Gas corrosion of elements of boiler equipment. Corrosion of hot water boilers and heat exchange equipment

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

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

If water remains in the system for one reason or another, then severe parking corrosion can 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 different intensity is quite often observed, despite the same shutdown modes of the system and the quality of the water contained in them; devices with significant thermal accumulation are subject to more severe corrosion than devices that have the dimensions of a furnace and a heating surface, since the boiler water in them cools faster; its temperature falls below 60-70°C.

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

The development of parking corrosion is greatly facilitated by the sludge that accumulates on the surfaces of the boiler, which usually retains moisture. In this regard, significant corrosion holes are often found in aggregates and pipes along the lower generatrix and at their ends, i.e., in areas of the greatest accumulation of sludge.

Methods of conservation of equipment in reserve

The following methods can be used to preserve equipment:

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

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

c) filling technological system nitrogen.

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

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

There are two types of short-term downtime:

a) scheduled, associated with the withdrawal to the reserve on weekends due to a drop in load or withdrawal to the reserve at night;

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

Depending on the purpose long downtime can be divided into the following groups: a) withdrawal of equipment to the reserve; b) current repairs; c) capital repairs.

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

When the system is placed on standby or when it is idle for a long time without performing repair work conservation is advisable to carry out by filling with a solution of nitrite or sodium silicate. In these cases, nitrogen conservation can also be used, necessarily taking measures to create a tightness of the system in order to prevent excessive gas consumption and unproductive operation of the nitrogen plant, as well as to create safe conditions for equipment maintenance.

Preservation methods by creating excess pressure, filling with nitrogen can be used regardless of the design features of the heating surfaces of the equipment.

To prevent parking corrosion of metal during major and current repairs only conservation methods are applicable that 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 emptying and depressurization of the system are inevitable. Validity protective film on the metal surface after treatment with sodium nitrite can reach 3 months.

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

Methods of preservation of hot water and steam boilers low pressure, as well as other equipment of closed technological circuits of heat and water supply, differ in many respects from the methods currently used to prevent parking corrosion at TPPs. The following describes the main methods for preventing corrosion in the idle mode of the equipment of such apparatuses. circulation systems according to the nature of their work.

Simplified preservation methods

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

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

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

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

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

When implementing various ways protection against parking corrosion, the following should be kept in mind.

1. For all types of conservation, preliminary removal (washing) of deposits of easily soluble salts (see above) is necessary in order to avoid increased parking corrosion in certain areas of the protected unit. It is mandatory to carry out this measure during contact conservation, otherwise intense local corrosion is possible.

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

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

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

The choice of desiccants is determined by the relative availability of the reagent and the desirability of obtaining the highest possible specific moisture content. The best desiccant is granular calcium chloride. Quicklime much worse than calcium chloride, not only due to 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 is covered with a layer of calcium carbonate, which prevents further absorption of moisture.

For the first time, external corrosion of screen pipes was found at two power plants near high-pressure boilers TP-230-2, which worked on coal of the ASh grade and sulfurous fuel oil and were previously in operation for about 4 years. The outer surface of the pipes was subjected to corrosive attack from the side facing the furnace, in the zone of 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 closed with each other, as a result of which the damaged surface of the pipes was uneven, bumpy. Fistulas appeared in the middle of the deepest ulcers, and jets of water and steam began to escape through them.

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

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

External corrosion of screen pipes was found in boilers TP-240-1 after 4 years of operation, operating at a pressure in the drums of 185 at. 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 highest heat load of the wall tubes. The peculiarity of the corrosion process was that the pipes were destroyed both from the side facing the furnace and from the side facing the lining (Fig. 62).

These facts show that the corrosion of screen pipes depends primarily on their surface temperature. In medium pressure boilers, water evaporates at a temperature of about 240 ° C; for boilers designed for a pressure of 110 atm, the calculated 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 screen pipes usually exceeds the boiling point by about 30-40 ° C.

Can. 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 on the fire 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 the corrosion of screen pipes from the brickwork 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 combustion were taken there.

