This corrosion in size and intensity is often more significant and dangerous than the corrosion of boilers during their operation.
When leaving water in systems, depending on its temperature and air access, a wide variety of cases of parking corrosion can occur. First of all, it should be noted the extreme undesirability of the presence of water in the pipes of the units when they are in reserve.
If water remains in the system for one reason or another, then severe parking corrosion can occur 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, parking corrosion of different intensity is quite 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 that have the dimensions of a furnace and a heating surface, since the boiler water in them cools faster; its temperature falls below 60-70°C.
At a water temperature above 85–90°C (for example, during short-term shutdowns of the apparatus), the overall corrosion decreases, and the corrosion of the metal of the vapor space, in which increased vapor condensation is observed in this case, can exceed the corrosion of the metal of the water space. Parking corrosion in the steam space is in all cases more uniform than in the water space of the boiler.
The development of parking corrosion is greatly facilitated by the sludge that accumulates on the surfaces of the boiler, which usually retains moisture. In this regard, significant corrosion holes are often found in aggregates and pipes along the lower generatrix and at their ends, i.e., in areas of the greatest accumulation of sludge.
Methods of conservation of equipment in reserve
The following methods can be used to preserve equipment:
a) drying - removal of water and moisture from aggregates;
b) filling them with solutions of caustic soda, phosphate, silicate, sodium nitrite, hydrazine;
c) filling the process system with nitrogen.
The method of conservation should be chosen depending on the nature and duration of downtime, as well as on the type and design features of the equipment.
Equipment downtime can be divided into two groups by duration: short-term - no more than 3 days and long-term - more than 3 days.
There are two types of short-term downtime:
a) scheduled, associated with the withdrawal to the reserve on weekends due to a drop in load or withdrawal to the 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) capital repairs.
In case of short-term downtime of the equipment, it is necessary to use conservation by filling with deaerated water with the maintenance of excess pressure or the gas (nitrogen) method. If an emergency shutdown is required, then the only acceptable method is conservation with nitrogen.
When the system is put into reserve or long-term downtime without repair work, it is advisable to carry out conservation by filling it with a solution of nitrite or sodium silicate. In these cases, nitrogen conservation can also be used, necessarily taking measures to create a tightness of the system in order to prevent excessive gas consumption and unproductive operation of the nitrogen plant, as well as to create safe conditions for equipment maintenance.
Preservation methods by creating excess pressure, 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 allow creating a protective film on the metal surface that retains its properties for at least 1–2 months after draining the preservative solution, since emptying and depressurization of the system are inevitable. The duration of the protective film on the metal surface after treatment with sodium nitrite can reach 3 months.
Preservation methods using water and reagent solutions are practically unacceptable for protection against parking corrosion of intermediate superheaters of boilers due to the difficulties associated with their filling and subsequent cleaning.
Methods of preservation of 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 methods currently used to prevent parking corrosion at TPPs. The following describes the main methods for preventing corrosion in the idle mode of the equipment of such apparatuses. circulation systems according to the nature of their work.
Simplified preservation methods
These methods are useful for small boilers. They consist in the complete removal of water from the boilers and the placement of desiccants 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 for room temperatures below and above zero. In rooms heated in winter time, one of the contact methods of preservation 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.), which ensures the complete stability of the protective film on the metal surface even when the liquid is saturated with oxygen.
Usually used solutions containing from 1.5-2 to 10 kg/m 3 NaOH or 5-20 kg/m 3 Na 3 P0 4 depending on the content of neutral salts in the source water. Smaller values refer to condensate, larger ones 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 economy 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 a protective solution (or gas) or moisture ingress. In addition, in many cases, preliminary cleaning of surfaces from various deposits (salts, sludge, scale) is useful.
When implementing various methods of protection against parking corrosion, the following should be borne in mind.
1. For all types of conservation, preliminary removal (washing) of deposits of easily soluble salts (see above) is necessary 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 conservation.
3. If the fittings are unreliable, it is necessary to disconnect the standby equipment from the operating units using plugs.
Leakage of steam and water is less dangerous with contact preservation, but is unacceptable with dry and gas protection methods.
The choice of desiccants is determined by the relative availability of the reagent and the desirability of obtaining the highest possible specific moisture content. The best desiccant is granular calcium chloride. Quicklime is much worse than calcium chloride, not only due to 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 is covered with a layer of calcium carbonate, which prevents further absorption of moisture.
2.1. heating surfaces.
The most characteristic damages of pipes of heating surfaces are: cracks in the surface of screen and boiler pipes, corrosive erosion of the outer and inner surfaces of pipes, ruptures, thinning of the walls of pipes, cracks and destruction of bells.
The reasons for the appearance of cracks, ruptures and fistulas: deposits in the pipes of boilers of salts, corrosion products, welding flash, which slow down circulation and cause overheating of the metal, external mechanical damage, violation of the water-chemical regime.
Corrosion of the outer surface of pipes is divided into low-temperature and high-temperature. Low-temperature corrosion occurs at blower installations when, as a result of improper operation, condensation is allowed to form on soot-covered heating surfaces. High-temperature corrosion can take place in the second stage of the superheater when burning sulphurous fuel oil.
The most common corrosion of the inner surface of pipes occurs when corrosive gases (oxygen, carbon dioxide) or salts (chlorides and sulfates) contained in boiler water interact with pipe metal. Corrosion of the inner surface of pipes is manifested in the formation of pockmarks, ulcers, shells and cracks.
Corrosion of the inner surface of pipes also includes: oxygen parking corrosion, under-sludge alkaline corrosion of boiler and screen pipes, corrosion fatigue, which manifests itself in the form of cracks in boiler and screen pipes.
Pipe damage due to creep is characterized by an increase in diameter and the formation of longitudinal cracks. Deformations in the places of pipe bends and welded joints can have different directions.
Burnouts and scaling in pipes occur as a result of their overheating to temperatures exceeding the calculated one.
The main types of damage to welds made by manual arc welding are fistulas that occur due to lack of penetration, slag inclusions, gas pores, and non-fusion along the edges of the pipes.
The main defects and damages of the surface of the superheater are: corrosion and scale formation on the outer and inner surfaces of the pipes, cracks, risks and delamination of the pipe metal, fistulas and ruptures of pipes, defects in pipe welds, residual deformation as a result of creep.
Damage to the fillet welds of the coils and fittings to the headers, causing a violation of the welding technology, have the form of ring cracks along the fusion line from the side of the coil or fittings.
Typical malfunctions that occur during the operation of the surface desuperheater of the boiler DE-25-24-380GM are: internal and external corrosion of pipes, cracks and fistulas in welded
seams and bends of pipes, shells that may occur during repairs, risks on the mirror of flanges, leakage of flanged joints due to misalignment of flanges. When hydraulic testing the boiler, you can
determine only the presence of leaks in the desuperheater. To identify hidden defects, an individual hydraulic test of the desuperheater should be carried out.
2.2. Boiler drums.
Typical damages of the boiler drums are: cracks-tears on the inner and outer surfaces of the shells and bottoms, cracks-tears around pipe holes on the inner surface of the drums and on the cylindrical surface of the pipe holes, intergranular corrosion of the shells and bottoms, corrosion separation of the surfaces of the shells and bottoms, ovality of the drum, oddulins (bulges) on the surfaces of the drums facing the furnace, caused by the temperature effect of the torch in cases of destruction (or loss) of individual lining parts.
2.3. Metal structures and lining of the boiler.
Depending on the quality of preventive work, as well as on the modes and periods of operation of the boiler, its metal structures may have the following defects and damage: breaks and bends of racks and connections, cracks, corrosion damage to the metal surface.
As a result of prolonged exposure to temperatures, cracking and violation of the integrity of the shaped brick, fixed on pins to the upper drum from the side of the furnace, as well as cracks in the brickwork along the lower drum and the hearth of the furnace, take place.
The destruction of the brick embrasure of the burner and the violation of the geometric dimensions due to the melting of the brick are especially common.
3. Checking the condition of the boiler elements.
Checking the condition of the elements of a boiler that has been brought out for repair is carried out based on the results of a hydraulic test, external and internal inspection, as well as other types of control carried out to the extent and in accordance with the program of the expert examination of the boiler (section "Program of the expert examination of boilers").
3.1. Checking heating surfaces.
Inspection of the outer surfaces of tubular elements should be especially carefully carried out in places where pipes pass through the lining, sheathing, in areas of maximum thermal stress - in the area of burners, hatches, manholes, as well as in places where screen pipes are bent and at welds.
To prevent accidents associated with thinning of the pipe walls due to sulfur and parking corrosion, it is necessary during the annual technical examinations carried out by the administration of the enterprise to inspect the pipes of the heating surfaces of boilers that have been in operation for more than two years.
