Wednesday, March 11, 2020

WEATHERING OF POLYETHYLENE PIPES


2.1       Introduction:
When exposed outdoors to weathering, the properties of polymers are subject to change. Primarily, the effect is one of photo-oxidation brought about by incident wavelengths within the ultraviolet range, although additional factors may contribute. In the case of plastics, photo-oxidation may result in loss of impact strength, resistance to slow crack growth, and thermal stability. This is a surface effect, and the bulk polymer is not necessarily affected. Pigment fade may also be a consideration.
2.2       End-user Requirements:
  1. Water (WSAA) and gas (AGA) industries require pipes to be suitable for use after two years of outdoors exposure during storage.
  2. In the case of black pipes, WSAA allows permanent exposure for above ground PE sewers. Other industries, such as mining, require up to 15 years of exposure during service.
2.3       Means of protection:
Protective additives such as carbon black and hindered amine light stabilizers (HALS) are used in order to minimize degradation. The use of carbon black optimizes UV stabilization and thus black pipes dominate general usage.
For colored compounds, stabilization has not been as effective as carbon black, but the introduction of HALS (hindered amine light stabilizers) has enabled greatly improved UV resistance, albeit still not to the level of carbon black.
2.4       Carbon Black
Carbon black confers excellent long-term retention of properties, hence this additive shows predominant use for PE pipes. For stabilization, the key attributes of the carbon black are particle size and distribution, plus volatile content.
Although actual long-term performance is not well documented quantitatively, there are sufficient data and long-term history of use for black PE pipe grades to assume suitability.
Chevron Phillips Chemical Company LP [7] have exposed black HDPE and MDPE pipe materials conforming to AS/NZS 4131, plus materials with non-conforming carbon black, for over nine years and seven years respectively in Singapore, where incident energy is about 80% of that in Nth. Queensland. These pipes, whilst exhibiting surface oxidation, showed undiminished hydrostatic pressure resistance, even for non-conforming carbon black (50 nm particle size) used for comparative purposes.
Gaz De France [8]. Reported exposure of pipes for 38 months in the South of France (approx. equal to 24 months in Nth. Queensland) - these were black MDPE pipe grades. Oxidation occurred to a depth of approximately 50μm, which was described as the general order of magnitude for black MDPE gas pipes. Taking into account that surface layers are removed prior to fusion welding, there is a “universal” allowance of 10% of pipe wall for surface damage during handling, and installation, the small amount of surface oxidation is negligible.
2.5       HALS
The development of HALS stabilizers has resulted in significantly improved long-term resistance to the effects of UV radiation on colored compounds. Such compounds are used as pipe identification and may be used as co extrusions on pipes or for the pipes themselves.
The performance of HALS stabilizers is well documented and acknowledged within AS/NZS 4130 by acceptance of 0.2% of HALS as being adequate for exposure during storage.
According to Ciba Specialty Chemicals, addition of 0.2% HALS Tinuvin 783 provides life of more than 16000 hrs, compared with less than 1000 hrs with nil stabilization, as measured by carbonyl absorbance of the sample exposed to weathering in Weatherometer.
Based on Ciba Specialty Chemicals data, 16000 hours in the Weatherometer is approximately equivalent to 12.5 years. This approximates to more than 10 years of exposure [9]. Notwithstanding the fact that artificial weathering cannot be accurately correlated with natural exposure, it can be seen that 16000 hours provide a large margin for two years exposure. Further, Cytec Industries Inc. have shown that the addition of HALS additive UV-3346 at 0.2% provides in excess of 30 months to 50% of original elongation to break, compared with 6-9 months for unstabilised material.
In pipe Standards, such as AS/NZS 4130, AS/NZS 4131, and ISO 4437, it is usual to expose pipe specimens, whereas in developmental work on polymers, small specimens are usually chosen, so that the degraded area only is evaluated.
