Probability of Failure Analysis due to Internal Corrosion in Cast-Iron Pipes

Abstract

In cast-iron pipes, chlorine consumption due to internal corrosion is high compared with consumption caused by other water-quality factors in such systems and may be considered as an approximate indicator of the rate of internal corrosion. Herein, a relationship is developed that determines the rate of internal corrosion as a function of the chlorine concentration, the chlorine decay constant, and the velocity of the water in a pipe segment with a given length and diameter. These factors may be considered uncertain at any given time and the probability of mechanical failure due to the thinness of the pipe wall caused by internal corrosion throughout the pipe life is estimated using Monte Carlo simulation, the first-order reliability method, and the second-order reliability method to account for these uncertainties. The results indicate that the likelihood of failure is nearly 50% by the 80th year for a cast-iron pipe length of 6 m and a diameter of 203 mm, and that the affect of internal corrosion on the pipe wall thickness and thus mechanical failure is lower than, but on the same order of magnitude as, the effect of external corrosion.