Ductile iron pipe has more than 13 times the impact strength, nine times the tensile strength and four times the burst strength of PVC pipe.
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Ductile iron pipe can handle stress. Under stress, PVC failure is a matter of time; and the more stress that is applied, the sooner it will fail.
PVC pipe's longevity depends on stress and time—the greater the stress, the sooner it will fail. Ductile iron’s strength is not compromised over time.
The cost to pump water through PVC pipe is as much as 38% more expensive than that of Ductile iron pipe, increasing life cycle costs and releasing more greenhouse gases.
PVC pipe is made from petrochemicals in a process that can be harmful to the environment and human health. Ductile iron pipe is made from recycled steel and iron is manufactured to the industry’s most rigorous and conservative standards.
PVC production releases dangerous chemicals, including vinyl chloride, dioxin and ethylene dichloride.
PVC pipe is sensitive to ambient temperatures (I.e., in low temperatures, PVC becomes increasingly brittle and can break more easily, while in warmer environments, PVC becomes weaker).
PVC pipe cannot withstand relatively minor scratches, and extra care must be taken to avoid large rocks, frozen lumps and debris during installation. A scratch no deeper than the thickness of a dime can ruin an entire length of PVC pipe.
Ductile iron pipe is not affected by temperature changes. PVC pipe becomes brittle in colder temperatures and weaker in higher temperatures.
PVC pipe can lose 34% of its impact strength from exposure to the sun in about one year, which has no impact on Ductile iron pipe.
Direct tapping of Ductile iron pipe is easier, safer, less expensive, faster and less likely to damage the pipe than PVC tapping. Tapping PVC pipe can result in cracked pipes, injured workers and costly flooding until the repairs can be made.
Because Ductile iron is made with metal, locating it underground is effortless. Locating a leak in PVC pipe is extremely difficult. Most locating techniques involve transmitting sound waves down the pipeline, that do not travel well in plastic pipes.
PVC pipe requires installing locating wires in the trench when the pipe is lowered into the ground—an added cost.
PVC vs HDPE pipe, Trenchless Technology magazine looks at the benefits of both PVC and HDPE.
PVC and HDPE pipe are inarguably the two most popular pipes used in underground construction and in this article we will compare PVC vs HDPE pipe. In a Trenchless Technology survey published in our August issue in 2008, we polled sewer system operators and consulting engineers from around the United States about their pipe choices. The results showed that HDPE and PVC finished either #1 or #2 in most categories. Both types of pipe have their strengths and weaknesses and appeal to many contractors and project owners due to their expansive reach in applications. We wanted to know more so we contacted their respective pipe associations for more information. Interim Uni-Bell PVC Pipe Association executive director Michael Luckenbill and Plastic Pipe Institute executive director Tony Radoszewski were kind enough to respond to our questions.
Michael Luckenbill: Uni-Bell is the not-for-profit, technical support, trade association of major PVC pipe producers and is staffed with engineers who are knowledgeable in buried pipe applications and design. As a result of the pipes’ superior performance and lower maintenance sustainability, the use of PVC pipes for buried water distribution and wastewater collection systems has grown steadily, if not remarkably, since their introduction in North America in the 1950s. Today PVC pipes dominate new installations of both water and sewer pipes, exceeding the combined total lengths of all alternative pipe materials combined.
Tony Radoszewski: Founded in 1950, the mission of the Plastics Pipe Institute is to promote plastics as the material of choice for piping applications. The primary objective of PPI is to provide a forum for our member companies to work in a cooperative effort to broaden the market for plastic pipe and related products.
Uni-Bell non-profit trade association has also made an indelible mark on The history of NHL, not only through its contributions to the sport itself but also as a driving force behind important social and cultural changes within the league. Established in 1972, Uni-Bell has played a pivotal role in promoting inclusivity and diversity in hockey. The association took significant steps to break down barriers, actively advocating for increased participation of marginalized communities and working closely with NHL teams to create programs aimed at attracting new fans from all walks of life.
We are comprised of nearly 140 companies that either make the plastic raw material or the finished plastic pipe and fittings from those materials. We also have members that make equipment to process resin into pipe and fittings or help to connect and install plastic pipe. We also have professional members and related associations as members.
