Taking a Second Look: HDPE Pipe for Large Diameter Applications

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Let’s Take a Second Look: HDPE Piping for Large Diameter Applications David Landing, P.E.1; and Marcel P. Dulay, Ph.D., P.E.2 1

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Trenchless Design Specialist, Florida Water, Jacobs Engineering Group, Jacksonville, FL. Email: david.landing@jacobs.com 2 Senior Hydraulics Engineer, Florida Water, Jacobs Engineering Group, Jacksonville, FL. Email: marcel.dulay@jacobs.com ABSTRACT High density polyethylene (HDPE) piping has been around since the late 1950s; however, it was not until the 1990s that its use became more widespread in large diameter pipes for municipal water and sewer applications. As its usage has become more common practice, coupled with improvements in the technology, the more recent resins (the building block of HDPE pipe) being produced have created a HDPE pipe product with thinner walls of a certain pressure rating than the previous resin types. In addition, newer technologies in the production process can now produce overall thicker wall HDPE pipe. These two advancements, the strength of the underlying resin and the ability to make overall thicker wall HDPE pipe during the extrusion process, now permit the industry to produce even larger diameter HDPE pipes at higher pressure rating classes that were previously unavailable.

INTRODUCTION This paper will cover HDPE pipe applications for nominal diameters of larger than 36 inches for pressurized conveyance piping in the municipal water and sewer industries. In the current world climate, aging infrastructure has created the constant need for routine rehabilitation and replacement of existing large diameter pipes. This existing infrastructure has either reached the end of its life expectancy or has been prone to frequent failures and/or repairs. Population growth and increased regulation on the industry has further accelerated this need to deliver and serve the needs of the ever-expanding public. The purpose of this analysis is to highlight the many reasons why HDPE pipe should further be considered in large capacity conveyance scenarios. In addition, HDPE is the most common solution for horizontal directional drilling (HDD) installations making it worth a second look when larger diameters and capacities are required. The usage of HDPE pipe can be compared to other piping materials such as ductile iron, polyvinyl chloride (PVC) and steel to find the best solution and product for a given scenario. These varying conditions can include the corrosivity of the soil the pipe is to be placed in or the fluid material to be conveyed, the installation method such open cut or trenchless, site restrictions, impact to the surrounding community, availability, handling concerns, historical performance of existing and legacy pipe materials, current and long term capacity of the pipe, life expectancy requirements, hydraulics and cost among others. Due to the nature of fused HDPE pipe being a flexible, durable, cost effective, fully restrained and easily handled material, it can check a lot of the boxes to be the ideal candidate over other pipe materials once project site conditions have been considered.

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NEW RESINS AND HDPE PIPE SIZES AVAILIBLE IN THE LAST DECADE The continued innovations in the molecular design of high-density polyethylene resins have resulted in the development of a new 4k series specification. These high-density resins have improved the physical attributes of polyethylene pipe such as increased durability, longer design life and increased pressure rating for HDPE plastic piping systems. This higher performing pipe product can be applied immediately to projects to add flexibility of plan options and cost savings.

To understand this better the predecessor of the 4k series resin, the 3k series, was weaker and more prone to slow crack growth (SCG). A direct improvement to this new strength classification is to achieve a pressure rating (PR) with thinner wall HDPE pipes than previously seen. For example, under the 3k series specification a dimension ratio (DR), the ratio of the overall outside diameter (OD) of the pipe over the pipe wall thickness, of 11 was previously rated for 160 psi while now this is achieved with a DR rating of 13.5. Similarly, the new 4k series resin has a PR of 200 psi for a DR 11 HDPE pipe.

While most water and sewer application needs are met with a test and surge pressure of 150 psi pressure class (PC), very few municipalities updated their standard specification requirements by way of DR to consider this new strength. In many cases, they do not even call for the new 4k series class. In discussions, modern manufacturers of HDPE pipe shared that they do not produce pipes with this classic resin but still stamp the pipe as such to fill orders because of these legacy specification requirements contained within existing client construction standards. The table below compares the PC rating of the old 3k resin with the new 4k resin.

Table 1. HDPE Pipe Pressure Classes Comparing 3k to 4k Specifications (Source: Plastic Pipes Institute-Handbook for Polyethylene Pipe, 2018)

From the table above you can see that a legacy specification referring to the 3k series HDPE pipe could have been updated when the new resin was made available to save cost of the pipe, handling/shipping weight and fusing times to a thinner wall pipe while achieving the same required strength and meeting project needs.

