For the first time, external corrosion of wall tubes was detected at two power plants near high-pressure boilers TP-230-2, which operated on ASh grade coal and sulphurous fuel oil and had been in operation for about 4 years before. The outer surface of the pipes was subjected to corrosive corrosion from the side facing the furnace, in the zone of the maximum flame temperature. 88

Mostly the pipes of the middle (in width) part of the furnace, directly above the incendiary, were destroyed. belt. Wide and relatively shallow corrosion pits had irregular shape and often joined each other, as a result of which the damaged surface of the pipes was uneven and bumpy. Fistulas appeared in the middle of the deepest ulcers, and jets of water and steam began to escape through them.

A characteristic feature was the complete absence of such corrosion on the wall tubes of the medium-pressure boilers of these power plants, although the medium-pressure boilers were in operation there for a much longer time.

In subsequent years, external corrosion of wall tubes also appeared on other high-pressure boilers operating on solid fuels. The zone of corrosive destruction sometimes extended to a considerable height; in some places, the thickness of the pipe walls as a result of corrosion decreased to 2-3 mm. It has also been observed that this corrosion is practically absent in high-pressure boilers operating on heavy fuel oil.

External corrosion of wall tubes was found in TP-240-1 boilers after 4 years of operation, operating at a pressure in the drums of 185 atm. These boilers burned brown coal near Moscow, which had a moisture content of about 30%; fuel oil was burned only during kindling. In these boilers, corrosion damage also occurred in the zone of the greatest heat load of the wall tubes. The peculiarity of the corrosion process was that the pipes collapsed both from the side facing the furnace and from the side facing the lining (Fig. 62).

These facts show that the corrosion of wall tubes depends primarily on the temperature of their surface. In medium pressure boilers, water evaporates at a temperature of about 240 ° C; for boilers designed for a pressure of 110 atm, the design boiling point of water is 317 ° C; in TP-240-1 boilers, water boils at a temperature of 358 ° C. The temperature of the outer surface of the wall tubes usually exceeds the boiling point by about 30-40 ° C.

Can. to assume that intense external corrosion of the metal begins when its temperature rises to 350 ° C. For boilers designed for a pressure of 110 atm, this temperature is reached only from the firing side of the pipes, and for boilers with a pressure of 185 atm, it corresponds to the temperature of the water in the pipes ... That is why corrosion of wall tubes from the lining side was observed only in these boilers.

A detailed study of the issue was carried out on TP-230-2 boilers operating at one of the mentioned power plants. Samples of gases and heat were taken there.

Particles from the torch at a distance of about 25 mm from the wall tubes. Near the front screen, in the zone of intense external pipe corrosion, the flue gases contained almost no free oxygen. Near the rear screen, in which external pipe corrosion was almost absent, there was much more free oxygen in the gases. In addition, the check showed that in the area of ​​corrosion formation more than 70% of gas samples

It can be "assumed that in the presence of excess oxygen, hydrogen sulfide burns out and corrosion does not occur, but in the absence of excess oxygen, hydrogen sulfide enters into chemical compound with metal pipes. In this case, iron sulfide FeS is formed. This corrosion product has indeed been found in shield tube deposits.

Not only carbon steel is exposed to external corrosion, but also chromium-molybdenum steel. In particular, in TP-240-1 boilers, corrosion affected the wall tubes made of 15XM steel.

Until now, there are no proven measures to completely prevent the described type of corrosion. Some reduction in the rate of destruction. metal was reached. after adjusting the combustion process, in particular when the excess air in the flue gases increases.

27. SCREEN CORROSION AT EXTREME PRESSURE

This book briefly describes the working conditions of metal steam boilers in modern power plants. But the progress of energy in the USSR continues, and now it comes into operation big number new boilers designed for more high pressures and steam temperature. In these conditions great importance has practical experience in operating several boilers TP-240-1, operating from 1953-1955. at a pressure of 175 atm (185 atm in a drum). Very valuable,> in particular, information about the corrosion of their screens.

The shields of these boilers were subject to corrosion from both the outside and inside... Their external corrosion is described in the previous paragraph of this chapter, but the destruction of the inner surface of the pipes is not similar to any of the types of metal corrosion described above.

