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.