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Probabilistic FFP approach of a heat exchanger
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ECN-C--96-005 ECN rapport
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A probabilistic model for a DSM (Dutch chemical industry) heat exchangerE102 was developed and probability-based FFP (Fitness for Purpose) analyses were compared with conventional deterministic FFP analyses based on PD 6493 (1991): Level 2. The probability of vessel failure at the mean working pressure was calculated and uncertainty and sensitivity analyses were carried out. Also the median failure load was predicted. A procedure to calibrate the safety factors of a PD 6493 (1991) analysis was outlined and a strategy to implement probabilistic UP models in the general FFP model was briefly discussed. A great advantage of the probabilistic model was the power to explore the uncertainty in the failure load. The model appeared to be very helpful to find the type of distribution for the predicted failure load and the cause of the uncertainty in the predicted failure load. The case study showed that at the moment the use of probabilistic analysis models is attractive and powerful to support the current deterministic defect acceptance rules. It is possible to compute the probability of failure at the maximum allowable load level according to a deterministic FFP analysis. First and Second Order Reliability Methods (FORM/SORM) and the so-called hybrid method, which is a combination of FORM and numerical integration are expected to be able to predict the probability of failure rapidly and efficiently on the present generation of PCs for most conditions. The use of these methods enables the performance of sensitivity and uncertainty analysis quite easily. Fracture of any welded steel structure with defects that break the surface or are embedded can be analysed with a general probabilistic model based on a suitable failure assessment code. 18 figs., 20 tabs., 23 refs., 1 appendix

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