Another more modern development, in only the last few years, is the fabrication technology of the HDPE pipe at the production plant. HDPE pipe production includes extrusion, cooling, hot embossing, and cutting. The raw material used is HDPE pellets made from virgin polyethylene granulates and recycled HDPE. [PPI, 2008] Previously, it was believed that the wall thickness had a

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fabrication limit of approximately 3.75 inches due to earlier production equipment and techniques.

In cases where larger diameters were needed to meet the capacity of higher demand systems, the possibility of HDPE as a selection material had to be eliminated as an option because the DR associated with the required PC’s just weren’t available on the market. For example, historically for a system that required a pressure of 150 psi as a minimum for a given product, along with the a production limit that tops out at a 3.75 inch wall thickness under a 3k series specification pipe sizes larger than 36 inches, with a DR of 11, weren’t available on the market. Now pipe diameters over 60 inches can be produced with HDPE and coupled with the 4k series resins ability to achieve a PC of over 150 psi can be done with a DR of 13.5 a great many options utilizing HDPE pipe ranging from 36 to 60 plus inches are available that weren’t so previously.

Part of the design process should include coordination with pipe manufactures including ISCO and organizations such as The Plastics Pipes Institute (PPI) to check with vendors regarding fabrication, availability and order fill lead time as these can be concerns with utilization of these larger diameters. In most cases, if a production plant has the capability for these larger sizes, production of these increased sizes can be done relatively easy by switching over the extrusion tooling on the production equipment to fill the order. Production of larger diameters of HDPE pipe can be done with a very minimal increase in the cost of the HDPE pipe being produced as the cost of HDPE pipe relies heavily on the weight of the HDPE and not the frequency the desired diameter is requested. In other words, aside from relatively small orders, a ton of 24-inch HDPE pipe has a comparable cost to that of a ton of 54-inch HDPE pipe.

HYRDRAULIC CONSIDERATIONS AND HDPE vs PVC/DIP NOMINAL PIPE SIZES Currently in the industry it is often believed when a pipe material in a given scenario switches material from ductal iron pipe (DIP) or PVC to that of HDPE the pipe diameter needs to upsized to a larger OD HDPE pipe due to the increased wall thickness associated with HDPE. The belief is that a huge capacity loss will take place because of the small reduction in pipe inside diameter (ID). Not to eliminate upsizing the HDPE pipe diameter to a larger size as a real and viable solution that should not be overlooked this may not always be necessary.

So how much does a small diameter reduction in large diameter pipe installations really matter when it comes to hydraulics? Ironically, not much thought is given when converting a given installation length from DIP to PVC or steel even though at large diameters this could be a difference of three or more inches of ID even in these non-HDPE conversions. There is much more stigma applied when switching a given installation length over to HDPE from DIP or PVC. In most cases of HDPE pipe HDD installations are often routinely automatically “ups-sized” over the length of the HDD without any thought to hydraulic analysis or requirements of the system. This is to ensure the HDD has at least the same minimum ID for the HDPE pipe segment as the standard municipality’s preferable product pipe on either side connecting to the HDD crossing.

What though are the real-world effects on the hydraulics of the pipe system and more specifically how does this effect the capacity the system can deliver? In truth and when examination is performed, the effect on the system is often negligible or near so. It is important to understand the effects of head loss on a system, rather the pressure loss due to friction associated with moving a volume of a liquid through a given pipe length. All conveyance piping systems see some form of head loss during operation no matter how they are designed. Pumps, booster pumps and water towers that feed a system are designed with this head loss accounted for to deliver the capacity requirements a given system demands.

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To illustrate the effects of the concept, consider this example: compare a 42” PC 200 DIP, a 42” PC 165 DR25 PVC pipe and a 48” IPS DR 13.5 HDPE (PR 160) pipe for a replacement project over a 1,000 ft length. All three of these pipes have similar capacities and have similar ID’s while recognizing the DIP has about 3 inches of ID more than the PVC and HDPE being compared. The pipe inside diameters in these three scenarios would be approximately 43.7 inches, 40.7 inches and 40.4 inches respectively. Utilizing the Hazen-William’s Equation, a common equation used in the industry to estimate pressure loss, this would equate to pressure loss of 0.10 psi, 0.13 psi and 0.13 psi respectively for a system operating with a design flow of 20 cubic feet per second. The flows in the three piping systems being in the 2 feet per second range with actual flow velocities of 1.92 feet/second, 2.21 feet/second and 2.24 feet/second respectively to achieve the same desired flow.

