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en: <br />SEWER RATE s rUDY j City of Santa Ana, CA <br />Probability of Failure Analysis (PoF) <br />The Probability of Failure (PoF) criteria (factors) are used to assess the likelihood that a particular asset <br />will fail. This is done by considering the physical properties of the asset itself as well as its surrounding <br />environment and /or operational history. <br />Three criteria were used for evaluating the likelihood of failure of the City's sewer main segments. These <br />criteria include Cohort Rank, Raw QSR Grade, and Overall QSR Segment Score. Each factor was scored <br />on a scale of 0 or 1 to 10 where a 10 is most likely to fail. <br />Cohort Rank <br />Some materials have been shown to be more prone to failure than others. Likewise, the older the asset <br />is, typically the more likely it is to fail. Often it is the combination of these considerations that impacts <br />the asset's life expectancy. By combining pipe material and vintage (installation year), groups of similarly <br />performing pipes can be identified. These are considered pipe "cohorts." <br />Figure B 19 looks at the percentage of pipe having Raw QSR Grades 3 or above shown by pipe <br />installation year groups. This helps to normalize the data to gain a better understanding of what vintage <br />of pipe is more structurally problematic. An average line has been included here to provide a reference. <br />Despite a few years of anomalies (minimal pipe installed), this graph suggests that there is a clear <br />demarcation point in approximately 1946. Pipelines installed prior to 1946 were found to have more <br />significant structural defects than average, while pipelines installed after 1946 were found to have less <br />significant defects than average. <br />Figure B -19: Percent of Mains with High Raw QSR Grades by Pipe Install Year <br />1111111 <br />Average <br />City staff indicated they often observe that the older VCP pipe is thinner walled and more brittle. <br />Actually wall thickness commonly varies by manufacturer. Getting specifications for older pipe is difficult <br />due to the fact that some manufacturers are no longer in business and several different classes or <br />65B -249 <br />NOVEMBER 2014 <br />