Particles from the flare at a distance of about 25 mm from the screen tubes. Near the front screen in the zone of intense external corrosion of pipes, the flue gases almost did not contain free oxygen. Near the rear screen, in which there was almost no external corrosion of the pipes, 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 a chemical combination with the metal of the pipes. In this case, iron sulfide FeS is formed. This corrosion product was indeed found in deposits on screen pipes.

Not only carbon steel, but also chromium-molybdenum steel is subject to external corrosion. In particular, in TP-240-1 boilers, corrosion affected screen pipes made of 15KhM steel.

Until now, there are no proven measures to completely prevent the described type of corrosion. Some decrease in the rate of destruction. metal was achieved. after adjustment of the combustion process, in particular, with an increase in excess air in the flue gases.

27. CORROSION OF SCREENS UNDER ULTRA-HIGH PRESSURE

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

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

Corrosion occurred mainly from the fire side of the upper part of the inclined pipes of the cold funnel and was accompanied by the appearance of corrosion pits (Fig. 63a). 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 coating of loose sludge 0.1-0.2 mm thick, which consisted mainly of iron and copper oxides. The increase in metal corrosion damage 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 screen pipes.

The regime of purely phosphate alkalinity was maintained in the boiler water. Phosphates were introduced into the boiler not continuously, but periodically.

Of great importance was the fact that the temperature of the metal of the pipes rose sharply from time to time and sometimes was above 600°C (Fig. 64). The zone of the most frequent and maximum increase in temperature coincided with the zone of the greatest destruction of the metal. Reducing 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 maximum value this temperature. The reasons for such a periodic increase in the temperature of the fire side of inclined pipes are cold. funnels have not yet been studied in detail.

This book deals with specific issues related to the operation of steel parts of a steam boiler. But in order to study these purely practical issues, it is necessary to know general information relating to the structure of steel and its "properties. In diagrams showing the structure of metals, atoms are sometimes depicted as balls in contact with each other (Fig. 1). Such diagrams show the arrangement of atoms in a metal, but in them it is difficult to clearly show the arrangement of atoms relative to each other friend.

Erosion is the gradual destruction of the surface layer of the metal under the influence of mechanical stress. The most common type of erosion of steel elements - a steam boiler is their abrasion by solid particles of ash moving along with flue gases. With prolonged abrasion, there is a gradual decrease in the thickness of the walls of the pipes, and then their deformation and rupture under the action of internal pressure.

MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR

MAIN SCIENTIFIC AND TECHNICAL DEPARTMENT OF ENERGY AND ELECTRIFICATION

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

RD 34.26.105-84

SOYUZTEKHENERGO

Moscow 1986

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

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

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

Deputy Head D.Ya. SHAMARAKOV

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

RD 34.26.105-84

Expiry date set
from 01.07.85
until 01.07.2005

These Guidelines apply to low-temperature heating surfaces of steam and hot water boilers (economizers, gas evaporators, air heaters various types etc.), as well as on the gas path behind the air heaters (gas ducts, ash collectors, smoke exhausters, chimneys) and establish methods for protecting heating surfaces from low-temperature corrosion.

The Guidelines are intended for thermal power plants operating on sour fuels and organizations designing boiler equipment.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Application

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Table 1

Table 2

Up to 0.1

Sulfur content in fuel oil S p , %	Corrosion rate (mm/year) at wall temperature, °С
Sulfur content in fuel oil S p , %	75 - 95	96 - 100	101 - 110	111 - 115	116 - 125
Less than 1.0	0,10	0,20	0,30	0,20	0,10
1 - 2	0,10	0,25	0,40	0,30	0,15
More than 2	131 - 140	Over 140
Up to 0.1	0,10	0,15	0,10	0,10	0,10
St. 0.11 to 0.4 incl.	0,10	0,20	0,10	0,15	0,10
Over 0.41 to 1.0 incl.	0,15	0,25	0,30	0,35	0,20	0,30	0,15	0,10	0,05
St. 0.11 to 0.4 incl.	0,20	0,40	0,25	0,15	0,10
Over 0.41 to 1.0 incl.	0,25	0,50	0,30	0,20	0,15
Over 1.0	0,30	0,60	0,35	0,25	0,15

6. For coals with a high content of calcium oxide in the ash, the dew point temperatures are lower than those calculated according to paragraphs of these Guidelines. For such fuels it is recommended to use the results of direct measurements.

