Abstract: Some problems that are easily overlooked in the design and application of centrifuges are introduced. The effects of several corrosion phenomena and their hazards are proposed. The structural design and other protection aspects are analyzed for discussion by relevant parties.
In recent years, China's national economy has continued to develop, new technologies and new processes have emerged, and the application range of centrifuges has been expanded and expanded. However, in the process of centrifuge production, most manufacturers are more concerned about centrifuges. The separation performance, strength requirements, and the impact on corrosion factors are often not enough, which brings some negative effects. This paper makes some analysis on this issue for everyone to explore.
As we all know, the danger of corrosion is extremely huge. There are many examples in the world that the centrifuge is a high-speed rotating equipment. The safety requirements are as important as pressure vessels. Most centrifuge manufacturers are designing, selecting and applying. In the middle, more consideration is given to the influence of uniform corrosion on strength parts, neglecting the adaptability of structural design and processing technology to corrosive environment, resulting in some serious consequences. Not only the parts tend to corrode, but also the materials that are separated from the pollution; even more serious, even the machine is destroyed.
1. Centrifuge environment Centrifuges can be divided into different types according to their functions and structures. However, whether it is a filter type, a separation type or a sedimentation type, the basic structure has the following common features:
(1) The drum is a high speed rotating member;
(2) Forming a separation space with the drum as the main body;
(3) The drum has a profiled structure;
(4) The material to be separated is a fluid;
(5) There are other metal or non-metal couplings or fittings in the drum.
These characteristics of the drum indicate that the core part of the centrifuge is a composite stress member, and its profiled geometry results in a multi-zone of stress distribution of the part; the cooperation of multiple parts makes the opportunity to form a galvanic pair become Possibly, this is some of the things that are worth noting at design time.
2. Possible Corrosion The metal corrosion pattern is mainly divided into two categories: uniform corrosion and local corrosion. The former is easy to find because of the apparent phenomenon, and it is clearly stated in most manual data. It will not be described in detail here. The latter did some discussion.
Localized corrosion only occurs locally, which is an extremely serious and harmful damage, such as pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion, wear corrosion, etc. In centrifuges, local corrosion is widespread and should be Focus on the analysis of its formation mechanism, take measures to overcome. Figures 1-3 are a set of corrosion and test coupons. The parts in Figure 1 have formed a slight surface corrosion in the atmospheric environment due to the uneven microstructure of the forged structure; Figure 2 shows that the drum parts have been severely rusted in the separation environment, which is a composite corrosion phenomenon; Figure 3 is the material in the standard soaking solution. The test of the hanging piece in the case.
2.1 Pitting corrosion Pitting is a highly corrosive phenomenon, mainly in easily passivated metals, such as stainless steel. Due to possible local defects, there are active ions in the solution that can destroy the passivation film (such as halogen). The group ion), the passivation film is partially destroyed, thereby forming a pair of galvanic couples, causing pitting corrosion. After the pitting corrosion is formed, due to the centrifugal force, the dynamic process of the galvanic couple in the hole will be accelerated, thereby ensuring the continuity of the hole corrosion until the perforation, which is completely different from the pitting phenomenon under static conditions. Centrifuges are in continuous operation under many conditions, and once a pitting defect is formed, serious consequences will result. Severe pitting defects are formed to a certain extent, which greatly affects the strength of the stressed parts. For plastic materials, under the influence of high-speed centrifugal force field, instantaneous cracking will occur, causing serious damage to the centrifuge.
2.2 Intergranular Corrosion Intergranular corrosion causes the part to lose strength and extensibility, causing the parts to be brittle. It is a harmful damage that develops from the surface along the grain boundary and has no signs of corrosion on the outer surface. This kind of corrosion, in daily maintenance, it is difficult to find abnormalities, but when the corrosion reaches a certain level, the internal structure of the parts is changed, resulting in severe attenuation of the physical and chemical properties of the parts, resulting in sudden bursting or breaking, and the consequences are very serious.
