Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don't impose corrosion or safety concerns.

A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.

Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that can vary from a fraction of an ohm to several ohms but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints normally segment the pipe, restricting its electrically continuous length, and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare with only a standard 1-mil asphaltic coating and therefore are effectively grounded for their entire length, which further prevents magnetic induction on the pipeline.

During construction of Ductile Iron pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., "limit of approach" regulations governing construction equipment, grounding straps, chains attached to rubber-tired vehicles to provide a ground, grounding mats, etc., especially if safety concerns are heightened due to the use of joint bonding and dielectric coatings.

Repair is achieved by first cutting out the defective or damaged lining to the metal so that the edges of the lining not removed are reasonably perpendicular to the pipe wall or slightly undercut. A stiff mortar is then prepared, containing not less than one part of cement to two parts of sand, by volume. This mortar is applied to the cutout area and troweled smooth with adjoining lining. To provide for proper curing of patches by preventing too rapid of a moisture loss from the mortar, the patched area is normally seal-coated immediately after any surface water evaporates, or alternatively the area is kept moist (e.g. with wet rags or burlap over the area or with the ends of the pipe or fitting taped over with plastic film, etc.). Of course, in potable water-related applications, no patch or curing components should be used in the repair that would negatively affect health or water quality.

Yes, Ductile Iron products can be successfully Glass lined. Glass lined pipe and fittings have been specified and utilized as a deterrent to interior build-up and clogging of problematic sludge and scum piping systems in wastewater and sewage treatment facilities for over 40 years. Not only is the excellent non-stick characteristic effective in combating the build-up of grease, sludge, and scum, but has been found to be the only deterrent to Struvite and Vivionite build-up as well.

No. The asphaltic coating is applied to the outside of Ductile Iron pipe in accordance with ANSI/AWWA C151/A21.51 to minimize atmospheric oxidation for aesthetic reasons. If soils are determined to be corrosive when tested in accordance with Appendix A of ANSI/AWWA C105/A21.5, DIPRA and its member companies recommend that polyethylene encasement in accordance with the AWWA C105 standard be installed for corrosion protection.

The ANSI/AWWA C150/A21.50 procedure used for calculating truck loads on buried Ductile Iron pipe, which is based on the teachings of Spangler and others, employs the same methods used in ANSI A21.1, the older design standard for Cast Iron pipe. The approach for calculating truck loading is adequate at any depth of cover. However, depths of cover less than 2.5 feet are generally not recommended under roads and highways due to the possibility of high dynamic loading. When 2.5 feet or more of cover cannot be provided, the procedure in ANSI/AWWA C150/A21.50 can still be applied. However, if impact factors higher than 1.5, which is incorporated in the standard, are anticipated, then such impact factors should be employed. Further, in those shallow covers, maintenance of the road surface over the pipe may be more of a concern than serviceability of the pipe.

Although there are differing opinions on this subject, a conservative maximum velocity for design purposes is 7 fps (feet per second).

The AWWA (American Water Works Association) standard for thickness design of ductile iron pipe is C150. The exercise for calculating the required thickness based on internal pressure includes a 100 psi allowance for surge pressure and a 2:1 safety factor. The surge pressure allowance is based on a 50 psi pressure rise for each foot of extinguished velocity, and the fact that most domestic water systems operate at approximately 2 fps.

Ductile Iron pipe may be rated as high as 350 psi service. A pipeline operating at 7 fps velocity could account for a 350 psi pressure surge (7fps X 50 psi/fps). Adding a potential surge pressure equal to the pressure rating of the pipe encroaches significantly on the safety factor. Exceeding 7 fps velocity could produce potentially damaging surge pressure.

The "service allowance" used in the design of Ductile Iron pipe is a holdover from the old Gray Iron pipe days. During that early period, it was called a "corrosion allowance" to offset any initial corrosion or minor surface imperfections that might occur.

With the advent of Ductile Iron pipe and polyethylene encasement for corrosion control, the corrosion allowance was retained for similar general conservatism but renamed as a service allowance.

The addition of a 0.08-inch service allowance, which is unique to Ductile Iron pipe, ensures that the actual wall thickness will always exceed the design thickness, thereby providing an additional margin of safety and dependability.

The external trench load in ANSI/AWWA C150/A21.50 consists of earth load plus truck load. The earth load on pipe increases as the depth of cover increases; the truck load increases as the depth of cover decreases. Therefore, the maximum depth of cover normally is limited by the earth load and the minimum depth of cover is limited by the truck load. For lower pressure classes of pipe in sizes 14 inches and larger installed in a Type 1 trench, this band of allowable depth of cover is limited, or even non-existent. Also, for higher pressure classes of pipe in sizes 14 inches and greater, it would normally be more economical to specify a better trench and a lower pressure class of pipe than a higher pressure class of pipe and a Type 1 trench. Improved bedding is desirable, particularly in larger pipe sizes, to improve uniformity of axial support under the haunches.

