HDPE vs. Ductile Iron

With an increased demand for infrastructure systems and supply chain issues across multiple industries, you may be faced with evaluating the use of different pipe products, including WL Plastics’ HDPE pressure pipe.

There are certain applications that are better suited for certain pipe materials. Ductile iron is suited for burial in areas with large rocks or cobbles where it is either impossible or cost prohibitive to bring in screened backfill due to its high tensile strength and elastic modulus. HDPE pipe is more flexible and durable, making it the material of choice for trenchless installation, for seismic areas, or for climates where freeze/thaw and drought are issues resulting in possible ground movement. Both pipe materials serve different purposes and provide different advantages. We will go over all these benefits in this detailed comparison.

While cast iron pipes have been around for centuries, ductile iron pipes are a relatively “new/improved” derivative of traditional cast iron piping with its first AWWA C151 publication in 1965. HDPE pipe had its first AWWA C906 publication in 1967. The difference between ductile and cast iron pipes is the addition of graphite, copper, and tin, which creates their spheroidal-shaped molecular structure and improves the cast iron’s fatigue and impact resistance. While these changes created a more ductile product, it also made the new ductile iron more susceptible to localized corrosion. 

When choosing a material for your piping application, it is important to understand the differences between ductile iron vs. HDPE, along with their advantageous properties and challenges.

HDPE Pipe and Ductile Iron Pipe Difference 

HDPE pipes differ from ductile iron pipes in several key ways. The main benefit of HDPE pipe vs. ductile iron is corrosion resistance. HDPE pipe doesn’t rust, corrode, or tuberculate and is more flexible and durable than iron pipes. Other distinct differences between HDPE and ductile iron are as follows:

• HDPE pipes are made from ethylene, a derivative of natural gas, while ductile iron pipes are created from ferric scrap metals. HDPE pipes are manufactured at much lower temperatures and require less energy for their production. 
• HDPE pipes don’t require thrust blocks at directional changes due to their fully self-restrained heat fusion joints. 
• HDPE pipe can handle much higher flow velocities than ductile iron pipe, up to two times its pressure class for occasional surge.  

Benefits and Advantages of HDPE Pipe 

Easy Handling 

HDPE pipes are light and flexible and easy to handle and install compared to heavier ductile iron. This offers significant cost savings in pipe and constructability. HDPE pipe is about an eighth the weight of metallic pipes, which often eliminates the need for heavy-lifting machinery during installation. HDPE also offers increased flexibility, allowing them to bend easily around obstacles.

Minimizes Installation Cost 

HDPE pipes can be welded into seamless, leak-free pipe lengths using minimal fittings, saving money on both labor and installation equipment. Using heat fusion joining eliminates the potential issues of a typical bell and spigot joined piping system such as improper insertion, over bending, and rolled gaskets. 

Heat-Fused Joints 

Welded joints using electrofusion, socket fusion, and most common butt fusion techniques on HDPE creates exceptionally strong joints that can exceed the strength of the pipe itself. These leak free heat fusion joints are easy to assemble and easy to inspect to ensure a long design life for the entire pipe system. 

Chemical and Corrosion Resistance 

HDPE pipe doesn’t rust, corrode, tuberculate, or support biological growth of any kind. HDPE pipe also has superior chemical resistance compared to traditional metallic pipe materials. For environments where exposure to salts, corrosives, acids, and bases are frequent, HDPE piping is the superior choice. 

Excellent Surge Resistance 

HDPE pipes have phenomenal surge resistance. Surge pressure or “water hammer” occurs when the flow rate of fluid inside a pipe changes. This can occur when a valve opens or closes or a pump starts or stops. The amount of surge experienced inside the pipe has a direct correlation to the elastic modulus of the pipe. HDPE pipes, with its lower elastic modulus act as a giant shock absorber, resulting in very little surge pressure traveling down the water column. A 2 foot per second velocity change in ductile iron pipe results in 100 psi of surge pressure. The same velocity change in HDPE pipe results in just 22 psi of surge pressure. 

Superior Flow Characteristics 

HDPE pipe has a Hazen Williams C-Factor of 150 and is smoother than ductile iron pipes. This creates less turbulence and friction in the water column against the pipe wall. With a higher C-Factor, in some cases you can get the same amount, or more water through HDPE with a smaller diameter vs. ductile iron pipe. This slick surface with a Roughness Coefficient of 0.000005 eliminates scaling, pitting, tuberculation and preserves the hydraulic characteristics of HDPE pipe for the design life of the system. 

HDPE pipes are the ideal piping solution for applications such as water, wastewater, power and communications, and oil and gas gathering. At WL Plastics, we produce HDPE pipes that meet our customers’ needs and comply with strict industry and safety regulations. Contact us today, and let us help you source quality HDPE piping for your application.