Industrial pressure vessel failure causes and patterns

Industrial production in use, the failure of pressure vessels is what reason, divided into what? The failure of pressure vessels is damage accumulation to a certain degree, strength, stiffness, or function of container can not meet the requirement of the use of state. So how is the damage? Damage is actually a process of container in external mechanical force, medium environment, such as heat, individually or jointly, the performance degradation of materials, structural discontinuity or carrying capacity decline, this is the damage. Injuries are not necessarily after failure and the failure occurred, there must be injury. Failure mode is the basis of pressure vessel design, design method, Criteria) Must be in view of the failure mode, a pressure vessel design steps should be sure the container has the possibility of failure mode; For class III pressure vessels, also when the design requirements of content including main failure modes, risk control risk assessment report. In addition to pressure vessel inspection results of evaluation, is established on the basis of failure mode. For boiler and pressure vessel common failure modes, failure modes in standard summarized as three categories. A, long-term failure mode 1, erosion, corrosion, 侵蚀,腐蚀) Pressure vessel materials under the effect of corrosive medium, such as carbon steel tank, as a result of uniform corrosion and material organization structure change of thinning or local corrosion pits, reduce material mechanics performance, container carrying capacity shortage occurring in the course of fracture. Pressure vessel corrosion mechanism is chemical corrosion and electrochemical corrosion. Corrosion morphology have uniform corrosion, pitting corrosion, intergranular corrosion, stress corrosion, crevice corrosion, hydrogen corrosion, bimetallic corrosion, etc. 2, the creep rupture ( 蠕变断裂) Pressure vessel at high temperature under load for a long time, as time slowly increase material of plastic deformation and plastic deformation caused by long-term accumulation thickness thinning or bulge deformation obviously, lead to vessel rupture. Pressure vessel creep occurs, general wall temperature at or above the material melting temperature of 25% ~ 35%. Creep rupture deformation depends on the toughness of the material, the fracture stress value is lower than the material use temperature ultimate strength. 3, the environment contribute to cracking, such as stress corrosion cracking, hydrogen cracking, etc. 4, creep - In the excessive deformation of mechanical joints or cause not allowed 5, creep buckling load transfer ( 不稳定蠕变) With the passage of time gradually accumulated deformation for creep deformation, creep deformation development to a certain degree of creep buckling will occur. Second, the short-term failure mode 1, ductile fracture ( Ductile rupture） Under pressure load, the stress value at or near to the wall material of ultimate strength of the fracture. Usually carbon steel pressure vessel of ductile fracture is the main reason of the wall thickness too thin, Design of wall thickness and thickness thinning due to corrosion) , inner pressure is too high, or improper selection, installation does not meet the safety requirements. 2, brittle fracture ( 脆性断裂) Container no obvious plastic deformation, and the wall of the stress value is far less than the ultimate strength of material and even lower than the material yield limit of fracture. The main cause of brittle fracture is material embrittlement ( Improper material selection, improper material processing technology, strain aging, running conditions) And the defects of material itself. 3, joint leakage caused by excessive deformation ( 泄漏关节由于过度变形) Containers of all kinds of interface failure or bulge over loose mouth leak sealing surface caused by the failure, leakage of medium may cause serious environmental pollution. 4, elasticity, plasticity, or elastic-plastic buckling ( Collapse) 5, excessive local strain cause of crack formation or ductile fracture under the effect of compressive stress, the rules of pressure vessels suddenly lost its original geometry caused failure called buckling failure. Container elastic instability is an important feature of elastic deflection and the load proportion, and the critical pressure has nothing to do with the strength of the material, mainly depends on the size of the container and the elastic properties of material, but when the level of stress in the container exceed the material yield point and inelastic instability occurs, the critical pressure is related to the strength of the material. Three, circulation failure mode 1, 2, extensibility, plastic deformation environment contributes to fatigue (3, alternating plasticity 4, elastic strain fatigue In the weeks and high cycle fatigue) Or elastic-plastic strain fatigue ( Low cycle fatigue) 。