2.1. heating surfaces.

The most typical 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 pipe walls, 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 the pipes also includes: oxygen parking corrosion, undersludge 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 pipes, cracks, risks and delamination of 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 the 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 bulges (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 parts of the lining.

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 the boiler, taken out for repair, is carried out according to the results of a hydraulic test, external and internal inspection, as well as other types of control carried out in the scope and in accordance with the program of expert examination of the boiler (section "Program of 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 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%.

Galustov V.S. Direct-flow sprayers in thermal power engineering (Document)

Filonov A.G. Water-chemical regimes of thermal power plants (Document)

Physical and chemical processes in the technosphere. Collection of tasks (Document)

Orlov D.S. Soil Chemistry (Document)

n1.doc

3.4. Corrosion of steam generator elements
3.4.1. 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 of 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 (in water economizers) have a particularly negative effect, 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 ax 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 enters steam generators when feed water containing an increased amount of ammonia, oxygen and free carbon dioxide aggressively acts on brass and copper pipes of 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. The copper ions present in the low pH feed water enter the steam generator under conditions alkaline environment also precipitate 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. Under-sludge corrosion occurs in the 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 sections of the metal under the layer of sludge. 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 ratios of the concentration of caustic soda and the sodium salts contained in the 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 content of chlorides 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 the 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 at high temperatures protective layer iron oxides to form 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.

Intergranular 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 a special case of electrochemical corrosion occurring along the grain boundaries of 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, 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, if the metal is exposed to thermal stresses simultaneously with a corrosive medium, which are variable in sign and magnitude, 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 of the drum walls 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 one 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 ultrahigh and supercritical pressure steam generators 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 proper 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 of the 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.

Means of protecting steam generators from parking corrosion during periods of their downtime in reserve and under repair are various methods conservation.
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 standby 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 blade apparatus of the turbine before it is idle for a long time.

Corrosion during the idle period usually has a relatively uniform character, under 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 steps 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 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 selection main, 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 exhaust air 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 for a given composition of 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 replenishment of cooling water losses in the circulating water supply systems of thermal power plants, fresh waters with increased mineralization, or contaminated with corrosive industrial and household waste.
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 water intake. hot water(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 cathodic process.

During the operation of heating equipment, crevice corrosion is often observed, i.e., selective, intense corrosion destruction of the metal in the crack (gap). A feature of the processes occurring in narrow gaps is the reduced oxygen concentration compared to the concentration in the bulk solution and the slow removal of corrosion reaction products. As a result of the accumulation of the latter and their hydrolysis, a decrease in the pH of the solution in the gap is possible.

With constant replenishment of a heat network with open water intake with deaerated water, the possibility of the formation of through holes in pipelines is completely excluded only under normal hydraulic conditions, 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. Corrosion of tubes of 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. Accelerated failure of heaters with brass tubes observed with increased salt content of water - more than 300 mg / dm 3, and chloride concentration - more than 20 mg / dm 3.

The average service life of heat exchanger tubes (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 with a closed heat supply scheme and communications made of 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 are subject to internal corrosion and significant contamination by its products. This leads to a decrease bandwidth 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 supply 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, 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 network of hot water supply pipelines, which, as a rule, are 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 corrosivity 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 a decrease in their 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 °С	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 impact of water quality on its corrosive properties, which is confirmed by data on the actual corrosive 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 causes their high corrosiveness.

When evaluating corrosivity surface water 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 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 basements in new residential buildings 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 on 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. The centralized anti-corrosion treatment of water at the central heating station did not receive widespread due to complexity of installations, high initial and operating costs and 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 inappropriate. 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 networks

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 decommissioning for a certain and indefinite period: decommissioning - for 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, determining the methods of conservation for various types of shutdowns and the duration of downtime of the 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 of reusing 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 all types of boilers, regardless of the water chemistry 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 preservation 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. Purge of the turbine plant with hot air prevents penetration into the internal cavities humid air and the course 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 conservation 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 heating temperature, taking into account the boiler pipes 40 ° C ( PTE 4.8.39).

With a closed heat supply system, 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 the heating networks in summer period 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
An aqueous solution of 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.

AT crystalline form has no 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)
Commodity 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.

