Some knowledge points about pressure vessels
Release Time:
2021-06-11 15:53
Source:
In addition to the quality of the steel, stress concentration caused by manufacturing and installation defects is also an important cause of low-temperature pressure vessels fracture. Especially at low temperatures, the large peak stress at the stress concentration point is superimposed on the overall thin film stress and bending stress of the equipment, causing the local area of the low-temperature pressure vessel to reach a high stress level. The steel's plastic deformation ability decreases, the self-limiting condition disappears at low temperatures, resulting in sudden brittle fracture of the steel.
In addition, during manufacturing, when the deformation rate of steel is too high under cold conditions, the phenomenon of work hardening will occur, increasing strength and hardness while reducing plasticity and toughness, and increasing the ductile-brittle transition temperature. If not eliminated, the risk of brittle fracture at low temperatures will increase. Therefore, at low temperatures pressure vessels during manufacturing and installation, measures should be taken to reduce the internal stress level and work hardening.
(1) Steel plate blanking should strictly follow the calculated layout dimensions to ensure accurate cylinder formation. The heads and spherical shell plates should be blanked a second time to ensure accuracy. During transportation, appropriate fixtures should be used to prevent deformation. Dimensions should be checked before installation, and plates exceeding standards should be re-pressed or rolled to ensure assembly quality. If the deviation exceeds the standard and forced assembly is performed, it will generate large assembly stress, which is pressure vessels very unfavorable for low-temperature applications. Therefore, when assembling the shell, forced assembly is prohibited.
(2) To avoid cold working hardening of steel, when forming or correcting low-temperature pressure vessel parts at normal temperature, the cold plastic deformation rate of the steel plate should be controlled at 2%, and hammering forming or correction is not allowed. Cold deformation treatment is not allowed when the ambient temperature is below -10°C. The small bending radius should be controlled, and multiple rolling is required. For spherical shell plates, suitable molds should be used for multiple-point pressing, while for elliptical heads, severe cold working hardening occurs in the transition zone and straight edge. To eliminate cold work hardening, the slab should be annealed before or during pressing to soften the structure and prevent fracture. After cold pressing, the same heat treatment as the original base material should be carried out to restore low-temperature toughness.
(3) During pairing, the deviation of each cylinder joint should be evenly distributed and not concentrated on one side or one section, so as to avoid excessive shape changes and reduce the degree of stress concentration. For butt joints with unequal thicknesses, the thick plate should be thinned and smoothly transitioned to the same thickness as the thin plate. For low-temperature spherical tanks, a piece-by-piece assembly method should be adopted to reduce uneven deviations and shape changes, ensuring that indicators such as misalignment, edges, and roundness meet the requirements.
(4) pressure vessels The surface of the shell and pressure-bearing parts should not be marked with steel stamps, only paint marks are allowed.
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