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:

	Very high impact strength
	Light weight
	Flexibility and ductility
	Chemical resistance
	Weatherabilit
	Wide temperature service range from 400oC to 600oC
	Smooth internal surface
	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 2100^oC 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:

	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.

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.