If the operating pressure of the system is 80 psi, these losses would be negligible to the overall pressure.

(1) The Hazen-Williams Equation One design feature of the Hazen-Williams Equation is the application of the “C” factor that corresponds to the pipe being analyzed wall smoothness. In design practice, to the benefit of plastic pipe materials, a common C factor for DIP is about 130-140 while plastic pipes use something in the range of 140-150. Much debate is given from differing entities and vendors in the industry as to which C factor should be applied and this factor can also be assumed to change over design life of the system as different pipe materials inner wall smoothness can be effected from corrosion, abrasion and loss of wall coatings. Regardless, plastic pipes often have smoother walls and better flow characteristics than that of DIP which is represented with the lower corresponding C factor being applied in the Hazen-Williams Equation.

An additional feature to consider is municipal systems are often not direct transmission of a single run of pipe rather set up on a grid system that includes treatment plants, booster pump stations and water towers scattered throughout the grid. In cases where systems are large and include these various design features losses due to the replacements of relatively short runs of pipe with similar ID losses as those in the example above these would almost disappear or become unmeasurable.

Furthermore, DIP is less resistant to corrosion than that of plastic pipes. To aid in exterior corrosion protection, polyethylene plastic is sometimes wrapped around the exterior of DIP installations especially in areas where stray electrical currents and electrical fields are produced from nearby buried electrical utilities which react specifically with metal pipe systems or exterior coatings are added due to the corrosivity adjacent soils. Alternatively, coatings are often also applied to the interior surface of the DIP; in the case of force main sewers due to the corrosivity or abrasiveness the fluids conveyed will have in the case of DIP. These applied coatings could be

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damaged during transport or installation which could mitigate much of their effectiveness. All this should be considered during short- and long-term system design.

It is important to note that HDPE pipes come in two pipe size specifications, Iron Pipes Sizes (IPS) and Ductile Iron Pipe Sizes (DIPS). This versatility is not readily available in other pipe materials. Having this unique information is useful as there are smaller graduations of sizes available that may not be available in other pipe products. For example, 42-inch nominal diameter IPS has an OD of 42.0 inches as compared to 42-inch nominal dimeter DIPS that has an OD of 44.5 inches. The next IPS pipe size available is 48-inch nominal with an OD of 48 inches. In other words, there is an intermediate real world 44.5-inches OD pipe size between 42 inches and 48 inches. This adds more flexibility to pipe diameter selection that can be considered during the design phase of the project.

WATER HAMMER, THE JOUKOWSKY EQUATION AND HDPE PIPE STRENGTH CONSIDERATIONS An important thing to consider for the design of pressure systems is the estimation of occasional surges, sometimes referred to as water hammer, that can occur in the system that would exceed normal operating pressures. The extensive study of water hammer dates back to the 19th century. [Bergant et al., 2006; Ghidaoui et al., 2005] Occasional surge (OS; emergency or transient) pressure is defined as short-term internal pressure events usually caused by emergency operations of the pipe network system (e.g., a rapid valve closure) or malfunction (e.g., power failure, component failure, etc.).[AWWA, 2019] The occasional surge events are only expected to happen a handful of times in the lifetime expectancy of a system. One accepted method for estimating pressure surges for immediate velocity change events is the Joukowsky Equation that for the purposes of this paper will be used to illustrate this phenomenon. The Joukowsky Equation relates the increase in piezometric head or pressure resulting from an instantaneous or near instantaneous reduction in velocity of fluid in a piping system. The Joukowsky Equation is not the final word for water hammer events as it has been built upon and further expanded through expansive research with additional formulas and analysis to understand it applicability and limitations but it does paint a picture as to how different pipe materials can affect the magnitude of water hammer in real world application.