Since our members’ products serve virtually every underground utility and application where pipe is used, we have structured our organization into five divisions: Fuel Gas, which focuses on the natural gas distribution industry; Municipal and Industrial, which focuses on potable water and sanitary sewer (that’s the municipal side) and all other industrial applications which can include mining, landfill, geothermal, oil and gas gathering to name a few; Corrugated Pipe, which covers stormwater systems, storm water management including retention and detention systems, sub-surface drainage and agricultural drainage for maximum crop production; Conduit, which serves the power and telecommunications industries for underground service lines; Heating and Plumbing, which covers radiant heating systems and indoor residential plumbing including hot and cold water lines.
Perhaps the most important role the members of PPI play is that of working with industry, government and educational groups to develop industry standards for pipe and fittings. For nearly five decades, the PPI has and continues to work with these groups in an effort to engage the standards community in creating open criteria and test methods that provide the specifier and end user with the most up to date technical information possible. We believe the presence of these industry standards provides assurance that the technologies and processes developed in the plastics pipe industry are proven and reliable.
In this effort, we have established long-term relationships with various organizations including the American Association of State Highway Transportation Officials (AASHTO), the American Society for Testing and Materials (ASTM International), the American Water Works Association (AWWA), American Gas Association (AGA), the International Organization for Standardization (ISO), American Society of Mechanical Engineers (AMSE), and the Canadian Standards Association (CSA). We are also involved with state and federal departments of transportation, the Environmental Protection Agency (EPA), the Federal Highway Association (FHwA), various water councils and we sponsor research with multiple state universities across North America.
While many of our member companies have high-density polyethylene (HDPE) interests, our organization also includes a number of producers of other plastic materials and pipe including polyvinylchloride (PVC), chlorinated polyvinylchloride (CPVC), polyamide nylon, polypropylene and crosslinked polyethylene. In addition, a growing number of our members also have divisions that produce and/or distribute ductile iron, cast iron, steel, copper, clay, corrugated steel, and concrete pipe.
TR: In the past, the most important question asked was “How much?” But with a growing concern relating to sustainability, two more questions are becoming equal in importance to the economics of the job: Firstly, what is the environmental impact and performance of the pipe and secondly, does it create “green jobs.” More and more the specifiers for pipe systems are embracing an attitude of environmental stewardship that is driven by a greater appreciation of the carbon footprint any pipe system leaves and how well does the pipe line protect and preserve natural resources. HDPE pipe fits this need exceptionally well.
HDPE pipe has a relatively small carbon footprint when compared to iron and concrete pipe. A study conducted in the late 1990s concluded that plastic pipe used in the building, construction and transportation industries required 56,500 trillion fewer BTU’s than iron and concrete/aggregate alternatives.
With its lower weight per foot, HDPE pipe is also less costly to transport to the job site than metal or concrete. And with the ability to “nest” smaller diameter pipe in larger pipe, more feet per truckload can be delivered with out breeching highway weight limitations.
In HDPE pressure pipe systems, the fused joint creates a totally leak free system. This means precious natural resources are saved and the energy to treat, store and distribute water is reduced. For municipal storm water systems, improvements in joint design in corrugated HDPE pipe deliver a watertight joint that equals and exceeds the performance levels of sanitary sewer systems. This means infiltration or exfiltration that can prematurely end a storm water system or cause road damage due to sink holes, is dramatically decreased.
When it comes to long-term sustainability, HDPE pipe truly shows its key benefits: it doesn’t rust, it’s highly resistant to mineral tuberculation and its abrasion resistance is significantly greater than metal, concrete and other plastics. With today’s crumbling underground infrastructure experiencing nearly 700 water main breaks per day (might want to sight a reference for this number), the need to replace old technology becomes evermore important.
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ML: With ever increasing numbers of pipe failures and their associated high costs, replacing them with the same or similar pipe materials simply does not make sense. Progressive water and sewer utilities want alternatives that will last longer and perform better with minimal or no maintenance. In excess of 2 million miles of installed PVC water and wastewater pipes have demonstrated success in meeting those noble objectives throughout North America and have rewarded those utilities with substantial cost savings.