For long term use of colored compounds in service, such as the use of white co extrusions for temperature minimization, the dosage level of HALS should be increased to maximum compatibility level. For example, Ciba Specialty Chemicals recommend inclusion of 0.4% Tinuvin 783, plus at least 2% of TiO2 retile [10]. In addition to providing the white color, TiO2 will impart improved UV resistance, although its effect is not as pronounced as with PVC. This will optimize performance for the 15 years required by the mining industry.
However, in order to quantify performance, natural and/or artificial weathering tests should be conducted.
2.6       Synergistic effects
It is known that stabilizers may act in combination with pigments and antioxidants to produce a combined and otherwise unpredictable effect on UV resistance.
For example, testing of HDPE by Chevron Phillips Chemical Company LP(3) has shown that for certain yellow pigments, 2% dosage is superior to 1% dosage without stabilization, but with stabilization the performance is reversed.
The synergistic effect demands exposure testing, by natural exposure and/or Weather-Ometer. It is recommended that in this context, Weather-Ometer testing be used only for comparative purposes against data established from natural exposure plus Weather-Ometer. In any case, all colored compounds should eventually be assessed by natural exposure testing.
2.7       Exposure Testing:
Where neither carbon black nor HALS is incorporated, compounds to AS/NZS 4130 and AS/NZS 4131 must be subjected to natural exposure testing.
Due to the potential synergistic effects, all colored pipe compounds should be evaluated by natural exposure testing to a total incident energy level of 14 GJ/m2, but the time period involved is likely to inhibit product development, so an interim acknowledgement of conformance could be granted on the basis of compounds that incorporate 0.2% min. of either Tinuvin 783 or Cytec UV-3346.
2.8       Means of Assessment
Following exposure, pipe specimens are tested to assess effect on key properties: resistance to slow crack growth (ESCR), impact strength, resistance to internal pressure, and thermal stability.
Resistance to slow crack growth and impact strength are usually assessed by some form of notched specimen testing. However, as the notching removes the affected layer, these tests are useless.
Fortunately, changes to both resistance to slow crack growth and impact strength can be indirectly assessed by one test, change in elongation to break. The general benchmark in industry is an allowable reduction in elongation to break of 50% of original and this is used for some of the data previously cited. AS/NZS 4131 and ISO 4437 require only that the elongation to break after exposure be  350%, rather than specify a change. An additionally, ISO 4437 requires the original elongation to break to be  350%.
Elongation to break of compounds that meet all other requirements of the Standards is typically greater than about 700%, so the requirement of 350% minimum is effectively 50% change.
As previously stated, the data used as the basis is developed from small specimens and thus conservative compared with exposure of pipes.
Resistance to internal pressure is best evaluated by short term 80°C testing, as this simulates long-term performance.
Thermal stability is considered to be of little relevance. Although it relates to pipe life and fusion welding,
the former is covered by the tensile and pressure testing, and for the latter, welding techniques require removal of the oxidized layer. Furthermore, it has been demonstrated that the OIT method specified in AS/NZS 4130 and ISO4437 is not appropriate for HALS stabilized materials after exposure and determination of carbonyl index should be made.
2.9       Conclusions:
  1. Black pipes to AS/NZS 4130 satisfy end-user requirements for both short and long-term use.
  2. Colored pipes, including co extrusions, to AS/NZS 4130 may be considered suitable for two years storage under exposure provided appropriate grades of HALS at dosage rate of 0.2% minimum are used.
Suitable grades include Ciba Specialty Chemicals Tinuvin 783 and Cytec Industries UV-3346.
Confirmation by natural exposure testing to a total energy level of 14 GJ/m2 should be conducted.
  1. For longer periods, such as 15 years, optimum performance is best obtained by incorporating HALS stabilizers up to polymer compatibility level, plus at least 2% of retile titanium dioxide.
Further quantification should be obtained by outdoors exposure testing.

