How to repair the shell-and-tube heat exchanger
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Key words: shell-and-tube heat exchanger, heat exchange tube, corrosion, repair, CLC, TQ05, document identification code, A, article number, 1672-3791(2009)08(a)-0097-02
There is a large amount of heat exchange between different media in the chemical industry, and a large part of the heat exchange is done by heat exchangers. The heat exchange equipment is widely used in chemical fertilizer, chemical industry, oil refining industry and many other industrial sectors. In the construction of chemical enterprises, heat exchange equipment accounts for a large proportion of total investment. Therefore, it is very important to ensure the safe operation of the heat exchange equipment for its maintenance and repair quality.
1. Types of shell-and-tube heat exchangers Common shell-and-tube heat exchangers have fixed tube type, floating head type and "U" type tube type.
(1) The fixed tube plate heat exchanger is to directly weld the tube sheets at both ends to the shell. It is mainly composed of main parts such as outer casing, tube sheet, tube bundle and head. A tube bundle is arranged in the casing, and the tube ends are welded, expanded or expanded, and the tube is fixed on the tube sheet, and the outer circumference of the tube sheet and the head flange are fastened by bolts. The fixed tube plate heat exchanger has the advantages of simple structure, low cost, easy manufacture, convenient pipe cleaning and maintenance, but difficulty in cleaning the shell side, and temperature difference stress exists after the tube bundle is manufactured. When the heat exchange tube and the housing have a large temperature difference, the housing should also be provided with an expansion joint.
(2) One end of the floating head heat exchanger is fixed between the casing and the pipe box, and the other end pipe plate can be freely moved inside the casing, that is, the casing and the tube bundle can be freely expanded. Therefore, there is no temperature difference stress between the tube bundle and the housing. Generally, the floating head is detachable, and the tube bundle can be freely extracted and loaded. This structure of the floating head heat exchanger can be used in a condition where the tube bundle and the casing have a large temperature difference. The cleaning and maintenance of the tube bundle and the casing are convenient, but its structure is relatively complicated and the sealing requirements are relatively high.
(3) U-tube heat exchanger is to shape the heat exchange tube into a U shape, and both ends are fixed on the same tube plate. Since the casing and the heat exchange tube are separated, the heat exchange tube bundle can be freely stretched, and no temperature difference stress is generated due to the temperature difference of the medium. U-tube heat exchanger has only one tube plate, no floating head, and the structure is relatively simple. The tube bundle can be freely extracted and loaded for convenient cleaning, and has the advantages of a floating head heat exchanger. However, since the heat exchange tubes are formed into U-shaped bends of different radii, the outer heat exchange tubes of the Zui can be replaced after being damaged, and other tubes are replaced. Damage can only block the pipe. At the same time, compared with the fixed tube plate heat exchanger, since the heat exchange tube is limited by the bending radius, there is a gap in the central portion of the tube bundle, and the fluid is easy to short-circuit, which affects the heat transfer effect.
2. Failure mode of shell-and-tube heat exchangers The common form of damage to heat exchangers is corrosion and leakage, and the casing is thinned. The corroded part is mainly at the junction of the heat exchange tube, the heat exchange tube and the tube sheet and the shell.
2.1 Corrosion of heat exchange tubes There are two cases of corrosion of heat exchange tubes. One is the corrosion of the tube wall by the inner and outer tubes, so that the entire tube wall is thinned. The other is local corrosion of the pipe wall, especially in the pipe section at the inlet of the heat exchange pipe. In addition, because the heat exchange tube has quality defects in the rolling process itself, such as slag inclusion, cracks, skin and pores, the material is uneven, causing pitting of the tube to the tube plate, and the defect is exposed after use, resulting in thinning of the heat exchange tube. leakage.
2.2 Corrosion of the connection between the pipe and the tube sheet The connection form of the heat exchange tube and the tube sheet is expanded, welded or expanded. After the heat exchanger has been in operation for a period of time, the seal weld is corroded or corroded to expose the internal defects of the seal weld and leak. In another case, due to the influence of cold and heat alternating stress during operation or poor quality, the expansion joint is slack and leakage occurs. Due to the stress of expansion and welding, it is easy to crack in the tube sheet and the expansion joint, especially in the case of stress corrosion. In addition, the tube mouth of the heat exchange tube is affected by the heat input of the material during cold expansion or the heat input of the tube and the tube plate sealing, and the corrosion resistance of the tube mouth is reduced to cause corrosion leakage. In addition, the heat exchange tube is improperly annealed due to the tube head or the plasticity of the heat exchange tube is not good. The tube head is cracked after the tube is expanded, and the defect is expanded after use to cause leakage.
