Influence of Nozzle Transition on Leak Before Break Parameters

Abstract

In the framework of leak before break a circumferential through-wall crack in a pipe with nozzle is investigated by means of finite elements and compared to a pipe only geometry. A range of crack angles (3–180°) and inner radius to wall thickness ratios (Ri/t = 5, 10, 20, 50 and 100) have been analysed in order to investigate parameter sensitivity. The nozzle and pipe models are compared for the leak before break parameters stress intensity factor (KI) and Crack Opening Area (COA). The results of the comparisons are presented as KI nozzle factor (KI nozzle / KI pipe) and COA nozzle factor (COAnozzle / COApipe). At small crack angles the KI nozzle factor is larger than unity, caused by the stress concentration at the nozzle to pipe intersection. At larger crack angles the KI nozzle factor decreases below unity, caused by the so-called bulging effect, which is apparent in the pipe only model. The COA nozzle factor is smaller than unity for all crack angles, caused by the increased stiffness of the nozzle. As the ratio Ri/t increases, both the KI and COA nozzle factors decrease since the nozzle stiffness relative to the pipe increases. For demonstration of leak before break the nozzle has a negative influence regarding the COA as smaller leaks are more difficult to detect. However, since the KI nozzle factor decreases at larger cracks the influence of nozzles regarding leak before break is not so straight forward.