Ductile iron pipe

Ductile iron pipe is a pipe commonly used for potable water distribution.[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 fluid being distributed, and various types of external coating are used to inhibit corrosion from the environment. Ductile iron pipe is a direct development of earlier cast iron pipe which it has superseded.[1] Ductile iron has proven to be a better pipe material, being stronger and more fracture resistant; however, like most ferrous materials, it is susceptible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 have done much to mitigate corrosion concerns, and a typical life expectancy is in excess of 75 years.

Contents

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. 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, 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

Pipe dimensions according to the American AWWA C-151

Pipe Size Outside Diameter
[in (mm)]
3 3.96 (100.584)
4 4.80 (121.92)
6 6.90 (175.26)
8 9.05 (229.87)
10 11.10 (281.94)
12 13.20 (335.28)
Pipe Size Outside Diameter
[in (mm)]
14 15.30 (388.62)
16 17.40 (441.96)
18 19.50 (495.3)
20 21.60 (548.64)
24 25.80 (655.32)
30 32.00 (812.8)

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 56 (2.205) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
50 66 (2.598) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
60 77 (3.031) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
65 82 (3.228) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
80 98 (3.858) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
100 118 (4.646) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
125 144 (5.669) 4.8 (0.189) 6.0 (0.236) 6.0 (0.236)
150 170 (6.693) 5.0 (0.197) 6.0 (0.236) 6.5 (0.256)
200 222 (8.740) 8.4 (0.331) 6.3 (0.248) 7.0 (0.276)
250 274 (10.787) 5.8 (0.228) 6.8 (0.268) 7.5 (0.295)
300 326 (12.835) 6.2 (0.244) 7.2 (0.283) 8.0 (0.315)
350 378 (14.882) 7.0 (0.276) 7.7 (0.303) 8.5 (0.335)
400 429 (16.890) 7.8 (0.307) 8.1 (0.319) 9.0 (0.354)
450 480 (18.898) - 8.6 (0.339) 9.5 (0.374)
DN Outside Diameter
[mm (in)]
Wall thickness
[mm (in)]
Class 40 K9 K10
500 532 (20.945) - 9.0 (0.354) 10.0 (0.394)
600 635 (25.000) - 9.9 (0.390) 11.1 (0.437)
700 738 (29.055) - 10.9 (0.429) 12.0 (0.472)
800 842 (33.150) - 11.7 (0.461) 13.0 (0.512)
900 945 (37.205) - 12.9 (0.508) 14.1 (0.555)
1000 1,048 (41.260) - 13.5 (0.531) 15.0 (0.591)
1100 1,152 (45.354) - 14.4 (0.567) 16.0 (0.630)
1200 1,255 (49.409) - 15.3 (0.602) 17.0 (0.669)
1400 1,462 (57.559) - 17.1 (0.673) 19.0 (0.748)
1500 1,565 (61.614) - 18.0 (0.709) 20.0 (0.787)
1600 1,668 (65.669) - 18.9 (0.744) 51.0 (2.008)
1800 1,875 (73.819) - 20.7 (0.815) 23.0 (0.906)
2000 2,082 (81.969) - 22.5 (0.886) 25.0 (0.984)

Australia

Australian and New Zealand pipes are sized to an independent specification, AS/NZS[2] 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 some knowledge of the pipe standard.

Nominal Size (DN) Outside Diameter
[mm (in)]
Nominal Wall Thickness
[mm (in)]
Flange Class
PN 20 PN 35
100 122 (4.803) - 5.0 (0.197) 7.0
150 177 (6.969) - 5.0 (0.197) 8.0
200 232 (9.134) - 5.0 (0.197) 8.0
225 259 (10.197) 5.0 (0.197) 5.2 (0.205) 9.0
250 286 (11.260) 5.0 (0.197) 5.6 (0.220) 9.0
300 345 (13.583) 5.0 (0.197) 6.3 (0.248) 10.0
Nominal Size (DN) Outside Diameter
[mm (in)]
Nominal Wall Thickness
[mm (in)]
Flange Class
PN 20 PN 35
375 426 (16.772) 5.1 (0.201) 7.3 (0.287) 10.0
450 507 (19.961) 5.6 (0.220) 8.3 (0.327) 11.0
500 560 (22.047) 6.0 (0.236) 9.0 (0.354) 12.0
600 667 (26.260) 6.8 (0.268) 310.3 (0.406) 13.0
750 826 (32.520) 7.9 (0.311) 12.2 (0.480) 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 U.S. flanges are either threaded or welded onto the pipe. In the European market flanges are usually welded on to the pipe. In the U.S. 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 is not buried.

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. 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 gaskin ring was pushed into te 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.