2.3 Wear Corrosion Wear and corrosion means that the surface of the part is subject to both wear and corrosion damage. During the normal operation of the centrifuge, due to the solid particles or mechanical impurities contained in the material, relative movement is formed at the surface of the contact part, resulting in wear. Since the distribution of the formed metal fibers is changed at the wear surface, breakage of the continuous fibers may occur, causing changes in polarity and changes in electronegativity, forming a local corrosive environment and causing corrosion. This form is mainly based on wear and can be easily observed.
2.4 Stress Corrosion Stress corrosion is a damage caused by a certain tensile stress in a corrosive environment. It has the following characteristics: residual tensile stress, applied tensile stress, corrosive infiltration environment, local defects. This image of corrosion is mainly due to the difference in potential generated by the difference in stress distribution. Under the action of the electrolyte liquid, a current is generated, and ion motion is generated, thereby causing corrosion. This corrosion is sometimes difficult to observe with the eyes. Figure 4 shows the sample in different heat treatment conditions soaked in artificial seawater for 2 days. The above four kinds of corrosion are extremely serious to the centrifuge and may cause serious consequences.
3 Measures 3.1 Response Measures Different materials and separation requirements require different models. After the model and main parameters are determined, according to the corrosion resistance of different materials in different environments, comprehensive physical and chemical properties, cost performance and other factors, the strength component materials are determined from the basic surface. From the material itself, the target material it targets is safe. Therefore, the selection of centrifuges should follow the following principles:
(1) There is no absolute universal anti-corrosion material. It is necessary to fully understand the application environment and distinguish the properties of the application environment, whether it is neutral, acidic or alkaline; whether it is oxidizing or reducing, etc., it should be targeted. If stainless steel is not suitable for concentrated sulfuric acid, carbon steel is suitable.
Stainless steel is passivated in air and other adaptive environments to form a stable protective layer on the surface of the stainless steel. Carbon steel is oxidized in an oxidizing environment, and an oxide film is formed on the surface thereof, which is also in a stainless state. Stainless steel in a reducing environment, such as a halogen liquid, has a strong penetrating ability due to a halogen ion such as Cl-, which destroys an oxide film formed on the surface of the stainless steel, thereby causing corrosion of the stainless steel part.
(2) In the same application environment, choose the same material or material with similar equilibrium potential as much as possible to avoid galvanic environment. Different materials have different equilibrium potentials, and the existence of potential differences, when in the through-flow field, will inevitably form a battery, forming a corrosive environment.
(3) Comprehensive cost performance, especially the importance of processing technology. The centrifuge is a high-speed rotating device. Under normal working conditions, the drum parts will generate a large centrifugal force, so these parts are required to have good plasticity and strength. When selecting materials, in addition to price factors, attention must be paid to their comprehensive properties, such as cold work characteristics and hot work characteristics.
3.2 Structural design An excellent design can extend the service life of the equipment and ensure the safety of the equipment. Regarding the structural design of the centrifuge, the design defects that zui easily accelerates corrosion are stress concentration tendency and gap environment. Sometimes, despite the metal soaking solution, the corrosion caused by the design environment is changed, and the chemical state changes (pH and potential drop). Will cause serious crevice corrosion, so the design should pay attention to:
(1) Try to avoid the occurrence of gaps; if necessary for structural design, avoid gaps as dead angles as much as possible, promote the balance of oxygen concentration, prevent the existence of oxygen concentration and reduce the damage of corrosion.
(2) In the same environment, try to use the same material and the same molding process; the main purpose is to prevent the generation of galvanic couples and avoid the formation of corrosive environments.
(3) Guarantee the uniformity of the parts or increase the compressive stress; use economical and reasonable processing methods to enhance the corrosion resistance of the parts.
(4) Avoid forming dead angles and ensure fluid flow. Prevents the formation of corrosive environments with oxygen concentrations.
(5) Design reasonable structure and reasonable processing methods. In the structural design, the uniformity and stability of the cloth and fluid flow are ensured as much as possible. The smoothness of the surface and the avoidance of stress concentration and residual stress are required during processing.