The control is carried out by external inspection with tapping of the previously cleaned outer surfaces of the pipes with a hammer weighing no more than 0.5 kg and measuring the thickness of the pipe walls. In this case, it is necessary to choose sections of pipes that have undergone the greatest wear and corrosion (horizontal sections, sections with soot deposits and covered with coke deposits).
Pipe wall thickness is measured with ultrasonic thickness gauges. It is possible to cut sections of pipes on two or three pipes of furnace screens and pipes of a convective beam located at the inlet and outlet of gases into it. The remaining thickness of the pipe walls must be at least the calculated one according to the strength calculation (attached to the Passport of the boiler), taking into account the allowance for corrosion for the period of further operation until the next survey and an increase in the margin of 0.5 mm.
The calculated wall thickness of the screen and boiler pipes for a working pressure of 1.3 MPa (13 kgf / cm 2) is 0.8 mm, for 2.3 MPa (23 kgf / cm 2) - 1.1 mm. The allowance for corrosion is accepted based on the results of measurements and taking into account the duration of operation between surveys.
At enterprises where, as a result of long-term operation, intensive wear of pipes of heating surfaces was not observed, control of the thickness of the walls of the pipes can be carried out during major repairs, but at least once every 4 years.
The collector, superheater and rear screen are subject to internal inspection. Mandatory opening and inspection should be subjected to the hatches of the upper collector of the rear screen.
The outer diameter of the pipes must be measured in the zone of maximum temperatures. For measurements, use special templates (staples) or calipers. On the pipe surface, dents with smooth transitions with a depth of not more than 4 mm are allowed, if they do not take the wall thickness beyond the limits of minus deviations.
Permissible difference in wall thickness of pipes - 10%.
The results of the inspection and measurements are recorded in the repair log.
3.2. Drum check.
Before identifying areas of the drum damaged by corrosion, it is necessary to inspect the surface before internal cleaning in order to determine the intensity of corrosion and measure the depth of metal corrosion.
Uniform corrosion is measured along the wall thickness, in which, for this purpose, a hole with a diameter of 8 mm is drilled. After measuring, install a plug in the hole and weld it on both sides or, in extreme cases, only from the inside of the drum. The measurement can also be made with an ultrasonic thickness gauge.
The main corrosion and pitting should be measured from the impressions. For this purpose, clean the damaged area of the metal surface from deposits and lightly lubricate with technical petroleum jelly. The most accurate imprint is obtained if the damaged area is located on a horizontal surface and in this case it is possible to fill it with molten metal with a low melting point. The hardened metal forms an exact cast of the damaged surface.
To obtain prints, use a tretnik, babbitt, tin, and, if possible, use plaster.
Impressions of damage located on vertical ceiling surfaces are obtained using wax and plasticine.
Inspection of pipe holes, drums is carried out in the following order.
After removing the flared pipes, check the diameter of the holes using a template. If the template enters the hole up to the stop ledge, then this means that the diameter of the hole has been increased beyond the norm. The measurement of the exact value of the diameter is carried out with a caliper and is noted in the repair log.
When checking the welded seams of drums, it is necessary to inspect the base metal adjacent to them for a width of 20-25 mm on both sides of the seam.
The ovality of the drum is measured at least every 500 mm along the length of the drum, in doubtful cases and more often.
Measuring the deflection of the drum is carried out by stretching the string along the surface of the drum and measuring the gaps along the length of the string.
The control of the surface of the drum, pipe holes and welded joints is carried out by external inspection, methods, magnetic particle, color and ultrasonic flaw detection.
Bumps and dents outside the zone of seams and holes are allowed (do not require straightening), provided that their height (deflection), as a percentage of the smallest size of their base, will not exceed:
towards atmospheric pressure (bulges) - 2%;
in the direction of steam pressure (dents) - 5%.
Permissible reduction in bottom wall thickness - 15%.
Permissible increase in the diameter of holes for pipes (for welding) - 10%.
A number of boiler houses use river and tap water with a low pH value and low hardness to feed heating networks. Additional treatment of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of 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 established water deaeration and possible increases in oxygen and carbon dioxide concentrations due to the lack of additional protective measures The thermal power equipment of CHPPs is subject to internal corrosion in heat supply systems.
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 the return water of the heating network - 3500 µg/l. After the first year of operation, cuttings were made from the network water pipelines, it turned out that 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 have been 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 arising under the influence atmospheric air on the wet surfaces of the boilers, continue to function during the operation of the boilers.
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3.4. Corrosion of steam generator elements3.4.1. Steam pipe corrosionandsteam generator drums
during their operation
Corrosion damage to the metals of steam generators is caused by the action of one or more factors: excessive heat stress on the heating surface, sluggish water circulation, stagnation of steam, stressed metal, deposition of impurities and other factors that prevent normal washing and cooling of the heating surface.
In the absence of these factors, a normal magnetite film is easily formed and retained in water with a neutral or moderately alkaline reaction of an environment that does not contain dissolved oxygen. In the presence of O 2 , on the other hand, inlet sections of water economizers, drums, and downpipes of circulation circuits can be exposed to oxygen corrosion. The low speeds of water movement are especially negative (in water economizers, since in this case the bubbles of the released air linger in the places of roughness of the inner surface of the pipes and cause intense local oxygen corrosion. Corrosion of carbon steel in the aquatic environment during high temperatures includes two stages: initial electrochemical and final chemical. According to this corrosion mechanism, ferrous ions diffuse through the oxide film to the surface of its contact with water, react with hydroxyl or water to form ferrous oxide hydrate, which then decomposes into magnetite and hydrogen according to the reaction:
| . | (2.4) |
Electrons passing along with iron ions through the oxide film are assimilated by hydrogen ions with the release of H 2 . Over time, the thickness of the oxide film increases, and diffusion through it becomes more difficult. As a result, the corrosion rate decreases with time.
nitrite corrosion. In the presence of sodium nitrite in the feed water, corrosion of the steam generator metal is observed, which in appearance is very similar to oxygen corrosion. However, in contrast to it, nitrite corrosion affects not the inlet sections of downcomers, but the inner surface of heat-stressed riser pipes and causes the formation of deeper pits up to 15–20 mm in diameter.
Nitrite accelerates the cathode process, and thus the corrosion of the metal of the steam generator. The course of the process during nitrite corrosion can be described by the following reaction:
| . | (2.5) |
Galvanic corrosion of steam generator metal. The source of galvanic corrosion of steam-generating pipes can be copper that gets into steam generators in those cases when feed water containing an increased amount of ammonia, oxygen and free carbon dioxide aggressively affects brass and copper pipes regenerative heaters. It should be noted that only metallic copper deposited on the walls of the steam generator can cause galvanic corrosion. When maintaining the pH value of the feed water above 7.6, copper enters the steam generators in the form of oxides or complex compounds that do not have corrosive properties and are deposited on the heating surfaces in the form of sludge. Copper ions present in the feed water with a low pH value, falling further into the steam generator, under alkaline conditions, are also deposited in the form of sludge-like copper oxides. However, under the action of hydrogen released in steam generators or an excess of sodium sulfite, copper oxides can be completely reduced to metallic copper, which, deposited on the heating surfaces, leads to electrochemical corrosion of the boiler metal.
Undersludge (shell) corrosion. Undersludge corrosion occurs in stagnant zones of the steam generator circulation circuit under a layer of sludge, which consists of metal corrosion products and phosphate treatment of boiler water. If these deposits are concentrated in heated areas, then intensive evaporation occurs under them, which increases the salinity and alkalinity of boiler water to dangerous values.
Subslurry corrosion spreads in the form of large pits up to 50–60 mm in diameter on the inner side of the steam pipes facing the furnace torch. Within the pits, a relatively uniform decrease in the thickness of the pipe wall is observed, often leading to the formation of fistulas. On the ulcers, a dense layer of iron oxides in the form of shells is found. The described destruction of the metal has received the name "shell" corrosion in the literature. Under-sludge corrosion caused by oxides of ferric iron and bivalent copper is an example of combined metal destruction; the first stage of this process is purely electrochemical, and the second is chemical, due to the action of water and water vapor on the overheated areas of the metal under the sludge layer. The most effective means of combating "shell" corrosion of steam generators is to prevent the occurrence of corrosion of the feed water tract and the removal of iron and copper oxides from it with feed water.
alkaline corrosion. The stratification of the steam-water mixture, which takes place in horizontal or slightly inclined steam-forming pipes, is known to be accompanied by the formation of steam bags, overheating of the metal, and deep evaporation of the boiler water film. The highly concentrated film formed during the evaporation of boiler water contains a significant amount of alkali in solution. Caustic soda, which is present in low concentrations in boiler water, protects the metal from corrosion, but it becomes a very dangerous corrosion factor if conditions are created on any parts of the surface of the steam generator for deep evaporation of boiler water with the formation of an increased concentration of NaOH.
