Introduction
Corrosion (from Latin corrosio - corrosion) is the spontaneous destruction of metals as a result of chemical or physical-chemical interaction with environment. In general, this is the destruction of any material - be it metal or ceramics, wood or polymer. The cause of corrosion is thermodynamic instability construction materials to the effects of substances in the environment in contact with them. Example - oxygen corrosion of iron in water:
4Fe + 2H 2 O + ZO 2 = 2 (Fe 2 O 3 H 2 O)
IN Everyday life For iron alloys (steels), the term “rusting” is more often used. Cases of corrosion of polymers are less known. In relation to them, there is the concept of “aging”, similar to the term “corrosion” for metals. For example, the aging of rubber due to interaction with atmospheric oxygen or the destruction of some plastics under the influence of precipitation, as well as biological corrosion. The rate of corrosion, like any other chemical reaction depends very much on temperature. An increase in temperature of 100 degrees can increase the corrosion rate by several orders of magnitude.
Corrosion processes are characterized by a wide distribution and variety of conditions and environments in which it occurs. Therefore, there is no single and comprehensive classification of corrosion cases encountered. The main classification is made according to the mechanism of the process. There are two types: chemical corrosion and electrochemical corrosion. This abstract examines chemical corrosion in detail using the example of small and large-capacity ship boiler plants.
Corrosion processes are characterized by a wide distribution and variety of conditions and environments in which it occurs. Therefore, there is no single and comprehensive classification of corrosion cases encountered.
Depending on the type of aggressive environment in which the destruction process occurs, corrosion can be of the following types:
1) -Gas corrosion
2) - Corrosion in non-electrolytes
3) -Atmospheric corrosion
4) -Corrosion in electrolytes
5) -Underground corrosion
6) -Biocorrosion
7) - Corrosion by stray current.
According to the conditions of the corrosion process, the following types are distinguished:
1) - Contact corrosion
2) - Crevice corrosion
3) -Corrosion during partial immersion
4) -Corrosion during full immersion
5) -Corrosion during alternating immersion
6) -Friction corrosion
7) -Stress corrosion.
By nature of destruction:
Complete corrosion covering the entire surface:
1) - uniform;
2) - uneven;
3) -selective.
Local (local) corrosion covering individual areas:
1) - spots;
2) - ulcerative;
3) - spot (or pitting);
4) - through;
5) - intercrystalline.
1. Chemical corrosion
Let's imagine metal in the process of producing rolled metal at a metallurgical plant: a red-hot mass moves along the stands of a rolling mill. Fiery splashes fly out from her in all directions. This is when particles of scale break off from the surface of the metal - a product of chemical corrosion resulting from the interaction of the metal with atmospheric oxygen. This process of spontaneous destruction of a metal due to the direct interaction of oxidizer particles and the oxidized metal is called chemical corrosion.
Chemical corrosion is the interaction of a metal surface with a (corrosive) environment, not accompanied by the occurrence of electrochemical processes at the phase boundary. In this case, the interactions of metal oxidation and reduction of the oxidizing component of the corrosive environment occur in one act. For example, the formation of scale when iron-based materials react at high temperatures with oxygen:
4Fe + 3O 2 → 2Fe 2 O 3
During electrochemical corrosion, the ionization of metal atoms and the reduction of the oxidizing component of the corrosive environment do not occur in one act and their rates depend on the electrode potential of the metal (for example, rusting of steel in sea water).
In chemical corrosion, metal oxidation and reduction of the oxidizing component of the corrosive environment occur simultaneously. Such corrosion is observed when metals are exposed to dry gases (air, fuel combustion products) and liquid non-electrolytes (oil, gasoline, etc.) and is a heterogeneous chemical reaction.
