scale processes can aggravate external corrosion (in a gaseous environment) due to the thermal resistance of the layer of scale and corrosion deposits, and, consequently, an increase in temperature on the metal surface.
External metal corrosion (from the side of the boiler furnace) depends on various factors, but, first of all, on the type and composition of the fuel burned.
Corrosion of gas-oil boilers
Fuel oil contains organic compounds of vanadium and sodium. If molten deposits of slag containing vanadium (V) compounds accumulate on the wall of the pipe facing the furnace, then with a large excess of air and/or a metal surface temperature of 520-880 oC, the following reactions occur:
4Fe + 3V2O5 = 2Fe2O3 + 3V2O3 (1)
V2O3 + O2 = V2O5 (2)
Fe2O3 + V2O5 = 2FeVO4 (3)
7Fe + 8FeVO4 = 5Fe3O4 + 4V2O3 (4)
(Sodium compounds) + O2 = Na2O (5)
Another corrosion mechanism involving vanadium (liquid eutectic mixture) is also possible:
2Na2O. V2O4. 5V2O5 + O2 = 2Na2O. 6V2O5 (6)
Na2O. 6V2O5 + M = Na2O. V2O4. 5V2O5 + MO (7)
(M - metal)
Vanadium and sodium compounds are oxidized to V2O5 and Na2O during fuel combustion. In deposits that adhere to the metal surface, Na2O is a binder. The liquid formed as a result of reactions (1)-(7) melts the protective film of magnetite (Fe3O4), which leads to oxidation of the metal under the deposits (melting temperature of deposits (slag) - 590-880 oC).
As a result of these processes, the walls of the screen pipes facing the firebox become evenly thinner.
The increase in metal temperature, at which vanadium compounds become liquid, is promoted by internal scale deposits in pipes. And thus, when the temperature of the metal’s yield point is reached, a pipe rupture occurs - a consequence of the combined action of external and internal deposits.
The fastening parts of the pipe screens, as well as the protrusions of the weld seams of the pipes, also corrode - the temperature rise on their surface accelerates: they are not cooled by the steam-water mixture, like pipes.
Fuel oil may contain sulfur (2.0-3.5%) in the form organic compounds, elemental sulfur, sodium sulfate (Na2SO4) entering oil from formation waters. On the metal surface under such conditions, vanadium corrosion is accompanied by sulfide-oxide corrosion. Their combined effect is most pronounced when 87% V2O5 and 13% Na2SO4 are present in the sediments, which corresponds to the content of vanadium and sodium in fuel oil in a ratio of 13/1.
In winter, when heating fuel oil with steam in containers (to facilitate draining), water in the amount of 0.5-5.0% additionally enters it. Consequence: the amount of deposits on the low-temperature surfaces of the boiler increases, and, obviously, the corrosion of fuel oil lines and fuel oil tanks increases.
In addition to the above-described scheme of destruction of boiler screen pipes, corrosion of steam superheaters, festoon pipes, boiler bundles, economizers has some peculiarities due to increased - in some sections - gas velocities, especially those containing unburned fuel oil particles and exfoliated slag particles.
Corrosion identification
The outer surface of the pipes is covered with a dense enamel-like layer of gray and dark gray deposits. On the side facing the firebox, there is a thinning of the pipe: flat areas and shallow cracks in the form of “scores” are clearly visible if the surface is cleaned of deposits and oxide films.
If the pipe is accidentally destroyed, then a through longitudinal narrow crack is visible.
Corrosion of pulverized coal boilers
In corrosion caused by the action of coal combustion products, sulfur and its compounds are of decisive importance. In addition, the course of corrosion processes is affected by chlorides (mainly NaCl) and alkali metal compounds. Corrosion is most likely when coal contains more than 3.5% sulfur and 0.25% chlorine.
Fly ash, containing alkaline compounds and sulfur oxides, is deposited on the metal surface at a temperature of 560-730 oC. In this case, as a result of the reactions that occur, alkali sulfates are formed, for example K3Fe(SO4)3 and Na3Fe(SO4)3. This molten slag, in turn, destroys (melts) the protective oxide layer on the metal - magnetite (Fe3O4).
