Signs of corrosive aggressiveness of water in boiler installations. Corrosion damage to screen pipes of gas-oil boilers

Owners of patent RU 2503747:

TECHNICAL FIELD

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

BACKGROUND OF THE ART

The operation of steam boilers is associated with simultaneous exposure to high temperatures, pressure, mechanical stress and an aggressive environment, which is boiler water. Boiler water and the metal of the boiler heating surfaces are separate phases complex system, which is formed upon their contact. The result of the interaction of these phases is surface processes that occur at their interface. As a result, corrosion and scale formation occur in the metal of the heating surfaces, which leads to a change in the structure and mechanical properties metal, and which contributes to the development of various damages. Since the thermal conductivity of scale is fifty times lower than that of iron heating pipes, there are losses of thermal energy during heat transfer - with a scale thickness of 1 mm from 7 to 12%, and with 3 mm - 25%. Severe scale formation in a continuous steam boiler system often causes production to be shut down for several days each year to remove the scale.

The quality of feed water and, therefore, boiler water is determined by the presence of impurities that can cause various types of corrosion of the metal of internal heating surfaces, the formation of primary scale on them, as well as sludge as a source of secondary scale formation. In addition, the quality of boiler water also depends on the properties of substances formed as a result of surface phenomena during water transportation and condensate through pipelines during water treatment processes. Removing impurities from feed water is one of the ways to prevent the formation of scale and corrosion and is carried out by methods of preliminary (pre-boiler) water treatment, which are aimed at maximizing the removal of impurities found in the source water. However, the methods used do not allow us to completely eliminate the content of impurities in water, which is associated not only with technical difficulties, but also 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 boilers of low and medium capacity.

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

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

US patent 1877389 proposes a method for removing scale and preventing its formation in hot water and steam boilers. In this method, the surface of the boiler is the cathode, and the anode is placed inside the pipeline. The method involves passing direct or alternating current through the system. The authors note that the mechanism of action of the method is that under the influence of an electric current, gas bubbles form on the surface of the boiler, which lead to the peeling off of existing scale and prevent the formation of a new one. The disadvantage of this method is the need to constantly maintain the flow of electric current in the system.

US Pat. No. 5,667,677 proposes a method for treating a liquid, particularly water, in a pipeline to slow down the formation of scale. This method is based on the creation of an electromagnetic field in pipes, which repels calcium and magnesium ions dissolved in water from the walls of pipes and equipment, preventing them from crystallizing in the form of scale, which allows the operation of boilers, boilers, heat exchangers, and cooling systems on hard water. The disadvantage of this method is the high cost and complexity of the equipment used.

Application WO 2004016833 proposes a method for reducing the formation of scale on a metal surface exposed to a supersaturated alkaline aqueous solution which is capable of forming scale after a period of exposure, comprising applying a cathodic potential to said surface.

This method can be used in various technological processes in which the metal is in contact with aqueous solution, in particular in heat exchangers. The disadvantage of this method is that it does not protect the metal surface from corrosion after removing the cathodic potential.

Thus, there is currently a need to develop an improved method for preventing scale formation of heating pipes, hot water boilers and steam boilers, which would be economical and highly effective and provide anti-corrosion protection to the surface for a long period of time after exposure.

In the present invention, this problem is solved using a method according to which a current-carrying electric potential is created on a metal surface, sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.

BRIEF DESCRIPTION OF THE INVENTION

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

Another objective of the present invention is to provide the possibility of eliminating or significantly reducing the need for descaling during operation of hot water and steam boilers.

Another objective of the present invention is to eliminate the need to use consumable reagents to prevent the formation of scale and corrosion of heating pipes of water heating and steam boilers.

Another object of the present invention is to enable work to begin to prevent the formation of scale and corrosion of heating pipes of hot water and steam boilers on contaminated boiler pipes.

The present invention relates to a method for preventing the formation of scale and corrosion on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale is capable of forming. This method consists in applying to the specified metal surface a current-carrying electric potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and ions to the metal surface.

According to some private embodiments of the claimed method, the current-carrying potential is set in the range of 61-150 V. According to some private embodiments of the claimed method, the above iron-containing alloy is steel. In some embodiments, the metal surface is the interior surface of the heating tubes of a hot water or steam boiler.

The method disclosed herein has the following advantages. One advantage of the method is reduced scale formation. Another advantage of the present invention is the ability to use a working electrophysical apparatus once purchased without the need to use consumable synthetic reagents. Another advantage is the possibility of starting work on dirty boiler tubes.

The technical result of the present invention, therefore, is to increase the operating efficiency of hot water and steam boilers, increase productivity, increase heat transfer efficiency, reduce fuel consumption for heating the boiler, save energy, etc.

Other technical results and advantages of the present invention include providing the possibility of layer-by-layer destruction and removal of already formed scale, as well as preventing its new formation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows the nature of the distribution of sediments on internal surfaces boiler as a result of applying the method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The method of the present invention involves applying to a metal surface subject to scale formation a current-carrying electrical potential sufficient to neutralize the electrostatic component of the adhesion force of colloidal particles and scale-forming ions to the metal surface.

The term "conducting electrical potential" as used in this application means an alternating potential that neutralizes the electrical double layer at the interface of the metal and the steam-water medium containing salts that lead to scale formation.

As is known to a person skilled in the art, the carriers of electric charge in a metal, slow compared to the main charge carriers - electrons, are dislocations of its crystal structure, which carry an electric charge and form dislocation currents. Coming to the surface of the heating pipes of the boiler, these currents become part of the double electrical layer during the formation of scale. The current-carrying, electrical, pulsating (i.e., alternating) potential initiates the removal of the electrical charge of dislocations from the metal surface to the ground. In this respect, it is a conductor of dislocation currents. As a result of the action of this current-carrying electrical potential, the double electrical layer is destroyed, and the scale gradually disintegrates and passes into the boiler water in the form of sludge, which is removed from the boiler during periodic purging.

Thus, the term “current-carrying potential” is understandable to a person skilled in the art and, in addition, is known from the prior art (see, for example, patent RU 2128804 C1).

As a device for creating a current-carrying electrical potential, for example, a device described in RU 2100492 C1 can be used, which includes a converter with a frequency converter and a pulsating potential regulator, as well as a pulse shape regulator. Detailed description of this device is given in RU 2100492 C1. Any other similar device may also be used, as will be appreciated by one skilled in the art.

The conductive electrical potential according to the present invention can be applied to any part of the metal surface remote from the base of the boiler. The place of application is determined by the convenience and/or effectiveness of using the claimed method. One skilled in the art, using the information disclosed herein and using standard testing techniques, will be able to determine the optimal location for application of the current-sinking electrical potential.

In some embodiments of the present invention, the current-sinking electrical potential is variable.

