Ductile iron pipe is pipe made of ductile cast iron commonly used for potable water transmission and distribution.Moser, A. P. and Folkman, Steven L. (2008) Buried Pipe Design (3rd edition) McGraw-Hill, New York, p. 336-337, This type of pipe is a direct development of earlier cast iron pipe, which it has superseded.
Description 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.Public Works April 15, 1995 Ductile iron mains; Water Supply and Treatment SECTION: Pg. pC34(4) Vol. V126 No. N5 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.
Dimensions 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.
North America In the US, nominal pipe sizes vary from 3 inches up to 64 inches, in increments of at least 1 inch, and are standardised according to the American AWWA C-151
Pipe Size Outside Diameter [in (mm)] 3 4 6 8 10 12 14 16 18 20 24 30
Europe
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.
DN Outside Diameter [mm (in)] Wall thickness [mm (in)] Class 40 K9 K10 40 50 60 65 80 100 125 150 200 250 300 350 400 450 – 500 – 600 – 700 – 800 – 900 – 1000 – 1100 – 1200 – 1400 – 1500 – 1600 – 1800 – 2000 –
Other European Standards give specifications on more dedicated products:
EN 15655:2009 – Ductile iron pipes, fittings and accessories – Internal polyurethane lining for pipes and fittings – Requirements and test methods
EN 877:1999/A1:2006 – Cast iron pipes and fittings, their joints and accessories for the evacuation of water from buildings – Requirements, test methods and quality assurance
CEN/TR 15545:2006 – Guide to the use of EN 545
CEN/TR 16017:2010 – Guide to the use of EN 598
EN 877:1999 – Cast iron pipes and fittings, their joints and accessories for the evacuation of water from buildings – Requirements, test methods and quality assurance
EN 877:1999/A1:2006/AC:2008 – Cast iron pipes and fittings, their joints and accessories for the evacuation of water from buildings – Requirements, test methods and quality assurance
EN 598:2007+A1:2009 – Ductile iron pipes, fittings, accessories and their joints for sewerage applications – Requirements and test methods
EN 12842:2012 – Ductile iron fittings for PVC-U or PE piping systems – Requirements and test methods
CEN/TR 16470:2013 – Environmental aspects of ductile iron pipe systems for water and sewerage applications
EN 14628:2005 – Ductile iron pipes, fittings and accessories – External polyethylene coating for pipes – Requirements and test methods
EN 15189:2006 – Ductile iron pipes, fittings and accessories – External polyurethane coating for pipes – Requirements and test methods
EN 14901:2014 – Ductile iron pipes, fittings and accessories – Epoxy coating (heavy duty) of ductile iron fittings and accessories – Requirements and test methods
EN 969:2009 – Ductile iron pipes, fittings, accessories and their joints for gas pipelines – Requirements and test methods
EN 15542:2008 – Ductile iron pipes, fittings and accessories – External cement mortar coating for pipes – Requirements and test methods
EN 545:2010 – Ductile iron pipes, fittings, accessories and their joints for water pipelines – Requirements and test methods
EN 14525:2004 – Ductile iron wide tolerance couplings and flange adaptors for use with pipes of different materials: ductile iron, Grey iron, Steel, PVC-U PE, Fibre-cement
Australia and New Zealand Australian and New Zealand pipes are sized to an independent specification, AS/NZS 2280, that is not compatible with European pipes even though the same nomenclature is used.
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.
Nominal Size (DN) Outside Diameter [mm (in)]Nominal Wall Thickness [mm (in)] Flange Class PN 20 PN 35 100 – 7.0 150 – 8.0 200 – 8.0 225 9.0 250 9.0 300 10.0 375 10.0 450 11.0 500 12.0 600 13.0 750 15.0
Joints
Individual lengths of ductile iron pipe are joined either by flanges, couplings, or some form of spigot and socket arrangement.
Flanges 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 . 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.
Current flange standards used in the water industry are ANSI B16.1 in the US, EN 1092 in Europe, and AS/NZS 4087 in Australia and New Zealand.
Spigot and socket 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.
Lifespan and corrosion 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.Moser, A. P. and Folkman, Steven L. (2008) Buried Pipe Design (3rd edition) McGraw-Hill, New York, p. 336-337, 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.
Over the last 100 years, the average thickness of iron pipes has decreased due to increased metal strength, through metallurgical advancements as well as improved casting technique.
Methods for mitigating corrosion 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.