In case of contact with the skin liquid glass 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 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 an alcohol-water solution with a mass fraction of the main substance of 1% is 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, batch 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 dangerous substances). MPC in the air working area for 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 a clear yellowish liquid with a sharp specific odor.

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 . Inhibitor IFKhAN-1 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 an IFKhAN-1 inhibitor, it is necessary to use agents personal protection- a suit made of cotton fabric (robe), rubber gloves.

Inhibitor IFKHAN-100, which is also a derivative of amines, is less toxic. Relatively safe exposure level - 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

1. 
   Types of corrosion processes.
2. 
   Describe chemical and electrochemical corrosion.
3. 
   Influence of external and internal factors for metal corrosion.
4. 
   Corrosion of the condensate-feeding path of boiler units and heating networks.
5. 
   Corrosion of steam turbines.
6. 
   Corrosion of the equipment of make-up and network paths of the heating system.
7. 
   The main methods of water treatment to reduce the intensity of corrosion of the heating system.
8. 
   The purpose of conservation of thermal power equipment.
9. 
   List the preservation methods.

a) steam boilers;

B) hot water boilers;

B) turbine plants;

D) heating networks.

10. Give a brief description of the chemicals used.

The owners of the patent RU 2503747:

FIELD OF TECHNOLOGY

SUBSTANCE: invention relates to thermal power engineering and can be used to protect heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems of residential buildings and industrial facilities from scale during current operation.

BACKGROUND OF THE INVENTION

The operation of steam boilers is associated with the simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the heating surfaces of the boiler are separate phases of a complex system that is formed when they come into contact. The result of the interaction of these phases are surface processes that occur at the interface between them. As a result, corrosion and scale formation occur in the metal of heating surfaces, which leads to a change in the structure and mechanical properties metal, and that contributes to the development of various damage. Since the thermal conductivity of the scale is fifty times lower than that of the iron of the 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 scaling in a continuous steam boiler system often results in production being stopped for several days a year to remove the scaling.

The quality of the feed water and therefore the boiler water is determined by the presence of impurities that can cause different kinds metal corrosion of internal heating surfaces, 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 the transportation of water, and condensate through pipelines, in water treatment processes. Removal of impurities from the 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 present in the source water. However, the methods used do not completely eliminate the content of impurities in water, which is associated not only with technical difficulties, but also with the economic feasibility of using pre-boiler water treatment methods. In addition, since water treatment is a complex technical system, it is redundant for small and medium capacity boilers.

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

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

US Pat. No. 1,877,389 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 in passing direct or alternating current through the system. The authors note that the mechanism of the method is that under the action of an electric current, gas bubbles form on the surface of the boiler, which lead to the exfoliation of the 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, in particular water, in a pipeline in order to slow down scale formation. This method is based on the creation in pipes electromagnetic field, 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 makes it possible to operate boilers, boilers, heat exchangers, and hard water cooling systems. The disadvantage of this method is the high cost and complexity of the equipment used.

WO 2004016833 proposes a method for reducing scale formation on a metal surface exposed to a supersaturated alkaline aqueous solution that is capable of scale formation after a period of exposure, comprising applying a cathode 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 cathode potential.

Thus, there is currently a need to develop an improved method for preventing scale formation of heating pipes, hot water and steam boilers, which is economical and highly effective and provides anti-corrosion protection of 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 electrical 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

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

Another object 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 for the use of consumable reagents to prevent the formation of scale and corrosion of the heating pipes of hot water and steam boilers.

Yet another object of the present invention is to enable work to be started to prevent scaling and corrosion of hot water and steam boiler heating pipes 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 in contact with a water-steam environment from which scale is capable of forming. Said method consists in applying to said metal surface a current-carrying electric potential sufficient to neutralize the electrostatic component of the force of adhesion of colloidal particles and ions to the metal surface.

According to some particular embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some particular embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the inner surface of the heating pipes 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 possibility of using once purchased a working electrophysical apparatus without the need for consumable synthetic reagents. Another advantage is the possibility of starting work on contaminated boiler tubes.

The technical result of the present invention, therefore, is to increase the 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 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 according to the present invention consists in applying to a metal surface subject to scale formation a conductive 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 "conductive electrical potential" in the sense in which it is used in this application means an alternating potential that neutralizes the electrical double layer at the interface between the metal and the steam-water medium containing salts that lead to the formation of scale.