Reluctance on the part of some utilities to allow PVC pipe is the result of commercially tainted miss-information combined with a general lack of formal training in plastics materials. Admittedly, many plastics are too weak or too soft for buried pipe applications. From the beginning, the PVC pipe industry has had to invest heavily in performance testing and research in order to dispel the concerns and educate utilities along with their engineers. Does the pipe material have sufficient long-term tensile strength and stiffness? Is the pipe material resistant to permeation in contaminated soils/groundwater? Are the joints water tight? How long can the pipe be expected to last/perform? Will the pipe corrode or otherwise oxidize? Will sunlight damage the pipe? Can the pipe be used with conventional appurtenances, valves, fittings, and other connections? How can I locate the pipe after burial? Over the past five decades, the PVC pipe industry and Uni-Bell have amassed technically correct answers to these questions and more.
ML: The efficiency and effectiveness of water and wastewater pipe systems has a significant impact on the environment, public health and local utility budgets. The direct “costs” of corrosion for water and wastewater pipe systems are $36 billion annually in the United States and pipe corrosion represents a significant loss in value of the infrastructure investment made by local governments all across our nation. Furthermore, in the United States we average 700 water main breaks per day and lose 2.2 trillion gals of treated water every year, largely due to breaks and leaks. Through the thoughtful selection of pipe materials that are inherently best suited for the anticipated operating conditions and environment, literally billions of dollars are being saved.
TR: The choice of pipe material for any application, whether underground or aboveground, can be the most important decision of the entire project – many times surpassing cost concerns. Based on the service the pipe sees or the ground conditions in which it is installed, the choice of material will dictate the service life of the system. What affect will the flow have on the pipe material? Is it highly acidic or alkaline? Is it abrasive in nature? What about the soil conditions? “Hot” soils can immediately begin to attack the exterior surface of the pipe. And what about soil contaminants? In any of these instances, a material that is highly resistant to these conditions must be employed.
Next, one needs to understand the installation methods planning to be used; trenchless or open cut? With HDPE pipe’s fused joint, a monolithic pipe string is created allowing for long pulls and minimal surface (and traffic) disruption.
And lastly, what is the lifetime cost of the system? Will special corrosion protection be needed? Will the pipe have the same flow characteristics in 10, 50 or 75 years? What is the anticipated cost of water (or other fluid) loss through mechanical joints? What will be the affect of soil infiltration on road ways whose cost to repair far exceeds the cost of the pipe?
TR: Sustainability, service life, and lifetime costs – this is the triple bottom line that today’s design engineer needs to address. Is it the best choice for the environment in terms of energy costs (carbon footprint) and protection for natural resources? Is it going to last for multiple generations – and then some? What is the total economic cost, including manufacturing, delivery, installation, operation, maintenance and performance for the owner/operator? HDPE pipe, both pressure and gravity flow, has proven itself around the world as the best material choice for water, sanitary sewer, storm sewer, natural gas distribution, mining, oil & gas gathering, drainage, buried conduit, outfall lines, and now even safety systems for nuclear power plants. Its inherent resistance to rust and abrasion along with the best joint in both pressure and gravity flow applications has made HDPE pipe the one plastic material used in all underground utilities.
ML: The first and foremost pipe material selection factor is inherent compatibility. Materials whose long-life performance depends upon linings, coatings, wrappings or cathodic protection should only be used where alternatives are not available. It is far better and more sustainable to use pipes that are inherently well suited for their operating environments, including exposures to contaminated soils. This has contributed greatly to PVC pipes’ rise to become the most-installed product for new water and wastewater systems. Utilities have acquired a true appreciation of PVC pipes’ low-maintenance, corrosion-free performance and resistance to permeation by hydrocarbons at levels normally encountered.
