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.

API 579的方法已被广泛接受为事实上的标准指南，适用于对压力设备中的裂纹状缺陷进行服务评估。根据不同的目的和需求，评估分为三个级别，从级别1的临界长度估计，级别2的故障分析图（FAD）的使用到级别3的有限元应力分析，从简化到准确。只要裂纹小于临界裂纹，通常认为是安全的，但实际上并非所有裂纹都是如此。因此，研究临界裂纹背后的物理意义并理解其保守性非常重要。

在这项研究中，根据API 579 1级确定的许多严重裂纹实例被放入2级分析中进行比较。将每个案例映射到2级分析的FAD上，以查看其位置和与安全区和危险区之间的边界线的距离。大量示例涵盖了大多数可能的情况，包括几何形状，厚度，内径，缺陷的压力和半长，裂纹深度，裂纹尖端和裂纹底部，总共有960例。被分析。

结果表明，圆柱状和球形容器圆周方向上的裂纹在远离FAD图边界的较小区域内紧密分布。在这些情况下，由于实际上大多数点都远离危险区域，因此该船实际上没有临界裂纹所暗示的危险。至于圆柱形容器的纵向裂缝，结果散落在周围，三个数据点位于允许区域之外。特殊的结果部分是由于在计算影响系数时插值过程的差异。通过这项研究，还发现，对于圆柱形容器中的纵向裂纹，估计临界裂纹长度的繁琐过程可以大大简化。