Corrosion occurred mainly from the firing side of the upper part of the inclined pipes of the cold funnel and was accompanied by the appearance of corrosion pits (Fig. 63, a). Subsequently, the number of such shells increased, and a continuous strip (sometimes two parallel. Stripes) of corroded metal appeared (Fig. 63.6). The absence of corrosion in the zone of welded joints was also characteristic.

Inside the pipes, there was a deposit of loose sludge 0.1-0.2 mm thick, which consisted mainly of iron and copper oxides. An increase in the corrosion destruction of the metal was not accompanied by an increase in the thickness of the sludge layer; therefore, corrosion under the sludge layer was not the main cause of corrosion of the inner surface of the wall tubes.

In the boiler water, the pure phosphate alkalinity regime was maintained. Phosphates were introduced into the boiler not continuously, but periodically.

Of great importance was the fact that the temperature of the pipe metal periodically rose sharply and sometimes exceeded 600 ° C (Fig. 64). The zone of the most frequent and maximum temperature rise coincided with the zone of the greatest destruction of the metal. A decrease in the pressure in the boiler to 140-165 atm (i.e., to the pressure at which new serial boilers operate) did not change the nature of the temporary increase in the temperature of the pipes, but was accompanied by a significant decrease in the maximum value of this temperature. The reasons for such a periodic increase in the temperature of the firing side of the inclined pipes are cold. funnels have not yet been studied in detail.

In marine steam boilers, corrosion can occur both from the steam-water circuit and from the side of fuel combustion products.

The inner surfaces of the steam-water circuit can be subject to the following types of corrosion;

Oxygen corrosion is the most dangerous species corrosion. Characteristic feature oxygen corrosion is the formation of local pitting foci of corrosion, reaching deep pits and through holes; The most susceptible to oxygen corrosion are inlet sections of economizers, collectors and downpipes of circulation circuits.

Nitrite corrosion - unlike oxygen corrosion, it affects inner surfaces heat-stressed lifting tubes and causes the formation of deeper ulcers with a diameter of 15 ^ 20 mm.

Intergranular corrosion is a special type of corrosion and occurs in places of the highest metal stresses (welded seams, rolling and flange joints) as a result of the interaction of the boiler metal with highly concentrated alkali. A characteristic feature is the appearance on the metal surface of a mesh of small cracks, gradually developing into through cracks;

Under-sludge corrosion occurs in places where sludge is deposited and in stagnant zones of boiler circulation circuits. The flow process is electrochemical in nature when iron oxides come into contact with a metal.

The following types of corrosion can be observed from the side of fuel combustion products;

Gas corrosion affects evaporative, superheating and economizer heating surfaces, casing lining,

Gas guide shields and other boiler elements exposed to high gas temperatures .. When the metal temperature of the boiler pipes rises above 530 ° C (for carbon steel), the destruction of the protective oxide film on the pipe surface begins, providing unhindered access of oxygen to the pure metal. In this case, corrosion occurs on the surface of the pipes with the formation of scale.

The immediate cause of this type of corrosion is a violation of the cooling regime of these elements and an increase in their temperature above the permissible level. For pipes of heating surfaces, the reasons for Ysh The temperature of the walls can be; formation of a significant layer of scale, disturbance of the circulation regime (stagnation, overturning, formation of steam plugs), water leakage from the boiler, uneven distribution of water and steam extraction along the length of the steam collector.

High-temperature (vanadium) corrosion affects the heating surfaces of superheaters located in the zone of high gas temperatures. When fuel is burned, vanadium oxides are formed. In this case, with a lack of oxygen, vanadium trioxide is formed, and with an excess of it, vanadium pentoxide is formed. Vanadium pentoxide U205, which has a melting point of 675 ° C, is corrosively dangerous. Vanadium pentoxide, released during the combustion of fuel oil, adheres to the heating surfaces, which have high fever, and causes active destruction of the metal. Experiments have shown that even vanadium contents as low as 0.005% by weight can cause dangerous corrosion.

Vanadium corrosion can be prevented by reducing permissible temperature the metal of the boiler elements and the organization of combustion with minimum excess air coefficients a = 1.03 + 1.04.