The concentration of caustic soda in the evaporated film of boiler water depends on:
A) on the degree of overheating of the wall of the steam-generating pipe compared to the boiling point at a given pressure in the steam generator, i.e. values?t s ;
B) the values of the ratio of the concentration of caustic soda and sodium salts contained in circulating water, which have the ability to greatly increase the boiling point of water at a given pressure.
If the concentration of chlorides in the boiler water significantly exceeds the concentration of NaOH in an equivalent ratio, then before the latter reaches dangerous values in the evaporating film, the chloride content in it increases so much that the boiling point of the solution exceeds the temperature of the overheated pipe wall, and further water evaporation stops. If the boiler water contains predominantly caustic soda, then at the value
?t s = 30 °C reaches 35%. Meanwhile, it has been experimentally established that already 5-10% solutions of sodium hydroxide at a boiler water temperature above 200 ° C are capable of intensively corroding the metal of heated areas and welds with the formation of loose magnetic ferrous oxide and simultaneous release of hydrogen. Alkaline corrosion has a selective character, moving deep into the metal mainly along pearlite grains and forming a network of intergranular cracks. A concentrated solution of caustic soda is also capable of dissolving the protective layer of iron oxides at high temperatures with the formation of sodium ferrite NaFeO 2, which is hydrolyzed to form alkali:
| | (2.6) |
| | (2.7) |
Due to the fact that alkali is not consumed in this circular process, the possibility of a continuous corrosion process is created. The higher the temperature of the boiler water and the concentration of caustic soda, the more intense the process of alkaline corrosion. It has been established that concentrated solutions of caustic soda not only destroy the protective magnetite film, but also inhibit its recovery after damage.
The source of alkaline corrosion of steam generators can also be sludge deposits, which contribute to the deep evaporation of boiler water with the formation of a highly concentrated corrosive alkali solution. A decrease in the relative proportion of alkali in the total salt content of boiler water and the creation of a predominant content in the latter of such salts as chlorides can dramatically weaken the alkaline corrosion of boiler metal. The elimination of alkaline corrosion is also achieved by ensuring the cleanliness of the heating surface and intensive circulation in all parts of the steam generator, which prevents deep evaporation of water.
intergranular corrosion. Intergranular corrosion appears as a result of the interaction of boiler metal with alkaline boiler water. A characteristic feature of intergranular cracks is that they occur in places of greatest stress in the metal. Mechanical stresses are made up of internal stresses arising during the manufacture and installation of drum-type steam generators, as well as additional stresses arising during operation. The formation of intergranular annular cracks on the pipes is facilitated by additional static mechanical stresses. They occur in pipe circuits and in the drums of the steam generator with insufficient compensation for thermal elongation, as well as due to uneven heating or cooling of individual sections of the body of the drum or collector.
Intercrystalline corrosion proceeds with some acceleration: in the initial period, the destruction of the metal occurs very slowly and without deformation, and then over time, its rate increases sharply and can take on catastrophic proportions. Intergranular corrosion of boiler metal should be considered primarily as special case electrochemical corrosion occurring along the grain boundaries of the stressed metal in contact with the alkaline concentrate of boiler water. The appearance of corrosive microgalvanic cells is caused by the difference in potentials between the bodies of crystallites, which act as cathodes. The role of anodes is played by collapsing grain edges, the potential of which is greatly reduced due to the mechanical stresses of the metal in this place.
Along with electrochemical processes, a significant role in the development of intergranular corrosion is played by atomic hydrogen, the discharge product
H + -ions on the cathode of corrosive elements; easily diffusing into the thickness of steel, it destroys carbides and creates large internal stresses in the metal of the boiler due to the appearance of methane in it, which leads to the formation of fine intergranular cracks (hydrogen cracking). In addition, during the reaction of hydrogen with steel inclusions, various gaseous products are formed, which in turn causes additional breaking forces and contributes to the loosening of the structure, deepening, expansion and branching of cracks.
The main way to prevent hydrogen corrosion of the boiler metal is to eliminate any corrosion processes that lead to the formation of atomic hydrogen. This is achieved by reducing sediment in the steam generator of iron and copper oxides, chemical cleaning of the boilers, improving water circulation and reducing local increased heat loads on the heating surface.
It has been established that intergranular corrosion of boiler metal in the joints of steam generator elements occurs only with the simultaneous presence of local tensile stresses close to or exceeding the yield strength, and with the concentration of NaOH in boiler water, which accumulates in leaks in the joints of boiler elements, exceeding 5–6%. For the development of intergranular destruction of boiler metal, it is not the absolute value of alkalinity that is essential, but the share of caustic soda in the total salt composition of boiler water. It has been experimentally established that if this proportion, i.e. the relative concentration of caustic soda in boiler water, is less than 10-15% of the total mineral soluble substances, then such water, as a rule, is not aggressive.
Steam corrosion. In places with defective circulation, where steam stagnates and is not immediately discharged into the drum, the walls of the pipes under the steam bags are subjected to strong local overheating. This leads to chemical corrosion of the metal of steam-generating pipes overheated to 450 °C and above under the action of highly superheated steam. The process of corrosion of carbon steel in highly superheated water vapor (at a temperature of 450 - 470 ° C) is reduced to the formation of Fe 3 O 4 and hydrogen gas:
| | (2.8.) |
Hence it follows that the criterion for the intensity of steam-water corrosion of the boiler metal is an increase in the content of free hydrogen in saturated steam. Steam-water corrosion of steam-forming pipes is observed, as a rule, in zones of sharp fluctuations in wall temperature, where heat changes take place, causing the destruction of the protective oxide film. This creates the possibility of direct contact of the overheated pipe metal with water or water vapor and chemical interaction between them.
Corrosion fatigue. In the drums of steam generators and boiler pipes, in the event that thermal stresses, which are variable in sign and magnitude, act on the metal simultaneously with the corrosive medium, corrosion fatigue cracks that penetrate deeply into the steel appear, which can be transcrystalline, intergranular or mixed. As a rule, the cracking of the boiler metal is preceded by the destruction of the protective oxide film, which leads to significant electrochemical inhomogeneity and, as a result, to the development of local corrosion.
In steam generator drums, corrosion fatigue cracks occur during alternating heating and cooling of metal in small areas at the junctions of pipelines (feed water, periodic blowing, phosphate solution inlet) and water-indicating columns with the drum body. In all these connections, the metal of the drum is cooled if the temperature of the feed water flowing through the pipe is less than the saturation temperature at the pressure in the steam generator. Local cooling walls of the drum with their subsequent heating with hot boiler water (at the moments of power failure) is always associated with the appearance of high internal stresses in the metal.
Corrosion cracking of steel is sharply enhanced under conditions of alternate wetting and drying of the surface, as well as in cases where the movement of the steam-water mixture through the pipe has a pulsating character, i.e., the speed of the steam-water mixture and its vapor content often and sharply change, as well as with a kind of stratification steam-water mixture into separate "plugs" of steam and water, following friend after another.
3.4.2. Superheater Corrosion
The rate of steam-water corrosion is determined mainly by the temperature of the steam and the composition of the metal in contact with it. The values of heat transfer and temperature fluctuations during the operation of the superheater, as a result of which the destruction of protective oxide films can be observed, are also of significant importance in its development. In a superheated steam environment with a temperature above
575 °C FeO (Wustite) is formed on the steel surface as a result of water-steam corrosion:
It has been established that pipes made of ordinary low-carbon steel, being exposed to highly superheated steam for a long time, are evenly destroyed with simultaneous degeneration of the metal structure and the formation of a dense layer of scale. In steam generators of ultrahigh and supercritical pressures at a steam superheat temperature of 550 °C and above, the most heat-stressed elements of the superheater (outlet sections) are usually made of heat-resistant austenitic stainless steels (chromium-nickel, chromium-molybdenum, etc.). These steels, under the combined action of tensile stresses and a corrosive environment, are prone to cracking. Most operational damages of superheaters, characterized by corrosion cracking of elements made of austenitic steels, are due to the presence of chlorides and caustic soda in the steam. The fight against corrosion cracking of parts made of austenitic steels is carried out mainly by maintaining a safe water regime of steam generators.