The process of chemical corrosion occurs as follows. The oxidizing component of the external environment, taking away valence electrons from the metal, simultaneously enters into a chemical compound with it, forming a film (corrosion product) on the metal surface. Further formation of the film occurs due to mutual two-way diffusion through the film of the aggressive medium towards the metal and metal atoms towards external environment and their interactions. Moreover, if the resulting film has protective properties, that is, it prevents the diffusion of atoms, then corrosion proceeds with self-inhibition over time. Such a film is formed on copper at a heating temperature of 100 °C, on nickel at 650, on iron at 400 °C. Heat steel products above 600 °C leads to the formation of a loose film on their surface. With increasing temperature, the oxidation process accelerates.
The most common type of chemical corrosion is the corrosion of metals in gases at high temperatures - gas corrosion. Examples of such corrosion are oxidation of furnace fittings, parts of internal combustion engines, grate bars, parts of kerosene lamps and oxidation during high-temperature processing of metals (forging, rolling, stamping). Other corrosion products may also form on the surface of metal products. For example, when exposed to sulfur compounds, sulfur compounds are formed on iron; on silver, when exposed to iodine vapor, silver iodide is formed, etc. However, most often a layer of oxide compounds is formed on the surface of metals.
Temperature has a great influence on the rate of chemical corrosion. As temperature increases, the rate of gas corrosion increases. The composition of the gas environment has a specific effect on the corrosion rate of various metals. Thus, nickel is stable in an oxygen environment, carbon dioxide, but is highly corrosive in a sulfur dioxide atmosphere. Copper is susceptible to corrosion in an oxygen atmosphere, but is stable in a sulfur dioxide atmosphere. Chrome has corrosion resistance in all three gas environments.
To protect against gas corrosion, heat-resistant alloying with chromium, aluminum and silicon is used, creating protective atmospheres and protective coatings aluminum, chromium, silicon and heat-resistant enamels.
2. Chemical corrosion in ship steam boilers.
Types of corrosion. During operation, the elements of a steam boiler are exposed to aggressive media - water, steam and flue gases. There are chemical and electrochemical corrosion.
Parts and components of machines operating at high temperatures, - piston engines and turbine type, rocket engines etc. The chemical affinity of most metals for oxygen at high temperatures is almost unlimited, since the oxides of all technically important metals are able to dissolve in metals and leave the equilibrium system:
2Me(t) + O 2 (g) 2MeO(t); MeO(t) [MeO] (solution)Under these conditions, oxidation is always possible, but along with the dissolution of the oxide, an oxide layer also appears on the surface of the metal, which can inhibit the oxidation process.
The rate of metal oxidation depends on the rate of the chemical reaction itself and the rate of diffusion of the oxidizing agent through the film, and therefore the protective effect of the film is higher, the better its continuity and the lower its diffusion ability. The continuity of the film formed on the surface of the metal can be assessed by the ratio of the volume of the formed oxide or some other compound to the volume of the metal spent on the formation of this oxide (Pilling-Badwords factor). Coefficient a (Pilling-Badwords factor) has different values for different metals. Metals that have a<1, не могут создавать сплошные оксидные слои, и через несплошности в слое (трещины) кислород свободно проникает к поверхности металла.
Continuous and stable oxide layers are formed at a = 1.2-1.6, but at large values of a the films are not continuous, easily separated from the metal surface (iron scale) as a result of internal stresses.
The Pilling-Badwords factor gives a very approximate estimate, since the composition of the oxide layers has a wide range of homogeneity, which is also reflected in the density of the oxide. So, for example, for chromium a = 2.02 (for pure phases), but the oxide film formed on it is very resistant to environmental influences. The thickness of the oxide film on the metal surface varies depending on time.
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 is local chemical corrosion caused by aggressive chemical compounds contained in ash deposits (sulfur, vanadium oxides, etc.).
Electrochemical corrosion, as its name indicates, is associated not only with chemical processes, but also with the movement of electrons in interacting media, i.e. with the appearance of electric current. These processes occur when the metal interacts with electrolyte solutions, which takes place in a steam boiler in which boiler water circulates, which is a solution of salts and alkalis that have disintegrated into ions. Electrochemical corrosion also occurs when the metal comes into contact with air (at normal temperature), which always contains water vapor, which condenses on the surface of the metal in the form of a thin film of moisture, creating conditions for electrochemical corrosion to occur.