Manufacture

Ductile iron pipe is produced by a technique known as centrifugal casting, originally developed for cast iron pipe in 1918. The molten ductile iron is poured into a rapidly spinning water-cooled mold. Centrifugal force results in an even spread of iron around the circumference.

Internal coatings

Ductile iron pipe is somewhat resistant to internal corrosion in potable water and less aggressive forms of sewage. However, even where pipe material loss and consequently pipe wall reduction is slow, the deposition of corrosion products on the internal pipe wall can dramatically reduce the effective internal diameter and effectively choke flow, increasing pumping costs and lowering system pressure, long before the pipe itself is at risk of failure. A variety of linings are available to reduce or eliminate corrosion, including cement mortar, polyurethane and polyethylene. Of these, cement mortar lining is by far the most common.

Cement mortar

The predominant form of lining for water applications is cement mortar centrifugally applied during manufacturing. The cement mortar comprises a mixture of cement and sand to a ratio of between 1:2 and 1:3.5. For potable water, portland cement is used; for sewage it is common to use sulfate resisting or high alumina cement.

Cement mortar linings have been found to dramatically reduce internal corrosion. A DIPRA survey has demonstrated that the Hazen-Williams factor of cement lining remains between 130 and 151 with only slight reduction with age.

External coatings

Unprotected ductile iron, similarly to cast iron, is intrinsically resistant to corrosion in most, although not all, soils. Nonetheless, because of frequent lack of information on soil aggressiveness and to extend the installed life of buried pipe, ductile iron pipe is commonly protected by one or more external coatings. In the U.S. and Australia, loose polyethylene sleeving is preferred. In Europe, standards recommend a more sophisticated system of directly bonded zinc coatings overlaid by a finishing layer be used in conjunction with polyethylene sleeving.

Polyethylene

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 1950s and first employed in the UK in 1965 and Australia in the mid-1960s.

Polyethylene sleeving comprises a loose sleeve of polyethylene sheet that completely wraps the pipe, including the bells of any joints. Sleeving inhibits corrosion by a number of mechanisms. It physically separates the pipe from soil particles, preventing direct galvanic corrosion. By providing an impermeable barrier to ground water, the sleeve also inhibits the diffusion of oxygen to the ductile iron surface and limits the availability of electrolytes that would accelerate corrosion. It provides a homogeneous environment along the pipe surface so that corrosion occurs evenly over the pipe. The sleeve also restricts the availability of nutrients which could support sulfate-reducing bacteria, inhibiting microbially-induced corrosion. Sleeving is not designed to be completely water-tight but rather to greatly restrict the movement of water to and from the pipe surface.[3] Water present beneath the sleeve and in contact with the pipe surface is rapidly deoxygenated and depleted of nutrients and forms a stable environment in which 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, its 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 where the pipe is installed at or below the water table, where there are additional artificial soil contaminants or where there are stray currents.[3] Because of the vulnerability of polyethylene to UV degradation, sleeving, or sleeved pipe should 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 UK and AS 3680 and AS 3681 in Australia.

Zinc

In Europe, ductile iron pipe is typically manufactured with a zinc coating overlaid by either a bituminous, polymer, or epoxy finishing layer. EN 545/598 mandates a minimum zinc content of 200 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).

No current AWWA standards are available for bonded coatings (zinc, coal tar epoxy, tape-wrap systems as seen on steel pipe) 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.[4]

Bituminous coatings

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 1 mil or 25 µm thick bituminous coating. This coating is not intended to provide protection once the pipe is installed.

Industry associations

In the United States ductile iron pipe is often promoted to municipalities and consulting engineers by the Ductile Iron Pipe Research Association.[5] Their focus is to promote the benefits of using ductile iron pipe on utility projects (water & sewer) over alternate products such as PVC, PCP, and HDPE.

Environmental

Ductile iron pipe in the developed world is normally manufactured exclusively from scrap steel. Ductile iron pipe can be recycled. In the U.S. with the growing environmental movement ductile iron pipe is in a natural position to regain market share lost to its largest competitor, the PVC industry, over the past 40 years.

Notes

  1. ^ a b c Moser, A. P. and Folkman, Steven L. (2008) Buried Pipe Design (3rd edition) McGraw-Hill, New York, p. 336-337, ISBN 978-0-07-147689-8
  2. ^ Standards New Zealand
  3. ^ a b IGN 4-50-03 - Operating Guidelines for the Use of Site-Applied, Factory Applied and Reinforced Factory Applied Polyethylene Sleeving on Ductile Iron Pipeline Systems [1]
  4. ^ AWWA Manual M41 - Ductile-Iron Pipe and Fittings
  5. ^ Iron Pipe Research Association

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