Characteristics which are normally considered while selecting
a grade are density, molecular weight distribution, comonomer
(type and content ), thermal stabilisation and UV stabilisation.
Density of a precompounded HDPE pipe grade ranges between 0.95
to 0.96gm/cc and the value depends upon the comonomer content
and the carbon black loading. Although molecular weight distribution
is not specified by the suppliers, generally HDPE grade with a
broad molecular weight distribution is used. Comonomer is used
to control the density and impact properties of the polymer. Comonomers
like propylene, butylene, hexene and octene are normally used.
Thermal stabilisation is achieved by adding antioxidant at the
polymer manufacturing stage. The thermal stabilisation is required
to protect the polymer from themal degradation while HDPE pipe
Processing. Thermal degradation is measured by determining the
+Oxidation Induction Time+ (OIT), which denotes the threshold
time limit beyond which the oxidative degradation of polymer will
set in at a given processing temperature. For a good HDPE pipe
grade, a minimum OIT value of 20 minutes at 2100 is required.
UV stabilisation of HDPE pipe grade is achieved by adding two
to three percent of carbon black by weight. Higher concentration
of carbon black (unlike other pigments/ colourants) is the most
difficult one to disperse in the polymer.
Effective dispersion can be obtained by using high shear twin
screw extruders at the granulation stage of the polymer. Therefore,
in order to get a good quality HDPE pipe, use of precompounded
HDPE is recommended.
Characteristics
HDPE pipes manufactured from quality HDPE pipe grade material
have the following important characteristics:
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Very high
impact strength |
|
Light weight |
|
Flexibility
and ductility |
|
Chemical
resistance |
|
Weatherabilit |
|
Wide temperature
service range from 400oC to 600oC |
|
Smooth internal
surface |
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Resistance
to rodent attack |
HDPE pipe system
The perception of many users about HDPE pipes is that these are
difficult to join and quality standard fittings are often not
available in the market. Actually, with its well-developed jointing
techniques and range of fittings, HDPE offers engineers the required
freedom in designing efficient and cost-effective pipe systems.
Jointing Techniques
The jointing techniques of HDPE pipes can be broadly classified
as permanent and detachable joints. The permanent jointing technique
employs the welding method to join the pipes. Butt fusion welding
and electrofusion welding are the two methods usually employed.
Butt fusion welding : This is simplest and the quickest
method of jointing HDPE pipes and can be used for all diameters.
The two ends of pipe to be joined are cleaned and brought together
and are lightly pressed against a heated tool. A hydraulic jack
is used to ensure the proper alignment of two pipe ends. Heating
temperature of 2100oC and pressure of 0.4 kg.cm2 to
1.2 kg/cm2 to get good quality welded joints. The most interesting
aspect of this method is that the strength of the joint is approximately
equal to the original pipe material. This can be checked by calculating
the weld factor, which generally varies between 0.9 to 1.
Electrofusion welding : This is a sophisticated technique
of jointing HDPE pipes and is usually employed for HDPE pipes
for gas distribution. Use of equipment fitted with time controller
for the heating operation and a transformer is made. Detachable
joints are used when the HDPE pipe requires frequent dismantling
and shifting. Three types of detachable joints are normally used.
They are:
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Flanged joints: HDPE pipe and slip-on flange (of matching dimensions)
are butt-welded to form a stopper-end and contact face. The two
flanged ends are then joined with standard nuts and bolts with
a proper gasket to ensure a leek-proof joint. |
|
Insert transition joints: Serrated hose nipples can be used
for connecting HDPE with threaded pipes with threaded pipes and
utilities. The required bore diameter can be formed by heating
the pipe end in an oil/glycerine bath maintained at 1300oC
till softening and then flaring it on a pre-sized wooden cone.
The serrated portion of the pipe/utilities and secured with a
jubilee clip/clamp to ensure a leak-proof joint. |
|
Compression couplers: These couplers can even be used for high-pressure
applications for pipe diameter of up to 160 mm OD. Compression
couplers are ideal for pipe systems requiring frequent dismantling
and shifting e.g. economic irrigation of larger areas by using
smaller pipe lengths |
HDPE pipes versus other materials
A good quality HDPE pipe exhibits the properties of a construction
material when compared with other conventional piping materials.
As discussed above, HDPE pipes have excellent physical properties
that are ideal for many industrial applications.
Strong and lightweight, HDPE pipes do not require heavy handling
and laying equipment which is common to steel and concrete pipes.