The corrosion rate is maximum at a metal temperature of 680-730 °C; as it increases, the rate decreases due to the thermal decomposition of corrosive substances.
The greatest corrosion occurs in the outlet pipes of the superheater, where the steam temperature is highest.
Corrosion identification
On screen pipes, you can observe flat areas on both sides of the pipe that are subject to corrosion damage. These areas are located at an angle of 30-45°C to each other and are covered with a layer of sediment. Between them is a relatively “clean” area exposed to the “frontal” influence of the gas flow.
The deposits consist of three layers: the outer layer is porous fly ash, the intermediate layer is whitish water-soluble alkaline sulfates, inner layer- shiny black iron oxides (Fe3O4) and sulfides (FeS).
On low-temperature parts of boilers - economizer, air heater, exhaust fan - the metal temperature drops below the “dew point” of sulfuric acid.
When burning solid fuel the gas temperature decreases from 1650 °C in the flare to 120 °C or less in the chimney.
Due to the cooling of the gases, sulfuric acid is formed in the vapor phase, and upon contact with a colder metal surface, the vapors condense to form liquid sulfuric acid. The “dew point” of sulfuric acid is 115-170 °C (it can be more - it depends on the content of water vapor and sulfur oxide (SO3) in the gas flow).
The process is described by the reactions:
S + O2 = SO2 (8)
SO3 + H2O = H2SO4 (9)
H2SO4 + Fe = FeSO4 + H2 (10)
In the presence of iron and vanadium oxides, catalytic oxidation of SO3 is possible:
2SO2 + O2 = 2SO3 (11)
In some cases, sulfuric acid corrosion when burning coal is less significant than when burning brown coal, shale, peat and even natural gas - due to the relatively greater release of water vapor from them.
Corrosion identification
This type of corrosion causes uniform destruction of the metal. Typically the surface is rough, with a slight coating of rust, and is similar to a non-corrosive surface. With prolonged exposure, the metal may become covered with deposits of corrosion products, which must be carefully removed during inspection.
Corrosion during breaks in operation
This type of corrosion occurs on the economizer and in those areas of the boiler where the outer surfaces are coated with sulfur compounds. As the boiler cools, the metal temperature drops below the “dew point” and, as described above, if there are sulfur deposits, sulfuric acid is formed. A possible intermediate compound is sulfurous acid (H2SO3), but it is very unstable and immediately turns into sulfuric acid.
Corrosion identification
Metal surfaces are usually coated with coatings. If you remove them, you will find areas of metal destruction where there were sulfur deposits and areas of uncorroded metal. This appearance distinguishes corrosion on a stopped boiler from the above-described corrosion of the economizer metal and other “cold” parts of a running boiler.
When washing the boiler, corrosion phenomena are distributed more or less evenly over the metal surface due to the erosion of sulfur deposits and insufficient drying of surfaces. With insufficient cleaning, corrosion is localized where sulfur compounds were.
Metal erosion
Under certain conditions, various boiler systems are subject to erosive destruction of metal, both from the inside and outside of the heated metal, and where turbulent flows occur at high speed.
Only turbine erosion is discussed below.
Turbines are subject to erosion from impacts from solid particles and steam condensate droplets. Solid particles (oxides) flake off the internal surface of superheaters and steam lines, especially during thermal transient conditions.
Droplets of steam condensate mainly destroy the surfaces of the blades of the last stage of the turbine and drainage pipelines. Erosion-corrosive effects of steam condensate are possible if the condensate is “acidic” - the pH is below five units. Corrosion is also dangerous in the presence of chloride vapor (up to 12% of the mass of deposits) and caustic soda in water droplets.
Erosion identification
Metal destruction from impacts of condensate drops is most noticeable at the leading edges of turbine blades. The edges are covered with thin transverse teeth and grooves (grooves); there may be inclined conical projections directed towards the impacts. There are protrusions on the leading edges of the blades and are almost absent on their posterior planes.
Damage from solid particles takes the form of tears, microdents and nicks on the leading edges of the blades. There are no grooves or inclined cones.