As is known to a person skilled in the art, electric charge carriers in a metal, which are 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 are part of the double electric layer during the formation of scale. The current-carrying, electric, pulsating (that is, alternating) potential initiates the removal of the electric charge of dislocations from the metal surface to the ground. In this regard, it is a current-carrying dislocation current. As a result of this conductive electrical potential, the electrical double 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 blowdowns.

Thus, the term "current-removing potential" is understandable to a specialist in this field of technology and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).

The device described in RU 2100492 C1, which includes a converter with a frequency converter and a pulsating potential controller, as well as a pulse shape controller, can be used as a device for creating a current-carrying electrical potential, for example. Detailed description this device is given in RU 2100492 C1. Any other similar device can also be used, as will be understood by a person 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 efficiency of the application of the claimed method. One skilled in the art, using the information disclosed herein and using standard test procedures, will be able to determine the optimal location for applying the current-dissipating electrical potential.

In some embodiments of the present invention, the conductive electrical potential is variable.

The conductive electrical potential according to the present invention may be applied for various periods of time. The potential application time is determined by the nature and degree of contamination of the metal surface, the composition of the water used, the temperature regime and the features of the operation of the heat engineering device, and other factors known to specialists in this field of technology. One skilled in the art, using the information disclosed in this specification and using standard test procedures, will be able to determine optimal time applications of the current-carrying electrical potential, based on the goals, conditions and state of the heat engineering device.

The value 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 on the basis of information known from the prior art, for example, from the book Deryagin B.V., Churaev N.V., Muller V.M. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the value 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. The values ​​of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the electrical double layer, which is the basis of the electrostatic component of the adhesion forces in the scale and, as a result, to the destruction of the scale. Current-removing potential values ​​below 61 V are insufficient for scale destruction, and at current-removing potential values ​​above 150 V, undesirable electroerosive 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 heat engineering devices: heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems for residential buildings and industrial facilities during current operation. This list is illustrative and does not limit the list of devices to which the method of the present invention may be applied.

In some embodiments, the iron-containing alloy from which the metal surface 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, magnico, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron alloys to which the method of the present invention may be applied. A person skilled in the art, on the basis of knowledge known from the prior art, will be able to such iron-containing alloys that can be used according to the present invention.

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

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

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 improve the efficiency of boilers, which significantly affects the economic performance of its work;

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

Allows you to abandon the processes of softening and deaeration of water, 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 heat supply systems, thermal desalination plants. sea ​​water, steam generators, etc.

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

IMPLEMENTATION EXAMPLE

The method claimed in the present invention was tested at the boiler plants "Admiralty Shipyards" and "Red Chemist". It has been shown that the method according to the present invention effectively cleans the internal surfaces of boilers from deposits. In the course of these works, an equivalent fuel saving of 3-10% was obtained, while the spread of savings values ​​is associated with varying degrees contamination of the internal surfaces of boilers. The aim of the work was to evaluate the effectiveness of the claimed method to ensure a reagent-free, scale-free operation of medium-sized steam boilers in conditions of high-quality water treatment, compliance with the water-chemical regime and a high professional level of equipment operation.

The test of the method claimed in the present invention was carried out on the 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 normative documents. The boiler is equipped with all the necessary means of monitoring its operation 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 capacity of the boiler was maintained at 18 t/h, the steam pressure in the boiler drum was 8.1...8.3 kg/cm 2 . The economizer worked in the heating mode. The source water was city water supply, which met the requirements of GOST 2874-82 "Drinking water". It should be noted that the amount of iron compounds at the input to the specified boiler room, as a rule, exceeds the regulatory requirements (0.3 mg/l) and amounts to 0.3-0.5 mg/l, which leads to intensive overgrowth of the internal surfaces with ferruginous compounds.

Evaluation of the effectiveness of the method was carried out according to the state of the internal surfaces of the boiler.

Evaluation of the influence of the method according to the present invention on the state of the internal heating surfaces of the boiler unit.

Prior to the start of the tests, an internal inspection of the boiler unit was carried out and the initial state of the internal surfaces was recorded. The 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 solid dark brown deposits with paramagnetic properties and, presumably, consisting of iron oxides. The thickness of deposits was up to 0.4 mm visually. In the visible part of the boiler pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of the pipe length with a size of 2 to 15 mm and a thickness of up to 0.5 mm visually).

