The real life expectancy of cast iron pipe is unknown, but is usually estimated at 100 years or more. The oldest operating cast iron main is that at Versailles, France, installed in 1664. In this country and in Canada there are more than 500 members of the Cast Iron Pipe Century Club, a unique organization composed of cities or utilities that have cast iron pipe still in service after 100 years.
Ductile iron pipe, a product of advanced metallurgy, offers unique properties for conveying water under pressure, and other piping uses. It combines the physical strength of mild steel with the long life of gray cast iron.
Ductile iron offers the greatest possible margin of safety against service failures due to ground movement and beam stresses. Virtually unbreakable in ordinary service, it also provides increased resistance to breakage caused by rough handling in shipping and installation.
Ductile iron is produced by adding a closely controlled amount of magnesium alloy to a molten iron of low phosphorous and low sulfur content. The magnesium alloy addition produces a remarkable change in the microstructure by causing the carbon in the iron to assume a spheroidal or nodular shape, (as contrasted to the flake form of graphite in gray cast iron), and at the same time producing a finer grained iron matrix in the surrounding ferrite structure. As a result of this remarkable change, a far stronger, tougher, and ductile material is obtained.
In addition to the benefits of long life, corrosion resistance, high structural strength, and tight joints, ductile iron is also readily machinable, an important requirement in any pipe that must be drilled, tapped or cut.
The McWane Companies manufacture ductile iron pipe in strict accordance with design criteria that have been developed by the American National Standards Institute, and which equals or exceeds the requirements of all published standards of the American Water Works Association.
Ductile iron pipe is pipe made of ductile cast iron commonly used for potable water transmission and distribution. This type of pipe is a direct development of earlier cast iron pipe, which it has superseded.
The ductile iron used to manufacture the pipe is characterized by the spheroidal or nodular nature of the graphite within the iron. Typically, the pipe is manufactured using centrifugal casting in metal or resin lined moulds. Protective internal linings and external coatings are often applied to ductile iron pipes to inhibit corrosion: the standard internal lining is cement mortar and standard external coatings include bonded zinc, asphalt or water-based paint. In highly corrosive environments loose polyethylene sleeving (LPS) to encase the pipe may also be used.
Life expectancy of unprotected ductile iron pipes depends on the corrosiveness of soil present and tends to be shorter where soil is highly corrosive. However, a lifespan in excess of 100 years has been estimated for ductile iron pipelines installed using "evolved laying practices", including use of properly installed LPS (polyethylene encasement). Studies of ductile iron pipe's environmental impact have differing findings regarding emissions and energy consumed. Ductile iron pipe manufactured in the US has been certified as a sustainable product by the Institute for Market Transformation to Sustainability.
Ductile iron pipe is sized according to a dimensionless term known as the Pipe Size or Nominal Diameter (known by its French abbreviation, DN). This is roughly equivalent to the pipe's internal diameter in inches or millimeters. However, it is the external diameter of the pipe that is kept constant between changes in wall thickness, in order to maintain compatibility in joints and fittings. Consequently, the internal diameter varies, sometimes significantly, from its nominal size.
Pipe dimensions are standardised to the mutually incompatible AWWA C151 (US Customary Units) in the United States, ISO 2531 / EN 545/598 (metric) in Europe, and AS/NZS 2280 (metric) in Australia and New Zealand. Although both metric, European and Australian are not compatible and pipes of identical nominal diameters have quite different dimensions.
European pipe is standardized to ISO 2531 and its descendent specifications EN 545 (potable water) and EN 598 (sewage). European pipes are sized to approximately match the internal diameter of the pipe, following internal lining, to the nominal diameter. ISO 2531 maintains dimensional compatibility with older German cast iron pipes. Older British pipes, however, which used the incompatible imperial standard, BS 78, require adapter pieces when connecting to newly installed pipe. Coincidentally, the British harmonization with European pipe standards occurred at approximately the same time as its transition to ductile iron, so almost all cast iron pipe is imperial and all ductile pipe is metric.
Australia adopted at an early point the imperial British cast iron pipe standard BS 78, and when this was retired on British adoption of ISO 2531, rather than similarly harmonizing with Europe, Australia opted for a "soft" conversion from imperial units to metric, published as AS/NSZ 2280, with the physical outer diameters remaining unchanged, allowing continuity of manufacture and backwards compatibility. Therefore, the inner diameters of lined pipe differ widely from the nominal diameter, and hydraulic calculations require specific knowledge of the pipe standard.