Owners of patent RU 2503747:
TECHNICAL FIELD
The invention relates to heat power engineering and can be used to protect heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems from scale residential buildings And industrial facilities during current operation.
BACKGROUND OF THE ART
The operation of steam boilers is associated with simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the boiler heating surfaces are separate phases of a complex system that is formed upon their contact. The result of the interaction of these phases is surface processes that occur at their interface. As a result of this, corrosion and scale formation occur in the metal of the heating surfaces, which leads to a change in the structure and mechanical properties of the metal, and which contributes to the development of various damages. Since the thermal conductivity of scale is fifty times lower than that of iron heating pipes, there are losses of thermal energy during heat transfer - with a scale thickness of 1 mm from 7 to 12%, and with 3 mm - 25%. Severe scale formation in a continuous steam boiler system often causes production to be shut down for several days each year to remove the scale.
The quality of feed water and, therefore, boiler water is determined by the presence of impurities that can cause various types of corrosion of the metal of internal heating surfaces, the formation of primary scale on them, as well as sludge as a source of secondary scale formation. In addition, the quality of boiler water also depends on the properties of substances formed as a result of surface phenomena during water transportation and condensate through pipelines during water treatment processes. Removing impurities from feed water is one of the ways to prevent the formation of scale and corrosion and is carried out by methods of preliminary (pre-boiler) water treatment, which are aimed at maximizing the removal of impurities found in the source water. However, the methods used do not allow us to completely eliminate the content of impurities in water, which is associated not only with technical difficulties, but also economic feasibility application of pre-boiler water treatment methods. In addition, since water treatment is complex technical system, it is redundant for boilers of low and medium productivity.
Known methods for removing already formed deposits use mainly mechanical and chemical methods cleaning. The disadvantage of these methods is that they cannot be produced during the operation of the boilers. In addition, chemical cleaning methods often require the use of expensive chemicals.
There are also known methods to prevent the formation of scale and corrosion, carried out during the operation of boilers.
US patent 1877389 proposes a method for removing scale and preventing its formation in hot water and steam boilers. In this method, the surface of the boiler is the cathode, and the anode is placed inside the pipeline. The method consists of passing a constant or alternating current through the system. The authors note that the mechanism of action of the method is that under the influence electric current Gas bubbles form on the surface of the boiler, which lead to the detachment of existing scale and prevent the formation of a new one. The disadvantage of this method is the need to constantly maintain the flow of electric current in the system.
US Pat. No. 5,667,677 proposes a method for treating a liquid, particularly water, in a pipeline to slow down the formation of scale. This method is based on the creation of an electromagnetic field in pipes, which repels calcium and magnesium ions dissolved in water from the walls of pipes and equipment, preventing them from crystallizing in the form of scale, which allows the operation of boilers, boilers, heat exchangers, and cooling systems on hard water. The disadvantage of this method is the high cost and complexity of the equipment used.
Application WO 2004016833 proposes a method for reducing the formation of scale on a metal surface exposed to a supersaturated alkaline aqueous solution which is capable of forming scale after a period of exposure, comprising applying a cathodic potential to said surface.
This method can be used in various technological processes, in which the metal is in contact with an aqueous solution, in particular in heat exchangers. The disadvantage of this method is that it does not protect the metal surface from corrosion after removing the cathodic potential.
Thus, there is currently a need to develop an improved method for preventing scale formation of heating pipes, hot water boilers and steam boilers, which would be economical and highly effective and provide anti-corrosion protection to the surface for a long period of time after exposure.
In the present invention, this problem is solved using a method according to which a current-carrying electric potential is created on a metal surface, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.
BRIEF DESCRIPTION OF THE INVENTION
An object of the present invention is to provide an improved method for preventing the formation of scale in heating pipes of hot water and steam boilers.
Another objective of the present invention is to provide the possibility of eliminating or significantly reducing the need for descaling during operation of hot water and steam boilers.
Another objective of the present invention is to eliminate the need to use consumable reagents to prevent the formation of scale and corrosion of heating pipes of water heating and steam boilers.
Another object of the present invention is to enable work to begin to prevent the formation of scale and corrosion of heating pipes of hot water and steam boilers on contaminated boiler pipes.
