PSI - Issue 60

P.A. Jadhav et al. / Procedia Structural Integrity 60 (2024) 631–654 P.A. Jadhav et. al./ Structural Integrity Procedia 00 (2019) 000 – 000

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Leak-before-break philosophy is applied to PT as a safety case. The qualification of the PT for LBB requires the value of “leakage size crack” (LSC). It is that length of the TWC, leakage through which needs to be detected by the sensors typically the Annular Gas Monitoring System. The TWC can get formed by DHC growth of an existing volumetric flaw, present on the inside surface of the PT. The statistics of such a flaw is generated in the present framework. While generating the statistics of the TWC thus obtained, several case studies are considered. The parameters used in each of this case study is listed in Table 3. In each of these cases, the root radius of the postulated flaw is kept as 30 microns and the considered is 50 ppm. The objective is to study the effect of mean of initial flaw length, a/t ratio and the on the TWC. All the random variables defined in Table 2 are used in Monte Carlo simulations to derive the values the crack length at the instance it becomes through the wall. The convergence is tested on the mean of the TWC.

Table 3: Case studies considered for deriving statistics of through wall crack and time to rupture Case 1 2 3 4 5 6 7 8 9 10 11 12 a/t % 4 4 4 4 4 4 10 10 10 10 10 10 2c (mm) 1.5 2.875 5 1.5 2.875 5 1.5 2.875 5 1.5 2.875 5 Residual Stress (MPa) 0 0 0 100 100 100 0 0 0 100 100 100

Fig. 16 and Fig. 17 shows the relative frequency histogram for the TWC obtained for Case 1 and Case 7. The histogram has a long tail on the higher side. Statistical tests show that the lognormal distribution is able to represent this data successfully. The 5, 50 and 95 percentile values of the size of the TWC for all cases is shown in Fig. 18. The CSA standard (2016) prescribes a value of 20 mm of the TWC for use as LSC, for demonstration of LBB behaviour. The observed 95 percentile values in all cases is near to 20 mm. The use higher values of LSC is conservative from the structural integrity consideration but is un-conservative in respect to the leakage detection capability.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

Relative Frequency

3-5 5-7 7-9 9-11 11-13 13-15 15-17 17-19 19-21 21-23 23-25

TWC length in mm

Fig. 16. Relative frequency histogram and statistics of the leakage size crack, Case 1