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Design stress of PVC pipe increases to 32 MPA by process technologies |
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For over six decades, high value thermoplastic pipes made from polyvinyl chloride (PVC-U) and polyethylene have been in use. Significant steps have been taken to improve the competitive position of PVC pipes via material savings in the past few years. The critical requirement for potable water and the incentive for water authorities and engineers to accept new pipe technologies has been the driving force behind the developments. Recently, design stress on PVC pipe has increased from 10 to 32 MPa along with the corresponding reduction in wall thicknesses and safety factors. The move to higher design stress, thinner wall pipes would probably have slowed were it not for the advances made in perfecting the in line production process for oriented PVC pipe. Oriented PVC pipe made using this process has an exceptional combination of strength and toughness properties that improves design stress.
One of the areas that PVC pipes have penetrated from a non existent share is mining in South Africa. South Africa has an abundance of minerals, especially gold and coal, with gold being mined from underground levels as deep as 5000 m. At these depths, working conditions are extremely difficult with temperatures approaching 50oC. For health and productivity reasons, cooling and fresh air ventilation is necessary. Cooling underground is done by pumping water at 5oC several kilometers down vertical shafts. Ventilation air is conveyed underground, cooled to reduce ambient temperatures and improve working conditions, while compressed air is necessary for operation of rock drills and other mining equipment. At the end of the process, the chilled water needs to be pumped back up to the surface for reuse. Since safety of miners is vital and all aspects of underground mining are subject to the most stringent safety rules; pipe failures cannot be tolerated. Steel pipes were the preferred carrier notwithstanding the high humidity underground and corrosive acidic conditions. It was in the mid-1970�s that it was decided to enter this market with PVC pipes.
PVC Mining Water Pressure Pipes - Design Considerations
PVC mine pipes offer superior thermal insulating properties Vs steel pipes. This translates into energy savings, as less power is consumed in re-cooling the return water to 5oC.
Polymer developments undertaken at that time and which have continued over the years, resulted in a material whose critical properties were essential to the later development of higher design stress, PVC-M pipes.
• The properties and consistency of PVC, in particular: K-value, apparent density, CPA (cold plasticizer absorption or porosity) and thermal stability.
• Various impact modifiers, taking into account pipe storage and outdoor weathering.
• The level of addition of these modifiers.
• Extrusion conditions to optimize processing and pipe toughness.
• The development of a suitable pipe joint.
• Testing procedures to verify quality in terms of pipe strength, toughness and consistency.
• A pipe standard for mining which had to be devised and accepted by the industry.
Chlorinated polyethylene (CPE) impact modifiers were selected as giving the best balance of properties and ease of processing. The first PVC pressure pipes supplied for chilled water contained 12 phr of modifier which was subsequently reduced to 10 phr in order to increase long-term strength and was later reduced still further, whilst maintaining acceptable toughness and a 50-year failure stress of 25 MPa. A special �pipe grade� polymer was developed and close tolerances set for pertinent polymer properties. Higher extrusion temperatures and shear are necessary to achieve correct levels of homogenization and gelling as it develops optimum toughness and leads to improvements in the heat stability of the polymer.
Mine pipes are suspended along haulage ways using metal S hooks and chains- The pipes are suspended and a pipe coupling resistant to end thrust was developed to handle the 16 bar and higher pressures used in mining. The coupling is compatible with steel pipes and uses standard fittings. The factory fitted Victaulic steel joint has serrated teeth on the inner surface which bonds to the pipe ends via mechanical interference, providing a watertight joint. Standard rubber seals and clamps are used to couple the pipes.
Standard impact tests are inadequate for determining toughness properties of these pipes and a high-speed (20m/second) impact test had to be developed so as to characterize pipe toughness properties. The test involves dropping steel strikers having masses up to 30 kg on pipes from a height of 20 m; impact energies are 600-5000J, depending on pipe size. Ductile �failure� is the requirement on this test. These special toughness properties are essential to prevent (brittle) failures of the pipes, in particular at the rigid steel ends, resulting from stress concentration effects.
Modified PVC pipes have been used successfully in extremely harsh mining environments for 25 years. The material�s versatility lends itself to formulations covering a variety of demanding applications. Sustainability of the product is demonstrated in several mines where PVC pipes are later removed for reuse; this is not generally the case with steel pipes because of deterioration through corrosion.
The design stress for modified PVC mine pipes has been raised to 12,5 MPa based on a long- term safety factor of 2, accepted by the industry.
PVC Mine Pipe for Compressed Air
Pipes conveying compressed air at 7 bar must be �fail-safe� under all conditions, especially considering stored energy effects and possible impact failure. A modified PVC pipe has been developed to meet this critical requirement. In early development testing pipes were burst using nitrogen at approximately 50 bar. In another test for ductility, the failure of the pipe containing 7 bar air pressure was initiated by an explosive detonator attached to the pipe wall.
A long term safety factor of 3 is used for pipe design in mining industry.
Oriented PVC Pressure Pipe
Oriented PVC pressure pipe has been used for almost 30 years but it is only with the advent of the in-line production process that the product has become competitive against pipes such as PVC-M, for the following main reasons:
• The greatly improved production output rates and reduced manufacturing costs.
• The remarkable combination of strength and toughness properties which allows for higher design stresses and material savings, especially in the higher pressure classes.
Oriented PVC pipe is manufactured by a patented process which orientates the long chain molecules of feed stock PVC-U pipe produced by conventional methods of extrusion. Increasing the diameter of the pipe during the process orientates the molecules in the hoop direction and leads to substantial improvements in the physical properties, both strength and toughness. The molecular orientation greatly increases the short and long-term strength of the material. Given the outstanding strength and toughness properties, 50 year safety factors of 1,6 or 1,4 can be applied for MRS 45 and MRS 50 PVC-O materials. Design stresses of 28 and 32 MPa are used for these oriented PVC pipe resulting in material savings of over 50% and 30% against the equivalent PVC-U and PVC-M products.
The process of molecular orientation produces a laminar or layered structure which is the key to the toughness and performance of oriented PVC pipe. If cracks do form, for example, due to defects or point loads, their passage through the material is hindered by the layered structure and propagation is effectively arrested by a reduction in stress concentration as the crack passes through each layer. Other advantages of oriented PVC pipe resulting from the higher design stress are increased hydraulic capacity and improved handling and installation characteristics.
Fatigue of plastics pipe is not usually associated with well-designed and operated pipelines and failures due to fatigue are extremely rare. Oriented PVC pie is extremely good resistance to fatigue failure and can be specified in those installations where some pressure cycling is anticipated and providing design engineers with greater confidence. This pipe also has excellent resistance to surge pressures and water hammer and is quite capable of withstanding pressure transients over twice the rated working pressure.
Applications include pumped water reticulation systems and rising sewer mains and installations where greater safety is required with respect to pressure surges, water hammer or pressure cycling. Oriented PVC pipe fitted with steel Victaulic joints is suitable in the mining environment. Having less than half the mass of modified PVC mine pipe provides advantages in handling and installations underground and in the higher pressure classes (16-25 bar) the product is competitive against traditional steel pipes. The products good performance in rapid crack propagation tests implies potential for conveyance of compressed air.
(Source Courtsey: M A Osry) |
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