Availability of longer lengths in HDPE pipes reduces the number
of joints and increases the speed of installation.
An HDPE pipe is flexible, allowing it to follow the contours
of rolling terrain, thus reducing the requirements of fittings.
It may be permanently cold-bent to a radius of 20 to 25 times
the pipe diameter without damage or effect upon physical properties.
In sections where a fitting is present, the bending radius should
be 100 times the pipe diameter. Unstable soils have little effect
on this flexible elastic piping system.
The smooth non-wetting bore of HDPE pipes offers low resistance
to flow fluids and slurries , and resists the adherence of scale
and deposits. The Hazen William factor indicates the amount of
energy lost when a fluid flows through a pipeline and can be quantified
using the formula:
Q = 1002 x C x D (2.65) x S (0.54)
HDPE pipes have a higher C value (150) than cast iron (120),
steel (120) and concrete pipes (100 to 120). Unlike other pipes,
on HDPE pipe retains its C value throughout its life span.
HDPE pipes are resistant to a broad range of corrosive chemicals
and does not support biological growth. These properties make it
ideal growth. These properties make it ideal piping material for
many chemical process pipelines and food industry.
Surge pressure, water hammer, subfreezing temperatures are well
tolerated by HDPE pipes. System pressures due to surge or water
hammer of up to 11/2 times the rated system operating pressure,
are well within the limits of the HDPE piping system, which has
burst pressure greater than four times the system pressure rating.
HDPE pipes, when compared in terms of unit water delivered cost,
are lower than any other pipe system. Rightly designed and properly
laid, HDPE pipe systems are practically maintenance-free and warrant
a minimum life of 50 years.
Applications
The HDPE pipe, given its several advantages, finds usage in a variety
of applications. Here are some of the industries using high-density
and their varied applications.
Industries: Agricultural (irrigation), chemical; dredging, fertilizers,
gas distribution, general industrial, landfill reclamation, metals
extraction, mining oil and gas production, power generation, pulp
and paper, sewage and water distillation.
Special services: Potable water supply; hazardous wastes, duct and
fire protection.
Applications: Submarine pipelines, acid lines, cold bed methane
drainage, corrosive wastes, de-watering, process waterlines, drainage
lines, fiber optic innerduct, fly ash disposal, relining salt water
intakes and sewer lines.
Some interesting applications are further detailed.
Submarine pipelines: This application is of special interest for
HDPE pipes. By virtue of its flexibility, an HDPE pipe automatically
accommodates itself within its minimum radius limits to the configuration
of the terrain. Submarine pipelines are used as pressure lines e.g.
drinking water lines, gravity flow lines and electrical cable conduits.
A submarine installation may consist of one or more pipelines laid
side by side or grouped together in one pipe bundle. Submarine pipelines
are installed by one of three different methods, depending on the
stretch of water to be crossed, the nature of the waterway bed and
safety requirements.
Laying on the waterbed: If conditions are suitable, the submarine
pipe is simply laid on the bed of the waterway. The individual pipe
sections are welded end-to-end on the blank, to the length required,
fitted with weights and launched into the water. The pipeline system
is then assembled, floated over the pipe route and lowered to the
bottom by flooding the lines.
Laying in dredged channels: The pipe is welded up from sections
on the bank to the length required and is then weighted. If the
water stretch to be crossed is suitable, the pipe is floated over
the dredged out channel and sunk into the position. Alternatively,
it could be drawn from one bank to the other on the bottom of the
channel, with suitable disposition of the sinker weights. After
the submarine pipe has been laid, the channel is back-filled or
dredged over. Back-filling up to the crown level of the pipe should,
as far as possible, be done with distributor tubes in order to avoid
local overloading of the crown during the discharge of the dredged
material.
Laying by the flushing-in method: In this method, the pipes, welded
up to the complete length required, are laid on the water without
weighting elements. Alternatively, depending on their diameter,
the pipes are wound in coils or on reels and carried on board the
laying vessel. The pipe, connected at one end to the land section
of the line or secured in position on the shore, is fed through
flushing lance, which is then lowered into a trench excavated at
the edge of the shore. After the flushing lance has been fastened
to the side of the ship, the laying operation is commenced. Nozzles
are fitted in the direction of travel in the base of the lance;
these are supplied with h. p. water at a delivery rate of 8,000-12000
liters/min and a pressure of 10-12 bar. Under this the bed of the
waterway is loosened along a narrow strip in the direction ahead,
enabling the flushing lance to channel its way along the bottom.