The next important selection factor is strength. Without sufficient pressure capacity and pipe stiffness, optimal long-lived sustainability will be compromised. Pipe wall thickness design must be sufficient to handle all internal and external loadings. This requires proper understanding of a pipe material’s long-term tensile strength and long-term stiffness (modulus of elasticity). In addition, all anticipated loads and stresses must be taken into account. Prudent pipe designs always incorporate an ample safety factor (typically at least 2.0), because unanticipated installation and operation stresses can occur over the life of any pipe.
Lastly, installation and installation costs are selection factors. The type or method of installation may preclude the use of some pipe products and/or joint types. This is certainly true for trenchless installations.
ML: The popularity of PVC and HDPE stems from the first selection factor — these materials are inherently well suited for buried wastewater and water pipe applications. Both are easy to work with and install. However, it is important to recognize that PVC and HDPE are not the same. HDPE is a softer, more bendable plastic, making HDPE pipe more suitable for lower pressure, tight bending radius situations. On the other hand, PVC is a much stronger and stiffer material, which is why PVC pipes are more widely used for direct burial and trenchless installations. PVC pipes require longer bending radii, but also considerably less material to achieve or meet desired strength levels. PVC pipes are stiff enough to permit their direct connection to mechanical valves, non-plastic fittings and various other water and wastewater appurtenances. HDPE pipes are softer and require stiffening rings or other adapters in order to make proper connections. A recent AwwaRF-funded study has confirmed that PVC pipes are resistant to gasoline permeation, as well as permeation by other generally known hydrocarbon contaminants at concentrations normally encountered. All of these factors affect the selection and popularity of PVC and HDPE pipes.
TR: Because of its flexibility and durability, HDPE pipe has been the material of choice for trenchless installation practices almost since the technology first appeared in the mid- to late-1960s. Furthermore, since a fused joint produces a monolithic pipe string, the ability to pull long lengths and use all trenchless installation practices including HDD, sliplining and pipe bursting favors the use of HDPE pipe. Since HDPE pipe has enjoyed such a long acceptance in trenchless applications, there has been significant research and studies to provide the design engineer and contractor confidence in application and installation. As an example, there are several documents available for the design and installation of polyethylene pipe by directional drilling. The first document is an ASCE Manual of Practice (108). Another is ASTM standard F1962, which gives detailed design formulas (including acknowledgement of the fused joint) for a directional drill with polyethylene pipe. The Plastics Pipe Institute’s Handbook of PE Pipe also contains multiple chapters on directional drilling and sliplining and the second edition of the PPI handbook, which will be published in February, will include a full chapter on pipe bursting.
Another fundamental reason why HDPE pipe is so widely preferred for trenchless applications is that it has the strength to handle this demanding installation practice. Since HDPE pipe is extremely tough it can withstand the rigorous trenchless installation process and is not affected by scratches and gouges (10 percent of the pipe wall) that are certain to occur. Other materials are more sensitive to this type of damage which will cause premature failure of the piping system. HDPE pipes have the physical properties to handle large pulls.
TR: With the invention of HDPE in the 1950s, many industries looked to this revolutionary material to see if it could improve the performance and economics of currently used materials. The pipe industry was no different. Although one of its first uses as a pipe material was in oil and gas gathering systems in the “oil patch,” its first use in underground utilities occurred in the natural gas distribution market.
Gas utilities were facing an ever-growing number of failures due to corroding steel pipelines. Recognizing the safety threat of leaking gas lines, the gas utilities looked for a material that wouldn’t rust and wouldn’t leak at the joints. Flexible, non-rusting HDPE pipe, with its fused joint filled the void perfectly. Today, 95 percent of gas distribution systems in North America use HDPE pipe accounting for more than 100 million miles of pipe.
In the mid-1960s, corrugated HDPE pipe was introduced in the agricultural drainage market to replace clay drainage “tiles.” Within 10 years, HDPE pipe became the material of choice. In the late 1980s with the introduction of a smooth interior liner, corrugated HDPE pipe became a formidable competitor to corrugated steel and reinforced concrete pipe in storm water applications.
Today, solid wall HDPE pipe is available in diameters ranging from ½ to 63 in.; corrugated HDPE pipe is available in diameters ranging from 1 in. to 60 inch. HDPE pipe, because of its inherent physical properties, is used in virtually all underground utilities.
