Ductile Iron Pipe: Why It's Best for Water and Wastewater
The unique strength of material, durability, and adaptability of ductile iron makes it the most reliable choice for water and wastewater pipelines. In addition to offering various engineering benefits, ductile iron pipes are also more energy-efficient and safer for the environment.
Water and wastewater pipelines are essential for all households, businesses, and industries. Therefore, it is crucial to know the properties and benefits of various pipe materials to ensure that these critical piping networks perform as expected.
Water and sewage pipelines can be constructed from various materials, such as cast iron, ductile iron, concrete, steel, plastics, and vitrified clay. However, among these, ductile iron is widely embraced as the material of choice for water and wastewater piping.
What is Ductile Iron?
Ductile iron is the successor to cast iron. Cast iron pipes were once frequently used in municipal utilities due to their cost-effectiveness and overall strength. However, this material was also heavy, prone to corrosion, and relatively brittle. To address these problems, ductile iron was introduced in the 1940s.
It was found that the addition of magnesium alloy to molten iron significantly changed the microstructure of the material and improved its mechanical properties. According to the ASTM standard for ductile iron casting, the chemical composition of ductile iron consists of 3.5-3.9% carbon and 2.25-3% silicon along with small percentages of manganese, magnesium, sulfur, phosphorus, etc.
Ductile iron maintains all the advantages of cast iron while improving on strength and ductility. This material also has a high strength to weight ratio, allowing lighter piping structures to be fabricated.
In this article, we will explore the benefits of ductile iron pipes and discuss why they are the best choice for water and wastewater pipelines. (Read also: What Equipment has Improved Wastewater Management?)
Advantages of Ductile Iron Pipes
Ductile iron pipes can be bent or cut without producing chip-breaking effects. The silicon and graphite content in ductile iron offers flex, lubrication, and abrasion resistance during manufacturing. Compared to steel pipes, the ductile iron pipes present several advantages during machining, including higher productivity, extended tool life, faster spindle speed, and reduced cost of operation.
Safety and Reliability
Other pipeline materials, such as PVC, can experience catastrophic failures due to ground movement and high pressures. (Read also: The Lifespan and Dangers of Old Piping Materials vs Plastics and Liners.) For potable water pipelines, this failure presents the risk of drinking water contamination. Ruptured wastewater pipes can also contaminate the surrounding environment.
Ductile iron pipes are known for their outstanding tensile strength, bending strength, and impact resistance. Pipes constructed from this material are usually manufactured with a minimum ultimate tensile strength of 60000 psi strength, a yield strength of 30000 psi, and a 10% minimum elongation. These strengths, combined with standard wall thicknesses, allow ductile iron pipelines to resist ground movement due to heaving, settlement and seismic activity.
Ductile iron pipes also have exceptional bursting strength, allowing them to withstand sudden high-pressure pressure surges, such as water hammer.
Various studies and field tests confirm that the longevity and corrosion resistance of ductile iron pipes exceeds that of cast iron. According to a report by the American Water Works Association (AWWA), the service life of a ductile iron pipe is expected to be over 100 years. This outlasts any other pipeline material used in water and wastewater infrastructure.
By contrast, steel pipes and concrete cylinders rely on external bonded coatings, e.g., cement-mortar for corrosion resistance. When bonded coatings are used, any damage to the coating can expose the pipe surface, making it susceptible to corrosion. Ductile iron pipes, on the other hand, are inherently resistant to corrosion in most environments. (Read also: Corrosion Mechanisms of Buried Pipes.)
In extremely harsh environments, ductile iron pipes may be encased in polyethylene for enhanced corrosion control.
Due to its flexibility and strength, the installation of ductile iron pipes is easier and less expensive than other pipe materials. For example, the strength and flexibility of ductile iron pipes allow them to be installed with less stringent bedding and backfill specifications. (Read also: How Loads Affect Buried Pipes.)
Other pipe materials, like PVC, concrete, and clay, require rigorous engineered fill and bedding specifications to minimize settlement and long-term creep. Although steel pipes also possess ductility, they may also need additional braces and external soil support during installation.
Additionally, since ductile iron pipes are machinable, they can be easily cut, drilled, and directly tapped in the field.
Standard ductile iron pipes also have internal diameters that are larger than other standard pipe materials. Additionally, the texture of the internal surface of the pipe remains consistent even after many years of service.
These factors contribute to increased and consistent flow capacity, as well as lower head loss. Consequently, the energy required for pumps to overcome these losses is also relatively low. Hence, ductile iron pipes can also reduce overall utility costs and also prolong the life of pumping equipment.
Because of the superior durability of ductile iron pipes, the maintenance required is negligible. In the event of a leak, inexpensive pipe sleeves can be used as a form of repair, eliminating the need for complete pipe replacement.
Last but not least, ductile iron pipes are environmentally friendly and recyclable. They can be produced from iron salvaged from recycled steel scraps, demolished buildings, or other sources.
Since the pipes themselves are also entirely recyclable, they produce zero net waste. This recyclability ultimately reduces environmental contamination and carbon dioxide emissions.
This is in addition to the environmental benefits of ensuring proper waste management while facilitating the distribution and disposal of water and wastewater, respectively.
As a result of its environmental, economic, and social benefits, ductile iron pipes have earned SMaRT certification from ‘The Institute for Market Transformation to Sustainability (MTS).’ This is the only pressure pipe available to achieve such certification to date.
The unique strength of material, durability, and adaptability of ductile iron makes it the most reliable choice for water and wastewater pipelines. In addition to offering various engineering benefits, ductile iron pipes are also more energy-efficient and safer for the environment. These characteristics have no doubt contributed to the increased popularity of ductile iron in both the water and wastewater industries.
Written by Krystal Nanan | Civil Engineer
Krystal is a civil engineer and project manager with an MSc in Construction Engineering and Management. Her experience includes the project management of major infrastructure projects, construction supervision, and the design of various infrastructure elements including roadway, pavement, traffic safety elements and drainage.