The current-sinking electric potential according to the present invention can be applied for various periods of time. The time of application of the potential is determined by the nature and degree of contamination of the metal surface, the composition of the water used, temperature conditions and the operating features of the heating device and other factors known to specialists in this field of technology. One skilled in the art, using the information disclosed herein and using standard testing techniques, will be able to determine optimal time application of current-carrying electrical potential, based on the goals, conditions and state of the heating device.

The magnitude of the current-carrying potential required to neutralize the electrostatic component of the adhesion force can be determined by a specialist in the field of colloid chemistry based on information known from the prior art, for example from the book B.V. Deryagin, N.V. Churaev, V.M. Muller. "Surface Forces", Moscow, "Nauka", 1985. According to some embodiments, the magnitude of the current-carrying electrical potential is in the range from 10 V to 200 V, more preferably from 60 V to 150 V, even more preferably from 61 V to 150 V. Values ​​of the current-carrying electrical potential in the range from 61 V to 150 V lead to the discharge of the double electrical layer, which is the basis of the electrostatic component of the adhesion forces in scale and, as a consequence, destruction of scale. Values ​​of the current-carrying potential below 61 V are insufficient to destroy scale, and at values ​​of the current-carrying potential above 150 V, unwanted electrical erosion destruction of the metal of the heating tubes is likely to begin.

The metal surface to which the method according to the present invention can be applied can be part of the following thermal devices: heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems of residential buildings and industrial facilities during ongoing operation. This list is illustrative and does not limit the list of devices to which the method according to the present invention can be applied.

In some embodiments, the iron-containing alloy from which the metal surface is made to which the method of the present invention can be applied may be steel or other iron-containing material such as cast iron, kovar, fechral, ​​transformer steel, alsifer, magneto, alnico, chromium steel, invar, etc. This list is illustrative and does not limit the list of iron-containing alloys to which the method according to the present invention can be applied. One skilled in the art, based on knowledge known in the art, will be able to identify such iron-containing alloys that can be used according to the present invention.

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

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

The method according to the present invention has the following advantages:

Does not require the use of reagents, i.e. environmentally friendly;

Easy to implement, does not require special devices;

Allows you to increase the heat transfer coefficient and increase the efficiency of boilers, which significantly affects economic indicators his works;

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

Allows you to abandon the processes of water softening and deaeration, which greatly simplifies technological scheme boiler rooms and makes it possible to significantly reduce costs during construction and operation.

Possible objects of the method can be hot water boilers, waste heat boilers, closed systems heat supply, installations for thermal desalination of sea water, steam conversion plants, etc.

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

EXAMPLE OF IMPLEMENTATION

The method claimed in the present invention was tested at the Admiralty Shipyards and Krasny Khimik boiler plants. The method according to the present invention has been shown to effectively clean the internal surfaces of boiler units from deposits. Savings were achieved during this work standard fuel 3-10%, while the spread in savings values ​​is associated with varying degrees of contamination of the internal surfaces of boiler units. The purpose of the work was to evaluate the effectiveness of the claimed method for ensuring reagent-free, scale-free operation of medium-power steam boilers under conditions of high-quality water treatment, compliance with the water chemistry regime and a high professional level of equipment operation.

The method claimed in the present invention was tested on steam boiler unit No. 3 DKVR 20/13 of the 4th Krasnoselskaya boiler house of the South-Western branch of the State Unitary Enterprise "TEK SPb". The operation of the boiler unit was carried out in strict accordance with the requirements of regulatory documents. The boiler is equipped with all the necessary means of monitoring its operating parameters (pressure and flow rate of generated steam, temperature and flow rate of feed water, pressure of blast air and fuel on the burners, vacuum in the main sections of the gas path of the boiler unit). The steam output of the boiler was maintained at 18 t/hour, the steam pressure in the boiler drum was 8.1...8.3 kg/cm 2 . The economizer operated in heating mode. As source water City water supply water was used, which met the requirements of GOST 2874-82 “Drinking water”. It should be noted that the amount of iron compounds entering the specified boiler room, as a rule, exceeds regulatory requirements (0.3 mg/l) and amounts to 0.3-0.5 mg/l, which leads to intensive overgrowing of internal surfaces with ferrous compounds.

The effectiveness of the method was assessed based on the condition of the internal surfaces of the boiler unit.

Assessment of the influence of the method according to the present invention on the condition of the internal heating surfaces of the boiler unit.

Before the start of the tests, an internal inspection of the boiler unit was carried out and the initial condition of the internal surfaces was recorded. A preliminary inspection of the boiler was carried out at the beginning of the heating season, a month after its chemical cleaning. As a result of the inspection, it was revealed: on the surface of the drums there are continuous solid deposits of a dark brown color, possessing paramagnetic properties and presumably consisting of iron oxides. The thickness of the deposits was up to 0.4 mm visually. In the visible part of the boiling pipes, mainly on the side facing the furnace, non-continuous solid deposits were found (up to five spots per 100 mm of pipe length with a size of 2 to 15 mm and a visual thickness of up to 0.5 mm).

The device for creating a current-carrying potential, described in RU 2100492 C1, was connected at point (1) to the hatch (2) of the upper drum on the back side of the boiler (see Fig. 1). The current-carrying electric potential was equal to 100 V. The current-carrying electric potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler unit, an almost complete absence of deposits (no more than 0.1 mm visually) was established on the surface (3) of the upper and lower drums within 2-2.5 meters (zone (4)) from the drum hatches (device connection points to create a current-carrying potential (1)). At a distance of 2.5-3.0 m (zone (5)) from the hatches, deposits (6) were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig. 1). Further, as you move towards the front, (at a distance of 3.0-3.5 m from the hatches) continuous deposits begin (7) up to 0.4 mm visually, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not evident. The current-carrying electric potential was equal to 100 V. The current-carrying electric potential was maintained continuously for 1.5 months. At the end of this period, the boiler unit was opened. As a result of an internal inspection of the boiler unit, an almost complete absence of deposits (no more than 0.1 mm visually) was established on the surface of the upper and lower drums within 2-2.5 meters from the drum hatches (attachment points of the device for creating current-carrying potential). At a distance of 2.5-3.0 m from the hatches, the deposits were preserved in the form of individual tubercles (spots) up to 0.3 mm thick (see Fig. 1). Further, as you move towards the front (at a distance of 3.0-3.5 m from the hatches), continuous deposits of up to 0.4 mm visually begin, i.e. at this distance from the connection point of the device, the effect of the cleaning method according to the present invention was practically not evident.

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

As a result of this stage of testing, it was concluded that the method according to the present invention, without the use of any reagents, can effectively destroy previously formed deposits and ensure scale-free operation of the boiler unit.

At the next stage of testing, the device for creating a current-carrying potential was connected at point “B” and the tests continued for another 30-45 days.

The next opening of the boiler unit was carried out after 3.5 months of continuous operation of the device.

