Ductile Iron Pipe Vs HDPE Pipe: Which One Is Better?

Ductile iron and HDPE are two common choices in pipe manufacturing. These two different types of pipes hold some significant differences in many aspects. Design properties, time and temperature, and external loading are examples of these differences. But, if you think these are the only factors helping to compare Ductile Iron Pipe vs HDPE pipe, you are in error. Crucial factors such as strength, constructability, and hydrostatic burst pressure are highly vital to be checked and compared. The total cost of these two types of pipes might be different in the initial and in the long run. Make sure to wisely estimate the number of the overall cost.

Design Properties

There is much to say about the properties of ductile iron pipe vs. HDPE pipe comparison.

• Ductile iron pipes have a hydrostatic design basis 26 times more than HDPE pipes. This means that ductile iron pipes can handle internal pressure much more than pipes made of HDPE. The strength of ductile iron pipes is 42,000 psi, while the number of HDPE pipes is almost 1600 psi, which makes ductile iron almost two and a half times stronger than HDPE.
• Also, HDPE pipes cannot handle sudden changes in fluid pressure. However, this is one of the ductile pipe’s benefits: it is resistant to damage and can handle pressure changes.
• Consider a 12-inch pipe that handles a pressure of 900 psi. The thickness of the wall of a ductile pipe must be 0.14 inches, whereas this number is 3.7 inches for HDPE.

To put it in a nutshell, the minimum thickness of a ductile iron pipe with a 12-inch length is 0.25 inches which provides proper resistance against 1590-psi pressure. On the other hand, HDPE provides resistance for only a 60-psi pressure with the same thickness.

External Loading

External forces certainly don’t prove any HDPE pipe advantages, for they aren’t designed to consider these types of forces. On the contrary, ductile iron pipes are perfectly designed to consider factors such as bending, deflection, and also internal pressure. For the buried pipelines, it is crucially important to be designed for external forces as these forces impact almost 99% of designs. HDPE ignores external forces, including the external force of live loads from the traffic above these pipes. On the other hand, ductile iron pipe designs adopt various factors to handle external forces, such as AASHTO H-20 tractor-trailer wheel loads.

Time and Temperature

There are many comparisons to make about ductile iron pipe vs HDPE. One of them is the difference between HDPE lifespan and ductile iron. Adding specific temperature conditions will highlight this difference even more.

• Ductile iron pipes generally don’t deteriorate with time and temperature. They can last for more than 100 years and maintain a steady temperature from -10°F to 150°F.
• HDPE pipes are mainly designed for the specific temperature of 73.4°F fixed. In a 1000 feet pipe, HDPE pipes expand and contrat 10 inches per 10°F change in temperature.
• It is significantly different for ductile iron pipes. These pipes are affected by a change of temperature 13 times less. The expansion is only 0.75 inches per 10°F in a 1000-feet long pipe.
• The last main difference in the durability of ductile iron pipe and HDPE is the length of service life. HDPE pipes are designed based on failure after 100,000 hours of service. Meanwhile, traditional gray iron pipes have shown resistance for 300 years, while ductile iron is even more resilient than traditional gray iron.

Strength

As ductile iron fittings are built with sturdy walls, the pipelines get strong and sturdy as well. On the other hand, due to seams in HDPE bends and tees, these fittings are way weaker. Thus, the maximum pressure they can handle is reduced from 254 psi to 190 psi. Also, the tensile strength of ductile iron is almost 24 times more than HDPE. This specification is a crucial basis property that ductile iron and HDPE can have. This is because tensile strength enables the material to resist the forces that are caused by the water hammer and internal hydrostatic pressure.

Constructability

The constructability and installation process for ductile iron pipes is different for ductile iron pipes vs. HDPE.

• Ductile iron pipes only require simple push-on rubber gasket joints. These joints are very easy to assemble or even to take apart when needed.
• HDPE pipes require special machinery and trained workers due to the critical process of melting and joining the pieces together. Even the weather conditions are important in the process of assembling HDPE joints. This is why most contractors have a third-party company do this jointing work for them.
• HDPE pipes generally come in a 40-foot length. Hence, safe trenches over 80 feet should be kept open for installation. However, this is often not possible in many construction projects.

Nevertheless, ductile iron pipes don’t face such problems related to handling or durability.

Hydrostatic Burst Pressure

Referring to Dipra, Ductile iron pipes can resist hydrostatic burst pressure up to 6.1 times more than HDPE pipes. Tests that followed ASTM D159912 guidelines included attaching gasketed, unrestrained end caps to pipe specimens and securing them in a hydrostatic test structure.

• With internal hydrostatic pressure increasing, the setup created stress, specifically in the pipe walls’ circumferential direction.
• 6 and 24-inch diameter ductile iron pipe specimens ruptured with a 15 to 41-inch long fracture.
• The 6—and 24-inch diameter HDPE specimens failed by ballooning, and some of them also bent and snaked. This led to the pipe detaching from the end closures and causing leakage at the test seals.

Total Cost

Cost comparison of ductile iron and HDPE pipes is essential to choosing the best option for projects. According to McWane Ductile, considering the operational costs of pressurized utility pipes is vital. Be careful that the initial cost may draw you in; however, the long-term costs will become considerable.

• As there are different pipe wall thicknesses, ductile iron pipes normally have larger internal diameters compared to HDPE or PVC pipes.
• In a 12-inch pipe, the additional cost of pumping water through a mile of HDPE pipe goes over 900,000$ during the first 25 years.
• Overall, if HDPE pipe is bought and installed for almost $175$ less per foot than ductile iron, it will be the more expensive option in the long run.

Though the above factors can only show one aspect of the overall expenses of materials, they are good enough to prove that ductile iron pipes are more efficient and reliable despite the higher initial cost.

 

The table below shows a good comparison of ductile iron pipe vs HDPE in terms of technical specifications.

 

Technical Specifications	Ductile Iron Pipes	HDPE Pipes
Pressure Rating	Higher pressure ratings (up to 350 psi)	Lower pressure ratings (up to 190 psi for certain pipes)
Wall Thickness	Thicker walls	Thinner walls
Joint Types	Restrained and flexible	Fused or flanged
Corrosion Resistance	High with the use of protective linings	Inherently corrosion-resistant
Temperature/UV Resistance	May need coatings for UV protection	Better performance in high temperatures and UV exposure
Flow Capacity	Smooth and good	Smooth and good
Lifespan	Long (with care)	Long (with care)

How to Choose the Best Option?

Choosing the best option as the applied utility goes further than factors like the initial costs. Ductile iron pipes might not always be the ideal option. However, they are an unquestionable choice for water and wastewater systems. The durability, strength, and corrosion resistance of ductile iron make it the wisest option. Ductile iron, owing its reputation especially to its exceptional function in water and wastewater systems, outperforms all the other alternatives.