Below we provide the answers to several frequently asked questions regarding Ductile Iron Pipe design.
Q: Are push-on and mechanical joints rated for the same pressure as the pipe or higher? What is the maximum rated pressure for push-on and mechanical joints?
A: Ductile Iron push-on and mechanical joints are covered in ANSI/AWWA C111/A21.11 “Rubber-Gasket Joints for Ductile-Iron Pressure Pipe and Fittings.” Section 4.2.2 of that standard states: “The mechanical and push-on joints shall have the same pressure rating as the pipe or fitting of which they are a part.” In other words, if the pipe is rated for 150 psi working pressure plus 100 psi surge (250 psi), so is the joint. If the pipe is rated for 350 psi working pressure plus 100 psi surge (450 psi), so is the joint.
This is not to say that Ductile Iron Pipe and push-on and mechanical joints cannot be rated above 350 psi working pressure plus 100 psi surge (450 psi). Footnotes under Table 7 in ANSI/AWWA C151/A21.51 “Ductile-Iron Pipe, Centrifugally Cast, for Water” state: “Ductile Iron Pipe for working pressures higher than 350 psi is available.” There are numerous Ductile Iron Pipelines operating at working pressures well in excess of 350 psi throughout the United States. Additionally, Ductile Iron’s push-on joints have been proven effective in actual tests and/or service with at least 1,000 psi internal pressure, 430 psi external pressure, and 14 psi negative air pressure with no leakage or infiltration.
(Issue: Spring/Summer 2002)
Q: ANSI/AWWA C150/A21.50 and ANSI/AWWA C600 standards recommend that for 14-inch and larger diameter Ductile Iron Pipe, consideration should be given to the use of laying conditions other than Type 1. What is the reason for this?
A: 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.
(Issue: Spring/Summer 2002)
Q: The highest modulus of soil reaction (E’) value in ANSI/AWWA C150/A21.50 is 700 psi for a Type 5 trench. If I know the installation will have a higher E’ value, can I use it in my design calculations for Ductile Iron Pipe?
A: Yes. During the development of the design procedures in the ANSI/AWWA C150/A21.50 Standard, a thorough investigation was conducted to establish conservative design criteria, metal stresses, and soil mechanics factors for use in applying Spangler equations to the design of Ductile Iron Pipe. One of the soil mechanics factors contained in the equations was the modulus of soil reaction (E’). This modulus, which is a measure of stiffness of the embedment material that surrounds the pipe, is dependent on the type of side fill and its degree of compaction. Therefore, it is not a pure material property.
In buried flexible pipe, the bending stress of trench load is reduced by the lateral soil reaction developed as the pipe deflects under trench load and the sides of the pipe push outward against the side fill. The horizontal pressure on each side would be proportional to the deflection of the pipe into the soil.
The E’ values listed for Trench Conditions 1 through 5 in ANSI/AWWA C150/A21.50 are very conservative and reflect realistic expectations of actual field installations. Higher E’ values can certainly be achieved and are appropriate for the design of Ductile Iron Pipe if they can be ensured through design and field verification.
(Issue: Fall/Winter 2001)
Q: What is the maximum service temperature for Ductile Iron Pipe?
A: The maximum service temperature for Ductile Iron Pipe is dependent on the type of elastomers used for gaskets, joint configurations, and linings utilized rather than the iron.
Generally, the maximum service temperature for Ductile Iron Pipe gaskets is 120°F to 300°F, depending on the type of elastomer and joint configuration. Standard SBR gaskets are generally rated up to 150°F for push-on joints and 120°F for mechanical joints.
The maximum service temperature for linings used for Ductile Iron Pipe is generally 120°F to 212°F. Seal-coated cement-mortar linings are rated for 150°F, and non-sealcoated cement-mortar linings are rated for 212°F.
For service temperatures greater than those above, consult the pipe manufacturer for specific recommendations. For gasket and lining data sheets, which include maximum service temperatures, contact DIPRA.
(Issue: Fall/Winter 2001)
Q: What is the purpose of the “service allowance” used in Ductile Iron Pipe wall thickness design?
A: 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.
(Issue: Spring/Summer 2001)
Q: What is the pressure and vacuum capacity of the push-on joint?
A: The push-on joint is rated for a water working pressure of 350 psi; however, for specific conditions the joint has been approved for much higher pressure ratings. The joint is designed and manufactured to close tolerances so that the gasket is self-centered, securely confined, and firmly compressed for a permanent, tight, trouble-free connection. It is bottle tight under vacuum and external pressure and becomes even tighter with the application of internal pressure. The design ensures effective sealing at low or high pressures and in straight or deflected joint alignment. It also eliminates any concerns of infiltration or root intrusion. The design also ensures positive sealing against negative pressure, thus preventing gasket “pullout” should a vacuum be created in the line.
Ductile Iron Pipe’s push-on joint systems have been proven effective in actual tests with up to 1,000 psi internal pressure, 430 psi external pressure, and 14 psi negative internal air pressure with no leakage or infiltration. Although the push-on joint is normally rated for 350 psi, currently there are Ductile Iron Pipe push-on joint pipelines with operating pressures in excess of 1,000 psi. For pressure ratings above 350 psi, contact the Ductile Iron Pipe manufacturers.
(Issue: Fall/Winter 2000)
Q: Is cyclic loading a concern for Ductile Iron Pipe?