2.3 Corrosion of the shell The corrosion of the shell has an overall thinning, local corrosion and pitting due to defects such as uneven material. When the material of the casing is in contact with other components of different materials such as baffles for a long time, electrochemical corrosion is likely to occur at the contact portion. In addition, the welded portion of the casing is inevitably prone to intergranular chromium deficiency for the stainless steel casing material, thereby increasing the possibility of intergranular corrosion. Furthermore, due to the presence of welding stress, stress corrosion is also likely to occur in stress-corroded media.
3. Inspection of heat exchanger tubes and seals leaks Commonly used test methods are hydraulic test, air pressure test, and ammonia leak test. In addition, some professional inspection units can also perform eddy current testing on pipes.
3.1 Hydraulic test After the heat exchanger is filled with water, it is found that the pipe itself leaks or has obvious leak points. It should be repaired before testing. Pay attention to the pressure test: (1) Check the equipment for damage and deformation, confirm that there is no abnormality, and keep the outer surface dry. A temporary pipeline for pressure test shall be provided to establish a pressure test system, which shall ensure water filling, complete venting and drainage. (2) When the hydraulic test of the heat exchange equipment is filled, the air should be drained from the height. (3) For the pressure test, two pressure gauges with the same range, verified, and valid during the period must be used. The range of the pressure gauge should be twice the test pressure. However, it shall not be less than 1.5 times and more than 3 times, the accuracy shall not be less than 1.5, and the dial shall not be less than 100 mm. (4) The pressure gauge should be installed at the high position of the heat exchange equipment and the low position of the Zui. The test pressure value should be based on the pressure gauge reading at the height of the Zui, and check with the pressure gauge reading at the lower part of the Zui. (5) During hydraulic pressure test, the pressure should rise slowly. After the test pressure is reached, the dwell time should not be less than 30 min, then the pressure should be reduced to the design pressure, and the welds should be kept long enough for all welds and inspections. (6) For stainless steel materials, the content of Cl ions in water should not be controlled to exceed 25 ppm. (7) Leakage sometimes occurs from the top to the bottom of a heat exchange tube joint, which may be an illusion, you must find a real leak.
3.2 Air pressure test method The air pressure test method is similar to the water pressure test method. Since the air pressure test is relatively high in risk compared with the water pressure test, in addition to the above precautions, it is also necessary to pay attention to the pressure test to be slowly boosted and boosted. During the process, it is necessary to closely monitor the appearance deformation of the equipment, with or without abnormal noise. The test pressure is lower than the hydraulic test. Then use soapy water to check the weld and the tube seal. The leak was found to be marked and the pressure relief was overhauled.
3.3 Ammonia Leakage Test Method For heat exchangers with high working pressure and tight sealing requirements or tube working pressure higher than shell side working pressure. The test pressure cannot be reached by using the hydraulic test housing. The method of ammonia leakage can be used for inspection. The shell should be replaced with nitrogen first, because the explosion limit of ammonia in air is 15% to 28% by volume, so the volume of the replacement gas is required to be 3 to 5 times the displacement space. After the replacement, the nitrogen gas was charged to 0.2 MPa, and the ammonia gas was recharged to 0.235 MPa for the ammonia leakage test, and the wet phenolphthalein test paper was attached to the tube sheet. When the test paper turns red, it indicates that there is ammonia leakage, and the mark is treated to the leaked part. Ammonia leakage test should pay attention to: (1) due to low pressure, for very small leakage, the inspection time (holding time) will be quite long, generally about 10 hours to 12 hours. (2) After the test is completed, it is still necessary to replace it with nitrogen. The discharged ammonia needs to be inserted into the water for absorption and cannot be directly discharged into the air to avoid environmental pollution or personal injury.
4. Heat exchanger maintenance 4.1 The pipe is subjected to a pressure test or an ammonia test to detect a heat exchange tube leaking due to corrosion. The simple method is to block the pipe. For tubes with relatively straight nozzles, the heat exchange tubes and tube sheets are welded to form a plug into a cylindrical shape, and one end is in the form of a blind hole, so that the welding deformation when the plug is welded to the tube sheet is mainly concentrated on the plug. Reduce the welding stress of the tube sheet. For the heat exchange tube and the tube sheet in the form of expansion joint, the tube mouth is not very regular due to corrosion or welding at the time of manufacture, the plug can be generally processed into a tapered wedge shape. The wedge plug should not be directly crushed to the tube plate by welding, especially for the expansion joint type tube, it should be noted that the welding can not be used. Because the tube is partially heated during welding, the shrinkage of the weld metal during cooling is easy to cause the surrounding tubes and tube sheets to be thermally stretched to cause stress deformation, thereby causing a pulling action on the tube to cause loosening of the surrounding expansion tube, when driving It is easy to leak. The taper of the plug should be as small as possible so that it does not slip out easily after the plug is tightened. In addition, the plug for plugging must pay attention to the material selection. In principle, the plug should be the same material as the tube to avoid electrochemical corrosion.