3.3 Surface protection measures A basic idea to control corrosion is to isolate the corrosive environment. In the design of centrifuges, surface treatment methods such as flange, galvanizing, chrome plating, electroless plating, composite lining, etc. are often used. These measures are effective in many environments; but for rotating parts, one must also pay attention to an important Phenomenon: the base material and the coating are two materials, and the coefficient of linear expansion is different. These structures will produce different deformation amounts when the rotating member is elastically deformed, thereby causing a large number of microcracks and peeling of the composite surface; The presence of the aforementioned phenomenon will accelerate the formation of corrosion. Therefore, the method of surface protection for rotating parts should be used with caution.
3.4 Corrosion inhibitor The centrifuge itself is multi-disciplinary. It is necessary to use the centrifuge. In addition to the above measures, it should also pay attention to the application environment. The application of corrosion inhibitors is also widely used in practical applications. For some harsh environments, the use of appropriate corrosion inhibitors can make the parts suitable. As long as the process permits, the use of corrosion inhibitors should be an effective way for centrifuge applications.
3.5 The correct process design is correct and the material selection is reasonable. It is only the precondition for solving the corrosion of the centrifuge; the correct application of the processing technology also affects the anti-corrosion ability of the centrifuge to some extent. Processing methods usually leave many defects on parts or equipment, such as misalignment, local stress, etc.; misalignment may cause gaps; improper positioning method may lead to eccentricity of parts; unreasonable pass may result in surface roughness; Appropriate, welding, heat treatment methods or measures are unreasonable, and the uneven force of the fastening bolts will cause local stress concentration. Therefore, finding the right process is also an effective way to prevent corrosion.
The corrosion problem of the centrifuge is a comprehensive and complex problem, because the working environment of the centrifuge is quite different from the data obtained by the general information and manual in the static. Therefore, in addition to the above analysis, in addition to the above analysis, consideration should also be given to the effects of dissolved oxygen, temperature, high-speed fluid and fluid state changes in the separation zone, and appropriate measures to ensure safe and reliable operation of the centrifuge.
In recent years, China's national economy has continued to develop, new technologies and new processes have emerged, and the application range of centrifuges has been expanded and expanded. However, in the process of centrifuge production, most manufacturers are more concerned about centrifuges. The separation performance, strength requirements, and the impact on corrosion factors are often not enough, which brings some negative effects. This paper makes some analysis on this issue for everyone to explore.
As we all know, the danger of corrosion is extremely huge. There are many examples in the world that the centrifuge is a high-speed rotating equipment. The safety requirements are as important as pressure vessels. Most centrifuge manufacturers are designing, selecting and applying. In the middle, more consideration is given to the influence of uniform corrosion on strength parts, neglecting the adaptability of structural design and processing technology to corrosive environment, resulting in some serious consequences. Not only the parts tend to corrode, but also the materials that are separated from the pollution; even more serious, even the machine is destroyed.
1. Centrifuge environment Centrifuges can be divided into different types according to their functions and structures. However, whether it is a filter type, a separation type or a sedimentation type, the basic structure has the following common features:
(1) The drum is a high speed rotating member;
(2) Forming a separation space with the drum as the main body;
(3) The drum has a profiled structure;
(4) The material to be separated is a fluid;
(5) There are other metal or non-metal couplings or fittings in the drum.
These characteristics of the drum indicate that the core part of the centrifuge is a composite stress member, and its profiled geometry results in a multi-zone of stress distribution of the part; the cooperation of multiple parts makes the opportunity to form a galvanic pair become Possibly, this is some of the things that are worth noting at design time.
2. Possible Corrosion The metal corrosion pattern is mainly divided into two categories: uniform corrosion and local corrosion. The former is easy to find because of the apparent phenomenon, and it is clearly stated in most manual data. It will not be described in detail here. The latter did some discussion.