The present invention relates to a method for preventing the formation of scale and corrosion on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale is capable of forming. This method consists in applying to the specified metal surface a current-carrying electric potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.
According to some private embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some private embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the interior surface of the heating tubes of a hot water or steam boiler.
Revealed in this description The method has the following advantages. One advantage of the method is reduced scale formation. Another advantage of the present invention is the ability to use a working electrophysical apparatus once purchased without the need to use consumable synthetic reagents. Another advantage is the possibility of starting work on dirty boiler tubes.
The technical result of the present invention, therefore, is to increase the operating efficiency of hot water and steam boilers, increase productivity, increase heat transfer efficiency, reduce fuel consumption for heating the boiler, save energy, etc.
Other technical results and advantages of the present invention include providing the possibility of layer-by-layer destruction and removal of already formed scale, as well as preventing its new formation.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows the distribution of deposits on the internal surfaces of the boiler as a result of applying the method according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The method of the present invention involves applying to a metal surface subject to scale formation a current-carrying electrical potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and scale-forming ions to the metal surface.
The term "conducting electrical potential" as used in this application means an alternating potential that neutralizes the electrical double layer at the interface of the metal and the steam-water medium containing salts that lead to scale formation.
As is known to a person skilled in the art, the carriers of electric charge in a metal, slow compared to the main charge carriers - electrons, are dislocations of its crystal structure, which carry an electric charge and form dislocation currents. Coming to the surface of the heating pipes of the boiler, these currents become part of the double electrical layer during the formation of scale. The current-carrying, electrical, pulsating (i.e., alternating) potential initiates the removal of the electrical charge of dislocations from the metal surface to the ground. In this respect, it is a conductor of dislocation currents. As a result of the action of this current-carrying electrical potential, the double electrical layer is destroyed, and the scale gradually disintegrates and passes into the boiler water in the form of sludge, which is removed from the boiler during periodic purging.
Thus, the term “current-carrying potential” is understandable to a person skilled in the art and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).
As a device for creating a current-carrying electrical potential, for example, a device described in RU 2100492 C1 can be used, which includes a converter with a frequency converter and a pulsating potential regulator, as well as a pulse shape regulator. Detailed description of this device is given in RU 2100492 C1. Any other similar device may also be used, as will be appreciated by one skilled in the art.
The conductive electrical potential according to the present invention can be applied to any part of the metal surface remote from the base of the boiler. The place of application is determined by the convenience and/or effectiveness of using the claimed method. One skilled in the art, using the information disclosed herein and using standard testing techniques, will be able to determine the optimal location for application of the current-sinking electrical potential.
In some embodiments of the present invention, the current-sinking electrical potential is variable.
The current-sinking electric potential according to the present invention can be applied for various periods of time. The time of application of the potential is determined by the nature and degree of contamination of the metal surface, the composition of the water used, the temperature regime and the operating characteristics of the heating device and other factors known to specialists in this field of technology. One skilled in the art, using the information disclosed herein and using standard testing techniques, will be able to determine optimal time application of current-carrying electrical potential, based on the goals, conditions and state of the heating device.
The magnitude of the current-carrying potential required to neutralize the electrostatic component of the adhesion force can be determined by a specialist in the field of colloid chemistry based on information known from the prior art, for example from the book B.V. Deryagin, N.V. Churaev, V.M. Muller. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the magnitude of the current-carrying electrical potential is in the range from 10 V to 200 V, more preferably from 60 V to 150 V, even more preferably from 61 V to 150 V. Values of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the double electrical layer, which is the basis of the electrostatic component of the adhesion forces in scale and, as a consequence, destruction of scale. Values of the current-carrying potential below 61 V are insufficient to destroy scale, and at values of the current-carrying potential above 150 V, unwanted electrical erosion destruction of the metal of the heating tubes is likely to begin.
The metal surface to which the method according to the present invention can be applied can be part of the following thermal devices: heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems of residential buildings and industrial facilities during ongoing operation. This list is illustrative and does not limit the list of devices to which the method according to the present invention can be applied.