Yes. Both ANSI/AWWA C150/A21.50 and ANSI/AWWA C151/A21.51 state that Ductile Iron pipe is available for water working pressure greater than 350 psi. These standards also list Pressure Class and Special Thickness Class Ductile Iron pipe. The Pressure Class designations (150 psi to 350 psi) in the standards are based on a 2.0 safety factor times the sum of working pressure and 100 psi allowance of surge. This establishes a net thickness to which a service allowance of 0.08-inch and a casting tolerance (which is dependent on the diameter of the pipe) is added. Based on the same design criteria, 6-inch Special Thickness Class 56 Ductile Iron pipe would be rated at 1,726 psi internal working pressure. Special Thickness Classes of Ductile Iron pipe are normally specified only because of high external loads due to deep bury, high dynamic loading, etc.; however, Special Thickness Class Ductile Iron pipe has also been specified and installed in systems with working pressures greater than 1,000 psi. For information and limitations, contact the manufacturers of Ductile Iron pipe.

Appendix A of ANSI/AWWA C151/A21.51, Ductile Iron Pipe, Centrifugally Cast, for Water, contains the minimum metal wall thickness required for 2, 3, and 4 threads for different diameter threaded outlets and different diameter pipe. Information is given for both threads conforming to Standard ANSI/ASME B1.20.1 (a.k.a. National Pipe Thread (NPT), Iron Pipe Thread (IP), or Standard Taper Pipe Thread) and AWWA C800 (a.k.a. Mueller Thread, cc thread, Corp Stop Thread). To assure adequate metal thickness for a particular pipe diameter and Pressure or Thickness Class, it is necessary to subtract the casting tolerance found in the Table in Section 4.4.2 from the Nominal Metal Wall thickness found in Table 1 of ANSI/AWWA C151/A21.51.

Concerning the security of a two engaged threads engagement, the Ductile Iron Pipe Research Association (DIPRA) conducted a study of ¾-inch and 1-inch corporation stops direct tapped into 6" Pressure Class 350 pipe. The tests were conducted on pipe sections with less than nominal metal wall thickness. After multiple corporation stops were installed in each piece of pipe under city line pressure, the installations were observed for leakage through the threads.  The water pressure was then raised to 1,000 psi in an effort to fail the 6" pipe and threaded connection. Leakage was not observed at the threaded connection. These tests were conducted with and without 3-mil thread sealing tape applied to the threads of the corporation stop. The installed corporation stops were then subjected to pull-out and cantilever load tests.  In the pull-out tests, the corporation stop failed at loads in excess of 6,500 pounds of force. The pipe threads were undamaged in each of the three tests. In the cantilever load tests, the corporation stops failed at bending moments in excess of 385 foot-pounds of force. Again the threads in the ductile iron pipe wall were undamaged. 

It can be clearly seen that work crews can direct tap service connections into Pressure Class Ductile Iron pipe under pressure, effecting structurally secure, watertight seals. It is recommended that two layers of 3-mil thread sealant tape be applied to the corporation stop threads to achieve a watertight service connection using a minimal tightening torque.

The results of this study have been published by the Ductile Iron Pipe Research Association under the title Direct Tapping of 6-inch Pressure Class 350 Ductile Iron Pipe and is available through the Web Site http://www.dipra.org.

Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, overhead AC power lines normally don't impose corrosion or safety concerns.

A consequence of AC power lines and buried pipelines sharing rights-of-way is that AC voltages and currents can be induced by magnetic induction on the pipelines. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the AC power line, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.

Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that may vary from a fraction of an ohm to several ohms, but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints segment the pipe and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare and are therefore essentially grounded for their entire length, which further prevents magnetic induction on the pipeline.

During construction of Ductile Iron pipelines in the vicinity of overhead AC power lines, certain safety precautions should be followed, e.g., "limit of approach" regulations governing construction equipment, grounding straps, or chains attached to rubber tired vehicles to provide a ground, etc.

No. Because buried Ductile Iron pipelines are electrically discontinuous and are essentially grounded for their entire length, underground electrical cables normally do not impose corrosion or safety concerns for Ductile Iron pipelines. A consequence of underground electrical cables and buried pipelines sharing the same right-of-way is that AC voltages and currents can be induced on the pipelines by the expansion and contraction of magnetic fields. The magnitude of the induced voltage and current on the pipeline is a function of a number of variables, including the length of pipeline paralleling the underground electrical cable, the longitudinal resistance of the pipeline, and the resistance of the pipeline coating.

Ductile Iron pipe is manufactured in nominal 18- and 20-foot lengths and employs a rubber-gasketed jointing system. These rubber-gasketed joints offer electrical resistance that may vary from a fraction of an ohm to several ohms, but nevertheless is sufficient for Ductile Iron pipelines to be considered electrically discontinuous. In effect, the rubber-gasketed joints segment the pipe and prevent magnetic induction from being a problem. Also, in most cases, Ductile Iron pipelines are installed bare and are therefore essentially grounded for their entire length which further prevents magnetic induction on the pipeline.

If, for some reason, a bonded-joint Ductile Iron pipeline parallels an underground or overhead high-voltage AC power line, additional investigation may be warranted depending on the pipe coating (if any), length of parallelism, etc. AC voltages induced on a pipeline pose a shock hazard rather than a corrosion concern.