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

This allowed us to draw the following conclusions:

The size of the zone within which scale-free operation of the boiler unit is ensured significantly exceeds the size of the zone of effective destruction of deposits, which allows subsequent transfer of the point of connection of the current-carrying potential to clean the entire internal surface of the boiler unit and further maintain its scale-free operation mode;

The destruction of previously formed deposits and the prevention of the formation of new ones is ensured by processes of different nature.

Based on the results of the inspection, it was decided to continue testing until the end of the heating period in order to finally clean the drums and boiling pipes and determine the reliability of ensuring scale-free operation of the boiler. The next opening of the boiler unit was carried out after 210 days.

The results of the internal inspection of the boiler showed that the process of cleaning the internal surfaces of the boiler within the upper and lower drums and boiling pipes resulted in almost complete removal of deposits. A thin, dense coating formed on the entire surface of the metal, black in color with a blue tarnish, the thickness of which, even in a moistened state (almost immediately after opening the boiler), did not visually exceed 0.1 mm.

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

The protective effect of the magnetite film lasted up to 2 months after disconnecting the device, which is quite enough to ensure the preservation of the boiler unit using the dry method when transferring it to reserve or for repairs.

Although the present invention has been described with respect to various specific examples and embodiments, it is to be understood that the invention is not limited thereto and that it may be practiced within the scope of the following claims.

1. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale can form, including applying a current-carrying electric potential to said metal surface in the range from 61 V to 150 V to neutralize the electrostatic component of the force adhesion between said metal surface and colloidal particles and ions forming scale.

The invention relates to heat power engineering and can be used to protect against scale and corrosion heating pipes of steam and hot water boilers, heat exchangers, boiler units, evaporators, heating mains, heating systems of residential buildings and industrial facilities during operation. A method for preventing the formation of scale on a metal surface made of an iron-containing alloy and in contact with a steam-water environment from which scale is capable of forming involves applying to said metal surface a current-carrying electric potential in the range from 61 V to 150 V to neutralize the electrostatic component of the adhesion force between the specified metal surface and colloidal particles and ions forming scale. The technical result is increasing the efficiency and productivity of hot water and steam boilers, increasing the efficiency of heat transfer, ensuring layer-by-layer destruction and removal of formed scale, as well as preventing its new formation. 2 salary f-ly, 1 ave., 1 ill.

A number of boiler houses use river and tap water with a low pH value and low hardness to feed heating networks. Additional treatment of river water at a waterworks usually leads to a decrease in pH, a decrease in alkalinity and an increase in the content of aggressive carbon dioxide. The appearance of aggressive carbon dioxide is also possible in connection diagrams used for large heat supply systems with direct water supply hot water(2000h3000 t/h). Softening water 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, thermal power equipment of thermal power plants is susceptible to internal corrosion.

When examining the make-up tract of one of the thermal power plants in Leningrad, the following data were obtained on the corrosion rate, g/(m2 4):

Installation location of corrosion indicators

In the make-up water pipeline after the heaters of the heating network in front of the deaerators, the 7 mm thick pipes thinned over the year of operation, in some places up to 1 mm, and through fistulas formed in some areas.

The causes of pitting corrosion of hot water boiler pipes are as follows:

insufficient removal of oxygen from make-up water;

low pH value due to the presence of aggressive carbon dioxide

(up to 10h15 mg/l);

accumulation of oxygen corrosion products of iron (Fe2O3;) on heat transfer surfaces.

Operating equipment on network water with an iron concentration of over 600 µg/l usually results in intensive (over 1000 g/m2) contamination of their heating surfaces with iron oxide deposits for several thousand hours of operation of hot water boilers. In this case, frequent leaks are noted in the pipes of the convective part. The content of iron oxides in sediments usually reaches 80–90%.

Start-up periods are especially important for the operation of hot water boilers. During the initial period of operation at one thermal power plant, oxygen removal was not ensured to the standards established by the PTE. The oxygen content in the make-up water exceeded these standards by 10 times.

The concentration of iron in the make-up water reached 1000 µg/l, and in the return water of the heating network - 3500 µg/l. After the first year of operation, cuttings were made from the network water pipelines; it turned out that their surface contamination with corrosion products was over 2000 g/m2.

It should be noted that at this thermal power plant, before putting the boiler into operation, the internal surfaces of the screen pipes and convective beam pipes were subjected to chemical cleaning. By the time the samples of screen pipes were cut out, the boiler had worked for 5300 hours. The sample of the screen pipe had an uneven layer of black-brown iron oxide deposits, firmly bound to the metal; height of tubercles 10x12 mm; specific pollution 2303 g/m2.

Sediment composition, %

The metal surface under the layer of deposits was affected by ulcers up to 1 mm deep. Convective beam tubes with inside were covered with iron oxide type deposits of black-brown color with the height of the tubercles up to 3-4 mm. The surface of the metal under the deposits is covered with ulcers various sizes depth 0.3x1.2 and diameter 0.35x0.5 mm. Some tubes had through holes (fistulas).

When hot water boilers are installed in old district heating systems in which significant amounts of iron oxides have accumulated, cases of deposits of these oxides in the heated boiler pipes are observed. Before turning on the boilers, it is necessary to thoroughly flush the entire system.

A number of researchers recognize the important role in the occurrence of sub-sludge corrosion of the process of rusting of pipes of water heating boilers during their downtime, when proper measures have not been taken to prevent standstill corrosion. Foci of corrosion that arise under the influence of atmospheric air on the wet surfaces of boilers continue to function during operation of the boilers.

2.1. Heating surfaces.

The most typical damage to heating surface pipes are: cracks on the surface of screen and boiler pipes, corrosion attacks on the outer and inner surfaces of pipes, ruptures, thinning of pipe walls, cracks and destruction of bells.

Reasons for the appearance of cracks, ruptures and fistulas: deposits in boiler pipes of salts, corrosion products, welding beads, which slow down circulation and cause overheating of the metal, external mechanical damage, disruption of the water chemistry regime.

Corrosion of the outer surface of pipes is divided into low-temperature and high-temperature. Low-temperature corrosion occurs in places where blowers are installed, when, as a result of improper operation, condensation is allowed to form on soot-covered heating surfaces. High temperature corrosion can occur in the second stage of the superheater when burning sour 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, cavities and cracks.

Corrosion of the inner surface of pipes also includes: oxygen stagnation corrosion, sub-sludge alkaline corrosion of boiler and screen pipes, corrosion fatigue, which manifests itself in the form of cracks in boiler and screen pipes.

Pipe damage due to creep is characterized by an increase in diameter and the formation of longitudinal cracks. Deformations in places where pipes are bent and welded joints can have different directions.

Burnouts and scaling in pipes occur due to their overheating to temperatures exceeding the design temperature.

The main types of damage to welds made by manual arc welding are fistulas that arise due to lack of penetration, slag inclusions, gas pores, and lack of fusion along the edges of pipes.

The main defects and damage to the surface of the superheater are: corrosion and scaling on the outer and inner surfaces of pipes, cracks, risks and delamination of pipe metal, fistulas and ruptures of pipes, defects in welded pipe joints, residual deformation as a result of creep.

