Q&A on Pressure Vessel Design (I)

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Question 1: How should the welding joint factor be indicated for vessels whose shells are made of seamless steel pipes? Answer: The welding joint coefficient should be indicated as 1. To avoid misunderstandings, it is sometimes necessary to specify separately the longitudinal welding joint coefficient and the circumferential welding joint coefficient. Question 2: How should the fixed end and the sliding end of a horizontal vessel with dual saddles be arranged? Answer: Under normal circumstances, the fixed end is placed at the end with the larger pipe diameter and greater constraints, while the sliding end is placed at the end with the smaller pipe diameter and fewer constraints. If pipeline stress calculations are conducted, the results of those calculations shall prevail. Question 3: What should be considered when dimensioning ear-type supports? Answer: Although ear-type supports use standard components, to facilitate the owner’s civil construction work, the dimensions of the base plate for the ear support should also be specified. Question 4: When selecting supporting bearings for design, apart from weight loads, what other factors should be taken into consideration? Answer: The effects of wind loads and seismic loads should also be considered, while paying attention to the constraints in the standards for L/DN≤5 and H0≤10. If it exceeds the above range, skirt supports should be considered. In addition to the weight of the body itself (including the weight of the material), the weight load must also take into account the weight of the water contained within. It should be noted that when considering the weight of the water held, the effect of seismic loads should not be taken into account; however, a certain amount of wind load must be considered, and the approach to solving this problem follows that used in tower design. When designing supported Type B supports, the effect of the reaction forces on the head must also be taken into account to determine whether the head meets the required specifications. If it does not, it is possible to increase the number of supports in order to reduce the reaction force on each support, or to increase the wall thickness of the head. Question 5: Question regarding the material selection for the skirt. Answer: The skirt shell (excluding the transition section) is designed to be a pressure-bearing component, and temperature is the main factor taken into account when selecting the material. According to NB/T 47041-2014 \"Tower-type Containers\", the design temperature for the skirt shell is determined as the lowest value of the average monthly minimum temperatures over the years in the area of use, plus 20°C; the material for the skirt is selected based on this design temperature. Question 6: What are the operating conditions for commercial-grade fasteners and specialty-grade fasteners? Answer: HG/T20613 specifies the use of fasteners, with commercial-grade fasteners being permitted for use only under specific conditions. 1. The operating conditions for commercial-grade hex bolts and Type I nuts shall meet the following requirements: (1) The nominal pressure class shall be less than or equal to PN16 ； (2) Non-toxic, non-flammable media and applications without severe cycling ； (3) Use non-metallic flat gaskets ； 2. The operating conditions for commercial-grade double-ended studs and Type I nuts shall meet the following requirements: (1) The nominal pressure class shall be less than or equal to PN40 ； (2) Non-toxic, non-flammable media and applications without severe cycling. 3. In addition to the conditions mentioned above, special-grade fully threaded studs and Type II nuts should be used. Question 7: What do low-carbon and ultra-low-carbon austenitic stainless steels mean? Answer: Austenitic stainless steels can be classified according to their carbon content; those with a C content of ≤0.08% are generally referred to as low-carbon stainless steels ； Austenitic stainless steels with C≤0.03% are referred to as ultra-low carbon stainless steels. The lower the carbon content in austenitic stainless steel, the better its resistance to intergranular corrosion. Thanks to the development of refining technologies at home and abroad, ultra-low carbon austenitic stainless steels that are inexpensive yet of high quality have been widely adopted; as a result, in applications requiring corrosion resistance, stainless steels with higher carbon content and stabilized by elements such as Ti and Nb are gradually being phased out. If stainless steel is to be used as a heat-resistant steel, a higher carbon content is desired. Question 8: What is the difference between steel pipes for fluid transport and steel pipes for structural use? Answer: Steel pipes used in structures are primarily employed in general metal structures, where it is necessary to ensure strength and stiffness. On the other hand, steel pipes used for fluid transport not only need to have the required strength and stiffness but also must maintain good sealing properties; therefore, each pipe is subject to a hydrostatic test. Pressure vessels must use steel pipes for fluid transportation. Question 9: What issues should be considered when carbon steel and carbon-manganese steel are used for extended periods at temperatures above 425°C? Answer: GB150 stipulates that when carbon steel and carbon-manganese steel are used for extended periods at temperatures above 425°C, attention should be paid to the graphitization tendency of the carbide phases in the steel. Under such conditions, the cementite in steel decomposes, as per Fe3C -> 3Fe + C (graphite). This decomposition causes part or all of the pearlite in the steel to disappear, resulting in a decrease in the material’s strength and ductility, as well as a greater reduction in its impact toughness; the steel becomes significantly more brittle.