Ductile iron pipe
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Ductile iron pipe is a pipe commonly used for potable water distribution and the pumping of sewage.[1] The predominant wall material is ductile iron, a spheroidized graphite cast iron, although an internal cement mortar lining usually serves to inhibit corrosion from the carried water and various types of external coating are used to inhibit corrosion from the environment. Ductile iron pipes are a direct development of earlier cast iron pipes which they have now completely superseded.[1] Ductile iron has proved to be a better pipe material, being stronger and more fracture resistant; however, like most ferrous materials, it is susceptible to corrosion, and retains some brittle characteristics.[1] Relatively recent developments such as polyethylene sleeving and/or zinc and polymer coatings and prior to that, cement mortar linings, have done much to mitigate corrosion concerns and a typical life expectancy is in excess of 75 years.
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, and consequently the internal diameter does vary, sometimes significantly, from its nominal size. Nominal pipe sizes vary from 3 inches up to 64 inches, in increments of at least 1 inch, in the USA.
Pipe dimensions are standardised to the mutually incompatible AWWA C151 (U.S. Customary Units) in the USA, EN 545/598 (metric) in Europe, and AS/NZS 2280 (metric) in Australia and New Zealand. Although both metric, whereas EN 545/598 defines the pipe internal diameter to closely match the nominal diameter, AS 2280 is a soft conversion from the original imperial dimensions and consequently there is typically a wide divergence between nominal and actual sizes.
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 due to dimensional variations in pipes sizes, and pressure requirements, but also due to independent standards development. In the U.S. flanges are 'threaded' onto the pipe. In the European market flanges are often welded on to the pipe. Flanges are available in a standard 125 lb. bolt pattern as well as a 250 lb. bolt pattern (steel bolt pattern). Both are usually rated at 250 PSI. A flanged joint is rigid and can bear both tension and compression as well as a limited degree of shear and bending. It is also dismantlable once constructed. Flanged joints cannot, however, be used for buried pipe due to the possibility of soil movement placing very large bending loads on the joint.
Current flange standards used in the water industry are ANSI B16.1 in the USA, 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. The most common are the Tyton joint, developed by U.S. Pipe, the Fastite, by the American Cast Iron Pipe Co., and the Rapid, by Saint-Gobain PAM, which is marketed outside the U.S. Restrained joint systems are available too. Each of the four U.S. manufacturers has their own proprietary restrained joint system that generally involves a "boltless system". Clow Water Systems has the Super-Lock joint, Pacific States Cast Iron Pipe Co. has the Thrust-Lock system, Griffin Pipe Products has the Snap-Lock joint, U.S. Pipe has the TR-Flex joint, and American Cast Iron Pipe has the Flex-Ring joint. Also available are locking gasket systems. Available for the standard 'push-joint' systems are the Sure Stop gasket by McWane, Field Lok by U.S. Pipe, and Fast Grip by American Cast Iron Pipe Co. These locking gasket systems work on the "Chinese Box" principle where you can push the pipe together, but will be unable to pull it apart (without using a special tool or blow torch on the gasket).
Manufacture
Ductile iron pipe is produced by pouring iron into a rapidly spinning water-cooled mold. Centrifugal force results in an even spread of iron around the circumference.
External Coatings
Unprotected ductile iron, similarly to cast iron, is intrinsically resistant to corrosion in most but not all soils. Nonetheless, due to common uncertainty in the agressiveness of soil conditions, and to increase the installed life time of buried pipe, ductile iron pipe is typically protected by one or more external coatings. In the U.S. and Australia, a loose polyethylene sleeving is recommended to beinstalled around the pipe following installation. In Europe, standards recommend a more sophisticated system of zinc coatings overlaid by fusion bonded epoxy (FBE). Only in highly aggressive soils, is an additional layer of polyethylene sleeving recommended.
Polyethylene Sleevings
Polyethylene sleeving was first developed by CIPRA (since 1979, DIPRA) in the U.S. in 1951 for use in highly corrosive soil in Birmingham, Alabama. It was employed more widely in the U.S. in the late 1950's and first employed in the U.K. in 1965 and Australia in the mid 1960's.