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

Typical malfunctions that arise during the operation of the surface desuperheater of the DE-25-24-380GM boiler are: internal and external corrosion of pipes, cracks and fistulas in welded

seams and pipe bends, cavities that may occur during repairs, risks on the face of flanges, leaks of flange connections due to flange misalignment. During a 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 carried out.

2.2. Boiler drums.

Typical damage to 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, intercrystalline corrosion of the shells and bottoms, corrosion separation of the surfaces of the shells and bottoms, drum ovality Oddulins (bulges) on the surfaces of the drums facing the furnace, caused by the temperature effect of the torch in cases of destruction (or loss) of individual parts of the lining.

2.3. Metal structures and boiler lining.

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 links, cracks, corrosion damage to the metal surface.

As a result of prolonged exposure to temperatures, cracking and damage to the integrity of the shaped bricks fixed on pins to the upper drum from the side of the firebox occur, as well as cracks in the brickwork along the lower drum and the hearth of the firebox.

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

3. Checking the condition of the boiler elements.

The condition of boiler elements taken out for repair is checked based on 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 boiler expert inspection program (section “Boiler Expert Inspection Program”).

3.1. Checking heating surfaces.

Inspection of the outer surfaces of pipe elements must be carried out especially carefully in places where pipes pass through the lining, casing, 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 pipe walls due to sulfur and static corrosion, it is necessary to inspect the heating surface pipes of boilers that have been in operation for more than two years during annual technical inspections carried out by the enterprise administration.

Control is carried out by external inspection by tapping the pre-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 sections of the pipes that have undergone the greatest wear and corrosion (horizontal sections, areas in soot deposits and covered with coke deposits).

The thickness of pipe walls is measured using ultrasonic thickness gauges. It is possible to cut out sections of pipes on two or three pipes of combustion screens and pipes of a convective beam located at the gas inlet and outlet. The remaining thickness of the pipe walls must be no less than the calculated one according to the strength calculation (attached to the Boiler Certificate), taking into account an increase for corrosion for the period of further operation until the next inspection and an increase in the margin of 0.5 mm.

The calculated wall thickness of screen and boiler pipes for an operating pressure of 1.3 MPa (13 kgf/cm2) is 0.8 mm, for 2.3 MPa (23 kgf/cm2) – 1.1 mm. The allowance for corrosion is taken based on the obtained measurement results and taking into account the duration of operation between surveys.

At enterprises where, as a result of long-term operation, intensive wear of heating surface pipes has not been observed, pipe wall thickness can be monitored during major repairs, but at least once every 4 years.

The collector, superheater and rear screen are subject to internal inspection. The hatches of the upper manifold of the rear screen must be subjected to mandatory opening and inspection.

The outer diameter of the pipes should be measured in the maximum temperature zone. For measurements, use special templates (staples) or calipers. Dents with smooth transitions with a depth of no more than 4 mm are allowed on the surface of the pipes, if they do not take the wall thickness beyond the limits of minus deviations.

The permissible difference in pipe wall thickness is 10%.

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

3.2. Checking the drum.

After 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.

Measure uniform corrosion along the thickness of the wall, in which a hole with a diameter of 8 mm is drilled for this purpose. After measuring, install a plug in the hole and scald 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.

Main corrosion and ulcers should be measured using impressions. For this purpose, clean the damaged area of ​​the metal surface from deposits and lightly lubricate it 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 impression of the damaged surface.

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

Impressions of damage located on vertical ceiling surfaces can be obtained using wax and plasticine.

Inspection of pipe holes and 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 protrusion, this means that the diameter of the hole is increased beyond the norm. The exact diameter is measured using a caliper and noted in the repair form.

When inspecting drum welds, it is necessary to check the adjacent base metal to 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, and in doubtful cases more often.

Measuring the drum deflection is carried out by stretching the string along the surface of the drum and measuring the gaps along the length of the string.

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.

Holes and dents outside the area 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, is no more than:

towards atmospheric pressure (outward) - 2%;

towards steam pressure (dents) - 5%.

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

The permissible increase in the diameter of holes for pipes (for welding) is 10%.

Identification of types of corrosion is difficult, and, therefore, errors are common in determining technologically and economically optimal measures to combat corrosion. The main necessary measures are taken in accordance with regulatory documents, which establish the limits of the main corrosion initiators.

GOST 20995-75 “Stationary steam boilers with pressure up to 3.9 MPa. Indicators of quality of feed water and steam" normalizes the indicators in feed water: transparency, that is, the amount of suspended impurities; general hardness, content of iron and copper compounds - prevention of scale formation and iron and copper oxide deposits; pH value - prevention of alkaline and acid corrosion and also foaming in the boiler drum; oxygen content - preventing oxygen corrosion; nitrite content - prevention of nitrite corrosion; content of petroleum products - preventing foam formation in the boiler drum.

The norm values ​​are determined by GOST depending on the pressure in the boiler (therefore, on the water temperature), on the power of the local heat flow and on the water treatment technology.

When investigating the causes of corrosion, first of all, it is necessary to inspect (where available) places of metal destruction, analyze the operating conditions of the boiler in the pre-accident period, analyze the quality of feed water, steam and deposits, analyze design features boiler

Upon external inspection, the following types of corrosion may be suspected.

Oxygen corrosion

: inlet sections of steel economizer pipes; supply pipelines when encountering insufficiently deoxygenated (above normal) water - “breakthroughs” of oxygen due to poor deaeration; feed water heaters; all wet areas of the boiler during its shutdown and failure to take measures to prevent air from entering the boiler, especially in stagnant areas, when draining water, from where it is difficult to remove steam condensate or completely fill with water, for example, vertical pipes of superheaters. During downtime, corrosion is enhanced (localized) in the presence of alkali (less than 100 mg/l).

Oxygen corrosion rarely (when the oxygen content in water is significantly higher than the norm - 0.3 mg/l) appears in the steam separation devices of boiler drums and on the drum wall at the water level boundary; in downpipes. Corrosion does not occur in riser pipes due to the deaerating effect of steam bubbles.

Type and nature of damage. Ulcers of varying depth and diameter, often covered with tubercles, the upper crust of which is reddish iron oxides (probably hematite Fe 2 O 3). Evidence of active corrosion: under the crust of the tubercles there is a black liquid sediment, probably magnetite (Fe 3 O 4) mixed with sulfates and chlorides. With extinct corrosion, there is a void under the crust, and the bottom of the ulcer is covered with deposits of scale and sludge.

At water pH > 8.5 - ulcers are rare, but larger and deeper, at pH< 8,5 - встречаются чаще, но меньших размеров. Только вскрытие бугорков помогает интерпретировать бугорки не как поверхностные отложения, а как следствие коррозии.

When the water speed is more than 2 m/s, the tubercles can take on an oblong shape in the direction of the jet movement.