Low-temperature (acidic) corrosion mainly affects tail heating surfaces. The combustion products of sulfurous fuel oils always contain water vapor and sulfur compounds, which form sulfuric acid when combined with each other. When gases are flushed with relatively cold tail heating surfaces, sulfuric acid vapors condense on them and cause metal corrosion. The intensity of low-temperature corrosion depends on the concentration of sulfuric acid in the moisture film deposited on the heating surfaces. In this case, the concentration of B03 in the combustion products is determined not only by the sulfur content in the fuel. The main factors affecting the rate of low-temperature corrosion are;

Conditions for the combustion reaction in the furnace. With an increase in the excess air ratio, the percentage of B03 gas increases (at a = 1.15, 3.6% of the sulfur contained in the fuel is oxidized; at a = 1.7, about 7% of sulfur is oxidized). With air excess coefficients a = 1.03 - 1.04 sulfuric anhydride B03 is practically not formed;

Condition of heating surfaces;

Boiler power supply too cold water causing a decrease in the temperature of the walls of the economizer pipes below the melancholy dew for sulfuric acid;

Concentration of water in fuel; when burning watered fuels, the dew point increases due to an increase in the partial pressure of water vapor in the combustion products.

Parking corrosion affects outer surfaces pipes and collectors, casing, combustion devices, fittings and other elements of the gas-air path of the boiler. The soot generated during fuel combustion covers the heating surfaces and the internal parts of the boiler gas-air path. The soot is hygroscopic, and when the boiler cools down it easily absorbs moisture that causes corrosion. Corrosion is ulcerative in nature when a film of sulfuric acid solution forms on the metal surface when the boiler cools down and the temperature of its elements drops below the dew point for sulfuric acid.

The fight against parking corrosion is based on the creation of conditions that exclude the ingress of moisture on the surface of the boiler metal, as well as the application anti-corrosion coatings on the surface of the boiler elements.

In case of short-term inactivity of the boilers after inspection and cleaning of the heating surfaces in order to prevent the ingress of atmospheric precipitation into the gas ducts of the boilers on chimney it is necessary to put on a cover, close the air registers, inspection holes. It is necessary to constantly monitor the humidity and temperature in the MCO.

To prevent corrosion of boilers during inactivity, various methods of boiler storage are used. There are two storage methods; wet and dry.

The main storage method for boilers is wet storage. It provides for the complete filling of the boiler. feed water passed through electron-ion exchange and deoxygenating filters, including a superheater and economizer. Boilers can be kept wet for no more than 30 days. In case of longer inactivity of the boilers, dry storage of the boiler is used.

Dry storage provides for the complete drainage of the boiler from water with the placement of coarse calico bags with silica gel in the collectors of the boiler, which absorb moisture. Periodically, the collectors are opened, the control measurement of the mass of the silica gel in order to determine the mass of absorbed moisture, and the evaporation of the absorbed moisture from the silica gel.

A number of power plants use river and tap water with low pH value and low hardness. Additional processing river water in a waterworks typically results in lower pH, lower alkalinity and higher 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 adjusted water deaeration and possible increases in oxygen and carbon dioxide concentrations due to the lack of additional protective measures in heat supply systems, pipelines are susceptible to internal corrosion, heat exchangers, storage tanks and other equipment.

It is known that an increase in temperature promotes the development of corrosion processes that occur both with the absorption of oxygen and the evolution of hydrogen. With an increase in temperature above 40 ° C, oxygen and carbon dioxide forms of corrosion increase sharply.

A special type of undersludge corrosion occurs in conditions of an insignificant content of residual oxygen (when the standards of the PTE 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 discovered under conditions of relatively weak heating and the absence of thermal loads. In this case, friable 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, that is, selective, intense corrosion destruction of the metal in the gap (gap). A feature of the processes occurring in narrow gaps is a lower oxygen concentration compared to the concentration in the volume of the solution and a slower removal of the products of the corrosion reaction. 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 heating network with an open water intake with deaerated water, the possibility of the formation of through holes in pipelines is completely excluded only in normal hydraulic mode, when an excess pressure above atmospheric pressure is constantly maintained at all points of the heat supply system.

The reasons for 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 products of oxygen corrosion of iron (Fe 2 O 3) on heat transfer surfaces. Increased content iron oxides in the network water contributes to the drift of the boiler heating surfaces by iron oxide deposits.