In some embodiments, the iron-containing alloy from which the metal surface is made to which the method of the present invention can be applied may be steel or other iron-containing material such as cast iron, kovar, fechral, transformer steel, alsifer, magneto, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron-containing alloys to which the method according to the present invention can be applied. One skilled in the art, based on knowledge known in the art, will be able to identify such iron-containing alloys that can be used according to the present invention.
The aqueous environment from which scale is capable of forming, according to some embodiments of the present invention, is tap water. The aqueous medium may also be water containing dissolved metal compounds. The dissolved metal compounds may be iron and/or alkaline earth metal compounds. The aqueous medium may also be an aqueous suspension of colloidal particles of iron and/or alkaline earth metal compounds.
The method according to the present invention removes previously formed deposits and serves as a reagent-free means of cleaning internal surfaces during operation of a heating device, subsequently ensuring its scale-free operation. In this case, the size of the zone within which the prevention of scale and corrosion is achieved significantly exceeds the size of the zone of effective scale destruction.
The method according to the present invention has the following advantages:
Does not require the use of reagents, i.e. environmentally friendly;
Easy to implement, does not require special devices;
Allows you to increase the heat transfer coefficient and increase the efficiency of boilers, which significantly affects the economic indicators of its operation;
Can be used as an addition to the applied methods of pre-boiler water treatment, or separately;
Allows you to abandon the processes of water softening and deaeration, which greatly simplifies the technological scheme of boiler houses and makes it possible to significantly reduce costs during construction and operation.
Possible objects of the method can be hot water boilers, waste heat boilers, closed systems heat supply, installations for thermal desalination of sea water, steam conversion plants, etc.
The absence of corrosion damage and scale formation on internal surfaces opens up the possibility of developing fundamentally new design and layout solutions for low- and medium-power steam boilers. This will allow, due to the intensification of thermal processes, to achieve a significant reduction in the weight and dimensions of steam boilers. Ensure the specified temperature level of heating surfaces and, consequently, reduce fuel consumption, the volume of flue gases and reduce their emissions into the atmosphere.
EXAMPLE OF IMPLEMENTATION
The method claimed in the present invention was tested at the Admiralty Shipyards and Krasny Khimik boiler plants. The method according to the present invention has been shown to effectively clean the internal surfaces of boiler units from deposits. In the course of these works, fuel equivalent savings of 3-10% were obtained, while the variation in savings values is associated with varying degrees of contamination of the internal surfaces of the boiler units. The purpose of the work was to evaluate the effectiveness of the claimed method for ensuring reagent-free, scale-free operation of medium-power steam boilers under conditions of high-quality water treatment, compliance with the water chemistry regime and a high professional level of equipment operation.
The method claimed in the present invention was tested on steam boiler unit No. 3 DKVR 20/13 of the 4th Krasnoselskaya boiler house of the South-Western branch of the State Unitary Enterprise "TEK SPb". The operation of the boiler unit was carried out in strict accordance with the requirements regulatory documents. The boiler is equipped with all the necessary means of monitoring its operating parameters (pressure and flow rate of generated steam, temperature and flow rate of feed water, pressure of blast air and fuel on the burners, vacuum in the main sections of the gas path of the boiler unit). The steam output of the boiler was maintained at 18 t/hour, the steam pressure in the boiler drum was 8.1…8.3 kg/cm 2 . The economizer operated in heating mode. City water supply water was used as the source water, which met the requirements of GOST 2874-82 “Drinking water”. It should be noted that the amount of iron compounds entering the specified boiler room, as a rule, exceeds regulatory requirements (0.3 mg/l) and amounts to 0.3-0.5 mg/l, which leads to intensive overgrowing of internal surfaces with ferrous compounds.
The effectiveness of the method was assessed based on the condition of the internal surfaces of the boiler unit.
Assessment of the influence of the method according to the present invention on the condition of the internal heating surfaces of the boiler unit.
Before the start of the tests, an internal inspection of the boiler unit was carried out and the initial condition of the internal surfaces was recorded. A preliminary inspection of the boiler was carried out at the beginning of the heating season, a month after its chemical cleaning. As a result of the inspection, it was revealed: on the surface of the drums there are continuous solid deposits of a dark brown color, possessing paramagnetic properties and presumably consisting of iron oxides. The thickness of the deposits was up to 0.4 mm visually. In the visible part of the boiling pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of pipe length with a size of 2 to 15 mm and a visual thickness of up to 0.5 mm).