Tuesday, January 7, 2020

HDPE USES AND APPLICATIONS


3.1       Introduction
The effect of installation procedures on the field performance of existing high-density polyethylene (HDPE) pipe used for drainage applications on highway projects was investigated. A total of 45 HDPE pipes were inspected at sites in South Carolina that were statistically selected based on geographical location, pipe diameter, use, and age. The condition of each pipe was not known prior to selection for inspection. Both the external and internal conditions of the pipe were evaluated with respect to AASHTO and ASTM specifications, measurements of pipe deflection with a mandrel set to 5% deflection, and visual inspections of the pipe interior using a video camera. The video camera inspections revealed circumferential cracks in 18% of the pipes, localized bulges in 20% of the pipes, and tears or punctures in 7% of the pipes. Deflections greater than 5% were observed in 20% of the pipes. Installation problems such as poor preparation of bedding soils, inappropriate backfill material, and inadequate backfill cover contributed to the excessive deflection and observed internal cracking in pipes with observed damage. Appropriate construction procedures are essential in achieving a proper installation.

·         Gas Gathering
·         Crude Oil Flow
·         Water Flood
·         Saltwater Disposal
·         Supply Water
·         Fuel Transfer
·         Main Lines

3.2       Fluid and Gas Flow

Polyethylene pipes are used extensively in gas distribution applications worldwide. In USA and Canada over 90% of the natural gas distribution system is in plastics pipes with polyethylene representing 99% of the installations. The use of polyethylene in natural gas distribution systems is growing rapidly.
PE is lightweight, flexible and available in long coils minimizing the number of joints. It is ideally suited for a wide range of service conditions requiring very little maintenance. It has good abrasion resistance, flexible not effected by soil shift and temperature fluctuations.
Polyethylene pipe is recommended by PIPA for use in compressed air installations.

3.3              Fluid Flow

Polyethylene has an extremely smooth surface resulting in a very low coefficient of friction and a minimal loss of head pressure due to frictional losses. This, combined with excellent corrosion and abrasion properties, results in excellent flow characteristics throughout the life of the pipe. for pressurized systems, a Hazen-Williams "C" factor of 150 is used.PE3408/3608 Extra High Molecular Weight (EHMW) Black Pipe - a premium quality, high density, extra high molecular weight, and polyethylene pipe specifically designed for the rigors of the oil field. It is produced from PE3408/3608 resin containing not less than two percent (2%) carbon black for superior resistance to UV degradation. This pipe offers outstanding environmental stress crack resistance (ESCR), the best chemical resistance of any polyethylene pipe and high impact resistance. Polyethylene® oil field products are available in diameters from 1/2" CTS to 6" IPS coiled and straight lengths from 1/2" through 65" IPS.

3.4              Oil Field

 

Moving fluids through pipe in the oil field demands the utmost in flexibility, reliability and performance. That is why Polyethylene is the best choice for the energy business. High-density polyethylene (HDPE) pipe provides superior flow characteristics, extended life, durability, and reduced maintenance than traditional piping materials, anywhere in the oil patch.
A wide selection of HDPE pipe can meet the needs for any oil field applications.
Polyethylene has products specifically for the oil and gas industry for gas gathering, crude transmission, water lines and auxiliary lines.
Polyethylene will not rust, rot, pit or corrode because of chemical, electrolytic or galvanic action. Chemicals that pose potentially serious problems for polyethylene are strong oxidizing agents or certain hydrocarbons. These chemicals may reduce the pressure rating for the pipe or be unsuitable for transport. Either can be a function of service temperature or chemical concentration. Continuous exposure to hydrocarbons can lead to permeation through the material or electrometric gaskets used at joints. The degree of permeation is a function of pressure, temperature, the nature of the hydrocarbons and the polymer structure of the piping material. The chemical environment may also be of concern where the purity of the fluid within the pipe must be maintained. Hydrocarbon permeation may affect pressure ratings and hinder future connections.
High Density Polyethylene (HDPE) is available for all pipe applications. Being non-chlorinated, requiring fewer additives, and having a much higher recycling rate, it is considered a more benign plastic than PVC. PVC is more resistant to combustion, but smolders at a lower temperature than HDPE and releases toxic hydrochloric gases before combustion. Cross-linked polyethylene (PEX) is a polyethylene similar in many characteristics to HDPE but with molecules cross-linked to improve its ability to handle higher temperatures. Copper is highly recyclable but copper leaching into water supplies can be harmful to aquatic life. Copper also has significant life cycle problems in its mining, manufacture. Concrete, iron and steel have significant embodied energy usage, and their manufacture is not environmentally benign. However, all of them (with the exception of ABS) are generally considered environmentally superior to PVC.  Aside from concrete, the primary PVC free alternatives are consistent with state government and professional association Environmentally Preferable Purchasing (EPP) guidelines (http://www.apwa.net/Documents/GovtAffairs/Policies/SolidWaste/solid-environpolicy.pdf). Steel, HDPE and copper pipe or conduit may all contain recycled content in the product. Quantities and post consumer content will vary with application and manufacturer. Alternative materials comparison issues The long-term durability of piping systems depends on many factors, including the soil environment, proper installation, material properties such as corrosion resistance, chemical resistance and strength and the performance of joints. Each of the primary PVC free materials has benefits that have kept them as significant market players.

