A methodology to estimate remaining service life of grey cast iron water mains

Abstract

The decision to repair, renew, or replace existing old grey cast iron mains is typically based on performance indicators such as structural integrity, hydraulic efficiency, system reliability, and water quality. Structural integrity (often quantified as the number of main breaks per kilometre or mile per year) is the most common performance indicator. However, these indicators represent past performance, rather than expected future performance. Decisions based on performance indicators may not, therefore, accurately meet the real needs of the utility owner of the water distribution system. A preferred approach to make decisions on pipe repair and replacement is to determine the expected remaining service (residual) life of each pipe segment and ensure that the necessary work is performed before failure occurs. Past efforts to estimate remaining service life of water mains have been based on corrosion pit depth and estimated corrosion rate with no regard to the influence of corrosion on the structural resistance capacity of water mains. This paper describes a methodology to estimate the remaining service life of grey cast iron mains that takes corrosion pit induced changes in the structural resistance capacity into account. The methodology combines the residual resistance capacity of grey cast iron mains, anticipated corrosion rates, and the measurement of corrosion pits by direct inspection or non-destructive evaluation technology to predict when the factor of safety of an individual pipe segment will fall below a minimum acceptable value set by the utility owner, i.e., remaining service life. The estimate of remaining service life may then be used to schedule appropriate maintenance or replacement of grey cast iron mains.Key words: water mains, remaining service life, residual life, repair, renew, or rehabilitate water mains, corrosion models, pit and spun grey cast iron.