3.4.3. Parking corrosion of steam generators
During downtime of steam generators or other steam-powered equipment in cold or hot standby or during repairs, the so-called parking corrosion develops on the metal surface under the action of atmospheric oxygen or moisture. For this reason, plant downtimes without adequate corrosion protection measures often result in serious damage, especially in steam generators. Steam superheaters and steam-generating pipes of transition zones of once-through steam generators suffer greatly from parking corrosion. One of the causes of parking corrosion of the inner surface of steam generators is filling them with oxygen-saturated water during downtime. In this case, the metal at the water-air interface is especially prone to corrosion. If the steam generator left for repair is completely drained, then a film of moisture always remains on its inner surface with simultaneous access of oxygen, which, easily diffusing through this film, causes active electrochemical corrosion metal. A thin film of moisture remains for quite a long time, since the atmosphere inside the steam generator is saturated with water vapor, especially if steam enters it through leaks in the fittings of steam generators operating in parallel. If chlorides are present in the water filling the reserve steam generator, then this leads to an increase in the rate of uniform corrosion of the metal, and if it contains a small amount of alkali (less than 100 mg / dm 3 NaOH) and oxygen, this contributes to the development of pitting corrosion.
The development of parking corrosion is also facilitated by the sludge that accumulates in the steam generator, which usually retains moisture. For this reason, significant corrosion shells are often found in drums along the lower generatrix at their ends, i.e., in areas of the greatest accumulation of sludge. Particularly susceptible to corrosion are areas of the inner surface of steam generators that are covered with water-soluble salt deposits, such as superheater coils and the transition zone in once-through steam generators. During downtime of steam generators, these deposits absorb atmospheric moisture and spread out with the formation of a highly concentrated solution of sodium salts on the metal surface, which has a high electrical conductivity. With free access of air, the corrosion process under salt deposits proceeds very intensively. It is very significant that parking corrosion enhances the process of corrosion of the boiler metal during the operation of the steam generator. This circumstance should be considered the main danger of parking corrosion. The formed rust, consisting of high-valence iron oxides Fe(OH) 3 , during the operation of the steam generator plays the role of a depolarizer of corrosive micro- and macrogalvanic couples, which leads to an intensification of metal corrosion during the operation of the unit. Ultimately, the accumulation of rust on the surface of the boiler metal leads to under-slurry corrosion. In addition, during the subsequent downtime of the unit, the reduced rust again acquires the ability to cause corrosion due to its absorption of oxygen from the air. These processes are cyclically repeated with the alternation of downtime and operation of steam generators.
Steam generators are protected from parking corrosion during periods of downtime in reserve and under repair using various conservation methods.
3.5. Steam turbine corrosion
The metal of the flow path of turbines can undergo corrosion in the steam condensation zone during operation, especially if it contains carbonic acid, cracking due to the presence of corrosive agents in the steam, and parking corrosion when the turbines are in reserve or under repair. The flow part of the turbine is especially subjected to parking corrosion in the presence of salt deposits in it. Salt solution formed during turbine downtime accelerates the development of corrosion. This implies the need for thorough cleaning of deposits from the turbine blade apparatus before long downtime her.
Corrosion during idle periods is usually relatively uniform, with adverse conditions it manifests itself in the form of numerous pits evenly distributed over the surface of the metal. The place of its flow is those stages where moisture condenses, which aggressively acts on the steel parts of the turbine flow path.
The source of moisture is primarily the condensation of steam that fills the turbine after it stops. The condensate partially remains on the blades and diaphragms, partially drains and accumulates in the turbine housing, since it is not discharged through the drains. The amount of moisture inside the turbine may increase due to steam leakage from the extraction and backpressure steam lines. The internal parts of the turbine are always colder than the air entering the turbine. The relative humidity of the air in the engine room is very high, so a slight cooling of the air is enough to set the dew point and release moisture on the metal parts.
To eliminate the parking corrosion of steam turbines, it is necessary to exclude the possibility of steam entering the turbines while they are in reserve, both from the side of the superheated steam pipeline and from the side of the extraction line, drainage lines, etc. To keep the surface of the blades, disks and rotor dry In this form, periodic blowing of the internal cavity of the reserve turbine is used with a stream of hot air (t = 80 h 100 ° C) supplied by a small auxiliary fan through a heater (electric or steam).
3.6. Turbine condenser corrosion
Under the operating conditions of steam power plants, there are often cases of corrosion damage to brass condenser tubes both from the inside, washed by cooling water, and from the outside. Intensively corrode the inner surfaces of the condenser tubes, cooled by highly mineralized, salt-lake waters containing a large amount of chlorides, or recycled circulating waters with high mineralization, and contaminated with suspended particles.
A characteristic feature of brass as a structural material is its tendency to corrosion under the combined action of increased mechanical stresses and a medium that has even moderate aggressive properties. Corrosion damage occurs in brass tube capacitors in the form of general dezincification, plug dezincification, stress corrosion cracking, impact corrosion and corrosion fatigue. The course of the noted forms of brass corrosion is decisively affected by the composition of the alloy, the technology for manufacturing condenser tubes, and the nature of the medium being contacted. Due to dezincification, the destruction of the surface of brass pipes can be of a continuous layered nature or belong to the so-called cork type, which is the most dangerous. Cork dezincification is characterized by pits that go deep into the metal and are filled with loose copper. The presence of through holes makes it necessary to replace the pipe in order to avoid suction of the cooling raw water into the condensate.
The studies carried out, as well as long-term observations of the state of the surface of condenser tubes in operating capacitors, have shown that the additional introduction of small amounts of arsenic into brass significantly reduces the tendency of brass to dezincification. Complicated in composition brass, additionally alloyed with tin or aluminum, also have increased corrosion resistance due to the ability of these alloys to quickly restore protective films when they are mechanically destroyed. Due to the use of metals that occupy different places in the potential series and are electrically connected, macroelements appear in the capacitor. The presence of a variable temperature field creates the possibility of the development of corrosive EMF of thermoelectric origin. The stray currents that occur when grounding near DC can also cause severe corrosion of capacitors.
Corrosion damage to condenser tubes from condensing steam is most often associated with the presence of ammonia in it. The latter, being a good complexing agent with respect to copper and zinc ions, creates favorable conditions for dezincification of brass. In addition, ammonia causes corrosion cracking of brass condenser tubes in the presence of internal or external tensile stresses in the alloy, which gradually widen the cracks as the corrosion process progresses. It has been established that in the absence of oxygen and other oxidizing agents, ammonia solutions cannot aggressively act on copper and its alloys; therefore, you can not be afraid of ammonia corrosion of brass pipes at an ammonia concentration in the condensate up to 10 mg / dm 3 and the absence of oxygen. In the presence of even a small amount of oxygen, ammonia destroys brass and other copper alloys at a concentration of 2–3 mg / dm 3 .
Steam-side corrosion can primarily affect the brass tubes of vapor coolers, ejectors, and air exhaust chambers of turbine condensers, where conditions are created that favor air ingress and local elevated ammonia concentrations in the partially condensed steam.
To prevent corrosion of condenser tubes on the water side, it is necessary in each specific case, when choosing a metal or alloys suitable for the manufacture of these tubes, to take into account their corrosion resistance at a given composition of the cooling water. Particularly serious attention should be paid to the choice of corrosion-resistant materials for the manufacture of condenser tubes in cases where the condensers are cooled by flowing highly mineralized water, as well as in conditions of replenishing cooling water losses in the circulating water supply systems of thermal power plants, fresh water with increased mineralization, or contaminated with corrosive industrial and domestic effluents.
3.7. Corrosion of make-up and network path equipment
3.7.1. Corrosion of pipelines and hot water boilers
A number of power plants use river and tap waters with low pH and low hardness to feed heating networks. Additional processing of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of corrosive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in acidification schemes used for large heat supply systems with direct hot water intake (2000–3000 t/h). Water softening according to the Na cationization scheme increases its aggressiveness due to the removal of natural corrosion inhibitors - hardness salts.
With poorly established water deaeration and possible increases in oxygen and carbon dioxide concentrations, due to the lack of additional protective measures in heat supply systems, pipelines, heat exchangers, storage tanks and other equipment are subject to internal corrosion.
It is known that an increase in temperature contributes to the development of corrosion processes that occur both with the absorption of oxygen and with the release of hydrogen. With an increase in temperature above 40 ° C, oxygen and carbon dioxide forms of corrosion increase sharply.
A special type of under-sludge corrosion occurs under conditions of a low content of residual oxygen (when the PTE standards are met) and when the amount of iron oxides is more than 400 μg/dm 3 (in terms of Fe). This type of corrosion, previously known in the practice of operating steam boilers, was found under conditions of relatively weak heating and the absence of thermal loads. In this case, loose corrosion products, consisting mainly of hydrated trivalent iron oxides, are active depolarizers of the cathode process.
