Heat exchanger is a device that can transfer the energy of cold and hot fluid to the fluid. In the petrochemical, coal chemical industry, salt chemical and thermal power plant and other industrial systems, the amount is very large. In the production process, the heat exchanger and the contact medium is more complex, often with high temperature and high pressure (near 1000 degrees) (2 500MPa), high velocity, strong corrosion resistance and other characteristics, so often occurs leakage leakage and corrosion erosion. And all kinds of leakage and corrosion, are directly or indirectly have a relationship. The leak has not only affected the smooth running of the production, but also has become one of the major equipment failures in these industries. The surface corrosion of heat exchange tubes is the most common phenomenon in the leak, which accounts for about 90% of the corrosion leakage. It is the result of the media's erosion and chemical corrosion in the medium.
1 reasons for failure of heat exchanger
The 1.1 heat exchanger surface corrosion and wear corrosion medium and the metal component surface, the relative velocity is larger, resulting in local surface component subjected to severe corrosion damage, this kind of corrosion is called corrosion wear, wear short. A flow medium that causes corrosion damage may be a gas, a liquid, or a solid particle, a gas bubble, etc.. Wear corrosion, is a high speed fluid on the metal surface has been generated by the mechanical erosion of the corrosion products and the corrosion of the new exposed metal surface.
Due to the production of medium in the petrochemical industry, often with a certain adhesion. In order to prevent the precipitation of the medium scale, the medium flow rate is greater than 2 m/s. High speed fluid, especially high speed fluid containing solid particles and air bubbles, can be washed and heat transfer surface, which can cause local surface pressure up to tens of MPA, which can cause fatigue and erosion of the metal surface. Although in the design, in order to prevent the fluid into the shell, the direct impact or erosion, erosion protection plate is arranged in the casing at the inlet of the tube, but due to fluid or long time scouring solid particles, the impingement plate will damage. In addition, due to vibration or micro vibration causes, also often make the baffle plate pipe hole is worn. The external characteristics of wear corrosion often appear horseshoe shaped groove or valley shape, according to the general direction of flow of fluid into the metal surface.
1.2 electrochemical corrosion caused by sediment
When the medium flow is uneven or retention, it is easy to form the deposit on the surface of heat exchange tube. Because the sediment is not continuous, not strong and uneven, in some parts of the formation of cracks and gaps, due to the differences in oxygen inside and outside the gap formed by electrochemical corrosion. Including: anodic oxidation, metal dissolution; cathodic reduction reaction, reduction (neutral or alkaline solution); cathodic reduction reaction, reduction (acid); at the same time, due to the presence of corrosion products, resulting in the electrochemical inhomogeneity of crevice corrosion, resulting in greater.
1.3 corrosion of water side of heat exchange tube
Because the heat exchanger is commonly used as the heat exchange medium, the corrosion of water can not be ignored. Water corrosion is mainly due to the reduction of pH value, water vapor permeability, the presence of dissolved oxygen, as well as harmful anions in water (Cl-, S2-, etc.) erosion, and caused by chemical or electrochemical corrosion. Therefore, the anti-corrosion requirements of the surface of heat exchanger tubes are: anti corrosion surface with good adhesion, thermal conductivity, temperature resistance and greater hardness. At the same time, it is required to have excellent chemical resistance, high resistance to water vapor permeability and a certain scale.
2 heat exchanger corrosion protection
2.1 sacrificial anode protection method
(1) the electrochemical corrosion of carbon steel. Carbon steel in electrolyte solution (such as water) will form a micro battery. The basic microstructure of carbon steel is ferrite and cementite (Fe) (Fe3C), the iron electrode potential body is low, as anode micro battery, cementite as cathode.
(2) sacrificial anode protection method. Galvanic corrosion occurs between cathode and anode in electrochemical corrosion. The electrode potential was lower than the body and the metal anticorrosion and corrosion contact corrosion current, using low potential metal, as high corrosion potential is the corrosion current, the corrosion prevention method of protection of sacrificial anode. The sacrificial anode material choice for carbon steel, carbon steel is the basic microstructure of ferrite (Fe), found in Fe than the standard low electrode potential material from the table. If the choice of aluminum as the anode, it is easy to form an oxide film, but does not produce corrosion current. Other materials are more expensive, so the choice of zinc as a sacrificial anode is more appropriate.
