MECHANICAL HANDZ: INTRODUCTION OF POLYETHYLENE

1.1 introduction of Polyethylene

Since its discovery in 1933, polyethylene (also known as polythene) has grown to become one of the world’s most widely used and recognized thermoplastic materials . The versatility of this unique plastic material is demonstrated by the diversity of its use. The original Application for polyethylene (PE) was as a substitute for rubber in electrical insulation during World War II. Polyethylene has since become one of the world’s most widely utilized thermoplastics. Today’s modern polyethylene resins are highly engineered for much more rigorous applications such as pressure-rated gas and water pipe, automotive fuel tanks and other demanding applications. Polythene’s use as a piping material was first developed in the mid 1950’s. In North America, its original use was in oil field production where a flexible, tough and lightweight piping product was needed to fulfill all the needs of a rapidly developing oil and gas production industry. The success of polyethylene pipe in these installations quickly led to its use in natural gas distribution where a coil able, corrosion-free piping material could be fusion joined in the field to assure a “leak free” method of transporting natural gas to homes and businesses. Polyethylene’s success in this critical application has not gone without notice and today it is the material of choice for the natural gas distribution industry. Sources now estimate that nearly 95% of all new gas distribution pipe installations in North America that are 12” in diameter or smaller are polyethylene piping

The performance benefits of polyethylene pipe in these original oil and gas related applications have led to its use in equally demanding piping installations such as potable water distribution, industrial and mining pipe, force mains and other critical applications where a tough, ductile material is needed to assure long-term performance. It is these applications, representative of the expanding use of polyethylene pipe that are the principal subject of this article. In the chapters that follow, we shall examine all aspects of design and use of polyethylene pipe in a broad array of applications. From engineering properties and material science to fluid flown and burial design; from material handling and safety considerations to modern installation practices such as horizontal directional drilling and/or pipe bursting; from potable water lines to industrial slurries, all these things have led to the growing use of polyethylene pipes in the world

1.2 Features and Benefits of HDPE Pipe

When selecting pipe materials, designers, owners and contactors specify materials that provide reliable, long-term service durability, and cost-effectiveness. Solid wall polyethylene pipes provide a cost-effective solution for a wide range of piping applications including gas, municipal, industrial, marine, mining, electrical and communications duct applications. Polyethylene pipe is also effective for above ground, buried, trench less, floating and marine installations. According to David A. Willoughby, P.O.E., “…one major reason for the growth in the use of the plastic pipe is the cost savings in installations, labor and equipment as compared to traditional piping materials. Add to this the potential for lower maintenance costs and increased service life and plastic pipe is a very competitive product.

Natural gas distribution was among the first applications for medium-density polyethylene (MDPE) pipe. In fact, many of the systems, currently in use, have been in continuous service since 1960 with great success. Today, polyethylene pipe represents over 95% of the pipe installed for natural gas distribution in diameters up to 12” in the U.S. and Canada. PE pipe has been used in potable water applications for almost 50 years and has been continuously gaining approval and growth in municipalities. The production, quality assurance and testing of PE gas pipes, including joints, are carried out according to international AWWA, NSF, and ASTM standards. The fear often expressed in the early days that HDPE would have insufficient resistance to the aromatics contained in natural gas (such as tetrahydrothiophene (THT), concomitant substances and condensates) has not been confirmed, either by laboratory tests, or by practical experience. Other material alternatives do not share PE’s advantages. For instance, there are about 23,000 fractures and corrosion failures of iron mains across the United Kingdom each year. Of these events, the majority are located and dealt with in a safe manner. However, on average, about 600 of these results in the leakage of gas into buildings and annually this results in 3 to 4 major incidents involving fire.

1.3 History of Polyethylene Pipe

The history of the polyethylene (PE) pipe began with early civilization's attempts to find a suitable transport medium that could move water and other fluids from one place to another. Concrete has, in some form or another, been around since the Assyrians, Babylonians and Egyptians, while steel was first patented in 1855. Plastic piping, on the other hand, beginning with polyvinyl chloride or PVC in 1926, dates back to the 1930s, when it was utilized for sanitary drainage. PE was first developed in 1933 as a flexible, low-density coating and insulating material for electrical cables.

