Spiral half tube repair of stainless steel reaction kettle

- May 04, 2019-

When the spiral half tube welded in the stainless steel reaction kettle is heated and cooled, cracks are likely to occur and leakage occurs. In the production of fine chemicals such as medicines and dyes, jacketed reactors are the most commonly used equipment. Their function is to pass cold or hot fluid into the jacket for cooling or heating of materials in the kettle. Most of the jacket forms are U-shaped or cylindrical. In recent years, spiral half-tube jackets have also been well applied. Compared with the conventional jacketed reaction vessel, the spiral half-tube jacket has the advantages of compact structure, good force of the kettle body, high heat transfer efficiency, low energy consumption and low steel consumption. The cracks occur in the heat affected zone of the butt joint of the half pipe and the corner joint of the half pipe and the cone, and there are many leak points. The crack is first removed by a sander and then welded by argon arc welding. After the welding is completed, it is found that the crack has expanded along the direction of the original crack, so the welding repair is stopped immediately.

Analysis of cracks in half pipe welds (1) Insufficient quality assurance system By consulting the quality assurance documents of the equipment, it was found that in order to catch up with the progress of the manufacturing process, the documentation and construction control were confusing, and the quality assurance system was almost out of control. (2) The welding sequence of the half pipe is incorrect. The order is correct: the butt weld between the half pipe is welded first, and then welded to the kettle body after passing the test. In fact, the manufacturer first welds the fillet welds on both sides of the half pipe to the kettle body, and then welds the butt welds between the half pipes, so that the half pipe after welding has no room for shrinkage, leaving a gap between the butt welds. Large assembly stress. This assembly stress is generally expressed as tensile stress, which is highly susceptible to weld cracking and reduces the tensile strength of the weld. (3) The manufacturing process of the half pipe is not standardized. The spiral half pipe of the cone part has both the change of the spiral radius and the change of the spiral lead angle. The manufacture and welding are much more difficult than the cylindrical part, with slight error. Fitting is prone to problems. The manufacturer did not process the uneven groove at the time of manufacture. During the welding process, the half pipe and the cone are closely attached by using an artificial external force and an increase in the height of the welding leg, thereby causing the half pipe to increase the internal stress again. (4) The half pipe welding process is unqualified. The half pipe is short in material feeding, the welding joint is more, the welding and cooling speed is fast, and the welding leg is high. Because the diffusion speeds of elements C and Cr are different, the mass fraction of Cr in the heat-affected zone decreases, and the high C mass fraction causes the toughness and plasticity of the material to decrease, the weldability decreases, and the tendency of welding cold cracks appears to be large. If the preheating and post-heating temperature are insufficient in the welding, the time is insufficient, the welding residual stress will be large, and the welding cold crack is prone to occur. 3 half pipe repair [3] Because the reactor is expensive and the quantity is large, it is unrealistic to rework and return to the factory, and it can only be repaired on site. The repairing scheme is that the original crack is not treated, and then the pre-made shield is attached to the crack and welded by argon arc welding.www.srchemicalequipment.com