3.5       Water SUPPLIES

The use of plastics pipes in potable water supply applications has been growing rapidly. Both PVC and Polyethylene pipe have major advantages over competitive materials and as polymer technology, keeps improving the choice of plastics pipes for water supply infrastructure projects keeps increasing.
Plastics pipes have design life in excess of 100 years during which they provide excellent performance and trouble free service life. They are corrosion resistant and because of their relatively lightweight are easy to handle, transport and install. Plastics pipes are flexible and fatigue resistant and can withstand repetitive pressure surges. Plastics pipes provide a smooth biological growth free bore through the life of the product eliminating flow restrictions common to other materials.
Water mains typically operate at pressures from 100 to 150 lbs per sq. in. (psi), while distribution lines operate between 40 and 100 psi. Service connection lines are usually a diameter of 1" or less and can be made of various materials: polyethylene, PVC, iron or copper pipe. Currently, PVC has a dominant share of the market for small diameter pipe in the water main (4” - 12”), sanitary sewer and storm sewer (4”-15”) markets, while traditional materials (ductile iron and concrete) continue to have majority market share in the larger diameter pipe. According to the Plastics News (July 16, 2001) the demand for large diameter pipe plastic pipe has increased 8.3% between 1990 and 2000.
The smaller tube sizes used for in building distribution are primarily split between PVC, copper, and iron. There is limited data on the breakdown of market share. Polyethylene is just beginning to penetrate the market for all sizes. The use of galvanized steel and Polyethylene has declined due to corrosion problems with galvanized and catastrophic failures with Polyethylene One of the key design concerns for drinking water infrastructure design and installation is leakage. When one turns on the tap for potable water, there is a cost associated with the acquisition, treatment, and supply (pumping) of the waster. If a water distribution system leaks, the lost water can become an extremely high cost. In arid areas, where costs to acquire water can be exorbitant, leaks can be an expensive proposition. A 4-inch leak in their 24-inch diameter iron pipe can result in the loss of 3 to 5 million gallons of water per day.
HDPE has a slight advantage in leak resistance over PVC. This is because it can be delivered in longer lengths, minimizing the quantity of joints. Furthermore, the butt or electro-fusion processes used to join HDPE provides stronger, tighter, more leak proof joints compared to the bell and spigot joints used in PVC pipe for mains or the solvent glue joints used for smaller distribution. The longer length of HDPE can require longer trenches to be open at a time, but its length and flexibility can allow for trench less procedure, particularly in sewer replacement. HDPE’s greater flexibility and resilience (particularly at lower temperatures) also make it less susceptible to surge and hammer shocks or to damage from digging. HDPE’s flexibility and resilience has made it increasingly popular in earthquake territory or other areas where soils can shift. For larger diameters, the fusion technique requires a fusion machine, which might be problematic in cramped spaces. For smaller diameter pipes, a handheld device can be used to weld/melt the pipe lengths together. Mechanical couplings are available for HDPE, though some of these couplings may be made of PVC.
PEX is another form of polyethylene that retains HDPE’s flexibility and chemical resistance while providing resistance to higher temperatures for which HDPE is not suitable. It is coupled with either fusion techniques or mechanical crimp couplings. Due to its higher temperature ratings it was initially used in radiant and district heating system applications, but is now also beginning to be used more widely in water supply and gas distribution systems.
Ductile Iron (DI) has significantly higher tensile strength, making it more capable of handling higher pressures, crushes and hammer than PVC. DI does not lose strength at high or low temperatures as PVC does. Ductile iron is impermeable to hydrocarbons and other groundwater contamination unlike PVC or other plastic pipe. “There has been much debate over the durability and expected lifespan of each of these materials. The life of a pipe system depends on not only the material, but also the installation and the surrounding environment. All these types of pipe have been on the market for over 30 years, and while there are examples of pipe failures for each of them, this study did not find conclusive evidence to suggest that one material has a significantly different lifespan from the other. When properly designed and installed, pipe systems of any of these materials can be sufficiently durable to withstand many decades of services.”

3.6       Sewerage and Drainage

The use of plastics pipes for both pressure and a gravity sewer is extensive. In addition, there is rapid growth in the use of plastics liners for repair of old and leaking sewer installations.
Availability of large diameter plastics pipes at competitive prices gives design engineers an opportunity to select products on cost and performance basis. Long life expectancy, low maintenance requirements are major advantages in the use of plastics pipes for sewage and drainage applications.

As in water main pipe, HDPE is a comparable alternative to PVC pipe in sewer systems. HDPE sewer pipes are also available in diameters ranging from 4 inches to 36 inches, although for storm sewer, much of the demand is for 10 to 15 inch, while for sanitary 8 to 12 inch are popular diameters. At larger diameters, the major market share is held by concrete, primarily due to cost.
Prior to the 1960s most sewer systems were combined sewers, that is, carried both sanitary and storm water. The system had to be designed to carry large volumes of water during rain events, but otherwise the capacity was little used. In addition, when it did rain the flood of relatively fresh water often negatively impacted water treatment. Design changed so that by the mid 1960s sanitary and storm systems were designed and constructed separately. Storm sewers collect water from roof drains, parking lots and streets. Unlike sanitary sewers, storm wastewater is not typically treated and the flow is directly discharged into a receiving body of water.

