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Cast Iron Pipe

Cast iron pipes can fail in many modes which in general can be summarized into two categories: loss of strength due to the reduction of wall thickness of the pipes, and loss of toughness due to the stress concentration at the tips of cracks or defects. Even in one category there can be many mechanisms that cause failure. The strength failure can be caused by hoop stress or axial stress in the pipes. A review of recent research literature (Sadiq et al., 2004; Moglia et al., 2008; Yamini, 2009; Clair and Sinha, 2012) suggests that current research on pipe failures focuses more on loss of strength than loss of toughness. As was mentioned in Section 3.3.7(b), the literature review also revealed that in most reliability analyses for buried pipes, multifailure modes are rarely considered although in practice this is the reality. Therefore the aim of this section is to consider multifailure modes in reliability analysis and service life prediction for ductile iron pipe. Both loss of strength and toughness of the pipe are considered. A system reliability method is employed in calculating the probability of pipe failure over time, based on which the service life of the pipe can be estimated. Sensitivity analysis is also carried out to identify those factors that affect the pipe behavior most.

Buried pipes are not only subjected to mechanical actions (loads) but also environmental actions that cause the corrosion of pipes. Corrosion related defects would subsequently cause fracture of cast iron pipes. In the presence of corrosion pit, failure of a pipe can be attributed to two mechanisms: (i) the stresses in the pipe exceed the corresponding strength; or (ii) the stress intensity exceeds fracture toughness of the pipe. Based on these two failure modes, two limit state functions can be established as follows.

Steel pipe is manufactured by the pit, horizontal or centrifugal method. In the vertical pit method, a mold is made by ramming sand around a pattern and drying the mold in an oven. A core is inserted in the mold and molten iron is poured between the core and the mold. In the horizontal method, a machine is used to ram sand around horizontal molds that have core bars running through them. The molten iron is poured into the molds from multiple-lipped ladle designed to draw the iron from the bottom to eliminate the introduction of impurities. In the centrifugal method (Figure 3.4), sand-lined molds are used that are placed horizontally in centrifugal casting machines. While the mold revolves, an exact quantity of molten iron is introduced, which, by action of the speed of rotation, distributes itself on the walls of the mold to produce pipe within a few seconds.

Many cast iron pipes made towards the end of the nineteenth century are still in use; their walls were relatively thick and not always of uniform, ‘Spun’ grey iron pipes were formed by spinning in a mould and produced a denser iron with pipes of more uniform wall thickness; they comprise a large proportion of the distribution mains in many countries. Three classes of such pipes were available: B, C, and D for working pressures of 60, 90, and 120 m respectively; classes B and C were more widespread. Carbon is present in the iron matrix substantially in lamellar or flaky form; therefore, the pipes are brittle and relatively weak in tension and liable to fracture. The manufacture of grey iron pipes has been discontinued in most countries, except for the production of non-pressure drainage pipes.

Since cast iron pipes are deteriorating rapidly and causing so many maintenance problems (Section 4.3.2), the distribution network is currently undergoing an extensive replacement scheme with old, leaking and corroded cast iron pipes being replaced by MDPE and uPVC. These new plastic pipe materials are thought to support fewer bacteria than the old hubless cast iron pipe. Their surface is smoother and therefore the surface area smaller and they are not subject to corrosion or biodeterioration.

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