The heating surfaces of tubular and regenerative air heaters, low-temperature economizers, as well as metal flues and chimneys are subject to low-temperature corrosion at metal temperatures below the dew point of the flue gases. The source of low-temperature corrosion is sulfuric anhydride SO 3, which forms sulfuric acid vapor in the flue gases, which condenses at the dew point temperatures of the flue gases. A few thousandths of a percent SO 3 in gases is enough to cause metal corrosion at a rate exceeding 1 mm/year. Low-temperature corrosion is slowed down by organizing the combustion process with small excess air, as well as by using fuel additives and increasing the corrosion resistance of the metal.
The combustion screens of drum and once-through boilers when burning solid fuel, steam superheaters and their fastenings, as well as the screens of the lower radiation part of supercritical pressure boilers when burning sulfur fuel oil are subject to high-temperature corrosion.
Corrosion of the inner surface of pipes is a consequence of the interaction of oxygen and carbon dioxide gases or salts (chlorides and sulfates) contained in the boiler water with the metal of the pipes. In modern supercritical steam pressure boilers, the content of gases and corrosive salts as a result of deep desalting of feed water and thermal deaeration is insignificant and the main cause of corrosion is the interaction of the metal with water and steam. Corrosion of the inner surface of pipes manifests itself in the formation of pockmarks, pits, cavities and cracks; the outer surface of damaged pipes may be no different from healthy ones.
Damage resulting from internal corrosion of pipes also includes:
oxygen stagnation corrosion, affecting any areas of the internal surface of pipes. The most intensively affected areas are those covered with water-soluble deposits (pipes of superheaters and the transition zone of once-through boilers);
sub-sludge alkaline corrosion of boiler and screen pipes, which occurs under the influence of concentrated alkali due to the evaporation of water under a layer of sludge;
corrosion fatigue, manifested in the form of cracks in boiler and screen pipes as a result of simultaneous exposure to a corrosive environment and alternating thermal stresses.
Scale forms on pipes due to their overheating to temperatures significantly higher than designed. Due to the increase in the productivity of boiler units, cases of failure of steam superheater pipes due to insufficient scale resistance to flue gases have recently become more frequent. Intense scaling is most often observed when burning fuel oil.
Wear of pipe walls occurs as a result of the abrasive action of coal and shale dust and ash, as well as jets of steam emerging from damaged adjacent pipes or blower nozzles. Sometimes the cause of wear and hardening of pipe walls is the shot used to clean heating surfaces. The locations and degree of pipe wear are determined by external inspection and measurement of their diameter. The actual pipe wall thickness is measured with an ultrasonic thickness gauge.
Warping of screen and boiler pipes, as well as individual pipes and sections of wall panels of the radiant part of once-through boilers occurs when pipes are installed with uneven tension, pipe fastenings are broken, water is leaking, and due to the lack of freedom for their thermal movements. Warping of coils and superheater screens occurs mainly due to burning of hangers and fastenings, excessive and uneven tension allowed during installation or replacement of individual elements. Warping of water economizer coils occurs due to burnout and displacement of supports and hangers.
Fistulas, bulges, cracks and ruptures can also appear as a result of: deposits in pipes of scale, corrosion products, process scale, welding beads and other foreign objects that slow down the circulation of water and contribute to overheating of the pipe metal; shot peening; discrepancies between steel grade and steam parameters and gas temperature; external mechanical damage; violations of operating conditions.
Corrosion of hot water boilers, heating systems, district heating systems are much more common than in steam and condensate systems. In most cases, this situation is explained by the fact that when designing a water heating system, less attention is paid to this, although the factors for the formation and subsequent development of corrosion in boilers remain exactly the same as for steam boilers and all other equipment. Dissolved oxygen, which is not removed by deaeration, hardness salts, carbon dioxide entering hot water boilers with feed water, cause various types of corrosion - alkaline (intercrystalline), oxygen, chelate, sub-sludge. It must be said that chelate corrosion in most cases is formed in the presence of certain chemical reagents, the so-called “complexons”.