Localized corrosion only occurs locally, which is an extremely serious and harmful damage, such as pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion, wear corrosion, etc. In centrifuges, local corrosion is widespread and should be Focus on the analysis of its formation mechanism, take measures to overcome. Figures 1-3 are a set of corrosion and test coupons. The parts in Figure 1 have formed a slight surface corrosion in the atmospheric environment due to the uneven microstructure of the forged structure; Figure 2 shows that the drum parts have been severely rusted in the separation environment, which is a composite corrosion phenomenon; Figure 3 is the material in the standard soaking solution. The test of the hanging piece in the case.
2.1 Pitting corrosion Pitting is a highly corrosive phenomenon, mainly in easily passivated metals, such as stainless steel. Due to possible local defects, there are active ions in the solution that can destroy the passivation film (such as halogen). The group ion), the passivation film is partially destroyed, thereby forming a pair of galvanic couples, causing pitting corrosion. After the pitting corrosion is formed, due to the centrifugal force, the dynamic process of the galvanic couple in the hole will be accelerated, thereby ensuring the continuity of the hole corrosion until the perforation, which is completely different from the pitting phenomenon under static conditions. Centrifuges are in continuous operation under many conditions, and once a pitting defect is formed, serious consequences will result. Severe pitting defects are formed to a certain extent, which greatly affects the strength of the stressed parts. For plastic materials, under the influence of high-speed centrifugal force field, instantaneous cracking will occur, causing serious damage to the centrifuge.
2.2 Intergranular Corrosion Intergranular corrosion causes the part to lose strength and extensibility, causing the parts to be brittle. It is a harmful damage that develops from the surface along the grain boundary and has no signs of corrosion on the outer surface. This kind of corrosion, in daily maintenance, it is difficult to find abnormalities, but when the corrosion reaches a certain level, the internal structure of the parts is changed, resulting in severe attenuation of the physical and chemical properties of the parts, resulting in sudden bursting or breaking, and the consequences are very serious.
2.3 Wear Corrosion Wear and corrosion means that the surface of the part is subject to both wear and corrosion damage. During the normal operation of the centrifuge, due to the solid particles or mechanical impurities contained in the material, relative movement is formed at the surface of the contact part, resulting in wear. Since the distribution of the formed metal fibers is changed at the wear surface, breakage of the continuous fibers may occur, causing changes in polarity and changes in electronegativity, forming a local corrosive environment and causing corrosion. This form is mainly based on wear and can be easily observed.
2.4 Stress Corrosion Stress corrosion is a damage caused by a certain tensile stress in a corrosive environment. It has the following characteristics: residual tensile stress, applied tensile stress, corrosive infiltration environment, local defects. This image of corrosion is mainly due to the difference in potential generated by the difference in stress distribution. Under the action of the electrolyte liquid, a current is generated, and ion motion is generated, thereby causing corrosion. This corrosion is sometimes difficult to observe with the eyes. Figure 4 shows the sample in different heat treatment conditions soaked in artificial seawater for 2 days. The above four kinds of corrosion are extremely serious to the centrifuge and may cause serious consequences.
3 Measures 3.1 Response Measures Different materials and separation requirements require different models. After the model and main parameters are determined, according to the corrosion resistance of different materials in different environments, comprehensive physical and chemical properties, cost performance and other factors, the strength component materials are determined from the basic surface. From the material itself, the target material it targets is safe. Therefore, the selection of centrifuges should follow the following principles:
(1) There is no absolute universal anti-corrosion material. It is necessary to fully understand the application environment and distinguish the properties of the application environment, whether it is neutral, acidic or alkaline; whether it is oxidizing or reducing, etc., it should be targeted. If stainless steel is not suitable for concentrated sulfuric acid, carbon steel is suitable.
Stainless steel is passivated in air and other adaptive environments to form a stable protective layer on the surface of the stainless steel. Carbon steel is oxidized in an oxidizing environment, and an oxide film is formed on the surface thereof, which is also in a stainless state. Stainless steel in a reducing environment, such as a halogen liquid, has a strong penetrating ability due to a halogen ion such as Cl-, which destroys an oxide film formed on the surface of the stainless steel, thereby causing corrosion of the stainless steel part.