4.2 The expansion of the expansion or repair welding tube or tube plate can be used to replenish the expansion. Generally, the roller expander is used to manually replenish the expansion. When the pipe and tube sheet seal welds leak, the original weld seam can be polished and then repaired. Repair welding should be carried out on a small scale, taking care not to affect the quality of other welds. In addition, for the repair welding which is leaked by the weld seam in the form of the pipe joint of the expansion welding, there is air, moisture and moisture in the corrosion gap, and due to the expansion during welding, they cannot overflow from the reverse surface and can only be flushed out from the molten pool. It is easy to cause the fluid metal in the molten pool to tumbling, thereby forming pores, so that the high temperature expansion stress in the near seam area of ​​the weld is partially loosened.
4.3 Changing the tube For the fixed tube plate heat exchanger, the blocking tube will affect the heat transfer effect. The tube can be changed while the site and equipment are allowed. First, the leaking tube is pulled out by using a special tool for extubation. Currently, there are many kinds of tools for extubation and tube exchange. If it is a large-area changeover, the weld metal of the heat exchanger can be drilled with a drill bit on the drill bed, so that the heat exchange tube can be easily extracted. For heat exchangers that cannot be moved in the field, the weld metal can be milled out with special tools, and the pipe can be pulled out with a hydraulic push or thread pull type special tool. After the tube is pulled out, the tube plate hole and the baffle hole should be cleaned, and the center line of the tube plate hole and the baffle hole should be measured to be skewed, and then the tube is passed. For the tube plate hole and heat exchange tube of the expansion joint structure, there should be no longitudinal scratches and pits. The tube replacement requirements for the heat exchanger should also be noted that the hardness of the tube material should be smaller than the hardness of the tube sheet, which is about HB30. . Otherwise, the annealing should be performed within 150mm to 200mm of the pipe end. For the sealing of welded structures, special attention should be paid to avoiding the generation of hydrogen holes in the weld. The hydrogen in the weld is mainly from the moisture in the welding rod, workpiece or air. At high temperatures, hydrogen decomposes into hydrogen atoms and dissolves a large amount of liquid metal. When cooled, the solubility of hydrogen in steel drops sharply due to the rapid cooling of the weld. The hydrogen dissolved in the metal does not overflow, thereby forming a hydrogen gas hole. Therefore, the heat exchange tube and the tube hole should be kept clean and dry, and the electrode rod should be dried according to the regulations and placed in the heat preservation barrel for use.
4.4 Excavation and replacement of the shell For local thinning of the shell or other local defects, the shell can be partially excavated. (1) The shape of the excavation may be a circle or a rectangle as the case may be. If the part to be dug is small, it is suitable to use a circle. If the part to be replaced is relatively large, a rectangle can be used, but the four corners of the rectangle should be processed into a circular arc. (2) The material of the re-welded sheet shall be consistent with the material of the original casing and shall be processed to conform to the curvature of the casing. (3) The excavation group shall open the V-shaped or U-shaped groove to the sight plate, and the weld shall adopt the full penetration structure of the full penetration or back pad. (4) When the group is correct, it cannot be strongly paired. The amount of misalignment and the angle of the weld should meet the requirements of GB150. (5) The weld is tested by 100% RT. (6) For the shell side pressure is not high, pay attention to the corrosion allowance of the shell during the design, the medium is a mild hazardous medium such as circulating water, the heat exchanger shell of magnesium nitrate can be supplemented with the same material as the shell. .
4.5 Other maintenance methods of the heat exchange tube For the fluid, the eddy current agitation is easily generated at the dead corner, so that the local end of the heat exchange tube is locally corroded, and the other parts of the tube are not obviously corroded and thinned, if all the heat exchange tubes are to be replaced It is bound to cause huge waste, especially for the more expensive heat exchange tube materials. In this case, some professional maintenance units can only replace the end of the pipe with a pipe butt joint, and the argon arc welding method in the pipe achieves good results.
In summary, due to the large number of applications of shell-and-tube heat exchangers in the chemical industry, the overhaul is also an important part of chemical overhaul. During the overhaul, the structural characteristics of the shell-and-tube heat exchanger should be understood, and practical and feasible inspection and repair methods should be selected according to its specific form. That is to say, in order to save the maintenance cost and strive for the maintenance time of the maintenance method.