During the operation of heating equipment, crevice corrosion is often observed, i.e., selective, intense corrosion destruction of the metal in the crack (gap). A feature of the processes taking place in narrow gaps is the reduced oxygen concentration compared to the concentration in the volume of the solution and the slow removal of corrosion reaction products. As a result of the accumulation of the latter and their hydrolysis, a decrease in the pH of the solution in the gap is possible.
With constant replenishment of the heating network with open water intake with deaerated water, the possibility of the formation of through holes in pipelines is completely excluded only in normal hydraulic mode, when excess pressure above atmospheric pressure is constantly maintained at all points of the heat supply system.
Causes of pitting corrosion of pipes of hot water boilers and other equipment are as follows: poor-quality deaeration of make-up water; low pH value due to the presence of aggressive carbon dioxide (up to 10–15 mg / dm 3); accumulation of oxygen corrosion products of iron (Fe 2 O 3) on heat transfer surfaces. The increased content of iron oxides in the network water contributes to the drift of the heating surfaces of the boiler with iron oxide deposits.
A number of researchers recognize an important role in the occurrence of under-sludge corrosion of the process of rusting of pipes of water-heating boilers during their downtime, when proper measures are not taken to prevent parking corrosion. The centers of corrosion that occur under the influence of atmospheric air on the wet surfaces of the boilers continue to function during the operation of the boilers.
3.7.2. Tube corrosion heat exchangers
The corrosion behavior of copper alloys depends significantly on temperature and is determined by the presence of oxygen in water.
In table. 3.1 shows the rates of transition of corrosion products of copper-nickel alloys and brass into water at high (200 μg / dm 3) and low
(3 μg / dm 3) oxygen content. This rate is approximately proportional to the corresponding corrosion rate. It increases significantly with increasing oxygen concentration and salinity of water.
In acidification schemes, the water after the calciner often contains up to 5 mg/dm
Table 3.1
The rate of transition of corrosion products into water from the surface
copper-nickel alloys and brass in a neutral environment, 10 -4 g / (m 2 h)
| Material | The content of O 2, mcg / dm 3 | Temperature, °C |
||||
| 38 | 66 | 93 | 121 | 149 |
||
| MN 70-30 MN 90-10 LO-70-1 | 3 | - | 3,8 | 4,3 | 3,2 | 4,5 |
Hard and soft deposits formed on the surface have a significant effect on the corrosion damage of tubes. The nature of these deposits is important. If deposits are able to filter water and at the same time can retain copper-containing corrosion products on the surface of the tubes, the local process of tube destruction is enhanced. Deposits with a porous structure (solid deposits of scale, organic) have a particularly unfavorable effect on the course of corrosion processes. With an increase in the pH of water, the permeability of carbonate films increases, and with an increase in its hardness, it sharply decreases. This explains that in schemes with starvation regeneration of filters, corrosion processes proceed less intensively than in Na-cationation schemes. The service life of the tubes is also shortened by the contamination of their surface with corrosion products and other deposits, leading to the formation of ulcers under the deposits. With the timely removal of contaminants, local corrosion of the tubes can be significantly reduced. An accelerated failure of heaters with brass tubes is observed with an increased salinity of water - more than 300 mg / dm 3, and chloride concentration - more than 20 mg / dm 3.
Average term The service life of tubes of heat exchangers (3–4 years) can be increased if they are made from corrosion-resistant materials. 1Kh18N9T stainless steel tubes installed in the make-up circuit at a number of thermal power plants with low-mineralized water have been in operation for more than 7 years without signs of damage. However, at present it is difficult to count on the widespread use of stainless steels due to their high scarcity. It should also be borne in mind that these steels are susceptible to pitting corrosion at elevated temperatures, salinity, chloride concentrations and fouling deposits.
When the salt content of make-up and network water is above 200 mg / dm 3 and chloride ions above 10 mg / dm 3, it is necessary to limit the use of brass L-68, especially in the make-up path to the deaerator, regardless of the water treatment scheme. When using softened make-up water containing significant amounts of aggressive carbon dioxide (over 1 mg / dm 3), the flow velocity in devices with a brass pipe system should exceed 1.2 m / s.
Alloy MNZh-5-1 should be used when the temperature of the make-up water of the heating system is above 60 °C.
Table 3.2
Metal tubes of heat exchangers depending
From the heating system make-up water treatment scheme
| Make-up water treatment scheme | Metal tubes of heat exchangers in the path to the deaerator | Metal tubes of network heat exchangers |
| Liming | L-68, LA-77-2 | L-68 |
| Na-cationization | LA-77-2, MNZH-5-1 | L-68 |
| H-cationization with starvation filter regeneration | LA-77-2, MNZH-5-1 | L-68 |
| Acidification | LA-77-2, MNZH-5-1 | L-68 |
| Soft water without treatment W o \u003d 0.5 h 0.6 mmol / dm 3, W o \u003d 0.2 h 0.5 mmol / dm 3, pH = 6.5 h 7.5 | LA-77-2, MNZH-5-1 | L-68 |
3.7.3. Assessment of the corrosion state of existingsystems
hotwater supply and causescorrosion
Hot water systems compared to others engineering structures(heating, cold water supply and sewerage systems) are the least reliable and durable. If the established and actual service life of buildings is estimated at 50–100 years, and for heating, cold water supply and sewerage systems at 20–25 years, then for hot water supply systems at closed scheme for heat supply and communications from uncoated steel pipes, the actual service life does not exceed 10 years, and in some cases 2-3 years.
Hot water pipelines without protective coatings subject to internal corrosion and significant contamination by its products. This leads to a decrease in the throughput of communications, an increase in hydraulic losses and disruptions in the supply of hot water, especially to the upper floors of buildings with insufficient pressure from the city water supply. In large hot water supply systems from central heating points, the overgrowing of pipelines with corrosion products violates the regulation of branched systems and leads to interruptions in the supply of hot water. Due to intense corrosion, especially of external hot water networks from central heating, the volume of current and major repairs is increasing. The latter are associated with frequent rearrangements of internal (in houses) and external communications, disruption of the improvement of urban areas within blocks, long-term interruption of hot water supply to a large number of consumers in case of failure of the head sections of hot water pipelines.
Corrosion damage to hot water pipelines from the central heating substation, if they are laid jointly with distributing heating networks, leads to flooding of the latter with hot water and their intense external corrosion. At the same time, great difficulties arise in detecting accident sites, a large amount of excavation work has to be carried out and the improvement of residential areas has to be worsened.
With insignificant differences in capital investments for the construction of hot and cold water supply and heating systems, the operating costs associated with frequent relocation and repair of hot water supply communications are disproportionately higher.
Corrosion of hot water systems and protection against it is of particular importance due to the scope of housing construction in Russia. The tendency to enlarge the capacities of individual installations leads to a branching of the hot water pipeline network, which, as a rule, is made of ordinary steel pipes without protective coatings. The ever-increasing shortage of water of drinking quality causes the use of new sources of water with high corrosive activity.
One of the main reasons affecting the state of hot water supply systems is the high corrosivity of heated tap water. According to VTI studies, the corrosiveness of water, regardless of the source of water supply (surface or underground), is characterized by three main indicators: the equilibrium saturation index of water with calcium carbonate, the content of dissolved oxygen, and the total concentration of chlorides and sulfates. Previously, in the domestic literature, the classification of heated tap water according to corrosivity, depending on the indicators of the source water, was not given.
In the absence of conditions for the formation of protective carbonate films on the metal (j
Observational data on existing hot water supply systems indicate a significant effect of chlorides and sulfates in tap water on corrosion of pipelines. Thus, even waters with a positive saturation index, but containing chlorides and sulfates in concentrations above 50 mg/dm3, are corrosive, which is due to the discontinuity of carbonate films and their decrease protective effect under the influence of chlorides and sulfates. When the protective films are destroyed, the chlorides and sulfates present in the water increase the corrosion of steel under the action of oxygen.
Based on the corrosion scale adopted in the thermal power industry and the experimental data of VTI, according to the corrosion rate of steel pipes in heated drinking water, a conditional corrosion classification of tap water at a design temperature of 60 ° C is proposed (Table 3.3). 