Flanges are flat rings around the end of pipes which mate with an equivalent flange from another pipe, the two being held together by bolts usually passed through holes drilled through the flanges. A deformable gasket, usually elastomeric, placed between raised faces on the mating flanges provides the seal. Flanges are designed to a large number of specifications that differ because of dimensional variations in pipes sizes and pressure requirements, and because of independent standards development. In the US, flanges are either threaded or welded onto the pipe. In the European market flanges are usually welded onto the pipe. In the US, flanges are available in a standard 125 lb bolt pattern as well as a 250 lb (and heavier) bolt pattern (steel bolt pattern). Both are usually rated at 250 psi (1,700 kPa). A flanged joint is rigid and can bear both tension and compression as well as a limited degree of shear and bending. It also can be dismantled after assembly. Due to the rigid nature of the joint and the risk of excessive bending moment being imposed, it is advised that flanged pipework not be buried.
Spigot and sockets involve a normal pipe end, the spigot, being inserted into the socket or bell of another pipe or fitting with a seal being made between the two within the socket. Normal spigot and socket joints do not allow direct metal to metal contact with all forces being transmitted through the elastomeric seal. They can consequently flex and allow some degree of rotation, allowing pipes to shift and relieve stresses imposed by soil movement. The corollary is that unrestrained spigot and socket joints transmit essentially no compression or tension along the axis of the pipe and little shear. Any bends, tees or valves therefore require either a restrained joint or, more commonly, thrust blocks, which transmit the forces as compression into the surrounding soil.
A large number of different socket and seals exist. The most modern is the "push-joint" or "slip-joint", whereby the socket and rubber seal is designed to allow the pipe spigot to be, after lubrication, simply pushed into the socket. Push joints remain proprietary designs. Also available are locking gasket systems. These locking gasket systems allow the pipe to be pushed together but do not allow the joint to come apart without using a special tool or torch on the gasket.
The earliest spigot and socket cast iron pipes were jointed by filling the socket with a mixture of water, sand, iron filings and sal-ammoniac (ammonium chloride.) A gasket ring was pushed into the socket round the spigot to contain the mixture which was pounded into the socket with a caulking tool and then pointed off. This took several weeks to set and produced a completely rigid joint. Such pipe systems are often to be seen in nineteenth century churches in the heating system.
In the late 1950s, ductile iron pipe was introduced to the marketplace, featuring higher strength and similar corrosion resistance compared to cast iron. According to a 2004 study, an expected lifespan of 100 years is likely for ductile iron pipe, based on test results, field inspections and in-service operations over 50 years. In 2012, the American Water Works Association reported that ductile iron pipes in benign soil or installed in more aggressive soils using "evolved laying practices" had an estimated life up to 110 years, based on a nationwide analysis of water pipes in the US.
Like most ferrous materials, ductile iron is susceptible to corrosion, therefore its useful life depends on the impact of corrosion. Corrosion can occur in two ways in ductile iron pipes: graphitization, the leaching away of iron content through corrosion leading to a generally weakened pipe structure, and corrosion pitting, which is a more localized effect also causing weakening of the pipe structure.
The potential for corrosion, leading to pipe failure, is significantly impacted by the corrosivity of soil. Unprotected pipes in highly corrosive soil tend to have shorter lifespans. The lifespan of ductile iron pipe installed in an aggressive environment without appropriate protection may be between 21 and 40 years. The introduction of corrosion mitigation methods for ductile pipe, including the use of polyethylene sleeving, can reduce corrosion by controlling the effect of corrosive soil on piping.
In the United States, the American National Standards Institute and American Water Works Association have standardized the use of polyethylene sleeving to protect ductile iron pipe from the effects of corrosion. A 2003 report by researchers from the National Research Council of Canada noted that "both good and poor performances" of polyethylene sleeving had been reported. However, a study in the Ductile Iron Pipe Research Association's Florida test site found that, compared with uncoated pipes exposed to a corrosive environment, pipes encased in loose polyethylene sleeving were "in excellent condition". 041b061a72