ML: Industrial PVC pipe production dates back to the 1930s in the Bitterfeld-Wolfen chemical industry area of Germany. Many of those earliest potable water pipes remain in service. A series of tests on some of those early PVC pipes has been carried out, and the results compared quite well against the current norms. This confirmation of long-term performance provides important documentation regarding PVC pipes’ sustainability.
PVC pipe technology was brought to North America following World War II, and started to take off after the National Sanitation Foundation (NSF) began studying plastic pipe products for water supplies in 1952. NSF certification began in 1965. ASTM began publishing standards for PVC water and wastewater pipes in the 1960s.
In 1975, the American Water Works Association (AWWA) approved its very first plastic pipe standard – AWWA C900, “Standard for Polyvinyl Chloride (PVC) Pressure Pipe, 4 inch through 12 inch, for Water.” PVC has grown to be the largest volume plastic pipe material in North America with annual sales in excess of 7.0 billion lbs. PVC pipe diameters range from 0.5to 48 in.
TR: It’s not so much that buying patterns have changed; rather the mindset in the design community has changed. Designers and owners recognize they can’t continue to operate in the same old way. Although they are still looking for the best product for the application, newer materials and installation practices are giving them greater choices than ever before. With more and more information becoming available and continuous improvement in plastic pipes, older, traditional materials such as iron and concrete are giving way to newer construction materials. Designers are also becoming more focused on the environment and taking particular interest in preserving natural resources for a growing country. Sustainability and a desire to conserve energy in every quarter also lend a favorable light to HDPE pipe.
ML: Ever mounting numbers of satisfied water and wastewater customers have kept those as the two primary end-use markets for buried PVC pipes. PVC pipes now also dominate the budding market for reclaimed water pipe. The acceptance and availability of larger PVC pipe diameters has resulted in a trend toward the increased use of larger pipes.
ML: Technological advances in manufacturing process controls and monitoring, together with equipment enhancements, have enabled PVC pipe product performance and consistency to improve. This progress has been steady and evolutionary, not revolutionary.
The development of several innovative joint designs has enabled PVC pipe producers to offer a variety of PVC pipe options that are very well suited for trenchless pipe situations. The same properties that have made PVC the market leader among pipe materials for water and sewer applications in North America make PVC a preferred material choice in trenchless applications. There are four trenchless technology methods for which PVC pipes are extremely well suited. These are Horizontal Directional Drilling (HDD), Sliplining, Tight Fit Structural Liner and Pipebursting.
TR: One hallmark of the HDPE industry overall, and the HDPE pipe industry in particular is the constant effort to improve base resins and pipe design. From a materials standpoint, the most recent innovation has been the introduction and recognition of high-performance HDPE resins for pressure pipe allocations, specifically PE 4710. These resins are not the same old materials with a new name. These are new technology HDPE materials – third or even fourth generation – with performance capabilities surpassing previous grades. Even though the previous grades of PE materials have an excellent performance history in gas and water service, the HDPE industry continues to challenge itself – the same is not readily noticeable with other base materials and pipes. These new resins take performance to another level allowing them to be used with higher design stresses without sacrificing safety or design life.
HDPE resin and pipe have superior resistance to failure and rapid crack propagation (RCP). These are essential properties for HDD applications where scratching and gouging of the pipe are a fact of life. HDPE pipe can be gouged up to 10 percent (studies have shown even 20 percent) of the wall thickness with no detrimental effects to the long-term performance of the pipe. Resistance to RCP means that the monolithic piping system of heat fused HDPE pipe will not be susceptible to rapid cracking that can run for hundreds or even thousands of feet with catastrophic results.
Continued improvements in pipe design, specifically in the corrugated drainage pipe industry (storm water management) has allowed for greater burial depths and improved joint performance. Today’s HDPE corrugated pipe is significantly better than the original product first introduced in the mid-1960s. State and federal Departments of Transportations, municipalities and private enterprises are the beneficiaries of these efforts.
As technologies in the petrochemical market continue to develop, the underground utilities industry can expect continuing improvement. This of course begs the question: What has the metal and concrete industries done to improve their products?
