The Production Process of Ductile Iron Pipe
In today’s modernized water and wastewater systems, Ductile iron pipe has become the industry standard. Since its introduction to the market in 1955, the appropriate standards committees have tested many different elements of Ductile iron throughout its production process.1 Because of such rigorous evaluations, Ductile iron piping has become the industry’s most comprehensive design standard, with extensive data on virtually every aspect of the pipe’s performance.
The success and popularity of Ductile iron piping is due to its basis of design standard being that when “subjected to internal pressure and underground loading conditions, it behaves as a flexible conduit and thus rebounds under pressure. Therefore, the pipe is designed separately to withstand external loads and internal pressure.” This comes with very strict standards, such as trench loading tests, strength tests, corrosion resistance, tapping strength, flow characteristics, impact resistance, lining, joint integrity, and more.
Alongside the strict standards for Ductile iron pipes is a very important design criteria. These are inclusive of load factors and design, calculations of ring bending stress and deflection, trench types, and internal pressure designs. Specifically, truck loads are based upon a single AASHTO H-20 truck with a 16,000-pound wheel load and an impact factor of 1.5 at all depths. Following, for the external load design, both calculations of ring bending stress and deflection are included where ring bending stress is limited to 48,000 psi, which provides a safety factor of 2.0 based upon minimum ultimate bending stress. The earth load is based upon the prism load concept for all pipe sizes. Additionally, deflection of the pipe ring is limited to a maximum of 3% for cement-lined pipe. To ensure a cost-effective trench section design, there are 5 different trench types that have been defined in the standards. Lastly, the internal pressure design is based upon working pressure plus a surge allowance of 100 psi, while a safety factor of 2.0 is also applied to this calculation. While minimum safety factors are set, actual total field service safety factors far outweigh these values.
The design procedure of Ductile iron pipes is done in a way to make it more conservative given its calculated thickness. This procedure consists of six steps, starting with the design of internal pressures, allowing for static pressure plus surge pressure. Next is the design implementation for bending stress due to external loads. Then manufacturers will select the larger resulting net wall thickness and add a 0.08-inch service allowance. Following that is a check for deflection, before finally adding a standard casting tolerance. Done correctly, this results in the total calculated design thickness, from which the appropriate pressure class can then be chosen. The standard casting tolerance addition alongside the 0.08-inch service allowance creates a larger margin of safety and dependability. To avoid corrosion, V-Bio enhanced polyethylene encasement is wrapped around the pipe to mitigate any such conditions, while cement-mortar lining on the inside increases the pipe’s resistance to corrosion as well. Lastly, Ductile iron pipe can be manufactured in 18- or 20-foot nominal laying lengths and with 3–64-inch diameters along a range of standard pressure classes. These design standards make way for five different laying conditions, which in turn gives the designer the flexibility to choose the most economical combination of pipe thickness and trench conditions.
The last piece of the production puzzle includes the various joint options, the numerous fittings, valves and hydrants, linings, and the polyethylene encasement. The common joints used in accordance with Ductile iron are the push-on joint, the mechanical joint, the restrained joint, flanged joint, ball-and-socket joint, and any other miscellaneous components. All of these are designed to be bottle-tight and easily assembled. The numerous Ductile iron fittings, combined with Ductile’s ability to be cut in the field, allow for installers to bypass unexpected obstacles during installation, which creates a competitive advantage. The various valves and hydrants are designed to assist in a variety of applications, such as fire protection, isolation, backflow prevention, flow control, and other special functions. These valves and hydrants can include metal-seated and resilient-seated gate valves, swing-check valves, or dry and wet barrel fire hydrants, and more. Linings in Ductile iron pipes are usually made up of cement-mortar to prevent tuberculation and to create a smooth surface on the inside, resulting in less friction and less head loss. Lastly, the polyethylene encasement can be added where the soil conditions are aggressive or unstable to protect from corrosion. The pipes are encased with V-Bio Enhanced Polyethylene at the job site, making damage far less likely than factory-applied coatings.
After production, Ductile iron pipes will exhibit tremendous tensile strength, beam strength, high impact resistance, as well as corrosion resistance. In addition to the ability to withstand crushing loads, Ductile iron also demonstrates great bursting strength, while also being easy to install and virtually maintenance-free.
