A pressure vessel is a closed vessel that can withstand pressure. The pressure vessel has a wide range of uses, and it plays an important role in many sectors such as industry, civil and military industry, as well as in many fields of scientific research. 

Machining deformation and prevention of pressure vessels

Stress deformation and prevention

1. Flame cutting deformation

(1) barrel section: When the short barrel section of the large diameter shell is blanked (the material is long and narrow), the flame cutting edge of the port is prone to deformation. Because of cutting after high temperature cooling, processing edge shrinkage, straight edge into the "arc" edge, cylinder roller round, its port is not in a horizontal plane, the error is large, can not meet the requirements of the group and welding. Symmetrical cutting or machining methods should be adopted to avoid deformation.

(2) head: forming head flame net material cut, the port surrounding will shrink, make the head diameter smaller. In serious cases, the diameter of the contracted head can not meet the size requirements. The overall molding of the head port processing, such as flame cutting, the molding mold design to consider the amount of shrinkage after cutting; For the port processing of the flap type combination head, such as flame cutting, the diameter of the head assembly should be properly enlarged to make up for the shrinkage after cutting. Mechanical processing can also be used to avoid deformation.

(3) machined parts blank (mainly steel plate blank) : 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 the processing quantity of the blank surface is not enough when it is serious. The blank board should be straightened after cutting. For the blank board that is difficult to be straightened, the processing allowance can be appropriately increased.

2. Machining instability and deformation

Processing instability deformation is often in the shape of the head or cylinder opening large holes (such as container loading and unloading holes), due to the opening area and its nearby stability weakened, resulting in the deformation of shell parts or parts. Try to avoid opening large holes directly in the separate cylinder section or the separate head, and then open large holes after assembling the shell into large sections or whole according to the situation; Before opening the large hole, strengthen the opening area with the reinforcement plate close to the shell, and remove the reinforcement plate when the shell is in the overall stable state after the assembly welding.

3. The welding deformation

Welding process is the technical requirements and operation regulations of container welding, including: welding method, welding groove, electrode type and diameter, welding process parameters, welding sequence, welding pass layer, pre-welding and post-welding treatment, welding environment requirements and anti-deformation measures, etc. The welding process must be qualified by process evaluation, and the technical requirements must be strictly implemented in the welding process.

According to the welding conditions and welding quantity of pressure vessels and large parts, analyze the size and shape of deformation to be generated by welding in advance, and formulate targeted control measures:

(1) For large pressure vessels with multiple welding passes, such as spherical vessels, they should be assembled and connected into a whole before welding. Welding should be carried out symmetrically and in accordance with the specified welding sequence.

(2) For large parts with multiple passes, such as melon flap type combination head and shell transition segment composed of flap pieces, in addition to the above requirements, mouth shape fixing fixtures should be provided at the welding site.

(3) For long and multi-section welded pressure vessels, the size of the cylinder joints should be properly released when the material is cut, so as to avoid the phenomenon of shell shortening after welding.

(4) For the assembly welding of pressure vessels, especially for the complex structure of pressure vessels, reasonable assembly sequence and welding anti-deformation measures should be taken to ensure that there is no deformation in the manufacturing.

(5) Reverse deformation measures: according to practical experience or calculation, in advance on the welding parts to welding deformation in the opposite direction to give deformation, welding after the pre-deformation is just offset, the specific approach is: the longitudinal joint of the pressure vessel cylinder section of the two ends of the head arc, in the welding deformation direction of the opposite direction set aside reverse deformation; The mold size of the combined flap head and ITD should be considered to offset the reverse deformation of welding deformation.

4. Preventive measures for deformation in heat treatment

(1) The heat treatment furnace must meet the specification requirements, the temperature in the furnace should be uniform and accurate, the flame nozzle on the furnace wall should be equipped with a firewall, and the flame is forbidden to directly contact or close to the heat treatment parts.

(2) After the pressure vessel with large length is put into the furnace, temporary support pad should be added, and the amount used depends on the specific scale of the vessel.

(3) The shell with larger diameter and thinner thickness should generally be strengthened internally.

(4) section prefabricated pressure vessels, section ports should be provided with strengthening support.

(5) The pressure vessel parts which are easy to lose stability due to high temperature should also be strengthened according to the specific situation.

Processing error deformation

1. Blanking error deformation

Due to the size of the material is not allowed, so that the shape of the formed parts beyond the standard provisions. Cutting size is not allowed mainly due to calculation or magnification sample error, in addition to improving the technical level of cutting personnel, should also be implemented cutting size proofreading system, and as far as possible to take the cutting size computer software management.

2. Forming error deformation

Pressure vessel parts in the process of molding, due to improper operation or non-standard mold deformation: hot molding head demold temperature has certain requirements, such as high temperature will lead to premature demold head shrinkage is larger, serious can make its geometric size out of standard; Mechanical roller or pressed container parts, due to improper operation caused by deformation; The mold design is not considered or wrong, so that the geometric dimensions of the pressure vessel components after molding do not meet the requirements. The main preventive measures are:

(1) Molding operation is carried out in strict accordance with the technical requirements of the process.

(2) Strictly control the shape of the workpiece by checking the sample.

(3) The mold design in accordance with the theoretical size and shape of the workpiece, fully consider the pressure vessel parts processing and forming changes after forming, cold forming mold to consider the springback of the forming part, hot forming mold to consider the shrinkage of the forming part after cooling.

3. Assembly error deformation

Pressure vessel shell assembly due to the wrong mouth or unstraightness error and other excessive deformation, called assembly deformation. Its preventive measures:

(1) Shell assembly should use positioning clamp, shell with larger diameter and thinner thickness, shell joint should be supported during assembly, strictly limit the wrong mouth on the butt side of the shell.

(2) The horizontal assembly of the shell should be carried out on the supporting roller, and its unstraightness should be checked with a straight line.

(3) section prefabricated pressure vessel, installation to set the clamping, and the theodolite to check its straightness.

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