(2) The Joukowsky Equation In this equation the “a” or wave speed is of particular interest as in a given situation the velocity and density of the fluid will be the same for comparison purposes between the different pipe materials the fluid is being conveyed in. This wave speed expresses the propagation velocity of a pressure wave in a pipe during these velocity changes. According to the equation there is a direct corresponding effect of the increase in wave speed to the increase of pressure seen in system. Some of the factors that can affect the wave speed include the pipe material, the fluid being conveyed, the DR rating of the pipe and how the pipe is restrained whether this be from actual restraint © ASCE Pipelines 2021

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features or the medium the pipe is buried in. Since wave speed, and the resulting pressure increase from a OS event can be hard to predict it is critical municipalities employ competent specialty engineers, that often use specially designed software such as Bentley’s Water Hammer[TM] to estimate OS effects and to determine the need, design and installation of risk mitigation measures.

Devices and monitoring equipment can also be added to a system to observe OS events to measure quick changes in pressure and the wave speeds found during these events for future mitigation measure implementation. Since the phenomenon is heavily dependent on the rigidity of the pipe carrying the material a pipe such as steel or ductile iron would see a pressure increase three to four times that of a softer material as those found in plastic pipe such as PVC or HDPE and in turn would see a similar increase in pressure.

These pressure increases also effect the more rigid materials differently. This can be seen in the way each material is handled in design for the analysis of water hammer events. Conveyance piping is expected to exhibit additional pressure capacity during short term surge event times than the normal operating pressure of a systems overall life cycle. In practice for OS events, DIP is typically expected to carry an additional 100 psi of capacity to the PC while PVC can carry 1.6 times the PC and HDPE can carry 2.0 times the PR. This increase due to OS would be added to the normal operating or situational pressures analyzed to estimate what effects this would have on the system.

For example, compare 42-inch PC 200 DIP to 42-inch PC 165 DR25 PVC to 48-inch IPS DR 13.5 HDPE (PR 160). All three of these pipes have similar capacities and have similar pressure ratings (the DIP has approximately 30% more than the PVC and HDPE). The wave speeds in these three systems would be approximately 3,260 ft/sec, 1,140 ft/sec and 880 ft/sec respectively for water conveyed in a given restraint condition. This would equate to pressure increases of approximately 290 psi, 100 psi and 80 psi respectively for an event of a 6.5 ft/sec velocity decrease.

If the operating pressure of the system is 80 psi the pressure in these three scenarios would jump to 370 psi, 180 psi and 160 psi respectively. The OS pressure of the three scenarios being 300 psi (200 psi + 100 psi for DIP), 264 psi (165 psi * 1.6 for PVC) and 320 psi (160 psi * 2.0 for HDPE) respectively. The plastic pipes would perform better during this surge event than that of the DIP. In addition, the higher pressures seen from the DIP system scenario could cause damage on other parts of the system, equipment and appurtences. While DIP has the highest PR of the three pipe materials, HDPE has the highest OS pressure allowance of the three.

So why is HDPE so resilient to these forces applied in the short term? It is because of the way HDPE behaves under stresses and strains. HDPE can see strains up to its yield strength and undergo relatively large dimension changes than that of other pipe materials and bounce back to its original length undamaged. 3k and 4k series HDPE pipe can be lengthened in most cases by over one percent with an applied force with no damage to the pipe, returning to its original length after the force is removed. Furthermore, HDPE has the unique property of exhibiting a different strength depending on the amount of time the force is applied. Aside from small variations other pipe materials such as steel, ductile iron and PVC will see similar strains for a force that is applied for one hour or fifty years while HDPE does not behave in this manner. This is referred as the Apparent Elastic Modulus. The Apparent Elastic Modulus for various specified HDPE pipes under different periods of loading are shown in the table below.

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Table 2. HDPE Pipe Specifications Apparent Elastic Modulus (Source: Plastic Pipes Institute-Handbook for Polyethylene Pipe, 2018)

The above table portrays that while the PC of a given HDPE pipe is built on a life expectancy of 50 years the HDPE has a strength greater than that if the loading is applied for shorter time periods such as those found during OS events. For the shortest duration of sustained loading condition shown in the table of half an hour the elastic modulus is approximately 2.8 times greater than that of the 50-year duration sustained loading condition.

HDD CONSIDERATIONS – TALE OF THE TAPE Due to the flexibility of HDPE pipe, much shorter HDD installations can be made than when compared to steel. HDD has become one of the most common and cost-effective trenchless installation methods on the market. Trenchless installation has become a common place due to minimization of impact on the environment such as installations required across wetland areas, lakes and rivers. It also decreases the disturbance to societal structures and movement; as in the case with heavily populated and developed urban centers such as high use interstates and travel ways.