Similar to water distribution use, PVC is dominant in the smaller size sewer pipe market with HDPE just beginning to seriously compete. These smaller lines are commonly used in the collection network of subdivisions. In this segment, the competing concrete pipe is non-reinforced concrete pipe in 8" and 10" sections. The smallest diameter reinforced concrete pipe is usually 12" pipe.

The flow formula for smooth pipe should be used to compute the gas flow rate through Polyethylene. It has been found that the Mueller formula for smooth wall pipe describes the flow characteristics of Polyethylene.

3.7       Plumbing

PVC pipes and fittings for plumbing and drainage applications is the choice of plumber’s word wide. Low cost, lightweight, long life expectancy usually for the life of the installation is the overwhelming advantages. PVC does not corrode internally or externally eliminating the possibility of pipe failures or blockages. Cross-linked polyethylene, polypropylene and Polyethylene pipes are used in hot and cold-water reticulation in domestic, commercial and industrial installations. Ease of installation using compression fittings is providing a cost advantage.
Polyethylene, like other plastics, has a thermal coefficient of expansion higher than metals. When subjected to a temperature change, unrestrained (not buried) polyethylene pipe will experience expansion and contraction.
The coefficient of thermal expansion/contraction for Polyethylene is 1.0 x 10-4 in/in/°F. As a general allowance, 1" per 100' of pipe per 10°F change in temperature.
Forces due to thermal expansion and contraction can be significant. Proper system design should be used to account for the compressive and tension stresses that can be generated.

When pipe is used in pressure applications, the longitudinal stress created by the sum of the bending radius, internal pressure and other stress loads on the pipe should not exceed the material’s design stress rating. Severe but acceptable bends in polyethylene pipelines should be buried or properly restrained.

3.8       Agriculture, Irrigation & Drainage

A variety of alternatives to PVC are used both for water delivery and for drainage. Irrigation sprinkler, drip and drainage systems have long been available in HDPE and have significant advantages in resilience against compression, shovel attack and ground movement. Corrugated steel, concrete and HDPE are all competitive alternatives for drainage.  HDPE drainage pipe is now available in formulations with high-recycled content. Plastic pipe has carved a hunk of the huge market previously dominated by concrete and steel. Highway drainage is a fast growing market for HDPE.  Recently, the Corrugated Polyethylene Pipe Association initiated a third party certification system, which allows for increased acceptance of their product by the American Association of State Highway and Transportation Officials. Footing and under slab drains are all available in HDPE.

3.9              Agricultural and Rural

Water is a lifeline for all farming operations and the security of water is essential. Plastics pipes are available for the wide range of farming applications. Pressure pipes for irrigation, plant watering and potable water reticulation. Non-pressure pipes for irrigation, stock watering, micro-irrigation and general water reticulation systems.
Low cost, wide range of pipe sizes, flexible and easy to handle and transport are all advantages important to the farmers.

3.10     Industrial and Chemical

Corrosion resistance and resistance to attack by many industrial chemicals make plastics pipes the obvious choice for chemical plant installations. Like with all materials used in the construction of chemical plants care must be taken in selecting the correct plastics pipes and fittings that will withstand the operating conditions.
The wide range of polymers used in manufacture of plastics pipes and fittings provide a good range of products from which to select the appropriate material. PVC piping systems are widely used in water, wastewater and chemical transfer. Polyethylene piping systems are well suited to installation in difficult industrial situations. Their high strength and ease of installation also makes them ideal for compressed air reticulation.

    Electrical and Communications

PVC is ideally suited for telecommunication and power conduits due to its high impact strength, smooth internal bore and large range of diameters. Flexibility and corrosion resistance characteristics of PVC conduits make them ideal for a wide range of installation conditions.

High-density polyethylene (HDPE) is ideal for pipe lining and cable encasing, which makes it perfect for communications cables. Although polypropylene pipes are used mainly for plumbing and sewerage applications, they can also be effectively used as conduits.

Saturday, January 4, 2020

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:
  • Pipe Size Based on Controlled Outside Diameter (DR)
  • Iron Pipe Size Inside Diameter, IPS-ID (SIDR)
  • Copper Tube Size Outside Diameter (CTS)
PE pipe is available in many forms and colors such as the following:
  • Single extrusion colored or black pipe
  • Black pipe with co extruded color striping
Black or natural pipe with a co extruded colored layer

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