Common Issues in Pressure Vessel Design – Structural Aspect 1

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Manhole placement: When the length of a horizontal container shell is large, the number of manholes is insufficient. Installing a manhole even when the horizontal container shell is long makes it difficult to discharge gases during welding ； Poor ventilation and inadequate lighting are present during can opening inspections, and it is difficult to evacuate personnel in case of emergencies. For example, when the length of the cylindrical body of a horizontal container is 6000 mm or more, it is necessary to consider providing two or more manholes. The issue of the manway location at the top of a cylindrical horizontal vessel supported by dual saddles. For cylindrical horizontal vessels supported by double saddles, the critical sections are the section at the midpoint between the two saddles and the sections at the saddles themselves. The axial stress is highest at the midpoint section, while the axial stress, circumferential stress, and shear stress all reach their extreme values at the saddle sections. For shells with A > 0.5Rm that lack rigidity and no reinforcement, buckling is likely to occur in the upper part of the sections at the two saddles. Therefore, try not to make holes in these areas. According to clause 6.3.2.1 of GB150.3, when using a reinforcement ring for strengthening, the following requirements must be complied with: (1) Containers made of low-alloy steel with a minimum standard tensile strength of Rm 540 MPa, or low-alloy steel of chromium-molybdenum type; (2) The thickness of the reinforcement ring must be more than 1.5 times the thickness of the container’s shell wall ； (3) Pressure vessels with a design pressure of 4.0 MPa or higher ； (4) Wall thickness δn of the container shell > 38 mm ； (5) Containers holding extremely hazardous and highly dangerous media with a shell wall thickness of 12 mm or more ； (6) Fatigued pressure vessels. Junction thickness: The wall thickness of the pipe selected when a fully penetrated weld is used for the connection between the pipe and the shell is too thin. The selection of the wall thickness for the nozzles on pressure vessel shells is a common issue in design, yet it often happens that the chosen wall thickness is not appropriate. This is particularly true for small-diameter nozzles. In cases where a fully penetrated weld joint between the nozzle and the shell is required, the wall thickness of the nozzle should be at least 1/2 of the shell’s wall thickness, or it should be at least 6 mm – whichever value is smaller. For welded joints with a large amount of weld metal at the groove, when the shell wall thickness is greater than 16 mm, the pipe fitting wall thickness should be greater than 8 mm ； When the shell wall thickness is large (wall thickness greater than or equal to 20 mm), a double-sided groove should be used for the weld joining the nozzle to the shell; see Table 15.4.1 in HG20583 for details. For low-temperature pressure vessels, the wall thickness of the pipe sections welded to the shell shall be not less than 5 mm; for short connections with Dn≤50 mm, thick-walled pipe stubs forged and then turned are preferred (the extended portions may use seamless steel pipes with normal wall thickness).