Failure modes of pressure vessels and preventive measures

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The failure modes of pressure vessels are divided into (1) short-term failure modes, (2) long-term failure modes, and (3) cyclic failure modes. Each failure mode can be further divided into various situations. However, not all failure modes need to be considered when designing pressure vessels; several failure modes that should be taken into account are listed at the end of the text. Short-term failure modes refer to sudden failures of pressure vessels under non-cyclic loading conditions, including: 1. Fracture: This refers to the breaking of a component without any significant plastic deformation under a single loading condition. The main reasons for the brittle fracture of containers during use are material embrittlement, inherent defects in the material, and the effects of stress. Preventive measures: Measures such as specifying requirements for the fracture toughness of materials, requiring adequate heat treatment after welding of materials, and setting a minimum temperature for hydrostatic tests can be taken. 2. Ductile fracture: It refers to the fracture that occurs when a component exhibits significant plastic deformation under a single loading condition. During use, the container may experience overpressure or uniform corrosion that reduces its wall thickness; the stresses within the component exceed the material’s yield limit and strength limit, resulting in significant plastic deformation until fracture occurs. This failure mode includes cracks or ductile tearing caused by excessive local strain, that is, cracks or fractures resulting from excessive strain in a particular area of the container. Preventive measures: Measures such as specifying safety factors for the yield strength and tensile strength of the materials can be adopted. 3. Joint leakage caused by excessive deformation: This refers to a situation where, under the action of internal pressure or external loads, excessive deformation occurs at the joints of a container, resulting in leakage of the internal medium and rendering the container unusable. The most common type of joint leakage is flange joint leakage, which may be caused by insufficient bolt preload, loss of resilience in the gaskets, or inadequate stiffness of the flanges. Preventive measures: Measures such as selecting appropriate gaskets and limiting the deformation of the joints can be taken. 4. Elastic instability or elastoplastic instability (buckling): This refers to a situation where the compressive stress generated in a component under load causes a change in its geometric shape; after the load is removed, the component’s geometry does not return to its original state on its own. Instability occurs when the compressive stress on the component exceeds the critical pressure. In elastic instability, the critical pressure is related to the dimensions of the component and the properties of the material; in elastoplastic instability, the critical pressure is related to the dimensions of the component, the properties of the material, and the strength of the material. Preventive measures: Measures such as specifying a safety factor for the critical load of the structure and limiting the geometric deviations of the container can be taken. Long-term failure modes refer to failures that occur in pressure vessels after exposure to non-cyclic loads over an extended period of time. These include: 1. Creep fracture: The process by which the metallic material of high-temperature vessels or their high-temperature parts undergoes slow plastic deformation over time under loads below the yield limit is known as creep. This failure mode refers to the contraction of the actual load-bearing cross-section of a component due to creep deformation, resulting in increased stress and eventual fracture. Preventive measures: Measures such as selecting appropriate materials and controlling stress levels can be taken.  2. Creep: refers to excessive deformation at mechanical joints or resulting in unacceptable load transfer. Preventive measures: Measures such as selecting appropriate materials and controlling stress levels can be taken.   3. Creep instability: refers to the instability or collapse of high-temperature vessels caused by creep deformation under compressive stress. Preventive measures: Measures such as selecting appropriate materials and controlling stress levels can be taken.  4. Erosion and corrosion: Refer to the loss of metal material due to the action of abrasive media and corrosion, which leads to a reduction in wall thickness, thereby decreasing the container’s load-bearing capacity or causing local perforations that result in leaks. Preventive measures: Measures such as selecting materials compatible with the medium and factoring in allowances for corrosion or abrasion in advance can be taken.  5. Environmental-induced cracking: refers to the cracking of materials under the action of corrosive media. For example, stress corrosion cracking, hydrogen-induced cracking, etc. Preventive measures: These include using materials suitable for the medium, employing appropriate manufacturing methods, and adding retarders. Cyclic failure mode refers to the failure of pressure vessels after being subjected to cyclic loads over an extended period of time. This includes: 1. Progressive plastic deformation: It occurs when, under repeated loading conditions, plastic deformation accumulates in certain parts of the vessel, leading to excessive deformation and subsequent failure. When a container is subjected to combined loads, some loads remain constant while others vary cyclically. These repeatedly changing loads can cause cumulative deformation, leading to failure. The most common form of this failure mode is thermal stress ratcheting. Preventive measures: Measures such as evaluation in accordance with the assessment criteria specified in the relevant standards can be taken.  2. Alternating plasticity: This refers to the failure that occurs when, under multiple loading conditions, certain parts of a container undergo repeated reverse plastic deformation. When the nominal stress of elasticity in certain parts of the container exceeds twice the material’s yield limit, cumulative reverse plastic deformation occurs during loading and unloading, leading to failure. Preventive measures: Measures such as ensuring that the elastic nominal stress remains below twice the yield limit should be taken.  3. Fatigue: refers to the failure of a container that occurs as a result of cracks forming in the material under cyclic loading, leading to unstable crack growth and fracture. Under alternating loads, the structure experiences localized damage accumulation at stress concentration areas, leading to crack initiation and propagation until the entire cross-section is penetrated and the structure fails. Preventive measures: These include conducting fatigue analysis in accordance with relevant standards, using designs that reduce stress concentration, and minimizing residual stresses generated during container manufacturing.  4. Environmental-induced fatigue: refers to the cracking and failure of containers under the combined action of cyclic loading and corrosive media. The combined effect of cyclic loading and corrosive media will cause premature failure of the container. Preventive measures: Measures such as selecting materials compatible with the medium, adopting structures that reduce stress concentration, and minimizing residual stresses generated during the container manufacturing process can be taken. Failure modes to be considered in design: When designing pressure vessels, it is not necessary to take all failure modes into account; generally, the following failure modes should be considered: 1) brittle fracture; 2) ductile fracture (including crack formation or ductile tearing caused by excessive local strain); 3) joint leakage due to excessive deformation; 4) elastic or elastoplastic instability (buckling).