The device for creating a current-removing potential, described in EN 2100492 C1, was attached at point (1) to the hatch (2) of the upper drum from the back of the boiler (see Fig.1). The current-carrying electrical potential was equal to 100 V. The current-carrying electrical 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, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the hatches of the drums (connection points of the device to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from hatches deposits (6) are 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 (7) 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 manifested. The current-carrying electrical potential was equal to 100 V. The current-carrying electrical 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, it was found that there were almost no deposits (no more than 0.1 mm visually) on the surface of the upper and lower drums within 2-2.5 meters from the hatches of the drums (the connection point of the device for creating a current-discharging 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 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 manifested.

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

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, effectively destroys previously formed deposits and provides a scale-free operation of the boiler.

At the next stage of testing, a 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 made after 3.5 months of continuous operation of the device.

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

This led to the following conclusions:

The size of the zone within which the 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 connection point of the current-removing potential to clean the entire internal surface of the boiler unit and further maintain its scale-free mode of operation;

The destruction of previously formed deposits and the prevention of the formation of new ones is provided by processes of various 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 boiler pipes and determine the reliability of ensuring the boiler's scale-free operation. 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 boiler pipes ended with almost complete removal of deposits. On the entire surface of the metal, a thin dense coating was formed, which had a black color with a blue tint, the thickness of which even in a wet state (almost immediately after opening the boiler) did not exceed 0.1 mm visually.

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

The protective effect of the magnetite film persisted for up to 2 months after the device was disconnected, which is quite enough to ensure the dry conservation of the boiler unit 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 to them and that it can 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 medium from which scale can form, including applying a current-carrying electrical potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and scale-forming ions.

SUBSTANCE: invention relates to thermal power engineering and can be used to protect heating pipes of steam and hot water boilers, heat exchangers, boiler plants, evaporators, heating mains, heating systems of residential buildings and industrial facilities from scale and corrosion 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 medium from which scale is capable of forming includes applying a current-removing electric potential in the range from 61 V to 150 V to the specified metal surface to neutralize the electrostatic component of the adhesion force between the specified metal surface and colloidal particles and scale-forming ions. The technical result is an increase in the efficiency and productivity of hot water and steam boilers, an increase in the efficiency of heat transfer, ensuring layer-by-layer destruction and removal of the formed scale, as well as preventing its new formation. 2 w.p. f-ly, 1 pr., 1 ill.

MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR

MAIN SCIENTIFIC AND TECHNICAL DEPARTMENT OF ENERGY AND ELECTRIFICATION

METHODOLOGICAL INSTRUCTIONS
BY WARNING
LOW TEMPERATURE
SURFACE CORROSION
HEATING AND GAS FLUES OF BOILERS

RD 34.26.105-84

SOYUZTEKHENERGO

Moscow 1986

DEVELOPED by the All-Union Twice Order of the Red Banner of Labor Thermal Engineering Research Institute named after F.E. Dzerzhinsky

PERFORMERS R.A. PETROSYAN, I.I. NADYROV

APPROVED by the Main Technical Directorate for the Operation of Power Systems on April 22, 1984.

Deputy Head D.Ya. SHAMARAKOV

METHODOLOGICAL INSTRUCTIONS FOR THE PREVENTION OF LOW-TEMPERATURE CORROSION OF HEATING SURFACES AND GAS DUTS OF BOILERS

RD 34.26.105-84

Expiry date set
from 01.07.85
until 01.07.2005

These Guidelines apply to low-temperature heating surfaces of steam and hot water boilers (economizers, gas evaporators, air heaters of various types, etc.), as well as to the gas path behind 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 sour fuels and organizations designing boiler equipment.

1. Low-temperature corrosion is the corrosion of tail heating surfaces, gas ducts and chimneys of boilers under the action of sulfuric acid vapors condensing on them from flue gases.

2. Condensation of sulfuric acid vapors, the volume content of which in flue gases during the combustion of sulfurous fuels is only a few thousandths of a percent, occurs at temperatures that are significantly (by 50 - 100 ° C) higher than the condensation temperature of water vapor.