MINISTRY OF ENERGY AND ELECTRIFICATION OF THE USSR

MAIN SCIENTIFIC AND TECHNICAL DEPARTMENT OF ENERGY AND ELECTRIFICATION

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

RD 34.26.105-84

SOYUZTEKHENERGO

Moscow 1986

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

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

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

Deputy Head D.Ya. SHAMARAKOV

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

RD 34.26.105-84

Expiry date set
from 01.07.85
until 01.07.2005

These Guidelines apply to low-temperature heating surfaces of steam and hot water boilers (economizers, gas evaporators, air heaters of various types, etc.), as well as to the gas path behind air heaters (gas ducts, ash collectors, smoke exhausters, chimneys) and establish methods for protecting surfaces heating from low temperature corrosion.

The Guidelines are intended for thermal power plants operating on sour fuels and organizations designing boiler equipment.

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

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

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

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

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

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

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

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

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

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

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

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

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

Application

The measured dew point temperature may differ from the theoretical value depending on the measurement technique. In these recommendations for flue gas dew point temperature tr The surface temperature of a standard glass sensor with 7 mm long platinum electrodes soldered at a distance of 7 mm from one another is assumed, at which the resistance of the dew film between the electrodes in the steady state is 107 Ohm. The measuring circuit of the electrodes uses low voltage alternating current (6 - 12 V).

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

3. When burning sulphurous solid fuels in a pulverized state, the dew point temperature of flue gases tp can be calculated from the reduced content of sulfur and ash in the fuel Sppr, Arpr and water vapor condensation temperature tcon according to the formula

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

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

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

Rice. 2. Differences in temperatures of the dew point of flue gases and condensation of water vapor in them, depending on the reduced sulfur content ( Sppr) and ash ( Arpr) in fuel

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

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

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

Qn- calorific value of gas in kJ/m3 (kcal/nm3);

FROM- coefficient equal to 4.187 if Qn expressed in kJ/m3 and 1.0 if in kcal/m3.

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

Table 1

	Corrosion rate (mm/year) at wall temperature, ºС



		0.5More than 2 0.20
St. 0.11 to 0.4 incl.
Over 0.41 to 1.0 incl.

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Types of corrosion. During operation, the elements of a steam boiler are exposed to aggressive media - water, steam and flue gases. Distinguish between chemical and electrochemical corrosion.

Chemical corrosion, caused by steam or water, destroys the metal evenly over the entire surface. The rate of such corrosion in modern marine boilers is low. More dangerous local chemical corrosion caused by aggressive chemical compounds contained in ash deposits (sulfur, vanadium oxides, etc.).

The most common and dangerous is electrochemical corrosion flowing in aqueous solutions electrolytes in the event of electric current, caused by the potential difference between individual sections of the metal, which differ in chemical heterogeneity, temperature or quality of processing.
The role of the electrolyte is performed by water (with internal corrosion) or condensed water vapor in deposits (with external corrosion).

The occurrence of such microgalvanic pairs on the pipe surface leads to the fact that metal ions-atoms pass into the water in the form of positively charged ions, and the pipe surface in this place acquires a negative charge. If the difference in the potentials of such microgalvanic pairs is insignificant, then a double electric layer is gradually created at the metal-water interface, which slows down the further course of the process.

However, in most cases, the potentials of individual sections are different, which causes the occurrence of an EMF directed from a larger potential (anode) to a smaller one (cathode).

In this case, metal ions-atoms pass from the anode into the water, and excess electrons accumulate on the cathode. As a result, the EMF and, consequently, the intensity of the metal destruction process are sharply reduced.