The device for creating a current-carrying potential, described in RU 2100492 C1, was connected at point (1) to the hatch (2) of the upper drum on the back side of the boiler (see Fig. 1). The current-carrying electric potential was equal to 100 V. The current-carrying electric potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler unit, an almost complete absence of deposits (no more than 0.1 mm visually) was established on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the drum hatches (device connection points to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from the hatches, deposits (6) were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig. 1). Further, as you move towards the front, (at a distance of 3.0-3.5 m from the hatches) continuous deposits begin (7) up to 0.4 mm visually, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not evident. The current-carrying electric potential was equal to 100 V. The current-carrying electric potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler unit, an almost complete absence of deposits (no more than 0.1 mm visually) was established on the surface of the upper and lower drums within 2-2.5 meters from the drum hatches (attachment points of the device for creating current-carrying potential). At a distance of 2.5-3.0 m from the hatches, the deposits were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig. 1). Further, as you move towards the front (at a distance of 3.0-3.5 m from the hatches), continuous deposits of up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not evident.
In the visible part of the boiling pipes, within 3.5-4.0 m from the drum hatches, an almost complete absence of deposits was observed. Further, as we move towards the front, non-continuous solid deposits are found (up to five spots per 100 linear mm with a size ranging from 2 to 15 mm and a visual thickness of up to 0.5 mm).
As a result of this stage of testing, it was concluded that the method according to the present invention, without the use of any reagents, can effectively destroy previously formed deposits and ensure scale-free operation of the boiler unit.
At the next stage of testing, the device for creating a current-carrying potential was connected at point “B” and the tests continued for another 30-45 days.
The next opening of the boiler unit was carried out after 3.5 months of continuous operation of the device.
An inspection of the boiler unit showed that the previously remaining deposits were completely destroyed and only a small amount remained in the lower sections of the boiler pipes.
This allowed us to draw the following conclusions:
The size of the zone within which scale-free operation of the boiler unit is ensured significantly exceeds the size of the zone of effective destruction of deposits, which allows subsequent transfer of the point of connection of the current-carrying potential to clean the entire internal surface of the boiler unit and further maintain its scale-free operation mode;
The destruction of previously formed deposits and the prevention of the formation of new ones is ensured by processes of different nature.
Based on the results of the inspection, it was decided to continue testing until the end of the heating period in order to finally clean the drums and boiling pipes and determine the reliability of ensuring scale-free operation of the boiler. The next opening of the boiler unit was carried out after 210 days.
The results of the internal inspection of the boiler showed that the process of cleaning the internal surfaces of the boiler within the upper and lower drums and boiling pipes resulted in almost complete removal of deposits. A thin, dense coating formed on the entire surface of the metal, black in color with a blue tarnish, the thickness of which, even in a moistened state (almost immediately after opening the boiler), did not visually exceed 0.1 mm.
At the same time, the reliability of ensuring scale-free operation of the boiler unit when using the method of the present invention was confirmed.
The protective effect of the magnetite film lasted up to 2 months after disconnecting the device, which is quite enough to ensure the preservation of the boiler unit using the dry method when transferring it to reserve or for repairs.
Although the present invention has been described in relation to various specific examples and embodiments of the invention, it should be understood that this invention is not limited thereto and that it may be practiced within the scope of the following claims
1. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale can form, including applying a current-carrying electric potential to said metal surface in the range from 61 V to 150 V to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and ions forming scale.
The invention relates to heat power engineering and can be used to protect against scale and corrosion heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems of residential buildings and industrial facilities during operation. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale is capable of forming involves applying to said metal surface a current-carrying electric potential in the range from 61 V to 150 V to neutralize the electrostatic component of the adhesion force between the specified metal surface and colloidal particles and ions forming scale. The technical result is increasing the efficiency and productivity of hot water and steam boilers, increasing the efficiency of heat transfer, ensuring layer-by-layer destruction and removal of formed scale, as well as preventing its new formation. 2 salary f-ly, 1 ave., 1 ill.