2.2 anticorrosive coating for heat exchanger and its application
A fairly thin layer of metal and inorganic coatings, which can provide an effective barrier between the metal and the environment, which is the main effect of this kind of coating (in addition to the sacrificial coating of zinc). Metal coating (or coating) construction methods are: electroplating, flame spraying, coating, hot dip and steam plating. The construction methods of inorganic coatings are: spraying, plating, or chemical conversion. After spraying, usually at high temperature baking. Metal coatings often show some deformation, while the inorganic coating is very crisp, two kinds of coating must be completely isolated, if there are pores or other defects due to galvanic effect, will cause accelerated localized corrosion of metal.
CH-784 anticorrosive coating epoxy urethane paint, its characteristics are: corrosion resistance, resistance to most water resistance, solvent resistance, acid and alkali, the temperature can reach 200 DEG C; physical and mechanical properties of the film, high hardness, smooth surface, strong adhesion, impact resistance, anti friction. Therefore, the water insoluble small friction, cooling water scale and away impurities are flowing, not attached to the wall. The film has obvious anti scaling effect; CH-784 anticorrosive paint primer paint containing metallic pigment, thermal conductivity, paint thin layer of 80~250&mu m. The paint layer thermal resistance is only 104M2h; 1.66× C / kcal, no rust, no scale, increase the flow rate of water, improve the heat transfer effect.
Because of the water quality, many factories use stainless steel, copper alloy, titanium alloy heat exchanger, but the cost is expensive. The corrosion protection coating is coated with carbon steel heat exchanger, the cost is only 1/4 ~ 1/5 of stainless steel heat exchanger, which is 1/11 of titanium heat exchanger. It can be used for 10 years.
2.3 CH-784 anticorrosive paint and its application
Trial manufacture of CH-784 anticorrosive paint. CH-784 anti-corrosion coating of epoxy amine based coatings, foreign Sakaphen-CH-784. The characteristics are: acid, alkali, oil, water, solvent resistance, heat resistance, abrasion resistance, heat conduction, shock resistance, hard and smooth, water vapor, good adhesion. Heat curing temperature 150 to 200 DEG C, CH-784 coating is a single package, convenient construction, heating and curing, can be made of light paint. The main film forming material of CH-784 anticorrosive coating is epoxy resin. Generally with 609 or 607 epoxy resin, epoxy resin and amino resin components to 3: 7 for the best.
2.4 construction technology requirements
(1) equipment, sandblasting derusting, degreasing, exposed metal color;
(2) coating thickness — — heat exchanger coating thickness is a key index, which requires both to achieve the purpose of corrosion protection, but also to meet the requirements of heat, the general coating thickness of 80 ~ 250μ M. Paint layer is too thick, it will affect the heat transfer, the film is brittle;
(3) the construction method is mostly used for soaking or using the heat exchanger itself as a shell, the inner wall of the pump circulating pipe is used, and the outer wall of the pipe is used for soaking;
(4) the number of &mdash coating; — primer finish 2 layers, 4 layers, each layer thickness of 25 ~ 40μ m;
(5) the heating method — — after the coating surface dry, the room heating curing. From 80 to start warming, slowly increased to 160 degrees Celsius temperature 2 h, and then naturally cool;
(6) the paint quality requirements of — — smooth, no pitting, pinholes, cracks, leakage of coating, uniform thickness.
All chemical devices, relatively close to the corrosion morphology of the heat exchanger, so it can be divided into: the types of corrosion electrochemical corrosion caused by sediment load; stress corrosion stress, thermal stress caused by the high velocity; erosion corrosion; corrosion of dissolved oxygen and material. In addition, according to the corrosion condition of the heat exchanger and the actual conditions of the site, different corrosion protection measures are adopted.
Source: China chemical equipment network