This phenomenon is called polarization. If the anode potential decreases as a result of the formation of a protective oxide film or an increase in the concentration of metal ions in the anode region, and the cathode potential remains practically unchanged, then the polarization is called anodic.

With cathodic polarization in solution near the cathode, the concentration of ions and molecules capable of removing excess electrons from the metal surface drops sharply. It follows from this that the main point of the fight against electrochemical corrosion is the creation of such conditions when both types of polarization will be maintained.
It is practically impossible to achieve this, since boiler water always contains depolarizers - substances that cause disruption of polarization processes.

Depolarizers include O 2 and CO 2 molecules, H +, Cl - and SO - 4 ions, as well as iron and copper oxides. Dissolved in water, CO 2 , Cl - and SO - 4 inhibit the formation of a dense protective oxide film on the anode and thereby contribute to the intensive course of anodic processes. Hydrogen ions H + reduce the negative charge of the cathode.

The influence of oxygen on the corrosion rate began to manifest itself in two opposite directions. On the one hand, oxygen increases the rate of the corrosion process, since it is a strong depolarizer of the cathode sections, on the other hand, it has a passivating effect on the surface.
Typically, boiler parts made of steel have a sufficiently strong initial oxide film that protects the material from oxygen exposure until it is destroyed by chemical or mechanical factors.

The rate of heterogeneous reactions (including corrosion) is regulated by the intensity of the following processes: supply of reagents (primarily depolarizers) to the surface of the material; destruction of the protective oxide film; removal of reaction products from the place of its occurrence.

The intensity of these processes is largely determined by hydrodynamic, mechanical and thermal factors. Therefore, measures to reduce the concentration of aggressive chemicals at a high intensity of the other two processes, as the experience of operating boilers shows, are usually ineffective.

It follows that the solution to the problem of preventing corrosion damage should be complex, when all factors influencing the initial causes of the destruction of materials are taken into account.

Electrochemical corrosion

Depending on the place of flow and the substances involved in the reactions, the following types of electrochemical corrosion are distinguished:

oxygen (and its variety - parking),
subsludge (sometimes called "shell"),
intergranular (alkaline brittleness of boiler steels),
slot and
sulfurous.

Oxygen corrosion observed in economizers, fittings, feed and downpipes, steam-water collectors and intra-collector devices (shields, pipes, desuperheaters, etc.). Coils of the secondary circuit of double-circuit boilers, utilizing boilers and steam air heaters are especially susceptible to oxygen corrosion. Oxygen corrosion proceeds during the operation of the boilers and depends on the concentration of oxygen dissolved in the boiler water.

The rate of oxygen corrosion in the main boilers is low due to effective work deaerators and phosphate-nitrate water regime. In auxiliary water-tube boilers, it often reaches 0.5 - 1 mm / year, although on average it lies in the range of 0.05 - 0.2 mm / year. The nature of the damage to boiler steels is small pits.

A more dangerous type of oxygen corrosion is parking corrosion flowing during the period of inactivity of the boiler. Due to the nature of the work, all marine boilers(especially auxiliary ones) are subject to intense parking corrosion. As a rule, parking corrosion does not lead to boiler failures, however, metal corroded during shutdowns, ceteris paribus, is more intensively destroyed during boiler operation.

The main cause of parking corrosion is the ingress of oxygen into the water if the boiler is full, or into the film of moisture on the metal surface if the boiler is dry. An important role is played by chlorides and NaOH contained in water, and water-soluble salt deposits.

If chlorides are present in water, uniform metal corrosion is intensified, and if it contains a small amount of alkalis (less than 100 mg/l), then corrosion is localized. To avoid parking corrosion at a temperature of 20 - 25 °C, the water should contain up to 200 mg/l NaOH.

External signs of corrosion with the participation of oxygen: local ulcers small size(Fig. 1, a), filled with brown corrosion products, which form tubercles over ulcers.

