Determination of the critical length of the crack-like flaws and its effect on safety. (March 2021)

• Record Type: Journal Article
• Authors: Tung, Fang-Yun; Lu, Yen-Ju; Wang, Chen-Hua
• Is Part Of: Journal of loss prevention in the process industries. Volume 69(2021)
• Publication Date: 2021-03
• Subjects: Crack-like flaws -- Critical crack length -- Failure analysis diagram; Chemical industries -- Safety measures -- Periodicals; 660.2804
• DOI: 10.1016/j.jlp.2020.104365
• Languages: English
• ISSNs: 0950-4230

Abstract

Abstract: Methodologies of API 579 have been widely accepted as the de-facto standard guideline for fitness for service assessment on crack-like flaws in pressure equipment. For different purposes and needs, the evaluation is divided into three levels, from critical length estimation in level 1, use of the failure analysis diagram (FAD) in level 2, to performing finite element stress analysis in level 3, that is from simplified to accurate. It is generally considered safe as long as the crack is smaller than the critical crack, but not all of them are actually the case. Therefore, it is very important to study the physical meaning behind the critical crack and understand the conservativeness of it. In this study, numerous examples of critical cracks determined according to API 579 Level 1 were put into Level 2 analysis for comparison. Each case is mapped onto the FAD of level 2 analysis to see its position and distance from the borderline between safe and danger zones. A huge amount of examples covering the majority of possible scenarios, including the geometry, thickness, inner diameter, pressure and half-length of the flaw, depth of the crack, crack tip and crack bottom, as a whole, there are 960 cases to be analyzed. Results show that cracks on the circumferential direction of the cylindrical and spherical vessels are distributed tightly in a small area far from the borderline on the FAD diagram. In these cases, the vessel is actually not as dangerous as the critical crack implies, since most of the points lie quite far from the danger zone. As for the longitudinal cracks of a cylindrical vessel, the results are scattered around, and three data points are located outside the allowable zone. The peculiar results are partly due to a discrepancy of interpolating procedure as the influence coefficient is being computed. Through this study, it is also found that the cumbersome process of estimating the critical crack length can be greatly simplified for longitudinal cracks in a cylindrical vessel. Highlights: The detected crack has potential to grow and lead to fracture on the equipment, if it is longer than the critical crack. The Level 1approach estimates the critical crack length, and Level 2 calculates toughness, load ratios and SIF. 5760 cases were created to confirm high consistency between Level 1 and Level 2 analysis. One special case that fails to pass the Level 2 criterion. FEM on the failed case indicates it is still acceptable. Longitudinal critical cracks on a cylindrical vessel are found to be near the unacceptable zone and needs care.