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Article
Publication date: 5 July 2013

Amarjit Singh and Stacy Adachi

The purpose of this paper is to analyze conditional failure rates, and prioritize water pipelines for replacement based on their expected failure rate where pipes are grouped…

439

Abstract

Purpose

The purpose of this paper is to analyze conditional failure rates, and prioritize water pipelines for replacement based on their expected failure rate where pipes are grouped based on age and pipe type. Thus, predictions can be made on the expected number of breaks in future years.

Design/methodology/approach

The time to failure of a pipe can be characterized by the stochastic properties of the population as a whole, from which the likelihood of component failure is derived. When the corresponding failure rate is plotted against time, a bathtub‐shaped curve results. The bathtub curve assists in determining maintenance schedules depending on the age of the pipe. Failure rates help determine whether the rates are more than an acceptable best practice threshold to signal replacement.

Findings

Ductile iron pipes had the highest failure rates, followed by asbestos cement pipes; PVC and concrete cylinder pipes had the lowest failure rates, but because concrete cylinder pipes are very time‐consuming to repair and very expensive to install, only PVC pipes are recommended on the basis of this study. Cast iron pipes fit the bathtub shape; ductile iron and asbestos concrete were somewhat bathtub shaped, though the early phase period was not apparent; the bathtub curve for concrete cylinder was fully inverted; while PVC pipes showed zero probability of failure during the middle period. The shapes of bathtub curves drawn on conditional failure rates were similar to those for the failure rates. The bathtub curves indicate that the general failure performance of pipe materials is somewhat contrary to general principles in manufacturing.

Practical implications

Analysis of failure serves a practical purpose for water utilities to allocate funds for pipe maintenance and prepare a schedule for pipe replacement, so as to provide the best quality services and safe drinking water to users of the utility.

Social implications

The proper prioritization of water supply pipes for repair and replacement is of great social importance to the public at large, which expends considerable funds to maintain their drinking water supply.

Originality/value

The study of bathtub curves has not been seen before in the analysis of water supply pipes. A unique discovery is that the traditional shape of the bathtub curve is not always applicable for water supply pipes.

Details

Built Environment Project and Asset Management, vol. 3 no. 1
Type: Research Article
ISSN: 2044-124X

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Article
Publication date: 8 July 2011

Amarjit Singh, Stacy Adachi and Megan Inouye

The purpose of this paper is to investigate performance times from a quality engineering perspective for response and repair of pipes at a public utility. The objective is to…

1902

Abstract

Purpose

The purpose of this paper is to investigate performance times from a quality engineering perspective for response and repair of pipes at a public utility. The objective is to scientifically determine the pipe that offers the most desirable downtime (DT) and time to repair (TTR).

Design/methodology/approach

Four types of water supply pipes – concrete cylinder (CC), cast iron (CI), ductile iron (DI), and polyvinyl chloride (PVC) – in prevalent use at the City and County of Honolulu Board of Water Supply were analyzed to determine the pipe type that is most consistently repaired to desired performance specifications. Data for mean downtime (MDT) and mean time to repair (MTTR) were used to evaluate the stability and capability of the repair processes for each pipe type. The analysis was completed through the use of control charts, operating characteristic (OC) curves, and process capability indices.

Findings

The results of the analysis indicated that CI pipes were the worst material in terms of DT and TTR. The control charts for MDT for all pipe types, and the MTTR for CI and CC pipes, were found to be out of statistical control, but the control charts for the MTTR of DI and PVC pipes were discovered to be in control. According to the OC curves, in which the hypothesis stated that the average MDT or MTTR was between the specification limits, there was a high tendency in all pipe types to accept the hypothesis when it was true. However, the probability of type I errors was high from operational standards at the USL level. Process capability analyses found that only CC pipes were able to meet performance design specifications; however, repair times are extremely large for CC pipes. Overall, it is recommended that CI pipes be replaced when the opportunity arises.

Practical implications

This investigation serves to address a major query in asset management at the public utility, that of which pipes should be selected during design and procurement from a maintenance perspective. In addition, the study helps to understand the trend of DT and TTR for the various pipes.

Social implications

Quality water supply is of paramount social importance in modern cities.

Originality/value

A quality engineering approach to asset management for pipe systems at public utilities that serves to add a new dimension to asset performance analysis is adopted.

Details

Built Environment Project and Asset Management, vol. 1 no. 1
Type: Research Article
ISSN: 2044-124X

Keywords

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