The conditions in which the elements of steam boilers are located during operation are extremely varied.
As numerous corrosion tests and industrial observations have shown, low-alloy and even austenitic steels can be subject to intense corrosion during boiler operation.
Corrosion of the metal heating surfaces of steam boilers causes premature wear and sometimes leads to serious problems and accidents.
Most emergency shutdowns of boilers occur due to through corrosion damage to the screen, grain economizer, steam superheating pipes and boiler drums. The appearance of even one corrosion fistula in a once-through boiler leads to the shutdown of the entire unit, which is associated with a lack of electricity production. Corrosion of high- and ultra-high-pressure drum boilers has become the main cause of failures in thermal power plants. 90% of operational failures due to corrosion damage occurred on drum boilers with a pressure of 15.5 MPa. A significant amount of corrosion damage to the screen pipes of the salt compartments occurred in areas of maximum thermal loads.
Inspections of 238 boilers (units with a capacity from 50 to 600 MW) conducted by US specialists revealed 1,719 unscheduled downtimes. About 2/3 of boiler downtime was caused by corrosion, of which 20% was due to corrosion of steam generating pipes. In the USA, internal corrosion was recognized as a serious problem after commissioning in 1955 large number drum boilers with a pressure of 12.5-17 MPa.
By the end of 1970, about 20% of the 610 such boilers were damaged by corrosion. Screen pipes were mostly susceptible to internal corrosion, while superheaters and economizers were less affected by it. With the improvement of the quality of feed water and the transition to a coordinated phosphating regime, with an increase in parameters on drum boilers of US power plants, instead of viscous, plastic corrosion damage, sudden brittle fractures of screen pipes occurred. “As of J970 t. for boilers with pressures of 12.5, 14.8 and 17 MPa, the destruction of pipes due to corrosion damage was 30, 33 and 65%, respectively.
According to the conditions of the corrosion process, a distinction is made between atmospheric corrosion, which occurs under the influence of atmospheric and also wet gases; gas, caused by the interaction of the metal with various gases - oxygen, chlorine, etc. - at high temperatures, and corrosion in electrolytes, in most cases occurring in aqueous solutions.
Due to the nature of corrosion processes, boiler metal can be subject to chemical and electrochemical corrosion, as well as their combined effects.
When operating the heating surfaces of steam boilers, high-temperature gas corrosion occurs in the oxidizing and reducing atmospheres of flue gases and low-temperature electrochemical corrosion of the tail heating surfaces.
Research has established that high-temperature corrosion of heating surfaces occurs most intensely only in the presence of excess free oxygen in the flue gases and in the presence of molten vanadium oxides.
High-temperature gas or sulfide corrosion in the oxidizing atmosphere of flue gases affects pipes of screen and convective superheaters, the first rows of boiler bundles, metal spacers between pipes, racks and suspensions.
High temperature gas corrosion in a reducing atmosphere was observed on the screen pipes of the combustion chambers of a number of high and supercritical pressure boilers.
Corrosion of heating surface pipes on the gas side is a complex physical and chemical process of interaction of flue gases and external deposits with oxide films and pipe metal. The development of this process is influenced by time-varying intense heat flows and high mechanical stresses arising from internal pressure and self-compensation.
On medium and low pressure boilers, the screen wall temperature, determined by the boiling point of water, is lower, and therefore this type of metal destruction is not observed.
Corrosion of heating surfaces from flue gases (external corrosion) is the process of metal destruction as a result of interaction with combustion products, aggressive gases, solutions and melts of mineral compounds.
Metal corrosion is understood as the gradual destruction of metal that occurs as a result of chemical or electrochemical exposure to the external environment.
\ The processes of metal destruction, which are a consequence of their direct chemical interaction with the environment, are classified as chemical corrosion.
Chemical corrosion occurs when metal comes into contact with superheated steam and dry gases. Chemical corrosion in dry gases is called gas corrosion.