Studies have concluded that AC current may cause corrosion at a rate that is only 1 percent or less than that of a similar electrical quantity of direct current. NACE RP0177-95, "Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems," considers 15 volts AC open circuit to constitute an anticipated shock hazard.

The linear expansion of ductile iron pipe as a result of thermal effects is very small. In fact, at .0000062 inches per inch per degree Fahrenheit, ductile iron expands less than concrete.

Because flange joints are bolted together in metal-to-metal contact, there might be some concern over linear expansion, with resultant growth of the pipeline. While a change in temperature of 30° F. could account for an expansion of less than a quarter inch per hundred feet of pipeline, it is a fact that most pipelines in buried service maintain a fairly constant temperature.

For rubber gasketed joints, such as TYTON JOINT®, TR FLEX®, and mechanical joint assembled in accordance with the manufacturer’s instructions, there is enough clearance from the face of the plain end of the pipe to the back of the bell to account for significant thermal growth before beginning to make metal-to-metal contact. Except for very long runs of exposed piping, particularly if flow is intermittent, thermal expansion is usually not a concern.

Yes. The use of select, non-corrosive material (such as sand or limestone) for bedding and backfill is referred to as "trench improvement." It is recognized that trench improvement generally provides good structural support and helps delay the onset of corrosion activity. However, experience has shown that trench improvement does not provide long-term protection to the pipe, particularly in highly aggressive soil environments. Permeation of native soil and moisture into the select backfill over time tends to make the select material take on corrosive properties. Therefore, trench improvement should not be used as the only method of corrosion control. Polyethylene encasement remains the most effective method for corrosion prevention of Ductile Iron pipe.

The design deflection limit for DIP is 3% of the outside diameter. This limit is well below the deflection limit that may damage cement lining and most polymeric linings. The tables for trench types and loading in the ANSI/AWWA C150/A21.5 standard "Thickness Design Of Ductile-Iron Pipe" are based on 3% deflection.

Cast iron is a generic name for any high carbon molten iron poured as a casting. When used to refer to pipe, cast iron (sometimes called gray iron) is a specific type in which the free graphite (Carbon) is in the shape of flakes. Cast Iron pipe were introduced into the United States in 1817.

Ductile Iron is a specific type of cast iron in which the free graphite is in the shape of nodules or spheroids. (Other names for ductile iron are nodular iron or spheroidal graphite iron.) Ductile Iron Pipe were introduced to the market in 1955.

Although nearly identical chemically, the two irons are quite different metallurgically. The now obsolete standard for Cast Iron Pipe (ANSI/AWWA A21.6/C106) required an iron strength of 18/40 (18,000 psi Bursting Tensile Resistance and 40,000 psi Ring Modulus of Rupture.) Although tensile testing was not a requirement of this standard, a tensile test of gray cast iron pipe would give a test result of approximately 20,000 psi Ultimate Tensile Strength, with no measurable Yield Strength or Elongation.

The current standard for Ductile Iron Pipe (ANSI/AWWA A21.51/C151) requires a minimum grade of 60-42-10 (60,000 psi Ultimate Tensile Strength, 42,000 psi Yield Strength, and 10% Elongation.) In addition, Ductile Iron Pipe manufactured under this standard are required to meet a minimum of 7 ft lbs impact resistance by the Charpy test. (Compare Gray Iron Pipe with an impact resistance of approximately 2 ft lbs or less.)

The difference in the physical properties of these two materials is attributable almost entirely to the difference in the shape of the free graphite. The shape of the graphite is determined at the instant of solidification and is made nodular by the addition of magnesium to the molten iron bath. Although Cast Iron was the best engineering material available for pipe production for nearly five hundred years, the development of Ductile Iron Pipe provides a far superior product.

ANSI/AWWA C600 "Installation of Ductile-Iron Water Mains and Their Appurtenances" requires that newly installed Ductile Iron water mains be hydrostatically tested at not less than 1.25 times the working pressure at the highest point along the test section and not less than 1.5 times the working pressure at the lowest point of testing.

After the air has been expelled and the valve or valves segregating the part of the system under test have been closed, pressure is then normally applied with a hand pump, gasoline-powered pump, or fire department pumping equipment for large lines. After the main has been brought up to test pressure, it is held at least two hours and the make-up water measured with a displacement meter or by pumping the water from a vessel of known volume. The make-up water is called the "testing allowance," and the allowable amount is a function of length of pipe tested, nominal diameter of the pipe, and the average test pressure. The hydrostatic pressure test helps to identify damaged or defective pipe, fittings, joints, valves, or hydrants, and also the security of the thrust restraint system.

The "testing allowance" is not a "leakage allowance." Properly installed Ductile Iron pipelines with properly assembled joints are bottle-tight and do not leak. The "testing allowance" is, however, a practical measure used to maintain the pressure, which might actually drop because of factors other than leakage, including trapped air, absorption of water by the cement lining, extension of restrained joints and other small pipe-soil movements, temperature variations during testing, etc.