How to repair the shell-and-tube heat exchanger?

- Aug 08, 2018-

How to repair the shell-and-tube heat exchanger

 

1. Blocking the tube-and-tube heat exchanger


      A simple method of testing a heat exchange tube that leaks due to corrosion after a pressure test or an ammonia test leak is to block the tube. 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.


 

2. Tube heat exchanger supplemental or repair welding


 

When the expansion of the tube or tube sheet is relaxed, the inflation can be performed. 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.


 

3. Tube and tube heat exchanger replacement


For the fixed tube plate heat exchanger, the blockage of the pipe will inevitably 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. Excavation and replacement of shell and tube heat exchanger shell


For local thinning of the casing or other local defects, the casing may 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. .

 

5. Other maintenance methods for shell-and-tube heat exchanger heat exchange tubes

 

For the fluid at the dead corner, vortex agitation is easy to occur, 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 will inevitably cause huge waste, especially for More expensive heat exchange tube material. 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.