Pressure Vessel Stress Deformation and Prevention

(1) Tube section: When the large diameter shell short tube section is blanked (the material is long and narrow), the flame cutting edge of the port is easily deformed. After the cutting is cooled at high temperature, the processing side shrinks, and the straight side becomes an "arc" side. After the cylinder section is rounded, its ports are not on the same level. When the error is large, it cannot meet the requirements of grouping and welding. Symmetrical cutting or machining should be adopted to avoid deformation.

(2) Head: After the forming head is cut with flame net material, the periphery of the port will shrink, making the diameter of the head smaller. In severe cases, the diameter of the shrunken head cannot meet the size requirements. For the port processing of the integrally formed head, if flame cutting is adopted, the shrinkage after cutting should be considered in the design of the forming mold; for the port processing of the flap-type combined head, if flame cutting is adopted, the diameter of the head should be taken into account when the head is assembled. It should be enlarged appropriately to compensate for the shrinkage after cutting. Machining can also be used to avoid deformation.

(3) Machined blanks (mainly steel sheet blanks): This blank is mostly used for large flanges or sealing rings on pressure vessels. After flame cutting, due to the uneven expansion and contraction of the steel plate, the blank surface is uneven, and in severe cases, the processing capacity of the blank surface is insufficient. After the blank plate is cut, it should be flattened and corrected. For the blank plate that is difficult to be corrected, the machining allowance can be appropriate.

2. Processing instability deformation

The processing instability deformation is often caused by opening large holes (such as container loading and unloading holes) in the formed head or cylinder section, resulting in partial or component deformation of the shell due to the weakening of the stability of the opening area and its vicinity. Try to avoid opening large holes directly on a single cylinder section or a single head. Assemble the shell into a large section or as a whole and then open a large hole according to the situation. Before opening a large hole, use a rib that is close to the shell in the opening area. Reinforcement is carried out, and when the shell is in an overall stable state after the welded pipe is assembled, the reinforcing plate is removed.

3. Welding deformation

Welding process is the technical requirements and operational regulations for vessel welding, including: welding method used, welding groove, electrode type and diameter, welding process parameters, welding sequence, number of bead layers, pre- and post-weld treatment, and welding environment requirements, as well as anti-deformation and anti-deformation measures. The welding process is qualified through process evaluation, and the process requirements are strictly implemented during the welding operation.

According to the welding conditions and welding volume of pressure vessels and large components, the size and shape of the deformation that will be generated by welding are analyzed in advance, and the control measures are formulated in a targeted manner:

(1) For large pressure vessels with multiple welds, such as spherical vessels, they should be assembled and connected as a whole before welding. The welding should be carried out symmetrically and the prescribed welding sequence should be followed.

(2) For large parts with multiple welding passes, such as the melon-shaped combined head and the shell transition section composed of flaps, in addition to the above requirements, a mouth-shaped fixing fixture should be installed at the welding site.

(3) For pressure vessels that are long and welded in multiple sections, the size of the barrel section should be appropriately released to reduce the welding shrinkage to avoid the phenomenon of shell shortening after welding.

(4) For the welding of pressure vessels, especially those with complex structures, a reasonable assembly sequence and welding deformation prevention measures should be taken to ensure that they are not deformed during manufacture.

(5) Anti-deformation measures: According to practical experience or calculation, pre-deform the welded parts in the opposite direction of the welding deformation, and this pre-deformation is just offset after welding. When the arc is pressed at both ends, the reverse deformation amount is reserved in the opposite direction of the welding deformation direction; the size of the combined petal head and the transition section mold should consider the reverse deformation amount to offset the welding deformation.

4. Preventive measures for heat treatment deformation

(1) The heat treatment furnace meets the requirements of the specification, and the temperature in the furnace is uniform and accurate. A fire wall should be installed at the flame nozzle of the furnace wall, and it is strictly forbidden for the flame to directly contact or approach the heat treatment parts.

(2) After the pressure vessel with larger length enters the furnace, temporary support pads should be added, and the quantity used depends on the specific size of the vessel.

(3) Shells with larger diameters and thinner thicknesses should generally be internally reinforced.

(4) For prefabricated pressure vessels in sections, reinforced supports shall be provided at the section ports.

(5) The pressure vessel parts that are easily destabilized by high temperature should also be reinforced according to the specific conditions.