Depending on the nature of the bed and specified requirements, the
depth to which the pipe is laid ranges from 2 to 6m beneath the
bed level.
Relining - New pipes in old
In renewing the existing defective pipelines, many successful examples
have proved that relining with HDPE pipe is a cost-saving method
of repair. It is used for sewers, drinking water lines and submarine
lines. The old pipe remains where it is - an empty pipe, as it were
to take the new line.
With the relining method, gravity flow line can be converted to
pressure lines thereby increasing transport capacity. In the same
way, low-pressure gas pipe systems can later be changed to medium
or high-pressure operation. Depending on the condition and route
of the of the old pipe, continuous lengths of lining pipe up to
400 m can be inserted. The length depends on the specific gravity
of the pipe, the permissible tensile stress, which should not exceed
10N/mm2, and coefficient of friction. Where pipe sections
are to be joined, it is necessary to open up the old pipe. The lining
pipe ends are then bolted together by means of loose flanges. The
selection of a HDPE linear that has an outside diameter of the pipe
to be relined, will generally serve two purposes. First, the size
differential usually provides adequate clearance to accommodate
the insertion process. Secondly, 100% or more of the original flow
capacity may be maintained. A differential of less than 10% may
provide adequate clearance in larger diameter piping structure.
To prevent length changes in the pipe due to temperature variations
and pressure fluctuations, the space between the old and new pipe
can be filled with low viscosity concrete.
Sewer pipes
Due to its good chemical resistance, large-bore HDPE pipes are ideally
suited for trade wastes and is being used to an increasing extent
as an underground sewer pipe. In submarine lines, pipe flexibility
is utilized to accommodate the pipe to the profile of the waterway
bed without the use of separate fittings. Sewer pipes can also be
laid under the beds of rivers, lakes, and etc. by well-established
flushing in method. Because of their characteristic properties,
buried HDPE sewer pipes are used to remove aggressive waste from
industrial areas, for pressure drainage and in the regions liable
to soil subsidence and inaccessible terrain.
Slurry transportation
HDPE pipes have proved superior to many other types of piping where
corrosion and erosion problems exist. In slurry pipelining, where
moderate pressures exist, and where large, heavy particles and high
water velocities are encountered, HDPE pipes shows exceptional water
characteristics. It has been laboratory-tested and field-proven
to outlast steel by at least three to one. In addition to its excellent
abrasion resistance, HDPE pipes offer other advantages over other
piping systems:
|
Lightweight and exceptional flexibility provide ease of
installation, even over difficult terrain. |
|
Excellent resistance to aggressive media and soils |
|
Low wall friction losses. |
|
Jointing either by simple heat fusion process or by mechanical
connectors. |
Irrigation
Irrigation pipes are very important in agriculture, and for several
years, HDPE pipes made for this purpose have proved highly successful.
Due to its lightness, an HDPE piping system saves labour in piping
system, transport and installation. Assembly is very simply affected
by means of specially developed quick action couplings. Since irrigation
lines are generally temporary, rapid assembly and disconnection
are essential. The excellent ESCR and UV stability of HDPE pipes
score over PVC pipes in outdoor applications like irrigation. Lower
friction of HDPE pipes to fluid flow results in savings to farmers
in terms of lower power consumption.
Domestic gas distribution
HDPE pipes are widely used all over the world for domestic gas distribution.
Their ductile nature and ability to withstand continuous pressure
makes them ideal piping material for this application. The electro
fusion welding of HDPE pipes provides leak-proof joints with weld
factor of 0.9 to 1.0, which is a must for applications like gas
distribution.
Apart from domestic gas distribution, HDPE pipes are also used for
conveyance of coal gas, biogas, compressed air supply etc.
Chemical process piping
HDPE pipes are resistant to a wide range of chemicals. Conveyance
of acids, alkalies and corrosive chemicals with HDPE pipes results
in uninterrupted operations as these pipes are corrosion and maintenance
free. However, before using HDPE pipes for particular operating
conditions in chemicals plants one should refer to a chemical resistance
chart of HDPE at various temperature for acids, alkalies and other
corrosive chemicals at different concentrations.
Considering the outstanding properties of HDPE pipes and its advantages
over other conventional piping material, it is no wonder that HDPE
pipes have attained popularity among engineers as a material of
construction. With savings in operating cost, ease of installation,
longer life and maintenance free operation, HDPE pipes are expected
to find more and more usage in Indian industry.
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