. Magnetic crusts are quite dense and could serve as a reliable barrier to the penetration of oxygen into the tubercles. But they are often destroyed as a result of corrosion fatigue, when the temperature of water and metal changes cyclically: frequent stops and starts of the boiler, pulsating movement of the steam-water mixture, stratification of the steam-water mixture into separate plugs of steam and water, following each other.

Corrosion increases with increasing temperature (up to 350 °C) and increasing chloride content in boiler water. Sometimes corrosion is enhanced by the thermal decomposition products of certain organic matter feed water.

Rice. 1. Appearance oxygen corrosion

Alkaline (in a narrower sense - intergranular) corrosion

Places of metal corrosion damage. Pipes in areas of high power heat flow (burner area and opposite the elongated torch) - 300-400 kW/m2 and where the metal temperature is 5-10 °C higher than the boiling point of water at a given pressure; inclined and horizontal pipes where water circulation is poor; places under thick sediments; zones near the backing rings and in the welds themselves, for example, in places where intra-drum vapor separation devices are welded; places near the rivets.

Type and nature of damage. Hemispherical or elliptical depressions filled with corrosion products, often including shiny crystals of magnetite (Fe 3 O 4). Most of the depressions are covered with a hard crust. On the side of the pipes facing the firebox, the recesses can connect, forming a so-called corrosion track 20-40 mm wide and up to 2-3 m long.

If the crust is not sufficiently stable and dense, then corrosion can lead - under conditions of mechanical stress - to the appearance of cracks in the metal, especially near the cracks: rivets, rolling joints, welding points of vapor separation devices.

Causes of Corrosion Damage. At high temperatures - more than 200 ° C - and a high concentration of caustic soda (NaOH) - 10% or more - the protective film (crust) on the metal is destroyed:

4NaOH + Fe 3 O 4 = 2NaFeO 2 + Na 2 FeO 2 + 2H 2 O (1)

The intermediate product NaFeO 2 undergoes hydrolysis:

4NaFeO 2 + 2H 2 O = 4NaOH + 2Fe 2 O 3 + 2H 2 (2)

That is, in this reaction (2) sodium hydroxide is reduced, in reactions (1), (2) it is not consumed, but acts as a catalyst.

When the magnetite is removed, caustic soda and water can react with the iron directly to release atomic hydrogen:

2NaOH + Fe = Na 2 FeO 2 + 2H (3)

4H 2 O + 3Fe = Fe 3 O 4 + 8H (4)

The released hydrogen is able to diffuse into the metal and form methane (CH 4) with iron carbide:

4H + Fe 3 C = CH 4 + 3Fe (5)

It is also possible to combine atomic hydrogen into molecular hydrogen (H + H = H 2).

Methane and molecular hydrogen cannot penetrate into the metal; they accumulate at the grain boundaries and, in the presence of cracks, expand and deepen them. In addition, these gases prevent the formation and compaction of protective films.

A concentrated solution of caustic soda is formed in places of deep evaporation of boiler water: dense scale deposits of salts (a type of sub-sludge corrosion); a crisis of nucleate boiling, when a stable vapor film is formed above the metal - there the metal is almost not damaged, but at the edges of the film, where active evaporation occurs, caustic soda is concentrated; the presence of cracks where evaporation occurs, which is different from evaporation in the entire volume of water: caustic soda evaporates worse than water, is not washed away by water and accumulates. Acting on the metal, caustic soda forms cracks at the grain boundaries directed into the metal (a type of intergranular corrosion - crevice).

Intergranular corrosion under the influence of alkaline boiler water is most often concentrated in the boiler drum.

Rice. 3. Intergranular corrosion: a - microstructure of the metal before corrosion, b - microstructure at the corrosion stage, formation of cracks along the grain boundaries of the metal

Such a corrosive effect on metal is possible only with the simultaneous presence of three factors:

• local tensile mechanical stresses close to or slightly exceeding the yield strength, that is, 2.5 MN/mm 2 ;
• loose joints of drum parts (indicated above), where deep evaporation of boiler water can occur and where accumulating caustic soda dissolves protective film iron oxides (NaOH concentration more than 10%, water temperature above 200 °C and - especially - closer to 300 °C). If the boiler is operated at a pressure lower than the rated pressure (for example, 0.6-0.7 MPa instead of 1.4 MPa), then the likelihood of this type of corrosion decreases;
• an unfavorable combination of substances in boiler water, which lacks the necessary protective concentrations of inhibitors of this type of corrosion. Sodium salts can act as inhibitors: sulfates, carbonates, phosphates, nitrates, cellulose sulfite liquor.

Rice. 4. Appearance of intergranular corrosion

Corrosion cracks do not develop if the following ratio is observed:

(Na 2 SO 4 + Na 2 CO 3 + Na 3 PO 4 + NaNO 3)/(NaOH) ≥ 5.3 (6)

where Na 2 SO 4, Na 2 CO 3, Na 3 PO 4, NaNO 3, NaOH are the contents of sodium sulfate, sodium carbonate, sodium phosphate, sodium nitrate and sodium hydroxide, respectively, mg/kg.

In currently manufactured boilers, at least one of the specified conditions for the occurrence of corrosion is absent.

The presence of silicon compounds in boiler water can also increase intergranular corrosion.

NaCl under these conditions is not a corrosion inhibitor. It was shown above: chlorine ions (Cl -) are corrosion accelerators; due to their high mobility and small size, they easily penetrate protective oxide films and produce highly soluble salts with iron (FeCl 2, FeCl 3) instead of poorly soluble iron oxides.

In boiler water, the values ​​of total mineralization are traditionally monitored, rather than the content of individual salts. Probably for this reason, standardization was introduced not according to the indicated ratio (6), but according to the value of the relative alkalinity of the boiler water:

Sh q rel = Sh ov rel = Sh ov 40 100/S ov ≤ 20, (7)

where Shk rel - relative alkalinity of boiler water, %; Shch ov rel - relative alkalinity of treated (additional) water, %; Shch ov - total alkalinity of treated (additional) water, mmol/l; S ov - mineralization of treated (additional) water (including chloride content), mg/l.

The total alkalinity of the treated (additional) water can be taken equal, mmol/l:

• after sodium cationization - the total alkalinity of the source water;
• after hydrogen-sodium cationization parallel - (0.3-0.4), or sequential with “hungry” regeneration of the hydrogen-cation exchange filter - (0.5-0.7);
• after sodium cationization with acidification and sodium chlorine ionization - (0.5-1.0);
• after ammonium-sodium cationization - (0.5-0.7);
• after liming at 30-40 °C - (0.35-1.0);
• after coagulation - (Sh about ref - D k), where Sh about ref is the total alkalinity of the source water, mmol/l; D k - dose of coagulant, mmol/l;
• after soda liming at 30-40 °C - (1.0-1.5), and at 60-70 °C - (1.0-1.2).

