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
·
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
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.
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