A key consideration in the design of a natural gas transmission pipeline is the location class of the pipeline. The location class is a geographic area along the pipeline classified primarily according to the number and proximity of residential buildings; in other words, the location class characterizes the population density along the pipeline. According to ASME B31.8 (ASME 2013), there are four location classes for gas pipelines, namely Class 1, Class 2, Class 3 and Class 4. The Class 1 represents sparsely populated areas such as wasteland, deserts and farmland; the Class 2 reflects fringe areas around cities and towns, industrial areas, ranch or country estates, etc.; the Class 3 reflects areas such as suburban housing developments, shopping centers, residential areas, etc., and the Class 4 represents city centers where multistory buildings (defined as having four or more floors above ground) are prevalent and traffic is heavy (ASME 2013).

DNV defines three safety classes, which are: 'low', 'normal' and 'high' for pipelines (Table below). For example, water injection lines may be classified as being a 'low' safety class while oil transportation lines may be considered as a 'high' safety class- for more detail see (Yasseri & Bahai, 2017)

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The question is whether there is a similar recommendation for a tolerable level of failure rates for water main pipelines in cities.

ASME 2013. ASME B31.8: Gas transmission and distribution piping systems. The American Society of Mechanical Engineers, Three Park Avenue, New York, NY

Sirous Yasseri1 and Hamid Bahai, Prognosis of Time to Failure of Corroding Pipelines IJCOE Vol.1/No. 2/Summer 2017 (11-20) available at: http://ijcoe.org/article-1-45-en.pdf

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