HDPE, however, is quite a bit different material from the PE used in the 1930s. LDPE was discovered in 1935 and it was not until nineteen years later in 1954 that commercially available quantities of HDPE appeared on the scene. As a relative newcomer in the piping industry, PE is constantly making its way into applications normally reserved for the older piping technologies. Since the late 1950s and early 1960s, PE has made its way into every corner of our lives launching a multi-billion dollar industry. It is currently the largest volume plastic in the world. This is partly due to the fact that there are certain characteristics (or combinations of characteristics) of HDPE that make it an attractive alternative. Whether it is an issue of installing a new piping system or rehabilitating an existing system, there are certain requirements placed on the piping material: that it be simple to install, that it doesn't leak or cost a lot to maintain, and will last a very long time.

1.4 What is Polyethylene

Polythene resins are milky white, translucent substances derived from ethylene (CH₂=CH₂). Its chemical formula is [─CH₂─CH₂─]_n (where n denotes that the chemical formula inside the brackets repeats itself to form the long chains of plastic molecules).

n CH₂=CH₂ ¾¾¾¾¾® [─CH₂─CH₂─]_n

When Hogan and Banks first created a reaction between ethylene and benzaldehyde using two thousand atmospheres of internal pressure, their experiment went askew when all the pressure escaped due to a leak in the testing container. On opening the tube, they were stunned to find a white waxy substance that looked a lot like some form of plastic. After repeating the experiment, they discovered that the loss of pressure was not due to a leak at all, but was a result of the polymerization process. The residue polyethylene (PE) resin was a milky white, translucent substance derived from ethylene (CH₂=CH₂). Polyethylene was produced with either a low or a high density.

Low-density polyethylene (LDPE) has a density ranging from 0.91 to 0.93 g/cm3 (0.60 to 0.61 oz/cu in). The molecules of LDPE have a carbon backbone with side groups of four to six carbon atoms attached randomly along the main backbone. LDPE is the most widely used of all plastics, because it is inexpensive, flexible, extremely tough, and chemical-resistant. LDPE is molded into bottles, garment bags, frozen food packages, and plastic toys.

High-density polyethylene (HDPE) has a density that ranges from 0.94 to 0.97 g/cm3 (0.62 to 0.64 oz/cu in). Its molecules have an extremely long carbon backbone with no side groups. As a result, these molecules align into more compact arrangements, accounting for the higher density of HDPE. is stiffer, stronger, and less translucent than low-density polyethylene. HDPE is formed into grocery bags, car fuel tanks, packaging, and, of course, piping.

1.5 Polyethylene Pipe

The history of the polyethylene (PE) pipe begins with early civilization's attempts to find a suitable transport medium that could move water and other fluids from one place to another. It is no secret that plastic is relatively a new kid on the block as a piping material. Concrete has, in some form or another, been around since the Assyrians, Babylonians and Egyptians, while steel was first patented in 1855. Plastic piping, on the other hand, beginning with polyvinyl chloride or PVC in 1926, dates back to the 1930s, when it was utilized for sanitary drainage. Polyethylene was first developed in 1933 as a flexible, low-density coating and insulating material for electrical cables. It played a key role during World War II -- first as an underwater cable coating and then as a critical insulating material for such vital military applications as radar insulations. Because of its lightweight, radar equipment was easier to carry on a plane, which allowed the out-numbered Allied aircraft to detect German bombers under difficult conditions such as nightfall and thunderstorms.

1.6 Polyethylene Time Line

1862 - Parkesine, the first synthetic plastic

1866 - Celluloid by John Wesley Hyatt

1891 - Rayon is used to make Cellophane

1900 - Celluloid is used for Film

1907 - Bakelite, the first thermosetting synthetic resin.

1918 - Polystyrene

1926 - PVC or Polyvinyl Acetate

1927 - Nylon - synthetic silk for stockings in 1939

1933 - Polyethylene

1935 - Low Density Polyethylene

1938 - Teflon

1951 - High Density Polyethylene

1957 - Velcro and Silly Putty

1.7 Life Cycle Cost Savings

For municipal applications, the life cycle cost of HDPE pipe can be significantly less than other pipe materials. The extremely smooth inside surface of HDPE pipe maintains its exceptional flown characteristics and butt fusion joining eliminates leakage. This has proven to be a successful combination for reducing total system operating costs.

1.8 LEAKS Free, Fully Restrained Joints HDPE

Heat fusion joining forms leak-free joints as strong as, or stronger than, the pipe itself. For municipal applications, fused joints eliminate the potential leak points that exist every 10 to 20 feet when using the bell and spigot type joints associated with other piping products such as PVC or ductile iron. As a result of this, the “allowable water leakage” for HDPE pipe is zero as compared to the water leakage rates of 10% or greater typically associated with other piping products. HDPE pipe’s fused joints are also self-restraining, eliminating the need for costly thrust restraints or thrust blocks while still insuring the integrity of the joint and the fl own stream. Notwithstanding the advantages of the butt fusion method of joining, the engineer also has other available means for joining HDPE pipe and fittings such as electro fusion and mechanical fittings. Electro fusion fittings join the pipe and/or fittings together using embedded electric heating elements. In some situations, mechanical fittings may be required to facilitate joining to other piping products, valves or other system appurtenances. Specialized fittings for these purposes have been developed and are readily available to meet the needs of most demanding applications.