A number of researchers admit important role in the occurrence of undersludge corrosion of the rusting process of pipes of hot water boilers during their downtime, when proper measures have not been taken to prevent parking corrosion. The centers of corrosion arising under the influence of atmospheric air on the humid surfaces of the boilers continue to function during the operation of the boilers.

2.1. Heating surfaces.

The most typical damage to the heating surface pipes are: cracks in the surface of the wall and heating pipes, corrosive corrosion of the external and internal surfaces of the pipes, ruptures, thinning of the pipe walls, cracks and destruction of bells.

Reasons for the appearance of cracks, ruptures and holes: deposits in the pipes of boilers of salts, corrosion products, welding burrs, slowing down circulation and causing overheating of the metal, external mechanical damage, violation of the water-chemical regime.

Corrosion of the outer surface of pipes is subdivided into low-temperature and high-temperature corrosion. Low-temperature corrosion occurs in areas where blowing devices are installed, when, as a result of improper operation, condensation is allowed to form on heating surfaces covered with soot. High-temperature corrosion can occur in the second stage of the superheater during the combustion of sulfurous 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 the metal of the pipes. Corrosion of the inner surface of pipes manifests itself in the formation of pockmarks, ulcers, shells and cracks.

Corrosion of the inner surface of pipes also includes: oxygen parking corrosion, undersludge alkaline corrosion of boiler and wall tubes, corrosion fatigue, which manifests itself in the form of cracks in boiler and wall tubes.

Creep damage to pipes is characterized by an increase in diameter and the formation of longitudinal cracks. Deformations in places of pipe bends and welded joints can have different directions.

Burnouts and scale formation in pipes occur due to their overheating to temperatures exceeding the design one.

The main types of damage welds made by manual arc welding - fistulas arising from lack of penetration, slag inclusions, gas pores, lack of fusion along the edges of the pipes.

The main defects and damage to 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, pipe holes and ruptures, defects in pipe welded joints, residual deformation as a result of creep.

Damage to the fillet welds of the welding of coils and fittings to the collectors, causing a violation of the welding technology, have the form of annular cracks along the fusion line from the side of the coil or fittings.

Typical malfunctions arising during the operation of the surface steam cooler of the DE-25-24-380GM boiler are: internal and external corrosion of pipes, cracks and holes in welded

seams and on pipe bends, sinks that may arise during repairs, risks on the flange mirror, leaks of flange connections due to skewed flanges. During the hydraulic test of 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 performed.

2.2. Boiler drums.

Typical damage to the boiler drums are: cracks-tears on the inner and outer surfaces of shells and bottoms, cracks-tears around pipe holes on the inner surface of drums and on the cylindrical surface of pipe holes, intergranular corrosion of shells and bottoms, corrosion separation of the surfaces of shells and bottoms, ovality of the drum oddulin (bulges) on the surfaces of 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. Steel structures and boiler lining.

Depending on the quality of preventive work, as well as on the modes and terms of operation of the boiler, its metal structures may have the following defects and damage: breaks and bends of struts and ties, 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 firebox, as well as cracks in brickwork along the bottom drum and bottom of the furnace.

Particularly common is the destruction of the brick burner embrasure and the violation of the geometric dimensions due to the melting of the brick.

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 hydraulic tests, external and internal inspection, as well as other types of control carried out to the extent 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 pipe elements must be especially carefully performed at the places where pipes pass through the lining, sheathing, in the zones 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 the thinning of the pipe walls due to sulfur and parking corrosion, it is necessary, during the annual technical inspections carried out by the administration of the enterprise, to control 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 examination with tapping 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, you should select the pipe sections that have undergone the greatest wear and corrosion (horizontal sections, areas in soot deposits and covered with coke deposits).

The measurement of pipe wall thickness is carried out with ultrasonic thickness gauges. It is possible to cut pipe sections on two or three pipes of the furnace walls and pipes of the convective bundle located at the gas inlet and outlet. The remaining pipe wall thickness must be at least calculated according to the strength calculation (attached to the boiler passport), taking into account the increase in corrosion for the period of further operation until the next survey and an increase in the margin of 0.5 mm.

The design wall thickness of the wall and boiler tubes 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. Corrosion allowance is taken according to the obtained measurement results and taking into account the duration of operation between surveys.