In the furnace and gas ducts of the boiler, gas corrosion of the outer surface of the pipes and superheater racks occurs under the influence of oxygen, carbon dioxide, water vapor, sulfur dioxide and other gases; the inner surface of the pipes - as a result of interaction with steam or water.
Electrochemical corrosion, unlike chemical corrosion, is characterized by the fact that the reactions occurring during it are accompanied by the appearance of an electric current.
The carrier of electricity in solutions are the ions present in them due to the dissociation of molecules, and in metals - free electrons:
The internal boiler surface is mainly subject to electrochemical corrosion. According to modern concepts, its manifestation is due to two independent processes: anodic, in which metal ions pass into solution in the form of hydrated ions, and cathodic, in which excess electrons are assimilated by depolarizers. Depolarizers can be atoms, ions, molecules, which are reduced.
Based on external signs, continuous (general) and local (local) forms of corrosion damage are distinguished.
With general corrosion, the entire heating surface in contact with the aggressive environment is corroded, evenly thinning on the inside or outside. With local corrosion, destruction occurs in individual areas of the surface, the rest of the metal surface is not affected by damage.
Local corrosion includes spot corrosion, ulcer corrosion, pitting corrosion, intergranular corrosion, stress-corrosion cracking, and metal corrosion fatigue.
A typical example of destruction from electrochemical corrosion.
Destruction from the outer surface of NRCh 042X5 mm pipes made of steel 12Kh1MF of TPP-110 boilers occurred in a horizontal section in the lower part of the lifting and lowering loop in the area adjacent to the bottom screen. On the back side of the pipe, an opening occurred with a slight thinning of the edges at the point of destruction. The cause of the destruction was the thinning of the pipe wall by approximately 2 mm due to corrosion due to deslagging with a jet of water. After stopping the boiler with a steam production of 950 t/h, heated by anthracite pellet dust (liquid slag removal), a pressure of 25.5 MPa and a superheated steam temperature of 540 °C, wet slag and ash remained on the pipes, in which electrochemical corrosion proceeded intensively. The outside of the pipe was coated with a thick layer of brown iron hydroxide. The internal diameter of the pipes was within the tolerances for pipes of high- and ultra-high-pressure boilers. Outer diameter dimensions have deviations beyond the minus tolerance: minimum outer diameter. amounted to 39 mm with a minimum allowable of 41.7 mm. The wall thickness near the point of corrosion failure was only 3.1 mm with a nominal pipe thickness of 5 mm.
The microstructure of the metal is uniform along the length and circumference. On the inner surface of the pipe there is a decarbonized layer formed during oxidation of the pipe during heat treatment. On outside there is no such layer.
Examination of the NRF pipes after the first rupture made it possible to find out the cause of the destruction. It was decided to replace the NRF and change the deslagging technology. In this case, electrochemical corrosion occurred due to the presence of a thin film of electrolyte.
Pit corrosion occurs intensely in individual small areas of the surface, but often to a considerable depth. When the diameter of the ulcers is about 0.2-1 mm, it is called pinpoint.
In places where ulcers form, fistulas can form over time. The pits are often filled with corrosion products, as a result of which they cannot always be detected. An example is the destruction of steel economizer pipes due to poor deaeration of feed water and low speeds of water movement in the pipes.
Despite the fact that a significant part of the metal of the pipes is affected, due to through fistulas it is necessary to completely replace the economizer coils.
The metal of steam boilers is subject to the following dangerous types of corrosion: oxygen corrosion during operation of the boilers and when they are under repair; intercrystalline corrosion in places where boiler water evaporates; steam-water corrosion; corrosion cracking of boiler elements made of austenitic steels; sub-sludge - howling corrosion. a brief description of the indicated types of corrosion of boiler metal are given in table. YUL.
During the operation of boilers, metal corrosion is distinguished - corrosion under load and standing corrosion.
Corrosion under load is most susceptible to heating. manufactured boiler elements in contact with a two-phase medium, i.e. screen and boiler pipes. The inner surface of economizers and superheaters is less affected by corrosion during boiler operation. Corrosion under load also occurs in an oxygen-free environment.
Parking corrosion occurs in undrained areas. elements of vertical superheater coils, sagging pipes of horizontal superheater coils
