Probability Failure Analysis Due To Internal Corrosion In Cast Iron Pipes

Abstract

Water distribution systems must be efficiently maintained, rehabilitated, and replaced to guarantee customers a sufficient supply of high quality and affordable water. Assessment of the system reliability and time to failure is required to support long-term planning of the renewal of pipes in water distribution systems. Internal and external corrosion in aging cast iron pipes can lead to mechanical failure in terms of water leakage and loss of hydraulic capacity due to buildup of corrosion products. While external corrosion has been shown to significantly affect the likelihood of mechanical failure, the risk of failure may be further heightened if internal corrosion is occurring. Chlorine consumption due to internal corrosion in aging cast iron pipes is a major factor in such systems, and has been used as an indicator of the rate of internal corrosion. This paper develops a methodology for estimating the probability of mechanical failure and the Factor of Safety (FOS) of cast iron pipes due to internal corrosion using a probabilistic analysis that incorporates the relationship between chlorine consumption and the rate of internal corrosion in a pipe. In addition to the length and diameter of the pipe, the relationship between chlorine consumption and the rate of internal corrosion is a function of the fraction of chlorine consumed within the pipe, the chlorine decay rate, and the velocity of the water and these factors may be considered uncertain at any given time. In this work, the probability of failure and FOS of pipes over a range of exposure times is estimated using the first order reliability method (FORM) to account for these uncertainties. Here mechanical failure is defined as the point at which the corrosion depth is more than the maximum acceptable decrease in pipe wall thickness set by the utility manager. The likelihood of failure increases to nearly 50% by the 100 th year of pipe age. The results indicate that the affect of internal corrosion on mechanical failure is lower than the effect of external corrosion, where the likelihood of failure is estimated at 50% in the 70 th year of pipe age. A sensitivity analysis is carried out to determine the sensitivity of the estimates of the failure probability to each of the uncertain factors. This paper was presented at the 8th Annual Water Distribution Systems Analysis Symposium which was held with the generous support of Awwa Research Foundation (AwwaRF).