A structural integrity assessment of an elbow subjected to internal pressure, system moments and operating at high temperature
Elbows are a common component in all the pipework and have been widely used in power plants and petrochemical plants. As they are highly stressed components, they are critical to the integrity of pipework and important to the safety of the plants. Therefore, many research papers have been published in the literature. In this paper, a structural integrity assessment of an elbow has been carried out assuming a defect initiated at the centre of the elbow under the loading conditions of internal pressure, system moments and high temperature. At the central section of the elbow, hoop and axial stresses under internal pressure and the resultant equivalent moment (applied in turn as in-plane, outof-plane and torsional moment) have been calculated using ASME III NB-3685. (1) Firstly, the maximum total hoop and axial stresses obtained above are identified, for each load, around the full circumference and the hoop and axial stresses at the section identified are linearised; (2) Secondly, the section of the maximum membrane hoop and axial stresses for each load are identified and the hoop and axial stresses at this section are linearised; (3) Finally, based on the stresses obtained in (1) and (2) above, membrane stresses due to internal pressure and system moment stresses can be taken to be primary (Table NB-3217-2 of ASME III) and 75％ of the through-wall bending stress induced by the bending moment can be treated as primary, as defined in ASME III NB-3685.4. In conjunction with this, ASME III NH-3223 (d) suggests a reduction factor on the through-wall bending stress induced by the bending moment to account for the reduction in extreme fibre bending stress due to the effect of creep (geometry dependent), which has been considered in this work. It should be noted that according to other reports, the membrane hoop and axial stresses are classified as primary and all the through-wall bending stresses in both the axial and circumferential directions are classified as secondary. However, in this paper calculations have been carried out using (a) 75％ of the through-wall bending stress induced by the bending moment as primary and (b) all the throughwall bending pressure stresses in both the axial and circumferential directions as secondary, respectively. Reference stresses are used in the limiting defect assessment, which has been conducted using R6, when calculating the plastic collapse parameter Lr. The reference stress for a defective bend is conservatively taken to be the higher of the reference stress calculated for the defect-free cylinder/bend and that calculated for the structure with the postulated defect. Furthermore, a local limit load solution is also presented in this paper. For creep damage prior to defect formation, the creep rupture reference stress includes a factor of l+0.13 (x-l) as specified in R5. Following crack formation, the appropriate stress for the evaluation of creep rupture damage is the greater of the uncracked rapture reference stress and the cracked body reference stress. The latter does not include the factor of 1+0.13(x-1). For the pipe bend, parent material properties are used in the assessment. There are no welding residual stresses at the central section of the bend. It is also assumed that no residual stresses exist after the production of the bends. Creep crack growth has been carried out using R5 Vol. 7. Appropriate combinations of creep deformation and creep crack growth used for the R5 assessment are detailed in the paper. Assessment results for an external circumferential defect are presented in this paper.
Structural integrity assessment Elbow ASME Ⅲ R6 R5 Reference stress Local limit load solution Creepcrack growth
Jinhua Shi
AMEC, Power & Process Europe, Rutherford House, Quedgeley, Gloucester GL2 4NF, UK
国际会议
2012 International Symposium on Structural Integrity 2012国际结构完整性学术研讨会 ISSI 2012
济南
英文
55-63
2012-10-31（万方平台首次上网日期，不代表论文的发表时间）