4. To prevent corrosion of heating surfaces during operation, the temperature of their walls must exceed the flue gas dew point temperature at all boiler loads.

For heating surfaces cooled by a medium with a high heat transfer coefficient (economizers, gas evaporators, etc.), the temperatures of the medium at their inlet must exceed the dew point temperature by about 10 °C.

5. For the heating surfaces of hot water boilers when they are operated on sulphurous fuel oil, the conditions for the complete exclusion of low-temperature corrosion cannot be realized. To reduce it, it is necessary to ensure the temperature of the water at the inlet to the boiler, equal to 105 - 110 °C. When using hot water boilers as peak boilers, this mode can be provided with full use of network water heaters. When using hot water boilers in the main mode, an increase in the temperature of the water entering the boiler can be achieved by recirculating hot water.

In installations using the scheme for connecting hot water boilers to the heating network through water heat exchangers, the conditions for reducing low-temperature corrosion of heating surfaces are fully provided.

6. For air heaters of steam boilers, the complete exclusion of low-temperature corrosion is ensured when the design temperature of the wall of the coldest section exceeds the dew point temperature at all boiler loads by 5-10 °C (the minimum value refers to the minimum load).

7. The calculation of the wall temperature of tubular (TVP) and regenerative (RAH) air heaters is carried out according to the recommendations of the “Thermal calculation of boiler units. Normative method” (M.: Energy, 1973).

8. When used in tubular air heaters as the first (by air) pass of replaceable cold cubes or cubes made of pipes with an acid-resistant coating (enamelled, etc.), as well as those made of corrosion-resistant materials, the following are checked for conditions for the complete exclusion of low-temperature corrosion (by air) metal cubes of the air heater. In this case, the choice of the wall temperature of cold metal cubes of replaceable, as well as corrosion-resistant cubes, should exclude intensive contamination of pipes, for which their minimum wall temperature during the combustion of sulfurous fuel oils should be below the dew point of flue gases by no more than 30 - 40 ° C. When burning solid sulfur fuels, the minimum temperature of the pipe wall, according to the conditions for preventing its intensive pollution, should be taken at least 80 °C.

9. In RAH, under conditions of complete exclusion of low-temperature corrosion, their hot part is calculated. The cold part of the RAH is made corrosion-resistant (enamelled, ceramic, low-alloy steel, etc.) or replaceable from flat metal sheets with a thickness of 1.0 - 1.2 mm, made of low-carbon steel. The conditions for preventing intense contamination of the packing are observed when fulfilling the requirements of clause of this document.

10. As an enameled packing, metal sheets with a thickness of 0.6 mm are used. The service life of enamelled packing, manufactured in accordance with TU 34-38-10336-89, is 4 years.

Porcelain tubes can be used as ceramic packing, ceramic blocks, or porcelain plates with ledges.

Given the reduction in fuel oil consumption by thermal power plants, it is advisable to use for the cold part of the RAH a packing made of low-alloy steel 10KhNDP or 10KhSND, the corrosion resistance of which is 2–2.5 times higher than that of low-carbon steel.

11. To protect air heaters from low-temperature corrosion during the start-up period, it is necessary to carry out the measures set out in the “Guidelines for the design and operation of power heaters with wire fins” (M.: SPO Soyuztekhenergo, 1981).

Kindling of the boiler on sulphurous fuel oil should be carried out with the air heating system turned on beforehand. The temperature of the air in front of the air heater in the initial period of kindling should, as a rule, be 90 °C.

11a. To protect the air heaters from low-temperature ("station") corrosion on a stopped boiler, the level of which is approximately twice as high as the corrosion rate during operation, before shutting down the boiler, it is necessary to thoroughly clean the air heaters from external deposits. At the same time, before shutting down the boiler, it is recommended to maintain the air temperature at the inlet to the air heater at the level of its value at the rated load of the boiler.

Cleaning of TVP is carried out with shot with a feed density of at least 0.4 kg/m.s (p. of this document).

For solid fuels, taking into account the significant risk of corrosion of ash collectors, the temperature of the flue gases should be selected above the dew point of the flue gases by 15–20 °C.

For sulphurous fuel oils, the flue gas temperature must exceed the dew point temperature at the rated load of the boiler by about 10 °C.

