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Innovations in PVC pipe systems drive their use for over 60 years |
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Conventionally PVC pipe systems are mainly used in the sewage, potable water and drainage sectors where they have successfully replaced traditional materials. Recently PVC pipe systems have witnessed several innovations, and offers better economics and provides more safe operation.
Molecular Oriented PVC Pipes
This innovation uses an orientation process to align the PVC molecules in the hoop direction, providing almost doubled hoop strength and significantly improved impact strength. Initially, a two-stage process was developed in which a thick-walled �feedstock� pipe was extruded and then cooled. In the second stage of the process, this pipe was subjected to internal pressure at a carefully controlled elevated temperature in a specially designed mould. This blew the pipe up to its final dimensions, causing orientation of the polymer molecules in the hoop direction as it did so. Rapid cooling then �froze in� this orientation and with it the increased mechanical properties. As a result, the pipes can be used at higher service pressures or, alternatively, at the same pressures, pipes with thinner walls can be used, giving material and cost savings. MO-PVC pipes have been in use for some years in the Europe, USA, Australia, South Africa and Japan. Lately the extrusion and orientation processes have been combined into a single operation. This avoids the need to re-heat the feedstock pipe and allows savings in energy and thus cost. Crack propagation in the radial or longitudinal directions is almost non-existent in MO-PVC, due to the orientation of the molecules in the hoop direction. As soon as a crack is initiated and starts to run in either the radial or longitudinal directions, it is deflected into the hoop direction. The crack front bifurcates repeatedly, absorbing the crack propagation energy in a very short distance, thus causing the crack to stop running. MO-PVC thus gives inherent resistance to point loads and notches without modification of the fracture toughness of the basic PVC mate�rial.
Impact Modified PVC Pipes
The basic PVC is modified by incorporating impact modifiers, usually chlorin�ated PE (CPE) or acrylics. The modifier acts as a rubber in the PVC matrix, giving the mate�rial excellent ductility and so providing additional toughness to the pipe and reducing notch sen�sitivity. This results in either higher safety (if the Safety Factor is unchanged) or allows material and cost savings (if a lower Safety Factor is used). Potable water supply companies in UK and Australia have already been using impact-modified pipes successfully for a number of years. Impact modified PVC pipes have also been used in the gas supply network of the Netherlands for 45 years. The total length of the network in the Netherlands is about 110,000 km and about 80% of this is the distribution net�work, which operates at < 200 mbar pressure. Higher pressures require PE or other materials. The failure and repair frequencies of the network has been closely monitored over the last 45 years and PVC pipe provides the positive performance of impact modified PVC pipes.
| Failure statistics for Dutch gas network |
| Material |
Steel |
Cast iron |
Asbestos-Cement |
PE |
PVC |
| Repairs / 100km / Year |
6,9 |
45,0 |
21,0 |
7,1 |
2,4 |
| (Source: Gastec) |
The data cover pipes of all ages, including some first generation PE of the late 60�s and PVC-U. Modern PE pipes perform on a par with ductile PVC. The ductile PVC piping system has been found to operate reliably and capable of withstanding the aggressive chemical components commonly present in natural gas. A recent Gastec study has shown the material and installa�tion costs of PVC are lower than competitive materials by up to 30% and the calculated total life cycle costs to be up to 8% lower. For over 35 year in South Africa, impact modified PVC pipes are used for compressed air, chilled water and aggressive slurries in harsh gold and coal mining environments. In deep level mining safety is a critical requirement and pipe failures cannot be tolerated. Impact modified PVC pipes have been used in the mining industry in South Africa for over 35 years.
PVC Pipe Systems for Pipeline Renovations
The main requirements for pipeline renovation systems are to combine an easy and safe installation with a long lifetime and a good cost performance. Two new PVC-close-fit in�novations fulfill these requirements in an excellent manner. Omega-Liner� renovation system is designed to fulfill these requirements by introducing a new structurally functional pipe into the old pipe, with an estimated lifetime of at least 50 years It is a trenchless close-fit relining technique with a preformed pipe for reno�vation of damaged pipes in the range of 150 � 450 mm diameter. It is preformed at its produc�tion site and sent to the construction site coiled on a drum. Due to the pre-folding, the diameter of the pipe is temporarily reduced by 30-40%. Therefore it is possible to install this pipe without the need for large pulling forces, via the manholes already in existence and without damaging the old pipe. A pre-warmed pipe length is pulled off the drum roller using a winch and is pulled into the pipeline to be renovated. Warm steam at low pressure induces the Omega-Liner� to regain its original round profile. The material is PVC modified using an ethylene copolymer, in effect, PVC �alloy�. The �alloy� for this system was developed to give optimized mate�rial characteristics (during manufacture, installation and use) and costs (mainly during installa�tion). The main parameters which influence the performance in manufacture, installation, use and overall costs are:-wall thickness of the PVC alloy formulation (type and amount of additives).
A higher wall thickness gives an improved stiffness performance in use, but directly increases the material costs and causes higher installation costs through a longer installation time (heating & cooling). It also makes the installation more difficult (higher pressures, higher weights, de-reeling). The formulation ingredients of the PVC alloy directly influence the material properties and pipeline costs. A low softening point allows the use of low temperatures and low pressures and therefore less costly equipment and shorter heating/cooling times. An increased E-modulus, obtained by adjusting the formulation ingredients, gives improved stiffness performance in use (allowing lower wall thickness), but also affects the installation stiffness performance (de-reeling, pulling in, temperatures, pressures).
The optimized E-modulus of the PVC alloy, in combination with the appropriate wall thickness gives well-balanced stiffness performance:
Above 45�C: - Low stiffness for folding and reeling (extrusion) and de-reeling (installation).
- Low wall thickness for low heating/cooling times and low expansion pressure.
Below 45�C: - Good stiffness required for performance in use.
DuralinerTMfor Renovation of Pipes in Potable Water, Gas and Sewage Sectors
This novel system was developed during 1994-1996 in USA and is used for the renovation of pipes in the potable water, gas and sewage sectors. It combines in a unique manner the advantages of molecular orientation described above with those of trenchless close-fit relining, to provide a fully structural pipe relining system. Thick walled �starting stock� PVC pipes, which have been extruded using conventional proc�esses, are butt fusion welded and fed into the host pipe, which is usually at least 50mm larger in diameter than the PVC pipe. Once the PVC pipe is pulled through, the ends of the run are capped off with fittings. Hot water at a closely controlled temperature is then introduced via the fittings and is pressurized, causing the PVC pipe to expand against the host pipe and the PVC molecules to become oriented in the hoop direction. Cold water is fed through the expanded pipe whilst the pressure is maintained, thus �freezing in� the orientation of the PVC molecules in a manner directly analogous to that described previously for the 2-stage MO-PVC process. The expansion fittings are removed and the PVC is trimmed to length. Once installed and reconnected, the system can be used at pressures up to 10bar. Projects already completed have demonstrated that considerable cost savings can be made using this method of pipeline rehabilitation.
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