1.9 Corrosion & Chemical Resistance

HDPE pipe will not rust, rot, pit, corrode, tube roulade or support biological growth. It has superb chemical resistance and is the material of choice for many harsh chemical environments. Although unaffected by chemically aggressive native soil, installation of PE pipe (as with any piping material) through areas where soils are contaminated with organic solvents (oil, gasoline) may require installation methods that protect the PE pipe against contact with organic solvents. Protective installation measures that assure the quality of the fluid being transported are typically required for all piping systems that are installed in contaminated soils

1.10 Fatigue Resistance and Flexibility HDPE

Pipe can be field bent to a radius of 30 times the nominal pipe diameter or less depending on wall thickness (12” HDPE pipe, for example, can be cold formed in the field to a 32-foot radius). Willoughby, D. A. (2002). Plastic Piping Handbook, McGraw-Hill Publications, New York.

1.11 Seismic Resistance

The physical attributes that allow HDPE pressure pipe to safely ac commodate repetitive pressure surges above the static pressure rating of the pipe, combined with HDPE’s natural flexibility and fully restrained butt fusion joints, make it well suited for installation in dynamic soil environments and in areas prone to earthquakes or other seismic activity.

1.12 Construction Advantages HDPE

Pipe’s combination of lightweight, flexibility and leak-free, fully restrained joints permits unique and cost-effective installation methods that are not practical with alternate materials. Installation method such as horizontal directional drilling, pipe bursting, slip lining, plow and plant, and submerged or floating pipe, can save considerable time and money on many installations. At approximately one-eighth the weight of comparable steel pipe, and with integral and robust joining methods, installation is simpler, and it does not need heavy lifting equipment. Polyethylene pipe is produced in straight lengths up to 50 feet and coiled in diameters up through 6”. Coiled lengths over 1000 feet are available in certain diameters. Polyethylene pipe can withstand impact better than PVC pipe, especially in cold weather installations where other pipes are more prone to cracks and breaks.

1.13 Durability OF Polyethylene

Polyethylene pipe installations are cost-effective, have long-term cost advantages due to the pipe’s physical properties, leak-free joints, and reduced maintenance costs. The polyethylene pipe industry estimates a service life for HDPE pipe to be, conservatively, 50-100 years if the system has been properly designed, installed and operated in accordance with industry established practice and the manufacturer’s recommendations. This longevity confers savings in replacement costs for generations to come. Properly designed and installed PE piping systems require little on-going maintenance. PE pipe is resistant to most ordinary chemicals and is not susceptible to galvanic corrosion or electrolysis.

1.14 Hydraulically Efficient

For water applications, HDPE pipe’s Hazen Williams C factor is 150 and does not change over time. The C factor for other typical pipe materials such as PVC or ductile iron systems declines dramatically over time due to corrosion and tuberculation or biological build-up. Without corrosion, tuberculation, or biological growth HDPE pipe maintains its smooth interior `all and its flown capabilities indefinitely to insure hydraulic efficiency over the intended design life.

1.15 Temperature Resistance

PE pipe’s typical operating temperature range is from -40°F to 140°F for pressure service. Extensive testing at very low ambient temperatures indicates that these conditions do not have an adverse effect on pipe strength or performance characteristics. Many of the polyethylene resins used in HDPE pipe are stress rated not only at the standard temperature, 73° F, but also at an elevated temperature, such as 140°F. Typically, HDPE materials retain greater strength at elevated temperatures compared to other thermoplastic materials such as PVC. At 140°F, polyethylene materials retain about 50% of their 73°F strength, compared to PVC which loses nearly 80% of its 73°F strength when placed in service at 140°F [5]

As a result, HDPE pipe materials can be used for a variety of piping applications across a very broad temperature range. The features and benefits of HDPE are quite extensive, and some of the more notable qualities have been delineated in the preceding paragraphs.