Depending on the sulfur content in the fuel oil, the calculated flue gas temperature at nominal boiler load should be taken as follows:

Flue gas temperature, ºС...... 140 150 160 165

When burning sulphurous fuel oil with extremely small excesses of air (α ≤ 1.02), the flue gas temperature can be taken lower, taking into account the results of dew point measurements. On average, the transition from small excesses of air to extremely small ones reduces the dew point temperature by 15 - 20 °C.

The conditions for ensuring reliable operation of the chimney and preventing moisture from falling on its walls are affected not only by the temperature of the flue gases, but also by their flow rate. The operation of the pipe with load conditions significantly lower than the design ones increases the likelihood of low-temperature corrosion.

When burning natural gas, the flue gas temperature is recommended to be at least 80 °C.

13. When the boiler load is reduced in the range of 100 - 50% of the nominal one, one should strive to stabilize the flue gas temperature, not allowing it to decrease by more than 10 °C from the nominal one.

The most economical way to stabilize the flue gas temperature is to increase the air preheating temperature in the heaters as the load decreases.

The minimum allowable temperatures for air preheating before RAH are taken in accordance with clause 4.3.28 of the Rules for the Technical Operation of Power Plants and Networks (M.: Energoatomizdat, 1989).

In cases where the optimum flue gas temperatures cannot be ensured due to insufficient RAH heating surface, air preheating temperatures should be taken at which the flue gas temperature will not exceed the values ​​given in paragraphs of these Guidelines.

16. Due to the lack of reliable acid-resistant coatings for protection against low-temperature corrosion of metal gas ducts, their reliable operation can be ensured by thorough insulation, ensuring the temperature difference between the flue gases and the wall is not more than 5 °C.

The currently used insulating materials and structures are not sufficiently reliable in long-term operation, therefore, it is necessary to periodically, at least once a year, monitor their condition and, if necessary, perform repair and restoration work.

17. When using on a trial basis to protect gas ducts from low-temperature corrosion of various coatings, it should be taken into account that the latter must provide heat resistance and gas tightness at temperatures exceeding the flue gas temperature by at least 10 ° C, resistance to sulfuric acid concentrations of 50 - 80% in the temperature range of 60 - 150 °C, respectively, and the possibility of their repair and restoration.

18. For low-temperature surfaces, RAH structural elements and boiler flues, it is advisable to use low-alloy steels 10KhNDP and 10KhSND, which are 2–2.5 times superior to carbon steel in corrosion resistance.

Absolute corrosion resistance is possessed only by very scarce and expensive high-alloy steels (for example, steel EI943, containing up to 25% chromium and up to 30% nickel).

Application

1. Theoretically, the dew point temperature of flue gases with a given content of sulfuric acid vapor and water can be defined as the boiling point of a solution of sulfuric acid of such a concentration at which the same content of water vapor and sulfuric acid is present above the solution.

The measured dew point temperature may differ from the theoretical value depending on the measurement technique. In these recommendations for flue gas dew point temperature t p the surface temperature of a standard glass sensor with 7 mm long platinum electrodes soldered at a distance of 7 mm from one another, at which the resistance of the dew film between for electrodes in steady state is equal to 10 7 Ohm. The measuring circuit of the electrodes uses low voltage alternating current (6 - 12 V).

2. When burning sulfurous fuel oils with excess air of 3 - 5%, the dew point temperature of flue gases depends on the sulfur content in the fuel Sp(rice.).

When burning sulphurous fuel oils with extremely low air excesses (α ≤ 1.02), the flue gas dew point temperature should be taken from the results of special measurements. The conditions for transferring boilers to the mode with α ≤ 1.02 are set out in the “Guidelines for the transfer of boilers operating on sulfurous fuels to the combustion mode with extremely small excess air” (M.: SPO Soyuztekhenergo, 1980).

3. When burning sulphurous solid fuels in a pulverized state, the dew point temperature of flue gases tp can be calculated from the reduced content of sulfur and ash in the fuel S p pr, A r pr and water vapor condensation temperature t con according to the formula

where a un- the proportion of ash in the fly away (usually taken 0.85).

Rice. 1. Dependence of flue gas dew point temperature on sulfur content in combusted fuel oil

The value of the first term of this formula at a un= 0.85 can be determined from Fig. .