At enterprises where, as a result of long-term operation, there was no intense wear of pipes of heating surfaces, control of the thickness of the pipe walls can be carried out at major overhauls, but at least once every 4 years.

The collector, superheater and rear screen are subject to internal inspection. The hatches of the upper collector of the rear screen must be opened and inspected.

The outer diameter of the pipes should be measured in the zone of maximum temperatures. For measurements, use special templates (staples) or a vernier caliper. Dents with smooth transitions with a depth of not more than 4 mm are allowed on the surface of the pipes, if they do not bring the wall thickness beyond the minus deviations.

The permissible wall thickness difference is 10%.

The results of the inspection and measurements are recorded in the repair form.

3.2. Drum check.

On the day of 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, measure the depth of metal corrosion.

Measure uniform corrosion along the wall thickness, in which a hole with a diameter of 8 mm is drilled for this purpose. 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 done with an ultrasonic thickness gauge.

Measure the main corrosion and pits by impressions. For this purpose, clean the damaged area of ​​the metal surface from deposits and lightly grease with petroleum jelly. The most accurate impression 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 accurate impression of the damaged surface.

To obtain prints, use a treetop, babbitt, tin, if possible, use plaster.

Damage impressions located on vertical ceiling surfaces should be obtained using wax and plasticine.

Inspection of pipe holes, drums is carried out in the following order.

After removing the flared pipes, check the hole diameter using a template. If the template enters the hole up to the stop protrusion, this means that the hole diameter is oversized. The exact size of the diameter is measured with a vernier caliper and noted in the repair form.

When inspecting the welded seams of drums, it is necessary to inspect the adjacent base metal for a width of 20-25 mm on both sides of the seam.

Drum ovality is measured at least every 500 mm along the drum length, in doubtful cases and more often.

Drum deflection is measured by stretching the string along the surface of the drum and measuring the gaps along the length of the string.

Inspection of the drum surface, pipe holes and welded joints is carried out by external inspection, methods, magnetic particle, color and ultrasonic flaw detection.

Allowed (do not require straightening) bumps and dents outside the zone of seams and holes, provided that their height (deflection), as a percentage of the smallest size of their base, is not more than:

towards atmospheric pressure (outlets) - 2%;

towards steam pressure (dents) - 5%.

The permissible reduction in the thickness of the bottom wall is 15%.

Allowed increase in the diameter of the holes for pipes (for welding) - 10%.

This corrosion, in size and intensity, is often more significant and dangerous than the corrosion of boilers during operation.

When leaving water in systems, depending on its temperature and air availability, a wide variety of cases of parking corrosion can occur. First of all, it should be noted that the presence of water in the pipes of the units is extremely undesirable when they are in reserve.

If water for one reason or another remains in the system, then strong parking corrosion can be observed in the steam and especially in the water space of the tank (mainly along the waterline) at a water temperature of 60-70 ° C. Therefore, in practice, parking corrosion of varying intensity is quite often observed, despite the same shutdown modes of the system and the quality of the water contained in them; devices with a significant thermal accumulation are subject to more severe corrosion than devices with the dimensions of a furnace and a heating surface, since boiler water it cools faster in them; its temperature drops below 60-70 ° С.

At a water temperature above 85-90 ° C (for example, during short-term shutdowns of apparatus), the general corrosion decreases, and the corrosion of the metal of the vapor space, in which in this case an increased condensation of vapors is observed, may exceed the corrosion of the metal of the water space. Standing corrosion in the steam space is in all cases more uniform than in the water space of the boiler.

Parking corrosion is strongly promoted by sludge accumulating on the boiler surfaces, which usually retains moisture. In this regard, significant corrosion pits are often found in aggregates and pipes along the lower generatrix and at their ends, that is, in the areas of the greatest accumulation of sludge.

Methods for conservation of equipment in reserve

The following methods can be used to preserve equipment:

a) drying - removal of water and moisture from the aggregates;

b) filling them with solutions of sodium hydroxide, phosphate, silicate, sodium nitrite, hydrazine;

c) filling technological system nitrogen.

The method of conservation should be selected depending on the nature and duration of the downtime, as well as on the type and design features equipment.

Equipment downtime can be divided into two groups in terms of duration: short-term — no more than 3 days and long-term — more than 3 days.

