Considerations for pressure vessel design

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Pressure vessels refer to enclosed devices used for holding gases or liquids and subjected to certain pressures. They include both fixed and mobile containers in which the maximum operating pressure is 0.1 MPa (gauge pressure) or higher, and the product of pressure and volume is 2.5 MPa·L or higher; such containers are used for gases, liquefied gases, and liquids whose maximum operating temperature is equal to or higher than their standard boiling point. 1. Structure of pressure vessels: 1. Single-layer type ； 2. Multi-layer type: High security, but with many production steps and low labor productivity ； 3. Laminated winding type: There is no need to wrap the laminates layer by layer nor weld the seams between each layer ； 4. Grooved coiled type: The grooved steel strips interlock layer by layer, allowing the layers of steel strips to bear a portion of the axial force of the container ； There are no circumferential welds that run through the entire wall thickness of the cylinder ； High safety in use ； However, special-rolled channel steel strips and dedicated machine tools are required ； 5. Heat-shrink type ； 6. Welded type: It serves as the main structural form for light water reactor pressure vessels, as well as for hydrogenation reactors and coal conversion reactors in the petroleum industry.   2. Pressure vessel design: Based on the given process design conditions, and in accordance with current standard specifications, materials are selected in an economical, appropriate, and reasonable manner while ensuring safety, followed by structural, strength, and sealing design. 1. Structural design: Determine a reasonable and economical structural format that meets the requirements for manufacturing, inspection, assembly, transportation, and maintenance ； 2. Strength design: Determine the structural dimensions to meet the requirements regarding strength, stiffness, and stability, thereby ensuring the safe and reliable operation of the container ； 3. Sealing design: Select appropriate sealing structures and materials to ensure good sealing performance.   3. Scope of pressure vessels: 1. Pressure-bearing components: Parts within the vessel that are directly subjected to pressure loads (including internal and external pressures), such as vessel shell components, opening reinforcement rings, and external pressure reinforcement rings ； 2. Unstressed components: Parts that are directly welded to the stressed components to meet operational requirements, forming a single unit, and that do not bear pressure loads (only bearing gravitational loads), such as supports, lugs, gaskets, etc ； 3. GB150 and the Code for Pressure Vessels clearly define the scope of pressure vessels, which refers to the shell and its components that are integrated as a whole and are under pressure (pressure elements).   4. Basic principles for the design of welded structures for pressure vessels: 1. Use butt joints as much as possible: This helps to ensure welding quality. Butt joints should be used for all longitudinal and circumferential welds, as well as for the welds on convex end caps; welded structures in other areas should also preferably use butt joints. Example: Replace fillet welds with butt welds [change Figure 1(a) to (b) and (c)]. It reduces force concentration, facilitates non-destructive testing, and helps ensure the internal quality of the joint. Figure 1: Fillet and butt joints in container fittings. 2. A fully penetrative weld structure should be preferred at all times; incomplete penetration defects are not allowed. Incomplete penetration refers to the condition where the base metal and weld metal do not fully fuse together, resulting in gaps. The initiation point of brittle failure caused by lack of penetration can also induce fatigue failure under alternating loads. Improvement: Select an appropriate groove shape, such as double-sided welding ； When the container diameter is small and it is not possible to clean the roots from inside the container, a butt joint with single-sided welding and double-sided formation should be used, such as by using TIG welding for the root pass, or by employing a groove with gussets. 3. Minimize stress concentration at welds: Joints are often the source of brittle failure and fatigue failure; therefore, it is necessary to minimize stress concentration when designing welded structures. Measure: Where possible, use welding with equal thicknesses. For the butt jointing of steel plates with different thicknesses, the thicker plate should be tapered at a certain angle before welding is carried out, in order to avoid sudden changes in shape and reduce stress concentration. Generally, when the thickness δ2 of the thin plate is not more than 10 mm and the difference in thicknesses between the two plates exceeds 3 mm ； Or when the thickness δ2 of the thin plate is greater than 10 mm, and the difference in thickness between the two plates exceeds 30% of the thickness of the thin plate or 5 mm, it is necessary to thin the edges of the thick plate as required in Figure 2. Figure 2: Butt joints with varying plate thicknesses. 5. Design of common welding structures for pressure vessels: Main contents: Selecting appropriate weld grooves to facilitate the penetration of welding materials (electrodes or wires) into the root of the groove, thereby ensuring full penetration. Factors for groove selection: 1. Minimize the amount of filler metal used ； 2. Ensure full penetration to avoid various welding defects ； 3. Facilitates welding and improves working conditions ； 4. Reduce welding deformation and residual deformation; for welding thicker components, it is advisable to use groove shapes that are symmetrical along the thickness, such as X-shaped grooves.   6. Problems caused by openings: weakening of the wall strength and generation of high local stresses.   7. Pressure vessels that meet one of the following conditions may not require inspection holes: 1. Pressure vessels with a cylinder diameter Di ≤ 300 mm. 2. The container is equipped with removable end caps, lids, or other covers that can be opened and closed, and their dimensions are not smaller than the specified values ； 3. Pressure vessels with no corrosion or only mild corrosion, requiring no internal inspection or cleaning ； 4. Pressure vessels for refrigeration units ； 5. Heat exchanger. If it does not fall under the above five situations. However, when it is not possible to create inspection holes for some special reason, the following measures should be taken: 1. Conduct 100% non-destructive testing on all the longitudinal and circumferential welds of the container ； 2. Indicate the calculated thickness on the design drawings, and conduct thorough thickness inspections during the use of the pressure vessel or during inspections ； 3 Correspondingly shorten the inspection cycle.   8. Sealing design for bolt-flange connections: The key aspects of bolt-flange connection design involve addressing two issues: 1. Ensuring that the connection is \"tight and leak-free\"” ； 2. The flange shall have sufficient strength to prevent failure under stress. In practical applications, leakage is the main issue; failure due to insufficient strength is rare. Sealing performance: compression surface, gasket.