A: No. As discussed in the DIPRA article “Cyclic Considerations in Force Main Design” (Spring/Summer 1998 Ductile Iron Pipe News), the design approach found in ANSI/AWWA C150/A21.50 standard for Ductile Iron Pipe wall thickness design eliminates cyclic fatigue as a concern for Ductile Iron Pipe. The design for internal pressure limits wall stress to 21,000 psi before allowances for service and casting are added. In practice, the stress is usually much less than 21,000 psi because of the extremely high pressure rating, conservative design, and casting practice of Ductile Iron Pipe. The cyclic fatigue limit for Ductile Iron has been reported in the literature to be between 28,000 psi and 35,000 psi. Therefore, when designed in accordance with the above standard, Ductile Iron Pipe can be expected to have an infinite life related to cyclic fatigue.
(Issue: Spring/Summer 1999)
Q: Can Ductile Iron Pipe be used for pressure applications in excess of the maximum Pressure Class (350 psi) listed in ANSI/AWWA C150/A21.50 and ANSI/AWWA C151/A21.51 Standards?
A: 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.
(Issue: Spring/Summer 1998)
Q: For very wide trenches, can Ductile Iron Pipe experience higher loadings than the prism load?
A: The Marston theory and the prism load are the two basic approaches used to estimate the soil-induced load on a buried pipe. The basic concept of the Marston theory is that the load due to the weight of the column of soil above a buried pipe is modified by the response of the pipe (in general, a flexible pipe reduces the load and a rigid pipe increases the load). The prism load is simply the weight of the column of soil directly over the pipe for the full height of the backfill. Research data indicate that the true effective load on a flexible conduit lies somewhere between the minimum predicted by Marston and the prism load. ANSI/AWWA C150/A21.50 “Thickness Design of Ductile-Iron Pipe” utilizes the conservative prism load in its design. The Marston equation for load on a trench conduit indicates that this load is a function of the width of the trench in which the conduit is laid; that is, the wider the trench, the greater the load on a conduit. Obviously, there is a limiting width for a given depth and size of pipe beyond which this principle does not apply. This is called the “transition width” and is a limiting value for calculating loads based on Marston’s trench formulas. At transition width and beyond, the loads can be calculated using Marston’s positive projection conduit or “embankment” equations. For wide trenches, Marston’s trench equations yield the soil prism load, which is consistent with Marston’s embankment equations that also yield the soil prism load when the product of the settlement ratio (rsd) and the projection ratio (p) is equal to zero. If (rsdp) is less than zero, then the soil induced load on the pipe is less than the prism load. If (rsdp) is greater than zero, then the soil induced load on the pipe is greater than the prism load. In general, there is very limited information available on appropriate settlement ratios for flexible pipe; however, under most conditions they are considered to be zero or a negative number. Therefore, unless (rsdp) is greater than zero, the prism load is the maximum load that will be imposed by the soil on a flexible pipe in virtually all cases.
(Issue: Spring/Summer 1998)
Q: If I specify a standard laying condition that, for general trench construction, the native soil will not support (e.g., Type 5 trench through native organic soils), what E’ value should I use? Can I obtain the Type 5 trench by specifying a wide trench, and if so, how wide?
A: In the case of a narrow trench installation there is a possibility, in very poor native soils, of the trench wall being too weak to sustain the loads transmitted through the bedding material surrounding the pipe. In such a case, one should conservatively assume the E’ value of the native soil. If the designed trench E’ value is used, a minimum width of embedment material is required to ensure that adequate embedment stiffness is developed to support the pipe without assistance from the sidewall. Generally, this minimum width of embedment is taken as approximately one pipe diameter from each side of the pipe to the trench wall. In some instances, soil stabilizing or segregating matting may be desired.
(Issue: Fall/Winter 1997)
Q: ANSI/AWWA C150/A21.50, “Thickness Design of Ductile-Iron Pipe,” Tables 50.1, 50.6, and 50.12 list depth of cover starting at 2.5 feet. Can Ductile Iron Pipe subject to truck loading be installed with less than 2.5 feet of cover and, if so, what design approach should be used?
A: 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.
DIPRA’s paper, “Truck Loads on Pipe Buried at Shallow Depths,” which provides more detailed information, is available on request.
(Issue: Spring/Summer 1993)
Q: Should negative pressures create a concern for Ductile Iron Pipe and/or its joints?
A: Under normal circumstances, no. For a given piping material, the lowest critical buckling pressure corresponds to the smallest thickness-diameter ratio. The smallest value of this ratio for domestically manufactured Ductile Iron Pipe corresponds to 64-inch Pressure Class 150 pipe, which, when exposed to the atmosphere and using the minimum manufacturing thickness, would have a safety factor of approximately 2.0 against failure due to buckling when subjected to a 10 psi internal vacuum. Higher pressure class 64-inch-diameter pipe and all smaller-diameter Ductile Iron Pipe would have even higher safety factors.
For buried applications, when calculating the allowable depth of cover due to buckling and assuming an internal vacuum of 10 psi, a water table even with the surface of the ground, and a safety factor of 2.0, in every case greater depths of cover are permissible than are allowed by current standard ANSI/AWWA C150/A21.50 design procedures. It is obvious, therefore, that a designer should not be concerned with buckling collapse of Ductile Iron Pipe in typical installations. There are extreme conditions, however, such as evacuation of large-diameter pipelines installed under great depths of water, where buckling design may be necessary. Additionally, Ductile Iron’s push-on joints have been proven effective in actual tests with up to 1,000 psi internal pressure, 430 psi external pressure, and 14 psi negative air pressure with no leakage or infiltration.
(Issue: Spring/Summer 1993)