There are two types of short-term downtime:

a) planned, related to the withdrawal to the reserve on weekends in connection with a drop in the load or withdrawal to the reserve at night;

b) forced - due to failure of pipes or damage to other equipment units, the elimination of which does not require a longer shutdown.

Depending on the purpose, long-term downtime can be divided into the following groups: a) putting equipment into reserve; b) current repairs; c) major repairs.

In case of short-term equipment downtime, it is necessary to use preservation by filling with deaerated water while maintaining overpressure or gas (nitrogen) method. If an emergency stop is required, then the only acceptable method is conservation with nitrogen.

When putting the system into reserve or for a long time idle without performing renovation works preservation is expediently carried out by filling with a solution of nitrite or sodium silicate. In these cases, nitrogen preservation can also be used, making sure to take measures to create a density of the system in order to prevent excessive gas consumption and unproductive operation of the nitrogen plant, as well as create a safe environment for equipment maintenance.

Methods of conservation by creating overpressure, filling with nitrogen can be used regardless of the design features of the heating surfaces of the equipment.

To prevent parking corrosion of metal during major and current repairs only methods of preservation are applicable, which make it possible to create a protective film on the metal surface that retains its properties for at least 1-2 months after draining the preservative solution, since the emptying and depressurization of the system is inevitable. Validity protective film on the metal surface after processing it with sodium nitrite can reach 3 months.

Preservation methods using water and reagent solutions are practically unacceptable for protection against parking corrosion of intermediate superheaters of boilers due to the difficulties associated with their filling and subsequent cleaning.

Methods for preserving hot water and steam boilers low pressure, as well as other equipment of closed technological circuits of heat and water supply differ in many respects from the methods of preventing parking corrosion at TPPs that are currently used. The following describes the main ways to prevent corrosion in the idle mode of equipment of devices of such circulation systems taking into account the specifics of their work.

Simplified preservation methods

These methods are useful for small boilers. They consist in the complete removal of water from the boilers and the placement of desiccants in them: calcined calcium chloride, quicklime, silica gel at the rate of 1-2 kg per 1 m 3 of volume.

This preservation method is suitable at room temperatures below and above zero. In rooms heated in winter time, one of the contact methods of conservation can be implemented. It boils down to filling the entire internal volume of the unit with an alkaline solution (NaOH, Na 3 P0 4, etc.), which ensures complete stability of the protective film on the metal surface even when the liquid is saturated with oxygen.

Usually used solutions containing from 1.5-2 to 10 kg / m 3 NaOH or 5-20 kg / m 3 Na 3 P0 4, depending on the content of neutral salts in the source water. Smaller values ​​refer to condensate, larger ones - to water containing up to 3000 mg / l of neutral salts.

Corrosion can also be prevented by overpressure, in which the steam pressure in the shutdown unit is constantly maintained at a level above atmospheric pressure, and the water temperature remains above 100 ° C, which prevents the access of the main corrosive agent - oxygen.

An important condition for the efficiency and economy of any method of protection is the maximum possible tightness of the steam-water fittings in order to avoid too rapid pressure drop, loss of protective solution (or gas) or moisture ingress. In addition, in many cases it is useful preliminary cleaning surfaces from various deposits(salts, sludge, scale).

When implementing different ways to protect against parking corrosion, the following should be borne in mind.

1. With all types of preservation, preliminary removal (flushing) of deposits of readily soluble salts (see above) is necessary to avoid intensification of parking corrosion in certain areas of the protected unit. It is imperative to carry out this measure during contact preservation, otherwise intense local corrosion is possible.

2. For similar reasons, it is desirable to remove before long-term conservation of all types of insoluble deposits (sludge, scale, iron oxides).

3. If the valves are unreliable, it is necessary to disconnect the backup equipment from the operating units using plugs.

The infiltration of steam and water is less dangerous with contact preservation, but unacceptable with dry and gas methods of protection.

The choice of desiccants is determined by the relative availability of the reagent and the desirability of obtaining the highest possible specific moisture content. The best desiccant is granulated calcium chloride. Quicklime is much worse than calcium chloride, not only due to the lower moisture capacity, but also the rapid loss of its activity. Lime absorbs not only moisture from the air, but also carbon dioxide, as a result of which it becomes covered with a layer of calcium carbonate, which prevents further absorption of moisture.