Polyethylene sleeving comprises a loose sleeve of polyethylene sheet that completely wraps the pipe, including the bells of any joints. The sleeve operates by a number of principles. It physically separates the pipe from soil particles. Further, by providing an impermeable barrier to ground water, the sleeve inhibits the diffusion of oxygen to the ductile iron surface and limits the availability of electrolytes that would accelerate corrosion. It also provides a homogeneous environment along the pipe surface so that corrosion occurs evenly over the pipe. Finally, the sleeve restricts the availability of nutrients required by sulfate-reducing bacteria reducing microbially-induced corrosion. Sleeving is not designed to be completely water-tight but rather to greatly inhibit the movement of water to and from the pipe surface[2]. Water present beneath the sleeve and in contact with the pipe surface is rapidly deoxygenated and depleted of nutrients and limited further corrosion occurs. An improperly installed sleeve that continues to allow the free flow of ground water is not effective in inhibiting corrosion.
Polyethylene sleeves are available in a number of materials. The most common contemporary compositions are linear low-density polyethylene film which requires an 8 mil or 200 µm thickness and high-density cross-laminated polyethylene film which requires only a 4 mil or 100 µm thickness. The latter may or may not be reinforced with a scrim layer.
Polyethylene sleeving does have limitations. In European practice, it's use in the absence of additional zinc and epoxy protective coatings is discouraged where natural soil resistivity is below 750 ohm/cm, where resistivity is below 1500 ohm/cm and the soil is frequently water logged, where there are additional artificial soil contaminants or where there are stray currents[2]. Due to the vulnerability of polyethylene to UV degradation, sleeving, or sleeved pipe should also not be stored in sunlight, although carbon pigments included in the sleeving can provide some limited protection.
Polyethylene sleeving is standardised according to ISO 8180 internationally, AWWA C105 in the U.S., BS 6076 in the U.K. and AS 3680 and AS 3681 in Australia.
Zinc Coatings
In Europe, ductile iron pipe is typically manufactured with a zinc coating overlaid by an either bituminous or polymer, normally epoxy, finishing layer. EN 545/598 mandates a minimum zinc content of 135 g/m2 (with local minima of 110 g/m2 at 99.99% purity), and a minimum average finishing layer thickness of 70 µm (with local minima of 50 µm) although some manufacturers, notably Saint-Gobain PAM considerably exceed these thicknesses.
No current AWWA standards are available for bonded coatings for ductile iron pipe, DIPRA does not endorse bonded coatings and AWWA M41 generally views them unfavourably, recommending they be used only in conjunction with cathodic protection [3].
Bituminous Coatings
As noted, zinc coatings are generally not employed in the U.S. and Australia. In order to protect ductile iron pipe prior to installation, pipe is instead supplied with a temporary bituminous coating. This coating is not intended to provide protection once the pipe is installed.
Producers
U.S.
In the United States ductile iron pipe is manufactured by McWane Inc.(consisting of four foundries - McWane Cast Iron Pipe Co., Clow Water Systems Company,Atlantic States Cast Iron Pipe Co. & Pacific States Cast Iron Pipe Co.), Griffin Pipe Products, U.S. Pipe & Foundry, and American Cast Iron Pipe Co. The primary headquarters for three of these four companies are based in Birmingham, AL.
Europe
Saint-Gobain PAM, a subsidiary of Saint-Gobain and the world's largest ductile iron pipe manufacturer, is predominant in Europe. Saint-Gobain PAM formed in 1970 following the merger of Saint-Gobain and the company Pont-à-Mousson (PAM). Saint-Gobain PAM's ductile iron pipe factory in the town of Pont-à-Mousson remains the world's largest.
Australia
In Australia, Tyco Flow Control Pacific, a subsidiary of Tyco International, is by a wide margin the largest Australian manufacturer of DICL, after having purchased Tubemakers Water and its single Yennora Manufacturing Facility in Sydney's west, from BHP in 1999.
Industry Associations
In the United States ductile iron pipe is often promoted to municipalities and consulting engineers by DIPRA, which is the Ductile Iron Pipe Research Association. Their focus is to promote the benefits of using ductile iron pipe on utility projects (water & sewer) over alternate products like PVC, PCCP, and HDPE.
Environmental
Ductile iron pipe in the developed world is normally manufactured exclusively from scrap steel.
Colloquialisms
As a commonly used construction material ductile iron pipe has assumed various colloquial shortened names. In America it is commonly referred to as 'ductile', in the UK, by the initials, 'DI', and in Australia as the acronym, DICL (Ductile Iron - Cement Lined), pronounced 'dickle'.