Ductile Iron Pipe (DIP)

Available in sizes 100–1350 mm (4–54 in.), DIP is widely used throughout the United States in water distribution systems. On the East Coast and in the Midwest, DIP is commonly used for both smaller distribution mains and larger transmission mains. On the West Coast, DIP is generally used for distribution pipelines 40 mm (16 in) and smaller, with alternative pipeline materials often selected for larger pipelines due to cost. Detailed descriptions of DIP, fittings, joints, installation, thrast restraint, and other factors related to design, as well as several important ANSI/AWWA specifications, are contained in the Ductile Iron Pipe Research Association (DIPRA) handbook (DIPRA, 1984).

3.4.1.1 Materials.

DIP is a cast-iron product. Cast-iron pipe is manufactured of an iron alloy centrifugally cast in sand or metal molds. Prior to the early 1970s, most cast-iron pipe and fittings were gray iron, a brittle material that is weak in tension. But now all cast-iron pipe, except soil pipe (which is used for nonpressure plumbing applications) is made of ductile iron. Ductile iron is produced by the addition of magnesium to molten low-sulfur base iron, causing the free graphite to form into spheroids and making it about as strong as steel. Regular DIP (AWWA C151) has a Brinell hardness (BNH) of about 165.

Tolerances, strength, coatings and linings, and resistance to burial loads are given in ANSI/AWWA C151/A21.51.

3.4.1.2 Available sizes and thicknesses. DIP is available in sizes from 100 to 1350 mm (4–

54 in). The standard length is 5.5 m (18 ft) in pressure ratings from 1380 to 2400 kPa (200–

350 lb/in²).

Thickness is normally specified by class, which varies from Class 50 to Class 56 (see DIPRA, 1984 or ANSI/AWWA C150/A21.50). Thicker pipe can be obtained by special order.

3.4.1.3 Joints. For DIP, rubber gasket push-on and mechanical are the most commonly used for buried service. These joints allow for some pipe deflection (about 2–5° depending on pipe size) without sacrificing water tightness. Neither of these joints is capable of resisting thrust across the joint and requires thrust blocks or some other sort of thrast restraint at bends and other changes in the flow direction.

Flanged joints (AWWA C115 or ANSI B16.1) are sometimes used at fitting and valve connections. Grooved end joints (AWWA C606) are normally used for exposed service and are seldom used for buried service. Flanged joints are rigid and grooved end joints are flexible. Both are restrained joints and do not typically require thrast restraint. Other types of restrained joints, such as restrained mechanical joints, are also available for buried service.

Various types of ductile iron pipe joints are shown in Fig. 3.1.

3.4.1.4 Gaskets. Gaskets for ductile iron push-on and mechanical joints, described in AWWA C111, are vulcanized natural or vulcanized synthetic rubber. Natural rabber is suitable for water pipelines but deteriorates when exposed to raw or recycled wastewater.

Gaskets for DIP flanges should be rubber, 3.2 mm (1/8 in) thick.

Gaskets for grooved end joints are available in ethylene propylene diene monomer (EPDM), nitrile (Buna-N), halogenated butyl rubber, Neoprene™, silicone, and fluorelastomers. EPDM is commonly used in water service and Buna-N in recycled wastewater.

3.4.1.5 Fittings. Some standard ductile or gray iron fittings are shown in Fig. 3.2. A list of standard and special fittings is also given in Table 3.8. Ductile iron fittings are normally

available only in standard configurations as described in AWWA C110. Greater cost and longer delivery times can be expected for special fittings. Fittings are designated by the size of the openings, followed (where necessary) by the deflection angle. A 90° elbow for 250 mm (10 in) pipe would be called a 250 mm (10 in) 90° bend (or elbow). Reducers, reducing tees, or reducing crosses are identified by giving the pipe diameter of the largest opening first, followed by the sizes of other openings in sequence. Thus, a reducing tee on a 300 mm (12 in) line for a 150 mm (6 in) fire hydrant run might be designated as a 300 mm×150 mm×300 mm (12 in×6 in×12 in) tee.

FIGURE 3.1 Couplings and joints for ductile iron pipe: (a) flexible coupling;

(b) mechanical joint; (c) push-on joint; (d) ball joint. Adapted from Sanks et al. (1989).

FIGURE 3.2 Ductile iron flanged fittings. Adapted from Sanks et al. (1989).

Standard ductile iron fittings are commonly available in flanged, mechanical joint, and push-on ends. It is considered good practice to include sufficient detail in construction plans and specifications to illustrate the type of joints that are expected at connections. The failure to detail a restrained joint when one is required by the design could result in an unstable installation.

3.4.1.6 Linings. Considering its low cost, long life, and sustained smoothness, cementmortar lining for DIP in water distribution systems is the most useful and common.

Standard thicknesses for shop linings specified in AWWA C104 are given in Table 3.9.

TABLE 3.8 Ductile Iron and Gray Cast-Iron Fittings, Flanged, Mechanical Joint, or Bell and Spigot*

*Size from 100 to 350 mm (4–54 in).

TABLE 3.9 Thickness of Shop-Applied Cement-Mortar Linings

*Single thickness per AWWA C104. Linings of double thickness are also readily available.

†Per AWWA C205.

Pipe can also be lined in place with the thicknesses given in Table 3.10. Because the standard, shop-applied mortar linings are relatively thin, some designers prefer to specify shop linings in double thickness. The designer should also be careful in specifying mortar lining thickness to match the pipe inside diameter (ID) with system valve IDs, particularly with short-body butterfly valves where the valve vane protrudes into the pipe. If the pipe ID is too small, the valve cannot be fully opened.

Although cement-mortar lining is normally very durable. it can be slowly attacked by very sott waters with low total dissolved solids content (less than 40 mg/L), by high-sulfate waters, or by waters undersaturated in calcium carbonate. For such uses, the designer should carefully investigate the probable durability of cement mortar and consider the use of other linings. Other linings and uses are shown in Table 3.11. In general, the cost of cement mortar is about 20 percent of that of other linings, so other linings are not justified except where cement mortar would not provide satisfactory

3.4.1.7 Coatings. Although DIP is relatively resistant to corrosion, some soils (and peat, slag, cinders, muck, mine waste, or stray electric current) may attack the pipe. In these TABLE 3.10 Thickness of Cement-Mortar Linings of Pipe in Place per AWWA C602

TABLE 3.11 Linings for Ductile Iron and Steel Pipe

applications, ductile iron manufacturers recommend that the pipe be encased in loose-fitting, flexible polyethylene tubes 0.2 mm (0.008 in) thick (see ANSI/AWWA C105/

A21.5). These are commonly known as “baggies.” An asphaltic coating approximately 0.25 min (0.001 in) thick is a common coating for ductile iron pipe in noncorrosive soils. In some especially corrosive applications, a coating, such as adhesive, hot-applied extruded polyethylene wrap, may be required.

In corrosive soils, the following coatings may be appropriate for protecting the pipe:

• Adhesive, extruded polyethylene wrap

• Plastic wrapping (AWWA C105)

• Hot-applied coal-tar enamel (AWWA C203)

• Hot-applied coal-tar tape (AWWA C203)

• Hot-applied extraded polyethylene [ASTM D 1248 (material only)]

• Coal-tar epoxy (MIL-P-23236)

• Cold-applied tape (AWWA C209)

• Fusion-bonded epoxy (AWWA C213)

Each of the above coatings is discussed in detail in the referenced specifications. Each coating system has certain limited applications and should be used in accordance with the NACE standards or as recommended by a competent corrosion engineer.