How to deal with corrosion problems in centrifuge applications
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In recent years, China's national economy has continued to develop, new technologies and new processes have emerged, and the scope of application of centrifuges is more extensive. However, in this process, most manufacturers are more concerned about the separation performance and strength of centrifuges. Requirements, the impact on corrosion factors is often not enough, resulting in some negative effects,
This article makes some analysis and explanation for 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, and its safety requirements are as important as pressure vessels. Most centrifuge manufacturers are designing, selecting and In the application, 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.
I. Centrifuge environment Centrifuges are divided into different types of equipment according to their functions and structures, but all have common characteristics:
1. The drum is a high speed rotating part;
2. The separation space is formed by the drum as the main body;
3. The rotating drum has a special-shaped structure;
4. There are other couplings or accessories in the drum.
These characteristics of the drum indicate that the core part of the centrifuge is a stress member, and its profiled geometry results in multiple stress distribution of the part, the cooperation of multiple parts, and the possibility of forming a pair of galvanic couples. These are some of the things that are worth noting.
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 and take measures to overcome it.
1. Pitting corrosion is a highly corrosive phenomenon, mainly in easily passivated metals, such as stainless steel. Due to possible local defects in the surface, there are active ions in the solution that can destroy the passivation film (such as halogen 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 different from the pitting phenomenon under static.
2. Intergranular corrosion will cause the parts to lose strength and extensibility, causing the parts to be brittle. It is a kind of harmful damage that develops from the surface along the grain boundary and has no signs of corrosion on the outer surface.
3. Wear and corrosion means that the surface of the part is subject to both wear and corrosion damage.
4. Stress corrosion is 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. The above four kinds of corrosion are extremely serious against the tk, which may cause serious consequences.
Third, measures 1, different materials and separation requirements, you need to choose 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 factors, cost performance and other factors are combined to determine the strength component materials from the basic surface. From the point of view of the material itself, the target material it targets is safe, therefore, the selection of materials should follow the following principles:
a, there is no absolute universal anti-corrosion material, you must fully understand the application environment, targeted materials, such as stainless steel is not suitable for concentrated sulfuric acid, carbon steel is suitable.
b. Stainless steel is not stainless, but it is not rusted in air and other adaptive environments.
c. In the same environment, choose the same material or material with similar equilibrium potential as much as possible to avoid galvanic environment.
d, comprehensive cost performance, pay special attention to the impact of processing technology.
2, structural design An excellent design can extend the service life of the equipment and ensure the safety of the equipment. For the structural design, the design defects that are easy to accelerate corrosion are stress concentration tendency and gap environment, sometimes the corrosion rate is small despite the metal soaking solution. However, the formation of a gap, due to changes in the chemical and electrochemical state of the internal solution (pH and potential drop), will cause severe crevice corrosion, so the design should pay attention to:
a, try to avoid the occurrence of gaps;
b. In the same environment, try to use the same material;
c. Ensure the uniformity of the parts or increase the compressive stress;
d. Avoid forming dead angles and ensure fluid flow.
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, and electroless plating are often used. These methods are effective in many environments, but for rotating parts, an important phenomenon must be noted: matrix The material and the coating are two materials, and their linear expansion coefficients are different, which will produce different deformation amounts when the rotating member is elastically deformed, thereby causing a large number of microcracks; if the above phenomenon exists, the corrosion formation will be accelerated. Therefore, the method of surface protection for rotating parts should be used with caution.
4, corrosion inhibitor centrifuge itself involves multi-discipline, the use of centrifuges, in addition to the above measures, should also pay attention to study its 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.
5, the correct process design is correct, the selection of materials is reasonable, only to solve the preconditions of centrifuge corrosion; the correct application of the processing technology also affects the anti-corrosion ability of the centrifuge to a certain extent. Processing methods usually leave many defects on parts or equipment, such as misalignment, local stress, etc. Misalignment may cause gaps. Unreasonable positioning methods may lead to eccentricity of parts, unreasonable pass may result in rough surface, and serious assembly. The afterburning nozzle is blocked, the valve is burnt, and the piston and cylinder liner are intensified. Therefore, when the diesel engine uses heavy oil for power generation, it is necessary to carry out necessary treatment on the heavy oil to reduce the solid impurities and moisture therein to adapt to the combustion of the diesel engine.
Third, choose what type of separation equipment.
The separation equipment used in the heavy oil separation industry is mainly divided into: all slagging type and partial slag type separation equipment according to the type of slag discharge; according to the type of separated materials, it is divided into: impurity type and water division type.
In general, the choice of models is not consistent: the customer's requirements for the separation equipment are mainly to exclude impurities in the heavy oil. Therefore, for heavy oil with high mechanical impurity content and water content less than 3%, the first choice is to ensure slagging. Separation equipment, that is, in the separation process, it is necessary to focus on adjusting the slagging performance of the separation equipment, so that the slag discharge is enlarged, and the removal of water is relatively unimportant, and if the water content is more than 3% heavy oil, At the same time, it is necessary to consider the use of a separate separation device for drainage, or to adjust the concentration to remove the moisture in the oil. Otherwise, it may cause power failure of the lower-level diesel engine, such as load shedding. Selection of treatment volume: Generally speaking, when burning heavy oil, the exhaust temperature of diesel engine is 15~20'12 higher than that of burning light diesel oil, and the fuel consumption rate is about 5%~8% higher. So when you know the rated throughput of the diesel engine, you need to know the separation equipment that needs to be processed.
In addition, it is important to note that the oil used by the user should be inquired in advance, and the corrosiveness of the material should be confirmed and noted to avoid disputes.
Fourth, how to better use the separation equipment Due to the characteristics of the heavy oil itself, when separating the materials, it will cause corrosion to the mechanical parts, including cold corrosion and hot corrosion. When the heavy oil is operated in the separation equipment, the separation temperature of the heavy oil is required to be high (generally required to be 95-98~C, the separation effect will be better, and the actual heavy oil temperature will be 2'12 when actually operating in the drum body. The temperature rise), the sulfur in the heavy oil produces a certain amount of sulfuric acid and sulfurous acid, which will condense on the surface of the part after the shutdown, forming cold corrosion; too high temperature of the heavy oil will cause the vanadium and sodium compounds to directly produce the material. Hot corrosion, in heavy oil separation, hot corrosion is much more serious than cold corrosion. In order to prevent the above situation from appearing in a large amount, the temperature reduction affects the separation effect. Therefore, there are only two solutions at present: one is to adjust the separation temperature according to the actual material condition, and is generally controlled at about 60 ° C ~ 75 '12. The premise of the method is that the slag content of the material is relatively small and the viscosity is low.
Another method is to change the material in the separation device that is in contact with the material depending on the condition of the material. These are some of my understanding of the heavy oil separation industry and the recommendations for the recommendation and use of heavy oil separation models. Inappropriate, improper welding, heat treatment, uneven force on the fastening bolts will cause local stress concentration. Therefore, finding the right processing method 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 in the static data of the general data sheet. 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.