The values ​​of relative alkalinity of boiler water according to Rostekhnadzor standards are accepted, %, no more than:

• for boilers with riveted drums - 20;
• for boilers with welded drums and pipes rolled into them - 50;
• for boilers with welded drums and pipes welded to them - any value, not standardized.

Rice. 4. Result of intergranular corrosion

According to Rostekhnadzor standards, Shch kv rel is one of the criteria safe work boilers It is more correct to check the criterion of potential alkaline aggressiveness of boiler water, which does not take into account the content of chlorine ion:

K sh = (S ov - [Cl - ])/40 Shch ov, (8)

where Ksh is a criterion for the potential alkaline aggressiveness of boiler water; S ov - mineralization of treated (additional) water (including chloride content), mg/l; Cl - - content of chlorides in treated (additional) water, mg/l; Shch ov - total alkalinity of treated (additional) water, mmol/l.

The value of K sch can be taken:

• for boilers with riveted drums pressure more than 0.8 MPa ≥ 5;
• for boilers with welded drums and pipes rolled into them with a pressure of more than 1.4 MPa ≥ 2;
• for boilers with welded drums and pipes welded to them, as well as for boilers with welded drums and pipes rolled into them with a pressure of up to 1.4 MPa and boilers with riveted drums with a pressure of up to 0.8 MPa - do not standardize.

Sludge corrosion

This name combines several different types of corrosion (alkali, oxygen, etc.). The accumulation of loose and porous deposits and sludge in different areas of the boiler causes corrosion of the metal under the sludge. The main reason: contamination of feed water with iron oxides.

Nitrite corrosion

. Screen and boiler pipes of the boiler on the side facing the firebox.

Type and nature of damage. Rare, sharply limited large ulcers.

. If there are more than 20 μg/l of nitrite ions (NO - 2) in the feed water, and the water temperature is more than 200 ° C, nitrites serve as cathodic depolarizers of electrochemical corrosion, being reduced to HNO 2, NO, N 2 (see above).

Steam-water corrosion

Locations of metal corrosion damage. The outlet part of superheater coils, superheated steam steam pipelines, horizontal and slightly inclined steam generating pipes in areas of poor water circulation, sometimes along the upper form of the outlet coils of boiling water economizers.

Type and nature of damage. Plaques of dense black iron oxides (Fe 3 O 4), firmly adhered to the metal. When the temperature fluctuates, the continuity of the plaque (crust) is disrupted and the scales fall off. Uniform thinning of metal with bulges, longitudinal cracks, breaks.

It can be identified as sub-sludge corrosion: in the form of deep ulcers with vaguely demarcated edges, most often near welds protruding into the pipe, where sludge accumulates.

Causes of corrosion damage:

• washing medium - steam in superheaters, steam pipelines, steam “pillows” under a layer of sludge;
• metal temperature (steel 20) more than 450 °C, heat flow to the metal section - 450 kW/m 2;
• violation of the combustion regime: slagging of burners, increased contamination of pipes inside and outside, unstable (vibrating) combustion, elongation of the torch towards the screen pipes.

The result: direct chemical interaction of iron with water vapor (see above).

Microbiological corrosion

Caused by aerobic and anaerobic bacteria, appears at temperatures of 20-80 ° C.

Locations of metal damage. Pipes and containers to the boiler with water at the specified temperature.

Type and nature of damage. The tubercles are of different sizes: diameter from several millimeters to several centimeters, rarely - several tens of centimeters. The tubercles are covered with dense iron oxides - a waste product of aerobic bacteria. Inside there is a black powder and suspension (iron sulfide FeS) - a product of sulfate-reducing anaerobic bacteria; under the black formation there are round ulcers.

Causes of damage. Natural water always contains iron sulfates, oxygen and various bacteria.

Iron bacteria in the presence of oxygen form a film of iron oxides, under which anaerobic bacteria reduce sulfates to iron sulfide (FeS) and hydrogen sulfide (H 2 S). In turn, hydrogen sulfide starts the formation of sulfurous (very unstable) and sulfuric acids, and the metal corrodes.

This type has an indirect effect on boiler corrosion: a water flow at a speed of 2-3 m/s tears off the tubercles, carries their contents into the boiler, increasing the accumulation of sludge.

In rare cases, this corrosion may occur in the boiler itself if, during a long shutdown of the boiler, the reserve is filled with water at a temperature of 50-60 o C, and the temperature is maintained due to random breakthroughs of steam from neighboring boilers.

Chelate corrosion

Locations of corrosion damage. Equipment in which steam is separated from water: boiler drum, steam separation devices in and outside the drum, also - rarely - in feedwater pipelines and economizer.

Type and nature of damage. The surface of the metal is smooth, but if the medium moves at high speed, then the corroded surface is not smooth, has horseshoe-shaped depressions and “tails” oriented in the direction of movement. The surface is covered with a thin matte or black shiny film. There are no obvious deposits, and there are no corrosion products, because the “chelate” (organic polyamine compounds specially introduced into the boiler) has already reacted.

In the presence of oxygen, which rarely happens in a normally operating boiler, the corroded surface is “invigorated”: roughness, islands of metal.

Causes of Corrosion Damage. The mechanism of action of the “chelate” was described earlier (“Industrial and heating boiler houses and mini-CHP”, 1(6)΄ 2011, p. 40).

“Chelate” corrosion occurs when there is an overdose of “chelate,” but it is also possible with a normal dose, since the “chelate” is concentrated in areas where intense evaporation of water occurs: nucleate boiling is replaced by film boiling. In steam separation devices, there are cases of particularly destructive “chelate” corrosion due to high turbulent velocities of water and steam-water mixture.

All of the described corrosion damage can have a synenergetic effect, so that the total damage from the combined action of different corrosion factors can exceed the sum of damage from individual types of corrosion.

As a rule, the action of corrosive agents enhances the unstable thermal regime of the boiler, which causes corrosion fatigue and initiates thermal fatigue corrosion: the number of starts from a cold state is more than 100, total number starts - more than 200. Since these types of metal damage occur rarely, cracks and pipe ruptures have an appearance identical to metal damage from various types of corrosion.

Usually, to identify the cause of metal destruction, additional metallographic studies are required: radiography, ultrasound, color and magnetic particle flaw detection.

Various researchers have proposed programs for diagnosing types of corrosion damage to boiler steels. The VTI program is known (A.F. Bogachev and co-workers) - mainly for power boilers high pressure, and developments of the Energochermet association - mainly for low and medium pressure power boilers and waste heat boilers.