Rice. 3.2. Dependence of the depth index P of corrosion of steel pipes in heated tap water (60 °C) on the calculated saturation index J:
1, 2, 3 - surface source 
; 4 - underground source 
; 5 - surface source 
On fig. 3.2. experimental data on the corrosion rate in samples of steel pipes with different quality of tap water are given. The graph traces a certain pattern of a decrease in the deep corrosion index (deep permeability) with a change in the calculated water saturation index (with chloride and sulfate content up to 50 mg / dm 3). With negative values of the saturation index, deep permeability corresponds to emergency and severe corrosion (points 1 and 2) ;
for river water with a positive saturation index (point 3) of acceptable corrosion, and for artesian water (point 4) - weak corrosion. Attention is drawn to the fact that for artesian and river water with a positive saturation index and a content of chlorides and sulfates less than 50 mg/dm3, the differences in the deep permeability of corrosion are relatively small. This means that in waters prone to the formation of an oxide-carbonate film on the pipe walls (j > 0), the presence of dissolved oxygen (high in surface water and insignificant in underground water) does not significantly affect the change in deep corrosion permeability. At the same time, test data (point 5) indicate a significant increase in the intensity of steel corrosion in water with a high concentration of chlorides and sulfates (about 200 mg / dm 3 in total), despite a positive saturation index (j = 0.5). The corrosion permeability in this case corresponds to the permeability in water, which has a saturation index j = – 0.4. In accordance with the classification of waters according to corrosivity, water with a positive saturation index and a high content of chlorides and sulfates is classified as corrosive.
Table 3.3
Classification of water by corrosivity
| J at 60 °C | Concentration in cold water, mg / dm 3 | Corrosion characteristic of heated water (at 60 °C) |
|
| dissolved oxygen O 2 | chlorides and sulfates (total)  |
||
 | Any | Any | highly corrosive |
 | Any | >50 | highly corrosive |
| | Any |  | Corrosive |
 | Any | >50 | slightly corrosive |
| | >5 | | slightly corrosive |
| |  | | non-corrosive |
The classification developed by VTI (Table 3.3) quite fully reflects the effect of water quality on its corrosion properties, which is confirmed by data on the actual corrosion state of hot water supply systems.
An analysis of the main indicators of tap water in a number of cities allows us to attribute most of the waters to the type of highly corrosive and corrosive, and only a small part to the type of slightly corrosive and non-corrosive. A large proportion of springs is characterized by an increased concentration of chlorides and sulfates (more than 50 mg/dm 3 ), and there are examples when these concentrations in total reach 400–450 mg/dm 3 . Such a significant content of chlorides and sulfates in tap water makes them highly corrosive.
When assessing the corrosivity of surface waters, it is necessary to take into account the variability of their composition during the year. For a more reliable assessment, one should use the data of not single, but possibly more water analyzes performed in different seasons for one or two last years.
For artesian sources, water quality indicators are usually very stable throughout the year. Usually, The groundwater are characterized by increased mineralization, a positive saturation index for calcium carbonate and a high total content of chlorides and sulfates. The latter leads to the fact that hot water systems in some cities that receive water from artesian wells are also subject to severe corrosion.
When there are several sources of drinking water in one city, the intensity and mass character of corrosion damage to hot water supply systems can be different. So, in Kyiv there are three sources of water supply:
R. Dnieper, r. Desna and artesian wells. Hot water supply systems in city districts supplied with corrosive Dnieper water are most susceptible to corrosion, to a lesser extent - systems operated on slightly corrosive Desnyanskaya water, and to an even lesser extent - on artesian water. The presence of districts in the city with different corrosion characteristics of tap water makes it very difficult to organize anti-corrosion measures both at the design stage and under the operating conditions of hot water supply systems.
To assess the corrosion state of hot water supply systems, they were surveyed in a number of cities. Experimental studies of the corrosion rate of pipes using tubular and plate samples were carried out in the areas of new housing construction in the cities of Moscow, St. Petersburg, etc. The results of the survey showed that the condition of pipelines is directly dependent on the corrosiveness of tap water.
A significant influence on the size of corrosion damage in the hot water supply system is exerted by the high centralization of water heating installations at central heating points or heat distribution stations (TPS). Initially, the widespread construction of central heating stations in Russia was due to a number of reasons: the lack of residential buildings basements suitable for accommodating hot water supply equipment; the inadmissibility of installing conventional (not silent) circulation pumps in individual heating points; the expected reduction in maintenance personnel as a result of the replacement of relatively small heaters installed in individual heating points with large ones; the need to increase the level of operation of central heating stations by automating them and improving maintenance; the possibility of building large installations for anti-corrosion treatment of water for hot water supply systems.
However, as the experience of operating central heating stations and hot water supply systems from them has shown, the number of maintenance personnel has not decreased due to the need to perform a large amount of work during the current and major repairs of hot water supply systems. Centralized anti-corrosion treatment of water at central heating stations has not become widespread due to the complexity of the installations, high initial and operating costs and the lack of standard equipment (vacuum deaeration).
In conditions where steel pipes without protective coatings are predominantly used for hot water supply systems, with high corrosive activity of tap water and the absence of anti-corrosion water treatment at the central heating station, further construction of the central heating station alone seems to be inexpedient. Construction in recent years of houses of new series with basements and the production of silent centrifugal pumps will facilitate the transition in many cases to the design of individual heating points (ITP) and increase the reliability of hot water supply.
3.8. Conservation of thermal power equipment
and heating systems
3.8.1. General position
Preservation of equipment is protection against the so-called parking corrosion.
Preservation of boilers and turbine plants to prevent corrosion of the metal of internal surfaces is carried out during routine shutdowns and put into reserve for a certain and indefinite period: decommissioning - in the current, medium, overhaul; emergency shutdowns, for a long-term reserve or repair, for reconstruction for a period of more than 6 months.
Based production instructions at each power plant, boiler house, a technical solution should be developed and approved for organizing the conservation of specific equipment, which determines the methods of conservation for various types of shutdowns and downtime technological scheme and auxiliary equipment.
When developing a technological scheme for conservation, it is advisable to use as much as possible standard installations for corrective treatment of feed and boiler water, installations for chemical cleaning of equipment, and tank facilities of a power plant.
The technological scheme of conservation should be as stationary as possible, reliably disconnected from the working sections of the thermal scheme.
It is necessary to provide for the neutralization or neutralization of waste water, as well as the possibility reuse preservative solutions.
In accordance with the adopted technical decision, an instruction for equipment conservation is drawn up and approved with instructions on preparatory operations, conservation and de-preservation technology, as well as safety measures during conservation.
When preparing and carrying out work on conservation and re-preservation, it is necessary to comply with the requirements of the Safety Rules for the operation of thermal mechanical equipment of power plants and heating networks. Also, if necessary, additional safety measures related to the properties of the chemicals used should be taken.
Neutralization and purification of spent preservative solutions of chemical reagents must be carried out in accordance with directive documents.
3.8.2. Methods for preservation of drum boilers
1. "Dry" shutdown of the boiler.
Dry shutdown is used for boilers of any pressure in the absence of rolling joints of pipes with a drum in them.
Dry shutdown is carried out during a planned shutdown for reserve or repair for up to 30 days, as well as during an emergency shutdown.
The dry stop technique is as follows.
After the boiler is stopped in the process of its natural cooling or cooling down, drainage begins at a pressure of 0.8 - 1.0 MPa. The intermediate superheater is devaporated onto the condenser. After draining, close all valves and valves of the steam-water circuit of the boiler.
Drainage of the boiler at a pressure of 0.8 - 1.0 MPa allows, after emptying it, to keep the temperature of the metal in the boiler above the saturation temperature at atmospheric pressure due to the heat accumulated by the metal, lining and insulation. In this case, the internal surfaces of the drum, collectors and pipes are dried.
2. Maintaining excess pressure in the boiler.
Maintaining a pressure above atmospheric pressure in the boiler prevents oxygen and air from entering it. Excess pressure is maintained when deaerated water flows through the boiler. Preservation while maintaining excess pressure is used for boilers of all types and pressures. This method is carried out when the boiler is taken into reserve or repair, not related to work on the heating surfaces, for a period of up to 10 days. On boilers with rolling joints of pipes with a drum, excessive pressure is allowed for up to 30 days.
3. In addition to the above preservation methods, the following are used on drum boilers:
Hydrazine treatment of heating surfaces at the operating parameters of the boiler;
Hydrazine treatment at reduced steam parameters;
Hydrazine “cooking” of boiler heating surfaces;
Trilon treatment of boiler heating surfaces;
Phosphate-ammonia "boiling";
Filling the heating surfaces of the boiler with protective alkaline solutions;
Filling the heating surfaces of the boiler with nitrogen;
Preservation of the boiler with a contact inhibitor.
3.8.3. Methods for conservation once-through boilers
1. "Dry" shutdown of the boiler.
Dry shutdown is used on all once-through boilers, regardless of the adopted water chemistry. It is carried out during any planned and emergency shutdowns for up to 30 days. The steam from the boiler is partially released into the condenser so that within 20-30 minutes the pressure in the boiler drops to
30–40 kgf/cm2 (3–4 MPa). Open the inlet manifolds and water economizer drains. When the pressure drops to zero, the boiler is evaporated to the condenser. The vacuum is maintained for at least 15 minutes.