HDPE pipe can achieve extremely low bending radii as compared to comparable steel pipe diameter sizes. Even the largest of HDPE pipe can achieve bending radii under 450 feet; where the rule of thumb for steel pipe is 1200 times the pipe diameter. For example, manufacturer recommendations and best practices dictate that for a 48-inch diameter steel pipe the minimum bending radius is 4,800 feet, over ten times the minimum bending radius of even a 54-inch diameter HDPE pipe. This impacts the minimum achievable overall HDD installation length as can be seen in the two figures below. This comparison was performed with a given set of “ground rules”. This included no compound curves (where a horizontal curve and vertical curve overlap) nor a single horizontal curve; an entry and exit angle of 12 degrees; a 50 foot distance between subsequent vertical curves as well as between the ground surface and the beginning of vertical curvature, and

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last a minimum depth of ground cover of 50 feet. In the current market fusible PVC doesn’t have a pipe diameter over 36 inches.

Figure 1. Scenario: Sample Profile Using HDPE Pipe Figure 1 shows that the HDPE pipe scenario can make this crossing in just under 689 feet. It should be stated that while this bending radius can theoretically be achieved with HDPE, pipe other considerations should be included when designing a minimum bending radius for HDD such as ground/site conditions, experience and ability of the selected contractor, is the small installation radius actually necessary, HDD drill rod capabilities and engineering best practices.

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Figure 2. Scenario: Sample Profile Using Steel Pipe For comparison, as seen in Figure 2, a 4,000-foot minimum bending radius was used which is less that the minimum bending radius for 42-inch steel pipe. This installation would require over 1,800 feet of installation length, almost three times the length of the comparable HDPE pipe installation scenario. This installation also would need an increased depth for the geometry and installation parameters to be achievable.

The minimum allowable bending radius would play a big part in the pipe string and staging area of the work site. Comparatively small or oddly shaped sites can be utilized when stringing out HDPE pipe as the pipe can curve back and forth to run along right of ways and set aside constructions areas that wouldn’t be possible with the more restrictive comparably sized steel pipe.

Fused HDPE pipe, while being fully restrained just as welded steel would be, has fusion and cool down times less than that of steel pipe weld and cooldown times. Plastic pipes do not require cathodic protection design and cathodic coatings to be applied in the field as in the field of steel pipe; thus, reducing overall construction time.

CONCLUSION Through the years, due to municipalities necessary adoption of standards, pipe material selection has been often overlooked as part of the design process. This is found true in the majority of cases except in the most complex and unique situations. These standards, once recorded, are at many times left as a legacy decision point even though technology and new systems have continued to evolve in the marketplace. The selection of pipe material, as it pertains to large © ASCE Pipelines 2021

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dimeter pipeline projects, should be given as much credence as other facets of organizing a solution to a conveyance problem. In some ways this selection could have more impact an overall projects implementation and success than these other project components and should not be overlooked.

With recent improvements in HDPE pipe materials and extrusion technology, there are more modern readily available options that just decades ago were not available. As engineers and utility owners we should investigate and consider all the tools available while building a more connected and sustainable world.

REFERENCES AWWA (American Water Works Association). (2019) Structural Classification of Pressure Pipe Linings: Suggested Protocol for Product Classification, AWWA Committee Report. AWWA, Denver, Colorado.

Bergant, A., Simpson, A. R., and Tijsseling, A. S. (2006). “Water hammer with column separation: A historical review”, Journal of Fluids and Structures. 22, 135-171.

DIPRA (Ductile Iron Pipe Research Association). (2016) General Ductile Iron Pipe, Last Revised May 2016. DIPRA, Birmingham, Alabama, USA. Ghidaoui, M. S., Zhao, M., McInnis, D. A., and Axworthy, D. H. (2005). “A Review of Water Hammer Theory and Practice”, Applied Mechanics Review. 58(1), 49-76.

NAPCO Pipe and Fittings. (2019). AWWA C900 Pressure Class Explanation, Technical Bulletin, Houston, Texas, USA. PPI (Plastic Pipes Institute). (2008). Handbook of Polyethylene Pipe, Plastic Pipes Institue, Second Edition Revised 2008, Irving, Texas, USA. Twyman, J. (2016). Wave Speed Calculation for Water Hammer Analysis, Obras Y Proyectos No.20 Concepción (December 2016), Rancagua, Chile.

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