Corrosion of steel in steam boilers, occurring under the influence of water steam, comes down mainly to the following reaction:

3Fe + 4H20 = Fe2O3 + 4H2

We can assume that the inner surface of the boiler represents a thin film of magnetic iron oxide. During operation of the boiler, the oxide film is continuously destroyed and formed again, and hydrogen is released. Since the surface film of magnetic iron oxide represents the main protection for steel, it should be maintained in a state of least permeability to water.
For boilers, fittings, water and steam pipelines, predominantly simple carbon or low-alloy steels are used. The corrosive medium in all cases is water or water vapor of varying degrees of purity.
The temperature at which the corrosion process can occur ranges from the temperature of the room where the inactive boiler is located to the boiling point of saturated solutions when the boiler is operating, sometimes reaching 700°. The solution may have a temperature significantly higher than critical temperature clean water(374°). However, high salt concentrations in boilers are rare.
The mechanism by which physical and chemical causes can lead to film failure in steam boilers is essentially different from the mechanism studied at lower temperatures in less critical equipment. The difference is that the corrosion rate in boilers is much greater due to the high temperature and pressure. The high rate of heat transfer from the boiler walls to the environment, reaching 15 cal/cm2sec, also increases corrosion.

POT CORROSION

Recognizing pitting corrosion

Determining the cause of the formation of corrosion damage of a certain type is often very difficult, since several causes can act simultaneously; in addition, a number of changes that occur when the boiler cools from high temperature and when water is drained sometimes masks the phenomena that took place during operation. However, experience greatly helps in recognizing pitting corrosion in boilers. For example, it was observed that the presence of black magnetic iron oxide in a corrosion shell or on the surface of a tubercle indicates that an active process was occurring in the boiler. Such observations are often used to check measures taken to protect against corrosion.
The iron oxide that forms in areas of active corrosion should not be mixed with black magnetic iron oxide, which is sometimes present as a suspension in boiler water. It must be remembered that neither the total amount of finely dispersed magnetic iron oxide, nor the amount of hydrogen released in the boiler can serve as a reliable indicator of the degree and extent of corrosion occurring. Ferrous hydrate entering the boiler from foreign sources, such as condensate tanks or boiler supply piping, may partly explain the presence of both iron oxide and hydrogen in the boiler. Ferrous hydroxide supplied with the feed water reacts in the boiler by reaction.

3Fe (OH)2 = Fe3O4 + 2H2O + H2.

Reasons influencing the development of pitting corrosion

Foreign impurities and stresses. Non-metallic inclusions in steel, as well as stress, can create anodic areas on the metal surface. Typically, corrosion pits come in different sizes and are scattered randomly across the surface. In the presence of stresses, the location of the shells obeys the direction of the applied stress. Typical examples include fin tubes where fins have cracked, as well as boiler tube flaring areas.
Dissolved oxygen.
It is possible that the most powerful activator of pitting corrosion is oxygen dissolved in water. At all temperatures, even in an alkaline solution, oxygen serves as an active depolarizer. In addition, oxygen concentration elements can easily occur in boilers, especially under scale or contamination, where stagnant areas are created. The usual measure to combat this type of corrosion is deaeration.
Dissolved carbonic anhydride.
Since solutions of carbonic anhydride have a slightly acidic reaction, it accelerates corrosion in boilers. Alkaline boiler water reduces the aggressiveness of dissolved carbonic anhydride, but the resulting benefit does not extend to steam-fed surfaces or condensate lines. Removal of carbonic anhydride along with dissolved oxygen by mechanical deaeration is common.
Recently, attempts have been made to use cyclohexylamine to eliminate corrosion in steam and condensate lines in heating systems.
Deposits on the walls of the boiler.
Very often, corrosion pits can be found along the outer surface (or under the surface) of deposits such as mill scale, boiler sludge, boiler scale, corrosion products, and oil films. Once started, pitting corrosion will continue to develop unless the corrosion products are removed. This type of local corrosion is enhanced by the cathodic (in relation to boiler steel) nature of the deposits or by the depletion of oxygen under the deposits.
Copper in boiler water.
If we take into account the large quantities of copper alloys used for auxiliary equipment(condensers, pumps, etc.), then it is not surprising that in most cases boiler deposits contain copper. It is usually present in a metallic state, sometimes in the form of an oxide. The amount of copper in deposits varies from fractions of a percent to almost pure copper.
The question of the significance of copper deposits in boiler corrosion cannot be considered resolved. Some argue that copper is only present during the corrosion process and does not affect it in any way; others, on the contrary, believe that copper, being a cathode in relation to steel, can contribute to pitting corrosion. None of these points of view has been confirmed by direct experiments.
In many cases, little (or even no) corrosion was observed despite the deposits throughout the boiler containing significant amounts of copper metal. There is also evidence that when copper comes into contact with low-carbon steel in alkaline boiler water at elevated temperatures, the copper is destroyed more quickly than the steel. Copper rings, crimping ends of flared pipes, copper rivets and screens of auxiliary equipment through which boiler water passes are almost completely destroyed even at relatively low temperatures. In view of this, it is believed that copper metal does not increase the corrosion of boiler steel. The deposited copper can be considered simply as the end product of the reduction of copper oxide by hydrogen at the time of its formation.
On the contrary, very strong corrosion pitting of boiler metal is often observed in the vicinity of deposits that are especially rich in copper. These observations led to the suggestion that copper, because it is cathodic to steel, promotes pitting corrosion.
The surface of boilers rarely presents exposed metallic iron. Most often, it has a protective layer consisting mainly of iron oxide. It is possible that where cracks form in this layer, a surface is exposed that is anodic to copper. In such places, the formation of corrosion pits increases. This can also explain, in some cases, accelerated corrosion in those places where a shell has formed, as well as severe pitting corrosion, sometimes observed after cleaning boilers with the use of acids.
Improper maintenance of idle boilers.
One of the most common reasons The formation of corrosion shells is caused by the lack of proper care of idle boilers. An idle boiler must be kept either completely dry or filled with water treated in such a way that corrosion is impossible.
The water remaining on the inner surface of an inactive boiler dissolves oxygen from the air, which leads to the formation of shells, which will later become centers around which the corrosion process will develop.
Common instructions for protecting idle boilers from corrosion are as follows:
1) draining water from a still hot boiler (about 90°); blowing the boiler with air until it is completely dry and kept dry;
2) filling the boiler with alkaline water (pH = 11), containing an excess of SO3 ions (about 0.01%), and storing under a water or steam seal;
3) filling the boiler with an alkaline solution containing chromic acid salts (0.02-0.03% CrO4").
When chemically cleaning boilers, the protective layer of iron oxide will be removed in many places. Subsequently, these places may not be covered with a newly formed continuous layer and shells will appear on them, even in the absence of copper. It is therefore recommended that immediately after chemical cleaning, the iron oxide layer be renewed by treating with a boiling alkaline solution (similar to what is done for new boilers coming into operation).

Corrosion of economizers

The general provisions regarding boiler corrosion apply equally to economizers. However, the economizer, heating the feed water and located in front of the boiler, is especially sensitive to the formation of corrosion pits. It represents the first high-temperature surface that experiences the destructive action of oxygen dissolved in the feed water. In addition, the water passing through the economizer generally has a low pH value and does not contain chemical retardants.
The fight against corrosion of economizers involves deaerating the water and adding alkali and chemical retarders.
Sometimes boiler water is treated by passing part of it through an economizer. In this case, sludge deposits in the economizer should be avoided. The effect of such boiler water recirculation on steam quality must also be taken into account.