In order to prevent the occurrence of corrosion in hot water boilers and its subsequent development, it is necessary to take seriously and responsibly the preparation of the characteristics of water intended for make-up. It is necessary to ensure the binding of free carbon dioxide and oxygen, bring the pH value to an acceptable level, and take measures to protect aluminum, bronze and copper elements of heating equipment and boilers, pipelines and heating equipment from corrosion.
Recently, special chemical reagents have been used for high-quality correction of heating networks, hot water boilers and other equipment.
Water is at the same time a universal solvent and an inexpensive coolant; it is beneficial to use in heating systems. But insufficient preparation can lead to unpleasant consequences, one of which is corrosion of hot water boilers. Possible risks are primarily associated with the presence of a large number of undesirable impurities in it. It is possible to prevent the formation and development of corrosion, but only if you clearly understand the reasons for its occurrence, and also be familiar with modern technologies.
Hot water boilers, as well as any heating systems that use water as a coolant, are characterized by three types of problems caused by the presence of the following impurities:
Each of the types of impurities listed can cause corrosion and failure of a hot water boiler or other equipment. In addition, they contribute to reducing the efficiency and performance of the boiler.
And if you use water that has not undergone special preparation in heating systems for a long time, this can lead to serious consequences - breakdown of circulation pumps, reduction in the diameter of the water supply system and subsequent damage, failure of control and shut-off valves. The simplest mechanical impurities - clay, sand, ordinary dirt - are present almost everywhere, both in tap water and in artesian springs. Also, coolants contain large quantities of corrosion products of heat transfer surfaces, pipelines and other metal elements of the system that are constantly in contact with water. It goes without saying that their presence over time provokes very serious problems in the functioning of hot water boilers and all thermal power equipment, which are mainly associated with corrosion of boilers, the formation of lime deposits, the removal of salts and foaming of boiler water.
The most common reason that causes corrosion of hot water boilers, these are carbonate deposits that occur when using water of high hardness, the removal of which is possible through. It should be noted that as a result of the presence of hardness salts, scale forms even in low-temperature heating equipment. But this is far from the only cause of corrosion. For example, after heating water to a temperature of more than 130 degrees, the solubility of calcium sulfate decreases significantly, resulting in the formation of a layer of dense scale. In this case, the development of corrosion of the metal surfaces of hot water boilers is inevitable.
a) Oxygen corrosion
Most often, steel water economizers of boiler units suffer from oxygen corrosion, which, due to unsatisfactory deaeration of the feed water, fail 2-3 years after installation.
The immediate result of oxygen corrosion of steel economizers is the formation of fistulas in the tubes, through which a stream of water flows out at high speed. Such jets directed at the wall of an adjacent pipe can wear it down to the point of forming through holes. Since the economizer pipes are located quite compactly, the resulting corrosion fistula can cause massive damage to the pipes if the boiler unit remains in operation for a long time with the resulting fistula. Cast iron economizers are not damaged by oxygen corrosion.
Oxygen corrosion the inlet sections of economizers are more often exposed. However, with a significant concentration of oxygen in the feed water, it penetrates into the boiler unit. Here, mainly drums and standpipes are exposed to oxygen corrosion. The main form of oxygen corrosion is the formation of depressions (ulcers) in the metal, which, when they develop, lead to the formation of fistulas.
An increase in pressure intensifies oxygen corrosion. Therefore, for boiler units with a pressure of 40 atm and above, even oxygen “slips” in deaerators are dangerous. The composition of the water with which the metal comes into contact is essential. The presence of a small amount of alkali enhances the localization of corrosion, while the presence of chlorides disperses it over the surface.
b) Parking corrosion
Boiler units that are idle are affected by electrochemical corrosion, which is called standstill corrosion. Depending on operating conditions, boiler units are often taken out of operation and placed in reserve or stopped for a long time.