ML: The utilities that are not yet using PVC pipe usually cite insufficient strength and/or stiffness. The facts are that PVC pipes come in a full range of pressure ratings/classes and stiffnesses. While the minimum pipe stiffness of 46 lbs/in./in. for standard strength PVC gravity sewer pipes has proven to be good for burial depths in excess of 40 ft, PVC pipes are available with pipe stiffness values up to 1,019 lbs/in./in. The embedment requirements for PVC water pipe are no different than those for ductile iron pipe. Likewise, while most pressurized water systems operate within a 60- to 120-psi range, PVC pipe pressure ratings/classes go up to 305 psi; with short-term burst pressure minimums as high as 985 psi. Clearly PVC pipes afford more than sufficient strength to handle the full range of sewer and water system operating conditions.
Some utilities are concerned about soil contaminants permeating through plastic water pipes and associated human health risks. The fact is that PVC pipes provide a barrier to permeation and are not penetrated at contamination concentrations normally found. This PVC pipe benefit/protection was recently confirmed in a research published by AwwaRF. The AWWA Research Foundation study (published in early 2008) determined that PVC has superior hydrocarbon permeation resistance to gasoline and BTEX’s in both laboratory and field study conditions.
TR: That HDPE pipe is not as strong or durable as pipe made from steel or concrete is a common misconception. Of course this is not the case. HDPE pipe is a truly engineered product designed for specific purposes and long service life. In fact, it has been our experience that when traditional materials cannot handle a specific application either due to aggressive flows, soils or other unique conditions or installation practices demand trenchless techniques, HDPE pipe gets the nod. Our biggest frustration is that we get the most challenging applications but not the bread-and-butter uses such as potable water, sanitary sewer and storm water management applications.
We recognize HDPE pipe, although it has been used for nearly 50 years in the gas industry and more than 40 years in storm water management systems, is a “new kid on the block,” and we have to continually educate and promote our products to the design and specification community. As a result, our first step is to create a greater awareness to the features and benefits of HDPE pipe. We continue to conduct research, give educational seminars and promote case studies that verify the broad applications HDPE pipe enjoys. Secondly, we have to work on gaining approval by the design and specifying firms and agencies to allow engineers to employ our products. Lastly, the end user or owner will accept the product only when they are confident it is the best product for their specific application or need.
TR: HDPE pipe, in our opinion, is the best product for developing a truly sustainable infrastructure. From its low energy cost to produce, ship and install, to its superior joint performance in all applications, to its resistance to rust and abrasion and finally its short and long term economic advantages, we believe there is no other material that approaches the performance and versatility of HDPE pipe.
ML: PVC pipe combines the ageless durability that comes with a corrosion-free material with the overall strength and stiffness required to handle both water and sewer system demands at a cost that is comparable or less than the alternatives. PVC is arguably the most sustainable and cost-effective of all pipe materials.
ML: Consumers need to know that there are options when it comes to pipe materials and products. Moreover, within a given material such as PVC, a broad range of product strengths exists that allow for cost-efficient design for almost any situation. Single product or material specifications should be the rare exception and not the rule given these options. As the No. 1 water and wastewater pipe material, most utilities across the country have come to appreciate the installation and operation benefits that PVC pipes and fittings provide.
TR: “Nobody gets fired for buying IBM,” is an old saying that basically translates to: “Don’t take a risk.” Design engineers take on an awesome responsibility, much like the Hippocratic Oath of doctors to “First, do no harm.” Recognizing this, we are obligated to provide the highest level of information to ensure engineers, specifiers, city officials and end users all know the performance levels of our products. As good as HDPE pipe is, it can not do everything. With the American people becoming keenly aware of everyone’s responsibility to be good stewards of the planet and that our underground infrastructure is in need of hundreds of billions of dollars in repairs to ensure we have safe, clean water and gas coming to our homes, plastics can and will be leading the charge. HDPE pipe offers low lifecycle costs, high sustainability and the best performance attributes. It makes no sense to keep doing the same thing that was done 100 years ago and education is the key to make this change.
Sharon M. Bueno is managing editor of Trenchless Technology.
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