BOILER WATER TREATMENT

CORROSION IN CONCENTRATED BOILER WATER

ALKALINE BRITTLESS OF STEEL

Stress required to cause alkali embrittlement

Practice shows a low probability of brittle fracture of conventional boiler steel if the stresses do not exceed the yield strength. Stresses created by steam pressure or uniformly distributed load from the structure’s own weight cannot lead to the formation of cracks. However, the stresses created by rolling sheet material intended for the manufacture of boilers, deformation during riveting or any cold working associated with permanent deformation can cause the formation of cracks.
The presence of externally applied stresses is not necessary for the formation of cracks. A boiler steel sample previously held under constant bending stress and then released may crack in an alkaline solution whose concentration is equal to the increased alkali concentration in the boiler water.

Alkali concentration

The normal concentration of alkali in the boiler drum cannot cause cracks, because it does not exceed 0.1% NaOH, and the lowest concentration at which alkali brittleness is observed is approximately 100 times higher than normal.
Such high concentrations may result from extremely slow percolation of water through a rivet seam or some other gap. This explains the appearance of hard salts on the outside of most rivet seams in steam boilers. The most dangerous leak is one that is difficult to detect. It leaves a residue of solid material inside the rivet joint where there are high residual stresses. The combined action of stress and a concentrated solution can cause the appearance of alkali brittleness cracks.

Alkali embrittlement detection device

A special device for monitoring the composition of water reproduces the process of water evaporation with increasing alkali concentration on a stressed steel sample under the same conditions in which this occurs in the area of ​​the rivet seam. Cracking of the control sample indicates that boiler water of this composition is capable of causing alkali embrittlement. Therefore, in this case, water treatment is necessary to eliminate its hazardous properties. However, cracking of the control sample does not mean that cracks have already appeared or will appear in the boiler. In rivet seams or other joints there is not necessarily both leakage (steaming), stress, and an increase in alkali concentration, as in the control sample.
The control device is installed directly on the steam boiler and allows you to judge the quality of the boiler water.
The test lasts 30 days or more with constant circulation of water through the control device.

Alkali Brittleness Crack Recognition

Alkali brittleness cracks in conventional boiler steel are of a different nature than fatigue or high stress cracks. This is illustrated in Fig. I9, which shows the intergranular nature of such cracks, forming a fine network. The difference between intergranular alkali brittleness cracks and intragranular cracks caused by corrosion fatigue can be seen by comparison.
In alloy steels (for example, nickel or silicon-manganese), used for locomotive boilers, cracks are also arranged in a grid, but do not always pass between crystallites, as in the case of ordinary boiler steel.

Alkali brittleness theory

Atoms in the crystal lattice of a metal located at the boundaries of crystallites experience less symmetrical influence from their neighbors than atoms in the rest of the grain mass. Therefore, they leave the crystal lattice more easily. One might think that with careful selection of an aggressive environment it will be possible to achieve such selective removal of atoms from crystallite boundaries. Indeed, experiments show that in acidic, neutral (with the help of a weak electric current, creating conditions favorable for corrosion) and concentrated alkali solutions, intergranular cracking can be obtained. If the solution causing general corrosion is modified by the addition of some substance that forms a protective film on the surface of the crystallites, the corrosion is concentrated at the boundaries between the crystallites.
The aggressive solution in this case is caustic soda solution. The sodium silica salt can protect the surfaces of crystallites without affecting the boundaries between them. The result of a combined protective and aggressive action depends on many circumstances: concentration, temperature, stressed state of the metal and composition of the solution.
There are also the colloidal theory of alkali brittleness and the theory of the action of hydrogen dissolving in steel.

Ways to combat alkaline embrittlement

One way to combat alkali brittleness is to replace boiler riveting with welding, which eliminates the possibility of leakage. Brittleness can also be eliminated by using steel that is resistant to intergranular corrosion or by chemically treating the boiler water. In riveted boilers currently used, the latter method is the only acceptable one.
Preliminary tests using a control sample are the best way to determine the effectiveness of certain water protective additives. Sodium sulfide salt does not prevent cracking. Sodium nitrogen salt is successfully used to protect against cracking at pressures up to 52.5 kg/cm2. Concentrated solutions of sodium nitrogen salt, boiling at atmospheric pressure, can cause stress corrosion cracks in mild steel.
Currently, sodium nitrogen salt is widely used in stationary boilers. The concentration of sodium nitrogen salt corresponds to 20-30% of the alkali concentration.

CORROSION OF STEAM HEATERS

Hydrogen as a measure of corrosion rate

Steam temperature in modern boilers approaches the temperatures used in the industrial production of hydrogen by direct reaction between steam and iron.
The rate of corrosion of pipes made of carbon and alloy steel under the influence of steam, at temperatures up to 650°, can be judged by the volume of hydrogen released. Hydrogen evolution is sometimes used as a measure of general corrosion.
IN Lately At power plants in the United States, three types of miniature installations are used to remove gases and air. They ensure complete removal of gases, and the degassed condensate is suitable for determining salts carried away by steam from the boiler. An approximate value of the total corrosion of the superheater during boiler operation can be obtained by determining the difference in hydrogen concentrations in steam samples taken before and after its passage through the superheater.

Corrosion caused by impurities in steam

The saturated steam entering the superheater carries with it small but measurable amounts of gases and salts from the boiler water. The most commonly encountered gases are oxygen, ammonia and carbon dioxide. When steam passes through the superheater, no noticeable change in the concentration of these gases is observed. Only minor corrosion of the metal superheater can be attributed to the action of these gases. It has not yet been proven that salts dissolved in water, dry, or deposited on superheater elements can contribute to corrosion. However, caustic soda, being the main integral part salts carried away by the boiler water can contribute to corrosion of the highly heated tube, especially if the alkali adheres to the metal wall.
Increasing the purity of saturated steam is achieved by thoroughly removing gases from the feed water. Reducing the amount of salts entrained in the steam can be achieved by thorough cleaning in the upper header, the use of mechanical separators, flushing the saturated steam with feed water, or suitable chemical treatment of the water.
Determination of the concentration and nature of gases entrained by saturated steam is carried out using the above-mentioned devices and chemical analysis. It is convenient to determine the concentration of salts in saturated steam by measuring the electrical conductivity of water or evaporation large quantity condensate
An improved method for measuring electrical conductivity is proposed, and appropriate corrections for some dissolved gases are given. The condensate in the miniature degassing units mentioned above can also be used to measure electrical conductivity.
When the boiler is idle, the superheater is a refrigerator in which condensation accumulates; In this case, normal underwater pitting is possible if the steam contained oxygen or carbon dioxide.

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