When the boiler unit is stopped in reserve, the pressure in it begins to drop and a vacuum arises in the drum, causing air to penetrate and enrich the boiler water with oxygen. The latter creates conditions for the occurrence of oxygen corrosion. Even when water is completely removed from the boiler unit, its internal surface is not dry. Fluctuations in air temperature and humidity cause the phenomenon of moisture condensation from the atmosphere contained inside the boiler unit. The presence of a film on the metal surface, enriched with oxygen when exposed to air, creates favorable conditions for the development of electrochemical corrosion. If there are deposits on the inner surface of the boiler unit that can dissolve in a film of moisture, the intensity of corrosion increases significantly. Similar phenomena can be observed, for example, in steam superheaters, which often suffer from standing corrosion.
If there are deposits on the inner surface of the boiler unit that can dissolve in a film of moisture, the intensity of corrosion increases significantly. Similar phenomena can be observed, for example, in steam superheaters, which often suffer from standing corrosion.
Therefore, when taking the boiler unit out of operation for a long period of downtime, it is necessary to remove existing deposits by washing.
Parking corrosion can cause serious damage to boiler units unless special measures are taken to protect them. Its danger also lies in the fact that the corrosion centers created by it during idle periods continue to act during operation.
To protect boiler units from parking corrosion, they are preserved.
c) Intergranular corrosion
Intergranular corrosion occurs in rivet seams and rolling joints of steam boiler units, which are washed off with boiler water. It is characterized by the appearance of cracks in the metal, initially very thin, invisible to the eye, which, as they develop, turn into large visible cracks. They pass between the grains of the metal, which is why this corrosion is called intergranular. In this case, the destruction of the metal occurs without deformation, therefore these fractures are called brittle.
Experience has established that intergranular corrosion occurs only when 3 conditions are simultaneously present:
1) High tensile stresses in the metal, close to the yield point.
2) Leaks in rivet seams or rolling joints.
3) Aggressive properties of boiler water.
The absence of one of the listed conditions eliminates the occurrence of brittle fractures, which is used in practice to combat intergranular corrosion.
The aggressiveness of boiler water is determined by the composition of the salts dissolved in it. The content of caustic soda is important, which at high concentrations (5-10%) reacts with the metal. Such concentrations are achieved in leaks in rivet seams and rolling joints, in which boiler water evaporates. This is why the presence of leaks can lead to brittle fractures under appropriate conditions. In addition, an important indicator of the aggressiveness of boiler water is relative alkalinity - Schot.
d) Steam-water corrosion
Steam-water corrosion is the destruction of metal as a result of chemical interaction with water vapor: 3Fe + 4H20 = Fe304 + 4H2
Metal destruction becomes possible for carbon steels when the pipe wall temperature increases to 400°C.
Corrosion products are hydrogen gas and magnetite. Steam-water corrosion has both a uniform and local (local) character. In the first case, a layer of corrosion products forms on the metal surface. The local nature of corrosion takes the form of ulcers, grooves, and cracks.
The main cause of steam corrosion is the heating of the tube wall to a critical temperature, at which the oxidation of the metal with water accelerates. Therefore, the fight against steam-water corrosion is carried out by eliminating the causes that cause overheating of the metal.
Steam-water corrosion cannot be eliminated by any change or improvement in the water chemistry of the boiler unit, since the causes of this corrosion lie in the combustion and intra-boiler hydrodynamic processes, as well as operating conditions.
e) Sludge corrosion
This type of corrosion occurs under a layer of sludge formed on the inner surface of the boiler unit pipe as a result of the boiler being fed with insufficiently purified water.
Metal damage that occurs during sludge corrosion is local (ulcerative) in nature and is usually located on the semi-perimeter of the pipe facing the furnace. The resulting ulcers look like shells with a diameter of up to 20 mm or more, filled with iron oxides, creating a “bump” under the ulcer.
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 one.
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 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 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%.
