Investigation of probability characteristic about thinning speed in actual plants and application for reliability analysis

Kurihara, Tomoyuki*; Kaida, Takuyo*; Okajima, Satoshi 

In Fitness-For-Service (FFS) assessment of pressure equipment, high reliability is needed in petroleum and petrochemical plant. Future Corrosion Allowance (FCA) is one of the most important factors in the metal loss assessment of FFS because it decides predicted value of the thickness during service periods. For the evaluation of FCA, the accurate corrosion rate from actual thinning data is needed and it is necessary to make a consideration of their variability of the thinning phenomena. In this study, the measured thickness data of the oil piping in the actual plant are collected and statistically analyzed as corrosion rate data which are classified by the operation condition. By the analysis, the probabilistic properties of corrosion rate are clarified in this piping system. Especially it is cleared that the corrosion rate of the air injection part is higher than that of the no air injection part. And the reliabilities of the protection of the failure by pressure and their change are quantitatively evaluated from the probabilistic properties of corrosion rate. From this case study, the procedure of FCA evaluation of FFS assessment was established and it's clear that the statistical analysis by the feature of thinning phenomena is important for accurate evaluation of corrosion data.