1.16 Ductility

Ductility is the ability of a material to deform in response to stress without fracture or, ultimately, failure. It is also sometimes referred to as trainability and it is an important performance feature of PE piping, both for above and below ground service. For example, in response to earth loading, the vertical diameter of buried PE pipe is slightly reduced. This reduction causes a slight increase in horizontal diameter, which activates lateral soil forces that tend to stabilize the pipe against further deformation. This yields a process that produces a soil-pipe structure that is capable of safely supporting vertical earth and other loads that can fracture pipes of greater strength but lower strain capacity. With its unique molecular structure, HDPE pipe has a very high strain capacity thus assuring ductile performance over a very broad range of service conditions. Materials with high strain capacity typically shed or transfer localized stresses through deformation response to surrounding regions of the material that are subject to lesser degrees of stress. Because of this transfer process, stress intensification is significantly reduced or does not occur, and the long-term performance of the material is sustained. Materials with low ductility or strain capacity respond differently. Strain sensitive materials are designed based on a complex analysis of stresses and the potential for stress intensification in certain regions within the material. When any of these stresses exceed the design limit of the material, crack development occurs which can lead to ultimate failure of the part or product. However, with materials like polyethylene pipe that operate in the ductile state, a larger localized deformation can take place without causing irreversible material damage such as the development of small cracks. Instead, the resultant localized deformation results in redistribution and a significant lessening of localized stresses, with no adverse effect on the piping material. As a result, the structural design with materials that perform in the ductile state can generally be based on average stresses, a fact that greatly simplifies design protocol. To ensure the availability of sufficient ductility (strain capacity) special requirements are developed and included into specifications for structural materials intended to operate in the ductile state; for example, the requirements that have been established for “ductile iron” and mild steel pipes. Similar ductility requirements have also been established for PE piping materials. Validation requirements have been added to PE piping specifications that work to exclude from pressure piping any material that exhibits insufficient resistance to crack initiation and growth when subjected to loading that is sustained over very long periods of time, i.e. any material that does not demonstrate ductility or strain ability. The PE piping material validation procedure is described in the chapter on Engineering Properties of Polyethylene.

1.17 Visco-Elasticity

Polyethylene pipe is a visco-elastic construction material [6]. Due to its molecular nature; polyethylene is a complex combination of elastic-like and fluid-like elements. As a result, this material displays properties that are intermediate to crystalline metals and very high viscosity fluids. The visco-elastic nature of polyethylene results in two unique engineering characteristics that are employed in the design of HDPE water piping systems, creep and stress relaxation. Creep is the time dependent viscous flown component of deformation. It refers to the response of polyethylene, over time, to a constant static load. When HDPE is subjected to a constant static load, it deforms immediately to a strain predicted by the stress-strain modulus determined from the tensile stress-strain curve. At high 12 introduction loads, the material continues to deform at an ever decreasing rate, and if the load is high enough, the material may finally yield or rupture. Polyethylene piping materials are designed in accordance with rigid industry standards to assure that, when used in accordance with industry recommended practice, the resultant deformation due to sustained loading, or creep, is too small to be of engineering concern. Stress relaxation is another unique property arising from the visco-elastic nature of polyethylene. When subjected to a constant strain (deformation of a specific degree) that is maintained over time, the load or stress generated by the deformation slowly decreases over time. This stress relaxation response to loading is of considerable importance to the design of polyethylene piping systems. As a visco-elastic material, the response of polyethylene piping systems to loading is time-dependent. The effective modulus of elasticity is significantly reduced by the duration of the loading because of the creep and stress relaxation characteristics of polyethylene. An instantaneous modulus for sudden events such as water hammer can be as high as 150,000 psi at 73°F. For slightly longer duration, but short-term events such as soil settlement and live loadings, the short-term modulus for polyethylene is roughly 110,000 to 120,000 psi at 73° F, and as a long-term property, the modulus is reduced to something on the order of 20,000-30,000 psi. As will be seen in the chapters that follow, this modulus is a key criterion for the long-term design of polyethylene piping systems. This same time-dependent response to loading also gives polyethylene its unique resiliency and resistance to sudden, comparatively short-term loading phenomena. Such is the case with polyethylene’s resistance to water hammer phenomenon, which will be discussed in more detail in subsequent sections of this article.

1.18 GENERAL

Polyethylene (PE) is a thermoplastic material produced from the polymerization of ethylene. PE plastic pipe is manufactured by extrusion in sizes ranging from ½" to 63". PE is available in rolled coils of various lengths or in straight lengths up to 40 feet. Generally small diameters are coiled and large diameters (>6" OD) are in straight lengths. PE pipe is available in many varieties of wall thicknesses, based on three distinct dimensioning systems: