Question – What is the difference between Ductile Iron (DI) and Polyvinyl Chloride (PVC)?
Answer – Many things, but some groups may attempt to “muddy the waters” with inaccurate information regarding the two materials.
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With the information that follows, we will take a closer look at what really separates Ductile iron pipe (DI pipe) from PVC pipe and why it matters.
Click the red play button in the image below to view a helpful video on the advantages of DI over PVC.
Ductile iron pipe is a flexible conduit - Ductile means flexible! Because DI pipe is flexible, this means it is less likely to shear under stress from normal soil pressure or ground shifts that occur throughout the life cycle of your water system.
Ductile iron is primarily made from recycled iron and steel scrap. DI pipe is recyclable and has no end of life.
PVC is made from chlorine (derived from salt) and carbon (predominantly derived from oil and gas via ethylene). PVC pipe is weak or fragile when compared to the strength of DI pipe. The recycled content of PVC is nearly zero and PVC can only be down-cycled. In addition, PVC is derived from vinyl chloride monomer which is a Group A Human Carcinogen.
It is made rigid by the addition of a stabilizer, and at one time, the PVC industry used lead as a stabilizer. Today, PVC manufacturers use organotins that may pose health risks, but some foreign manufacturers continue to use lead.
A recent water pipe project at a major university involved installing dual 36-inch DI pipelines underneath a critical building on campus. The lines were installed to transport water to a chiller system to cool other buildings.
The piping material selected for this project was Ductile iron because of its ability to withstand ground shift and heavy pressures from the structure that rests above it. PVC, being weak in nature, tends to burst into small pieces due to heavy pressures versus stronger materials such as DI that will not obliterate.
PVC failures also have a history of being catastrophic. That is, not just a loss of some water or water pressure but a TOTAL LOSS of water. This instability is a severe issue for seismic prone areas. Not only is there a threat of damage from earthquakes, but also vulnerability to destruction from fires that typically follow the earthquakes. If there’s no water, there’s no fire protection.
PVC pipe can also be adversely affected in areas prone to wildfires. For example, the City of Santa Rosa, California estimates the repair of the contaminated Fountain Grove water system could take up to two years. Test results conducted on PVC pipelines indicated they absorbed benzene and other chemicals, resulting in an advisory to residents to not drink or bathe in the water. Replacement costs are estimated at more than $43 million dollars.
Permeation
DI pipe is impermeable and protects the water supply from toxic infiltration. The C909-16 Molecularly Oriented Polyvinyl Chloride Standard states that polyvinyl chloride may be subject to permeation by low molecular weight organic solvents or petroleum products. Why take a chance with a permeable material?
Would you trust plastic on a bridge? Didn't think so. DI pipe is an excellent choice for high-profile projects such as bridge installations. DI pipe possesses expansion and contraction capabilities required for such constructions. There is no trench support on a bridge to handle PVC pipe versus DI pipe, which does not require trench support. There are many above-ground installations on bridges and piers where DI pipe excels.
Penstocks are installations involving the redirection of streams down a mountain to a power plant. These lines are often subject to extremely high internal pressures due to the elevation changes and head pressure. The physical properties of DI pipe shine again versus the weaker PVC pipe for high-profile installations.
Fire hydrants give us a sense of security, knowing brave firefighters have the equipment available to protect us. Have you ever seen a fire hydrant made of PVC? I didn't think so.
Fire hydrants are made of Ductile iron. The hydrant standpipe is made of Ductile iron as well. There are times when a section of pipe between the DI is installed with PVC with the presumption of “saving money.” However, PVC is typically not connected directly to the hydrant section due to potential thrust forces in that area.
DI pipe is non-combustible and can withstand the stress caused by fires. In an emergency, it can be counted on to provide a safe and reliable supply of water for firefighting.
On the other hand, PVC pipe is combustible. Polyvinyl Chloride, of all the plastic polymers, has been implicated primarily in causing the most severe problem in firefighting today because PVC releases hydrogen chloride gas when as it melts in a fire. Many plastics produce large volumes of pulmonary irritant gases when burned. The firefighter is at fatal risk if he does not know that plastic covered cable is involved in a fire. (U.S. EPA)
I wish I had a nickel for every time a Municipal Operator has mentioned how many PVC leaks must be repaired the same month the hydrants are exercised. Just a coincidence? No, it’s not.
Municipal Operators are trained to slowly open and close valves on the hydrants to reduce the potential for a surge. Firefighters – on the other hand – are clearly focused on THE FIRE. What are the odds that firefighters are going to carefully and gently open or close the valve on a fire hose during the fire? The same surges are present when opening or closing the hydrant valve and are detrimental to products that cannot handle a surge.
The maximum allowable surge pressure for PVC is based on an operating temperature of 73 degrees Fahrenheit. We all know waterlines will not always operate at 73 degrees Fahrenheit. The PVC material is more susceptible to surge as temperature varies. Allowance is also based on a minor velocity change of 1.0 feet/second. PVC also has a "Time to Failure Rate." That's correct, the clock is ticking as soon as PVC is placed into service. See our down-loadable comparison sheet for more details on this.
Ductile iron pipelines are the only material to be included in the International Seismic Design Standards. DI pipe, being a flexible conduit with high tensile strength, provides a product with the ability to flex during seismic activity versus brittle materials that are subject to shear during the same seismic event.
Expansion joints may also be installed to provide expansion and contraction of axial movements. Restraint systems available with DI pipe provide a system that is independently strong and does not rely on ground surfaces for stability.
For more Di Pipe Manufacturerinformation, please contact us. We will provide professional answers.
Another potential installation in areas susceptible to ground shifts is the Horizontal Directional Drilling (HDD) method. This process involves boring a path underground and installing DI pipe below an area that has been reclaimed or is subject to slipping.
For an efficient and easy way to check horizontal drilling calculations, download the McWane Pocket Engineer. We developed this handy app to allow engineers and contractors to do their job by quickly providing critical calculations in the field that would otherwise take valuable time to compute. It's FREE and designed to work on any mobile device.
Water systems achieve maximum flow by using Ductile iron pipe. The strength of DI pipe provides an opportunity for larger Inside Diameters (ID) that result in additional flow. Because it is a weaker material, a 30-inch PVC pipe is needed to match the same ID as a 24-inch DI pipe. Therefore, additional costs must be included for a larger trench and the up-sizing of valves and fittings.
The C Factor is a value used to indicate the smoothness of the interior of a pipe. The PVC industry contends that the C Factor is an issue for DI pipe. However, that is simply not the case with modern, cement-mortar lined DI pipe. Laminar Flow is the real key to flow, not the C factor. Essentially, Laminar Flow is when the fluid flows through the middle of the pipe, reducing the significance of the C Factor as a contributing factor.
The Type I or Type II trench is used for Ductile iron pipe installation versus a Type IV or V trench required for PVC pipe. Type IV or V trench is required for PVC pipe due to the compaction that must be achieved to support the weaker material. Compacting the soil takes extra time and additional equipment.
The soil must be compacted to 80 to 90 percent of initial soil density for a Type IV or V trench. It is very challenging to compact to 90 percent with optimal conditions. The same size trench must be excavated for an 8-inch pipe regardless of the pipe materials used.
Check out this helpful blog by my colleague Gary Gula, McWane Ductile Sales Representative, that discusses the various trench types for DI pipeline installation.
Ductile iron pipe reduces installation costs due to the ability to direct tap. Tapping saddles or sleeves are required to tap PVC, which adds more costs. A popular event at water industry conventions is the tapping contest. Operators race against the clock and compete to determine who has the best crew. It is an exciting event, and operators are proud to be victors.
What kind of pipe do you think is used for these tapping contests? That's right: DI pipe. To see this contest in action, hop on over to our ACE’18 Pipe Tapping Contest post where you can watch the AWWA’s teams compete for the fastest time.
Jerry Regula, "JR" is a product engineer with McWane Ductile. Jerry’s responsibility is to provide technical support for water professionals at all levels including engineers, private water companies, contractors, municipalities, and water districts. JR has been with McWane Ductile for more than 30 years, starting on the ground floor. Jerry has been involved in nearly all facets of the foundry, providing him with extensive experience in manufacturing, installation, and design of ductile iron water products. JR is a NACE Certified Corrosion Technician, Envision Specialist, and member of the American Society of Civil Engineers. He enjoys spending time with his church where he is a speaker and treasurer, as well as spending time with his wife Susan and their two daughters, Katelyn and Shannon on their farm in Ohio. “Many people do not realize how their water gets to them or how blessed we are to have clean drinking water,” says Regula. “I am honored as a water professional to do my part in ‘Building Iron Strong Utilities for Generations.’" Jerry’s favorite quote: "I can do all things through Christ who strengthens me.” ://www.linkedin.com/in/jerry-regula-6a87b/
A century ago, dedicated American engineers installed iron pipes to create the country's water supply system. This sturdy, safe and reliable product has withstood the test of time. Modern ductile iron pipes have a service life of more than 100 years. Due to their recyclable components, energy saving during service, durability, their own recyclability and the promise of the ductile iron pipe industry, it is a more environmentally friendly product. Ductile iron pipe is a key component of a clean energy future and modern, resilient water infrastructure. Ductile iron pipe has proven itself to stand the test of time.
Ductile Iron Pipes
• Environmentally Superior Pipe
Ductile iron pipes are natural,safe, and sustainable. Ductile ironpipes contain at least 90% recycledmaterials with the pipes themselvesbeing 100% recyclable.
• Better Value
Ductile iron pipe is a better value than plastic pipe because it lasts longer and saves money over time. According to a University of Michigan report, Ductile iron pipe is the more cost-effective material over a pipeline’s service life with lower operational and maintenance costs and lower energy costs. By increasing the flow capacity and reducing costs, significant energy savings are achieved during the life of the pipeline. Ductile iron saves money.
• Lower Emissions and Less Energy
Ductile iron pipe has better environmental performance due to its lower greenhouse gas emissions in both the production and operation phases. From cradle-to-grave, ductile iron pipe is superior environmentally, as it requires less energy to pump water and has a lower environmental impact.
• Resilient
Ductile iron pipe is resilient through extreme weather events, natural disasters, soil contamination, and unpredictable situations. With its strength and durability, Ductile iron is the pipe of choice to protect against wildfires, earthquakes, floods, hurricanes, extreme temperatures, and the regular, ongoing stresses facing water utility systems. It is strong enough to withstand the harshest conditions, from high pressure applications, heavy soil and traffic loads, to unstable soil conditions. It can resist corrosion in most soils, and usually only needs an effective and economical polyethylene encasement. This is a loose sheathing in harsh environments. The metallic nature of ductile iron pipe means that it can be easily positioned underground by conventional pipe locators. Due to its strength, durability and conservative design, ductile cast iron has been the pipe of choice for many years to prevent surges and increase pressure loads.
• Health and Safety
Production of Ductile iron pipe does not release dangerous chemicals like vinyl chloride, dioxin or ethylene dichloride. Ductile iron pipe does not absorb toxins like plastic pipe and provides better protection against drinking water contamination. Ductile iron pipe is safer to install and maintain than plastic pipe, which is more prone to catastrophic installation and operational failures causing injuries to workers and damage to property. For workers who cut and beat ductile iron pipes on site, the installation is simple and safe. The ductile iron pipe is strong and durable, and will not be damaged during operation and installation.
• Longer Life Cycle
Ductile iron pipe requires very little maintenance and has an expected service life of at least 100 years. Ductile iron pipe provides significant cost savings and benefits compared to plastic pipe, which has an average service life of 55 years.According to a recent report by the American Water Works Association, the expected service life of modern ductile iron pipes is at least 105 years. There are more iron pipes in service than any other pipe, and ductile iron pipes are currently the longest service life of any material on the market. A longer life cycle saves money and is better for the environment. It also means fewer interruptions, fewer replacements, and more peace of mind for local communities.
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