Q&A on Pressure Vessel Design (II)

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Question 1: How should welding materials be selected for dissimilar metal materials? Answer: Welding of austenitic stainless steels and ferritic steels (including low-carbon steel, low-alloy high-strength steel, pearlitic heat-resistant steel, and ferritic stainless steels). (1) When welding ferritic steels, welding materials of the same microstructure and composition should be used as much as possible. However, when using austenitic welding materials with the aim of eliminating post-weld stress relief heat treatment or improving the welding process, the following factors should be taken into account: 1) There is a significant difference in the linear expansion coefficients between austenitic steel and ferritic steel; in situations involving high temperatures or frequent temperature changes, the thermal stresses resulting therefrom must be considered ； 2) During the fusion welding process, the decarburized layer (on the ferritic steel base metal side) and the carburized layer (on the austenitic weld metal side), resulting from carbon migration at the fusion line between the austenitic weld metal and the ferritic steel base metal, lead to a decrease in high-temperature strength and ductility. (2) When welding ferritic steel base material with austenitic welding materials, the preheating temperature can be reduced by about 100°C compared to that when using welding materials with similar alloy compositions for welding ferritic steel, and post-weld heat treatment is generally not required. (3) For butt or fillet welding of austenitic stainless steels and ferritic steels, A302 electrodes are generally recommended. However, when the operating temperature is above 450–550°C and the welded joint is subject to high stress levels, high-nickel austenitic electrodes such as A502, A507, and A607 should be used. When the operating temperature is above 550°C and the joint is under high stress, nickel-based welding materials should be employed. For shielded metal arc welding, ENiCrFe-3 and ENiCrFe-2 that meet AWS A5.11 standards can be used. Question 2: What are the requirements for designing environments subject to hydrogen corrosion? Answer: (1) Corrosive environment: A chemical pressure vessel with a design temperature of 200°C or higher and in contact with a hydrogen atmosphere is considered to be in a hydrogen corrosion environment. In a hydrogen corrosion environment, when other factors such as high-temperature sulfur corrosion, high-temperature creep, the multiplicative effect of creep and hydrogen corrosion, and temper embrittlement are also present, it is necessary to consider the impact of these factors on the high-temperature mechanical properties of the steel. (2) Material requirements and restrictions 1) The condition of steel in a hydrogen corrosion environment shall comply with the standard specifications, and stress-relief heat treatment shall be carried out after welding ； 2) The operating temperature limit for ferritic steel in hydrogen corrosion environments is determined according to the Nelson curve, with a temperature safety margin of more than 20°C ； 3) Austenitic stainless steel will not suffer from hydrogen corrosion when used at the temperatures and hydrogen partial pressures indicated on the Nelson curve, and therefore no post-weld stress-relief heat treatment is required. Question 3: When is it necessary to carry out heat treatment to restore mechanical properties? Answer: To restore the mechanical properties of materials that have been lost due to cold working hardening, GB150.4 stipulates that compressed components meeting certain conditions should undergo heat treatment to restore their mechanical properties. Both heat treatment to restore mechanical properties and post-weld stress-relief heat treatment belong to stress-relief annealing (low-temperature annealing). Therefore, when necessary, the heat treatment to restore the mechanical properties of the raw materials and the post-weld stress-relief heat treatment for pressure vessel products can be carried out simultaneously. The requirements for heat treatment to restore the mechanical properties of materials are easily overlooked in the design of container manway splices. Question 4: What is post-weld hydrogen removal treatment? Answer: During welding, hydrogen from the electrode, flux, and moisture in the air is decomposed into atomic form at high temperatures and dissolves in the liquid metal. As the weld cools, the solubility of hydrogen in steel drops sharply; due to the rapid cooling of the weld, hydrogen does not have time to escape and remains in the weld metal. After some time, it accumulates in the weld or at the fusion line. When they accumulate to a certain extent, under the influence of welding stresses, cold cracks, that is, delayed cracks, may form in the weld or heat-affected zone. Therefore, it is required to preheat the welding rod first, and after welding, the weld seam should be post-heated to 200°C; the usual post-heating time is 16 hours. This helps to reduce the cooling rate of the weld seam, allowing hydrogen to escape properly, a process known as post-weld hydrogen removal treatment. Containers that require degassing treatment after welding can be exempted from this treatment if post-weld heat treatment is carried out immediately thereafter. Question 5: What about the medium density in the calculation of horizontal vessels? Answer: The medium density should be the density of the actual medium; it cannot be the density of water. Question 6: If austenitic stainless steel clad pressure vessels meet the conditions for stress-relief heat treatment as specified in GB150, how should such issues be addressed? Answer: If the equipment is made of austenitic stainless steel composite plates intended to resist intergranular corrosion caused by certain media, care should be taken before deciding whether to carry out stress-relief heat treatment. This is because the temperature used for stress relief (620 ℃ ±20℃) falls within the sensitization range of stainless steel (400 ℃ – 850℃), which can lead to a reduction in chromium content in the stainless steel’s crystal structure, thereby decreasing its resistance to intergranular corrosion. Therefore, stress-relief treatment is not recommended. During manufacturing, it is possible to consider using small electrodes, low current, and multiple pass welds. If it is only for cleanliness requirements, stress relief treatment can be considered. Question 7: Can metal wound gaskets be used on Type A flanges of pressure vessels? Answer: For Type A flat-face flanges, only non-metallic gaskets are permitted in accordance with the requirements for pressure vessel flanges; it is incorrect to use wound gaskets. Type B flanges and high-neck flanges can use wound gaskets. Like PL-type pipe flanges, A-type flanges have poor rigidity and poor sealing performance. If the medium contained is extremely or highly hazardous, or minor leaks are not permitted, a high-neck flange is more appropriate, to be used in conjunction with wound gaskets or metal-clad gaskets. Question 8: What are the structural design issues of vertical fixed-tube-sheet heat exchangers? Answer: Vertical fixed-tube-sheet heat exchangers should preferably use ear-type supports, which are to be installed on the shell side of the shell. The support plane of the ear-type supports in vertical fixed-tube-sheet heat exchangers should generally be above the equipment’s center of gravity and the expansion joints. For the tie rods of vertical fixed-tube-sheet heat exchangers, the fixed end should be located on the upper tube sheet, as long as the assembly requirements are met, regardless of whether the inlet for the shell-side fluid is positioned at the top or bottom. Question 9: What is the structural design of the support plates in the shell side of a horizontal fixed-tube-sheet condenser when the condensing medium flows through it? Answer: From a heat transfer perspective, a condenser with a condensing medium in the shell side does not require baffle plates. However, in order to increase the stiffness of the heat exchange tubes and prevent excessive deflection or tube vibration, when the unsupported span of the heat exchange tubes exceeds the values specified in the standards, a small number of support plates need to be installed. The support plates should be arranged vertically left and right, with a liquid inlet provided at the lowest point.