2. Hydrazine and oxygen treatment of heating surfaces at operating parameters of the boiler.
Hydrazine and oxygen treatment is carried out in combination with a dry shutdown. The procedure for carrying out hydrazine treatment of a once-through boiler is the same as that of a drum boiler.
3. Filling the heating surfaces of the boiler with nitrogen.
Filling the boiler with nitrogen is carried out at excess pressure in the heating surfaces. Preservation with nitrogen is used on boilers of any pressure at power plants that have nitrogen from their own installations!
4. Preservation of the boiler with a contact inhibitor.
Preservation of the boiler with a contact inhibitor is used for any types of boilers, regardless of the water-chemical regime used, and is carried out when the boiler is taken into reserve or repaired for a period of 1 month to 2 years.
3.8.4. Ways of preservation of hot water boilers
1. Preservation with calcium hydroxide solution.
The protective film remains for 2–3 months after the boiler has been emptied of the solution after 3–4 or more weeks of contact. Calcium hydroxide is used for the preservation of hot water boilers of any type at power plants, boiler houses with water treatment plants with lime economy. The method is based on highly effective inhibitory abilities of Ca(OH) 2 calcium hydroxide solution. The protective concentration of calcium hydroxide is 0.7 g/DM 3 and above. Upon contact with metal, its stable protective film is formed within 3–4 weeks.
2. Preservation with sodium silicate solution.
Sodium silicate is used for the conservation of hot water boilers of any kind when the boiler is taken into reserve for up to 6 months or when the boiler is taken out for repairs for up to 2 months.
Sodium silicate (liquid sodium glass) forms a strong protective film on the metal surface in the form of a Fe 3 O 4 FeSiO 3 compound. This film shields the metal from the effects of corrosive agents (CO 2 and O 2). When implementing this method, the boiler is completely filled with a solution of sodium silicate with a concentration of SiO 2 in the preservative solution of at least 1.5 g/DM 3 .
The formation of a protective film occurs when the preservative solution is kept in the boiler for several days or the solution circulates through the boiler for several hours.
3.8.5. Methods for conservation of turbine plants
Preservation with heated air. Purging the turbine plant with hot air prevents moist air from entering the internal cavities and the occurrence of corrosion processes. Especially dangerous is the ingress of moisture on the surface of the flow part of the turbine in the presence of deposits of sodium compounds on them. Preservation of a turbine plant with heated air is carried out when it is put into reserve for a period of 7 days or more.
Preservation with nitrogen. When filling the internal cavities of the turbine plant with nitrogen and subsequently maintaining a small excess pressure, the ingress of moist air is prevented. The supply of nitrogen to the turbine is started after the turbine is stopped and the vacuum drying of the intermediate superheater is completed. Preservation with nitrogen can also be applied to the steam spaces of boilers and heaters.
Preservation of corrosion with volatile inhibitors. Volatile corrosion inhibitors of the IFKhAN type protect steel, copper, brass by being adsorbed on the metal surface. This adsorption layer significantly reduces the rate of electrochemical reactions that cause the corrosion process.
To preserve the turbine plant, air saturated with the inhibitor is sucked through the turbine. Air is saturated with an inhibitor when it comes into contact with silica gel impregnated with an inhibitor, the so-called linasil. Linasil is impregnated at the factory. To absorb excess inhibitor at the outlet of the turbine, the air passes through pure silica gel. For preservation of 1 m 3 volume, at least 300 g of linasil is required, the protective concentration of the inhibitor in the air is 0.015 g/dm 3 .
3.8.6. Conservation of heating networks
During the silicate treatment of make-up water, a protective film is formed against the effects of CO 2 and O 2 . In this case, with direct analysis of hot water, the content of silicate in make-up water should be no more than 50 mg / dm 3 in terms of SiO 2.
When silicate treatment of make-up water, the maximum concentration of calcium should be determined taking into account the total concentration of not only sulfates (to prevent precipitation of CaSO 4), but also silicic acid (to prevent precipitation of CaSiO 3) for a given heating water temperature, taking into account the boiler pipes 40 ° C ( PTE 4.8.39).
At closed system heat supply, the working concentration of SiO 2 in the preservative solution can be 1.5 - 2 g / dm 3.
If you do not preserve with a solution of sodium silicate, then heating network during the summer period, they must always be filled with network water that meets the requirements of PTE 4.8.40.
3.8.7. Brief characteristics of the chemicals used
for conservation and precautions when working with them
Water solution hydrazine hydrate N 2
H 4
·N 2
O
A solution of hydrazine hydrate is a colorless liquid that easily absorbs water, carbon dioxide and oxygen from the air. Hydrazine hydrate is a strong reducing agent. Toxicity (hazard class) of hydrazine - 1.
Aqueous solutions of hydrazine with a concentration of up to 30% are not flammable - they can be transported and stored in carbon steel vessels.
When working with solutions of hydrazine hydrate, it is necessary to exclude the ingress of porous substances and organic compounds into them.
Hoses should be connected to the places of preparation and storage of hydrazine solutions to flush the spilled solution from the equipment with water. For neutralization and neutralization, bleach must be prepared.
The solution of hydrazine that has fallen on the floor should be covered with bleach and washed off with plenty of water.
Aqueous solutions of hydrazine can cause skin dermatitis and irritate the respiratory tract and eyes. Hydrazine compounds entering the body cause changes in the liver and blood.
When working with hydrazine solutions, it is necessary to use personal glasses, rubber gloves, a rubber apron, a KD gas mask.
Drops of hydrazine solution that come into contact with the skin and eyes should be washed off with plenty of water.
Aqueous ammonia solutionNH 4
(Oh)
An aqueous solution of ammonia (ammonia water) is a colorless liquid with a sharp specific odor. At room temperature and especially when heated, ammonia is abundantly released. Toxicity (hazard class) of ammonia - 4. The maximum permissible concentration of ammonia in the air - 0.02 mg / dm 3. Ammonia solution is alkaline. When working with ammonia, the following safety precautions must be observed:
- ammonia solution should be stored in a tank with a sealed lid;
– spilled ammonia solution should be washed off with plenty of water;
– if it is necessary to repair the equipment used for the preparation and dosing of ammonia, it should be thoroughly rinsed with water;
- Aqueous solution and ammonia vapors cause irritation of the eyes, respiratory tract, nausea and headache. Especially dangerous is the ingress of ammonia into the eyes;
– when working with ammonia solution, it is necessary to use protective goggles;
– Ammonia that has come into contact with the skin and eyes must be washed off with plenty of water.
Trilon B
Commodity Trilon B is a white powdery substance.
Trilon solution is stable, does not decompose during prolonged boiling. The solubility of Trilon B at a temperature of 20–40 °C is 108–137 g/dm 3 . The pH value of these solutions is about 5.5.
Commodity Trilon B is supplied in paper bags with a polyethylene liner. The reagent must be stored in a closed, dry place.
Trilon B does not have a noticeable physiological effect on the human body.
When working with commodity Trilon, it is necessary to use a respirator, gloves and goggles.
Trisodium phosphateNa 3
PO 4
12N 2
O
Trisodium phosphate is a white crystalline substance, highly soluble in water.
In a crystalline form, it does not have a specific effect on the body.
In a dusty state, getting into the respiratory tract or eyes irritates the mucous membranes.
Hot phosphate solutions are dangerous if splashed into the eyes.
When carrying out work accompanied by dusting, it is necessary to use a respirator and goggles. Use goggles when working with hot phosphate solution.
In case of contact with skin or eyes, rinse with plenty of water.
Sodium hydroxideNaOH
Caustic soda is a white, solid, very hygroscopic substance, highly soluble in water (at a temperature of 20 ° C, the solubility is 1070 g / dm 3).
Caustic soda solution is a colorless liquid heavier than water. The freezing point of a 6% solution is minus 5 °C, a 41.8% solution is 0 °C.
Caustic soda in solid crystalline form is transported and stored in steel drums, and liquid alkali in steel containers.
Caustic soda (crystalline or liquid) that has fallen on the floor should be washed off with water.
If it is necessary to repair the equipment used for the preparation and dosing of alkali, it should be washed with water.
Solid caustic soda and its solutions cause severe burns, especially if it comes into contact with the eyes.
When working with caustic soda, it is necessary to provide a first-aid kit containing cotton wool, a 3% solution of acetic acid and a 2% solution of boric acid.
Personal protective equipment when working with caustic soda - cotton suit, goggles, rubberized apron, rubber boots, rubber gloves.
