Safety Assessment For Nozzle Containing Through-wall Crack At The Shoulder

Cracks that tend to appear in weld joint between vessel and nozzle are bearing enormous risk to nozzle safety, thus quantitative investigations into stress intensity factor and limit load are of great significance to the safety rating of pressure vessel containing defects. Stress intensity factor, reflecting the stress field intensity near the tip zone of the crack, is one of the key parameters in the safety rating of weld joint cracks. And the limit load is another important parameter, the analysis of which can reveal a more authentic state of stress than conventional analysis.Based upon the theory of fracture mechanics,3D linear elastic FEA is adopted to study the stress intensity factor of nozzles with through-wall crack at the shoulder under the conditions of various internal pressure and moment. In the application of the elastic-plastic mechanics theory,3D elastic-plastic FEA is used to calculate and analyse the limit plastic load of the same structure. It is proposed the safety assessment method for vessels containing defects under two loads.The main contents and the corresponding conclusions are as follows:(1) The finite element model of piping containing axial wall-though crack is estabished then validated by calculating the stress intensity factor and limit load. APDL provided by ANSYS is applied to establish the parameterization model, loading and post-processing macro-programs, composing the finite element model of nozzle with through-wall crack at the shoulder.(2) KI (the type I stress intensity factor),p (limit load under pressure) and m (limit load under moment) are applied to rate the nozzle safety. Dimensionless numbers such as crack length a, diameter ratio of nozzle do/di, diameter ratio of vessel Do/Di and diameter ratio of nozzle and vessel di/Di are determined.(3) When subjected to internal pressure solely, KIP is proportional to P in conditions of different structure sizes. Longer crack, bigger hole and thinner wall make a larger KIp and a smaller p. When both do/di and Do/Di are larger than 1.1, KIP decreases unobviously with the increase of the thickness of nozzle and the thickness of vessel. The fitting equation is obtained when both do/di and Do/Di is larger than 1.1. The relationship table of KI, p, a, do/di, Do/Di and di/Di are presented when diameter ratio of vessel and diameter ratio of nozzle is smaller than 1.1, and safety assessment is made under internal pressure. (4) When subjected to internal pressure solely, KIM is proportional to M in conditions of different structure sizes. Longer crack, smaller hole and thinner wall make a larger KIM and a smaller m. When both do/di and Do/Di are larger than 1.1, KIM decreases unobviously with the increase of the thickness of nozzle and the thickness of vessel. The fitting equation is obtained when both do/di and Do/Di is larger than 1.1. The relationship table of KIM, m, a, do/di Do/Di and di/Di are presented when diameter ratio of vessel and diameter ratio of nozzle is smaller than 1.1, and safety assessment is made under moment.(5) When subjected to the combination of internal pressure and moment, stress intensity factor is simply the summation of the stress intensity factors that are subjected to internal pressure and moment respectively, KIS= P·KIP+M·KIM. The relation between limit pressure and limit moment fits the linearity equation, mB·P+pA·M-mB·pA= 0. And the fracture ratio and load ratio used to safety assessment under internal pressure and moment are given, which are And safety assessment is made under internal pressure and moment.