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