If alkali gets on the skin, it must be removed with cotton wool, rinse the affected area with acetic acid. If alkali gets into the eyes
it is necessary to wash them with a stream of water, and then with a solution of boric acid and contact the first-aid post.
Sodium silicate (liquid glass sodium)
Commercial liquid glass is a thick solution of yellow or gray color, the content of SiO 2 in it is 31 - 33%.
Sodium silicate comes in steel barrels or tanks. Liquid glass should be stored in dry enclosed spaces at a temperature not lower than plus 5 °C.
Sodium silicate is an alkaline product, it dissolves well in water at a temperature of 20 - 40 °C.
If a liquid glass solution comes into contact with the skin, it should be washed off with water.
Calcium hydroxide (lime mortar) Ca(OH) 2
Lime mortar is a clear, colorless and odorless liquid, non-toxic and slightly alkaline.
A solution of calcium hydroxide is obtained by settling milk of lime. The solubility of calcium hydroxide is low - no more than 1.4 g / dm 3 at 25 ° C.
When working with lime mortar, people with sensitive skin are advised to wear rubber gloves.
If the solution gets on the skin or in the eyes, wash it off with water.
contact inhibitor
Inhibitor M-1 is a salt of cyclohexylamine (TU 113-03-13-10-86) and synthetic fatty acids of fraction C 10-13 (GOST 23279-78). In its commercial form, it is a pasty or solid substance from dark yellow to brown. The melting point of the inhibitor is above 30 °C, the mass fraction of cyclohexylamine is 31–34%, the pH of the alcohol-water solution is mass fraction the main substance 1% is equal to 7.5–8.5; the density of a 3% aqueous solution at a temperature of 20 ° C is 0.995 - 0.996 g / dm 3.
Inhibitor M-1 is supplied in steel drums, metal flasks, steel barrels. Each package must be marked with the following data: name of the manufacturer, name of the inhibitor, lot number, date of manufacture, net weight, gross weight.
Commercial inhibitor refers to combustible substances and must be stored in a warehouse in accordance with the rules for the storage of combustible substances. The aqueous solution of the inhibitor is not flammable.
The inhibitor solution that has fallen on the floor must be washed off with plenty of water.
If it is necessary to repair the equipment used to store and prepare the inhibitor solution, it should be thoroughly rinsed with water.
The M-1 inhibitor belongs to the third class (moderately hazardous substances). MPC in the air of the working area for the inhibitor should not exceed 10 mg/dm 3 .
The inhibitor is chemically stable, does not form toxic compounds in the air and wastewater in the presence of other substances or industrial factors.
Persons involved in work with an inhibitor must have a cotton suit or dressing gown, gloves, and a headgear.
Wash hands after handling inhibitor. warm water with soap.
Volatile Inhibitors
Volatile atmospheric corrosion inhibitor IFKHAN-1(1-diethylamino-2 methylbutanone-3) is clear liquid yellowish color with a sharp specific smell.
The liquid inhibitor of IFKhAN-1, according to the degree of exposure, belongs to highly hazardous substances. MPC of inhibitor vapors in the air of the working area should not exceed 0.1 mg/dm 3 . The IFKhAN-1 inhibitor in high doses causes excitation of the central nervous system, irritating effect on the mucous membranes of the eyes, upper respiratory tract. Prolonged exposure of the inhibitor to unprotected skin may cause dermatitis.
The IFKhAN-1 inhibitor is chemically stable and does not form toxic compounds in the air and wastewater in the presence of other substances.
Liquid inhibitor IFKhAN-1 refers to flammable liquids. The ignition temperature of the liquid inhibitor is 47°C, the self-ignition temperature is 315°C. In case of fire, the following fire extinguishing agents are used: felt mat, foam fire extinguishers, OS fire extinguishers.
Cleaning of premises should be carried out in a wet way.
When working with the IFKhAN-1 inhibitor, it is necessary to use personal protective equipment - a suit made of cotton fabric (robe), rubber gloves.
Inhibitor IFKHAN-100, which is also a derivative of amines, is less toxic. Relatively safe level exposure - 10 mg / dm 3; ignition temperature 114 °C, self-ignition 241 °C.
Safety measures when working with the IFKhAN-100 inhibitor are the same as when working with the IFKhAN-1 inhibitor.
It is forbidden to carry out work inside the equipment until it is depreserved.
At high concentrations of the inhibitor in the air or if it is necessary to work inside the equipment after it has been depreserved, a brand A gas mask with a brand A filter box (GOST 12.4.121-83 and
GOST 12.4.122-83). The equipment must be ventilated beforehand. Work inside the equipment after depreservation should be carried out by a team of two people.
After finishing work with the inhibitor, wash your hands with soap and water.
In case of contact with the liquid inhibitor on the skin, wash it off with soap and water, in case of contact with the eyes, rinse them with a plentiful stream of water.
test questions
Types of corrosion processes.
Describe chemical and electrochemical corrosion.
Influence of external and internal factors on metal corrosion.
Corrosion of the condensate-feeding path of boiler units and heating networks.
Corrosion of steam turbines.
Corrosion of the equipment of make-up and network paths of the heating network.
The main methods of water treatment to reduce the intensity of corrosion of the heating system.
The purpose of conservation of thermal power equipment.
List the preservation methods.
B) hot water boilers;
B) turbine plants;
D) heating networks.
10. Give a brief description of the chemicals used.
Low-temperature corrosion affects the heating surfaces of tubular and regenerative air heaters, low-temperature economizers, as well as metal gas ducts and chimneys at metal temperatures below the dew point flue gases. The source of low-temperature corrosion is sulfuric anhydride SO 3 , which forms sulfuric acid vapor in flue gases, which condenses at flue gas dew point temperatures. A few thousandths of a percent of SO 3 in gases is enough to cause metal corrosion at a rate exceeding 1 mm/year. Low-temperature corrosion slows down when organizing a furnace process with small excesses of air, as well as when using fuel additives and increasing the corrosion resistance of the metal.
The furnace screens of drum and once-through boilers are exposed to high-temperature corrosion during combustion. solid fuel, superheaters and their fastenings, as well as screens for the lower radiation part of supercritical pressure boilers when burning sulfurous fuel oil.
Corrosion of the inner surface of the pipes is a consequence of the interaction with the metal of the pipes of gases of oxygen and carbon dioxide) or salts (chlorides and sulfates) contained in the boiler water. AT modern boilers supercritical steam pressure, the content of gases and corrosive salts as a result of deep desalination of feed water and thermal deaeration is insignificant, and the main cause of corrosion is the interaction of metal with water and steam. Corrosion of the inner surface of pipes is manifested in the formation of pockmarks, pits, shells and cracks; the outer surface of damaged pipes may not differ from healthy ones.
Damage due to internal pipe corrosion also includes:
oxygen parking corrosion affecting any parts of the inner surface of pipes. The areas covered with water-soluble deposits are most intensively affected (pipes of superheaters and the transition zone of once-through boilers);
under-sludge alkaline corrosion of boiler and screen pipes, which occurs under the action of concentrated alkali due to evaporation of water under a layer of sludge;
corrosion fatigue, which manifests itself in the form of cracks in boiler and screen pipes as a result of simultaneous exposure to a corrosive environment and variable thermal stresses.
Scale is formed on pipes as a result of their overheating to temperatures significantly higher than the calculated ones. In connection with the increase in the productivity of boiler units, cases of failure of superheater pipes due to insufficient scale resistance to flue gases have recently become more frequent. Intensive scaling is most often observed during the combustion of fuel oil.
Pipe wall wear occurs as a result of the abrasive action of coal and shale dust and ash, as well as steam jets coming out of damaged adjacent pipes or blower nozzles. Sometimes the cause of wear and hardening of the pipe walls is the shot used to clean the heating surfaces. The places and degree of wear of pipes are determined by external inspection and measurement of their diameter. The actual wall thickness of the pipe is measured with an ultrasonic thickness gauge.
Warping of screen and boiler pipes, as well as individual pipes and sections of wall panels of the radiation part of once-through boilers, occurs when pipes are installed with an uneven tightness, pipe fasteners are broken, water is lost, and due to the lack of freedom for their thermal movements. Warping of coils and screens of the superheater occurs mainly due to burning of hangers and fasteners, excessive and uneven tightness allowed during installation or replacement individual elements. Warping of the water economizer coils occurs due to burnout and displacement of supports and hangers.
Fistulas, bulges, cracks and ruptures can also appear as a result of: deposits in pipes of scale, corrosion products, technological scale, welding flash and other foreign objects that slow down the circulation of water and contribute to overheating of the pipe metal; shot hardening; non-compliance of steel grade with steam parameters and gas temperature; external mechanical damage; operational violations.
