7 major design criteria considered in pressure vessel design

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(1) Elastic failure criterion. According to the theory of elastic strength, when the equivalent stress at areas far from the edges of a container reaches its yield point, this represents the ultimate load-bearing state of the container. It specifies that the yield limit is the stress at which the container fails. After considering the safety factor, the actual stress in the container remains within the elastic range. The design formulas in GB150 for components such as internally pressurized cylinders and internally pressurized convex heads are established based on the principle of elastic failure. (2) Plastic failure criterion. The guideline states that when a certain point on the container reaches yield, it does not necessarily lead to container failure. Only when the whole structure yields does it reach the limit state of the container’s load-bearing capacity. It specifies that the full yield pressure is the highest pressure at which the container fails. After considering the safety factor, the strength check condition for the bending stress is found to be 1.5t. WeChat official account: Pressure Vessel Sharing Station. For brittle materials, although they are also subjected to bending stress, when the surface stress reaches σs, any further increase in external load prevents significant plastic deformation of the wall surface, which can lead to cracking. Therefore, merely considering the introduction of plastic failure criteria in pressure vessel design, material selection should also aim to exclude brittle materials with poor plasticity, or appropriate restrictive measures should be taken. JB4732-1995 \"Steel Pressure Vessels – Code for Analytical Design\" provides a design method based on plastic failure criteria. The design or stress calculation formulas for elements such as flat plates, and the necks of cylinders (or nozzles) connected by integral flanges (including any type of flange designed as an integral flange) are all formulated according to the principles of plastic failure. WeChat official account: Pressure Vessel Sharing Station. (3) Elasto-plastic failure criterion. The elastoplastic failure criterion is applicable to cyclic loading processes. According to the concept of stress classification, when a certain amount of local plastic deformation occurs in the peripheral areas of a vessel, it represents the limit state of its load-bearing capacity. It takes into account that when excessive plastic deformation occurs due to edge stresses, it will accelerate fatigue failure or cause brittle fracture. Due to this failure criterion, local plastic deformation is permitted in the structure; and because the stress distribution is uneven throughout the structure, the local plastic regions are surrounded by larger elastic regions, which is why it is called an elastoplastic failure criterion. The elastoplastic failure criterion is also not applicable to brittle materials. JB4732-1995 \"Steel Pressure Vessels – Code for Analytical Design\" provides a design method based on elasto-plastic failure criteria. The design formulas and charts in GB150 for connections such as those between internal pressure conical heads and cylinders, and between flangeless spherical heads and cylinders, are all established based on the principle of elastoplastic failure. (4) Fatigue failure criterion. According to this criterion, under alternating loads, the limiting state of a container’s load-bearing capacity is reached when the maximum alternating stress (for a given number of cycles) or the number of cycles (for a given maximum alternating stress) reaches the values specified by the fatigue design curve. When design specifications require consideration of the fatigue of containers, in addition to performing strength calculations on them, fatigue design is also necessary, that is, calculating the service life of the containers. “The container design code “Designed per analysis” includes fatigue design methods. Since fatigue design involves elastoplastic failure criteria, when incorporating fatigue design into the analytical design system, the JB/T4732-1995 standard “Steel pressure vessels – Analytical design standard” should be adopted. This standard is more stringent than the GB150 standard—which is used for rule-based design—in terms of material selection, design, structure, manufacturing, and inspection. WeChat official account: Pressure Vessel Sharing Station. Stress cycle counts exceeding 10^5 are considered high-cycle fatigue, while those ranging from 10^2 to 10^5 are classified as low-cycle fatigue. JB/T4732-1995 stipulates that for steel materials with a room-temperature tensile strength of o6≤550MPa, fatigue analysis can be omitted if the number of fatigue cycles is less than 1000. (5) Fracture failure criterion. It is a calculation criterion based on the concepts of fracture mechanics, using the stress or crack size that causes low-stress brittle fracture of the container as the critical condition. Such critical states and the corresponding fracture failure criteria include the critical stress intensity factor and the K-criterion, the critical crack opening displacement and the COD-criterion, and the critical J-integral and the J-integral criterion. Fracture failure criteria are generally applied to the assessment of in-service pressure vessels with excessive defects, in order to determine whether the vessel can continue to be used (under conditional supervised use) or should be scrapped. (6) Creep failure criterion. This is a design criterion for containers operating at high temperatures. Under the long-term effect of high temperatures and certain stresses, plastic deformation in the container will continue to accumulate. When its creep rate (or equivalent creep stress) reaches a certain value, it represents the ultimate load-carrying capacity of the container. When designing according to the creep failure criterion, the creep value of the vessel wall (or the equivalent stress calculated from the creep equation) should be kept within a specified allowable range. However, the rule design for high-temperature vessels involves selecting appropriate materials for use at high temperatures only up to certain limits, and determining the allowable stress based on the creep limit and endurance strength in order to control creep values. (7) Corrosion failure. Corrosion failure of chemical pressure vessels refers to the damage that occurs when the vessel walls in contact with the medium are eroded by corrosive substances. It can be categorized into two main types: uniform corrosion and localized corrosion. Controlling corrosion failure essentially means selecting the appropriate materials and suitable anti-corrosion measures based on the properties of the medium. WeChat official account: Pressure Vessel Sharing Station. Under normal circumstances, the limiting conditions for failure due to uniform corrosion are actually the same as those for elastic failure; that is, a corrosion margin is added to the shell thickness calculated based on the maximum principal stress ； Regarding localized corrosion, it is related not only to the material used for the container but also to its composition, as well as the temperature, pressure, and stress levels under which it is used; therefore, to date, there are no unified criteria for limiting localized corrosion failure.