Rice. 2. Differences in temperatures of the dew point of flue gases and condensation of water vapor in them, depending on the reduced sulfur content ( S p pr) and ash ( A r pr) in fuel

4. When burning gaseous sulphurous fuels, the flue gas dew point can be determined from fig. provided that the sulfur content in the gas is calculated as reduced, i.e. as a percentage by mass per 4186.8 kJ/kg (1000 kcal/kg) of the calorific value of the gas.

For gaseous fuels, the reduced mass percent sulfur content can be determined from the formula

where m- the number of sulfur atoms in the molecule of the sulfur-containing component;

q- volume percentage of sulfur (sulphur-containing component);

Q n- heat of combustion of gas in kJ / m 3 (kcal / nm 3);

FROM- coefficient equal to 4.187 if Q n expressed in kJ/m 3 and 1.0 if in kcal/m 3 .

5. The corrosion rate of the replaceable metal packing of air heaters during fuel oil combustion depends on the temperature of the metal and the degree of corrosivity of flue gases.

When burning sulphurous fuel oil with an excess of air of 3–5% and blowing the surface with steam, the corrosion rate (on both sides in mm/year) of RAH packing can be tentatively estimated from the data in Table. .

Table 1

Table 2 Up to 0.1 Sulfur content in fuel oil S p , % Corrosion rate (mm/year) at wall temperature, °С 75 - 95 96 - 100 101 - 110 111 - 115 116 - 125 Less than 1.0 0,10 0,20 0,30 0,20 0,10 1 - 2 0,10 0,25 0,40 0,30 0,15 More than 2 131 - 140 Over 140 Up to 0.1 0,10 0,15 0,10 0,10 0,10 St. 0.11 to 0.4 incl. 0,10 0,20 0,10 0,15 0,10 Over 0.41 to 1.0 incl. 0,15 0,25 0,30 0,35 0,20 0,30 0,15 0,10 0,05 St. 0.11 to 0.4 incl. 0,20 0,40 0,25 0,15 0,10 Over 0.41 to 1.0 incl. 0,25 0,50 0,30 0,20 0,15 Over 1.0 0,30 0,60 0,35 0,25 0,15 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 in hot water boilers, heating systems, heating systems is much more common than in steam condensate systems. In most cases, this situation is explained by the fact that less attention is paid to this when designing a water heating system, although the factors for the formation and subsequent development of corrosion in boilers remain exactly the same as for steam boilers and all other equipment. Dissolved oxygen, which is not removed by deaeration, hardness salts, carbon dioxide entering hot water boilers with feed water, cause various types of corrosion - alkaline (intercrystalline), oxygen, chelate, sludge. It must be said that chelate corrosion in most cases is formed in the presence of certain chemical reagents, the so-called "complexons".

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

Recently, special chemicals have been used for high-quality correctional heating networks, hot water boilers and other equipment.

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

For hot water boilers, however, as for any heating systems that use water as a coolant, three types of problems are characteristic due to the presence of the following impurities:

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

Each of these types of impurities can cause corrosion and failure of a hot water boiler or other equipment. In addition, they contribute to a decrease in the efficiency and productivity of the boiler.

And if for a long time you use water that has not undergone special preparation in heating systems, then this can lead to serious consequences - breakdown of circulation pumps, reduction in the diameter of the water supply and subsequent damage, failure of control and shutoff valves. The simplest mechanical impurities - clay, sand, ordinary dirt - are present almost everywhere, both in tap water and in artesian sources. Also in coolants in large quantities there are corrosion products of heat transfer surfaces, pipelines and other metal elements of the system that are constantly in contact with water. Needless to say, their presence over time provokes very serious malfunctions in the functioning of hot water boilers and all heat and power equipment, which are mainly associated with boiler corrosion, formation lime deposits, entrainment of salts and foaming of boiler water.

Most common cause, which gives rise to boiler corrosion, these are carbonate deposits that occur when using water of increased hardness, the removal of which is possible through. It should be noted that as a result of the presence of hardness salts, scale is formed even in low-temperature heating equipment. But this is not the only cause of corrosion. For example, after heating water to a temperature of more than 130 degrees, the solubility of calcium sulfate is significantly reduced, resulting in a layer of dense scale. In this case, the development of corrosion is inevitable. metal surfaces hot water boilers.