The removal of oxygen from the feed water is one of the important measures to reduce oxygen corrosion. Since 1986, the oxygen content in the feed water for marine auxiliary and waste boilers has been limited to 0.1 mg/l.

However, even with such an oxygen content of the feed water, corrosion damage to the boiler elements is observed in operation, which indicates the predominant influence of the processes of destruction of the oxide film and the leaching of reaction products from the corrosion centers. Most good example illustrating the effect of these processes on corrosion damage are the destruction of the coils of waste-heat boilers with forced circulation.

Rice. 1. Damage due to oxygen corrosion

Corrosion damage in case of oxygen corrosion, they are usually strictly localized: on the inner surface of the inlet sections (see Fig. 1, a), in the area of bends (Fig. 1, b), on the outlet sections and in the coil elbow (see Fig. 1, c), as well as in steam-water collectors of utilization boilers (see Fig. 1, d). It is in these areas (2 - the area of near-wall cavitation) that the hydrodynamic features of the flow create conditions for the destruction of the oxide film and intensive washing out of corrosion products.
Indeed, any deformation of the flow of water and steam-water mixture is accompanied by the appearance cavitation in near-wall layers expanding flow 2, where the formed and immediately collapsing vapor bubbles cause the destruction of the oxide film due to the energy of hydraulic microshocks.
This is also facilitated by alternating stresses in the film, caused by the vibration of the coils and fluctuations in temperature and pressure. The increased local flow turbulence in these areas causes active washing out of corrosion products.

On the direct outlet sections of the coils, the oxide film is destroyed due to impacts on the surface of water droplets during turbulent pulsations of the steam-water mixture flow, the dispersed-annular mode of motion of which passes here into a dispersed one at a flow velocity of up to 20-25 m/s.
Under these conditions, even a low oxygen content (~ 0.1 mg/l) causes intense destruction of the metal, which leads to the appearance of fistulas in the inlet sections of the coils of waste-heat boilers of the La Mont type after 2-4 years of operation, and in other areas - after 6-12 years.

Rice. Fig. 2. Corrosion damage to the economizer coils of the KUP1500R utilization boilers of the motor ship "Indira Gandhi".

As an illustration of the foregoing, let us consider the causes of damage to the economizer coils of two utilization boilers of the KUP1500R type installed on the Indira Gandhi lighter carrier (Alexey Kosygin type), which entered service in October 1985. Already in February 1987 due to damage economizers of both boilers were replaced. After 3 years, damage to the coils also appears in these economizers, located in areas up to 1-1.5 m from the inlet manifold. The nature of the damage indicates (Fig. 2, a, b) typical oxygen corrosion followed by fatigue failure (transverse cracks).

However, the nature of fatigue in individual areas is different. The appearance of a crack (and earlier, cracking of the oxide film) in the region of the weld (see Fig. 2, a) is a consequence of alternating stresses caused by the vibration of the tube bundle and design feature connection unit of coils with a collector (the end of a coil with a diameter of 22x2 is welded to a curved fitting with a diameter of 22x3).
The destruction of the oxide film and the formation of fatigue cracks on the inner surface of the straight sections of the coils, remote from the inlet by 700-1000 mm (see Fig. 2, b), are due to alternating thermal stresses that occur during the commissioning of the boiler, when the hot surface served cold water. At the same time, the effect of thermal stresses is enhanced by the fact that the finning of the coils makes it difficult for the pipe metal to expand freely, creating additional stresses in the metal.

Subslurry corrosion usually observed in the main water-tube boilers on the inner surfaces of the screen and steam-generating pipes of the inflow bundles facing the torch. The nature of undersludge corrosion is oval pits with a size along the major axis (parallel to the axis of the pipe) up to 30-100 mm.
There is a dense layer of oxides in the form of “shells” 3 on the ulcers (Fig. 3). oxide films.
A loose layer of scale and corrosion products is formed on top.
For auxiliary boilers, this type of corrosion is not typical, but under high thermal loads and appropriate water treatment modes, the appearance of undersludge corrosion in these boilers is not excluded.