(2) In the same application environment, choose the same material or material with similar equilibrium potential as much as possible to avoid galvanic environment. Different materials have different equilibrium potentials, and the existence of potential differences, when in the through-flow field, will inevitably form a battery, forming a corrosive environment.
(3) Comprehensive cost performance, especially the importance of processing technology. The centrifuge is a high-speed rotating device. Under normal working conditions, the drum parts will generate a large centrifugal force, so these parts are required to have good plasticity and strength. When selecting materials, in addition to price factors, attention must be paid to their comprehensive properties, such as cold work characteristics and hot work characteristics.
3.2 Structural design An excellent design can extend the service life of the equipment and ensure the safety of the equipment. Regarding the structural design of the centrifuge, the design defects that zui easily accelerates corrosion are stress concentration tendency and gap environment. Sometimes, despite the metal soaking solution, the corrosion caused by the design environment is changed, and the chemical state changes (pH and potential drop). Will cause serious crevice corrosion, so the design should pay attention to:
(1) Try to avoid the occurrence of gaps; if necessary for structural design, avoid gaps as dead angles as much as possible, promote the balance of oxygen concentration, prevent the existence of oxygen concentration and reduce the damage of corrosion.
(2) In the same environment, try to use the same material and the same molding process; the main purpose is to prevent the generation of galvanic couples and avoid the formation of corrosive environments.
(3) Guarantee the uniformity of the parts or increase the compressive stress; use economical and reasonable processing methods to enhance the corrosion resistance of the parts.
(4) Avoid forming dead angles and ensure fluid flow. Prevents the formation of corrosive environments with oxygen concentrations.
(5) Design reasonable structure and reasonable processing methods. In the structural design, the uniformity and stability of the cloth and fluid flow are ensured as much as possible. The smoothness of the surface and the avoidance of stress concentration and residual stress are required during processing.
3.3 Surface protection measures A basic idea to control corrosion is to isolate the corrosive environment. In the design of centrifuges, surface treatment methods such as flange, galvanizing, chrome plating, electroless plating, composite lining, etc. are often used. These measures are effective in many environments; but for rotating parts, one must also pay attention to an important Phenomenon: the base material and the coating are two materials, and the coefficient of linear expansion is different. These structures will produce different deformation amounts when the rotating member is elastically deformed, thereby causing a large number of microcracks and peeling of the composite surface; The presence of the aforementioned phenomenon will accelerate the formation of corrosion. Therefore, the method of surface protection for rotating parts should be used with caution.
3.4 Corrosion inhibitor The centrifuge itself is multi-disciplinary. It is necessary to use the centrifuge. In addition to the above measures, it should also pay attention to the application environment. The application of corrosion inhibitors is also widely used in practical applications. For some harsh environments, the use of appropriate corrosion inhibitors can make the parts suitable. As long as the process permits, the use of corrosion inhibitors should be an effective way for centrifuge applications.
3.5 The correct process design is correct and the material selection is reasonable. It is only the precondition for solving the corrosion of the centrifuge; the correct application of the processing technology also affects the anti-corrosion ability of the centrifuge to some extent. Processing methods usually leave many defects on parts or equipment, such as misalignment, local stress, etc.; misalignment may cause gaps; improper positioning method may lead to eccentricity of parts; unreasonable pass may result in surface roughness; Appropriate, welding, heat treatment methods or measures are unreasonable, and the uneven force of the fastening bolts will cause local stress concentration. Therefore, finding the right process is also an effective way to prevent corrosion.
The corrosion problem of the centrifuge is a comprehensive and complex problem, because the working environment of the centrifuge is quite different from the data obtained by the general information and manual in the static. Therefore, in addition to the above analysis, in addition to the above analysis, consideration should also be given to the effects of dissolved oxygen, temperature, high-speed fluid and fluid state changes in the separation zone, and appropriate measures to ensure safe and reliable operation of the centrifuge.
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