Water & Wastewater Treatment

WWT August 2017

Water & Wastewater Treatment Magazine

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Page 21 of 39

22 | AUGUST 2017 | WWT | www.wwtonline.co.uk pressure until the obstruction shi s which, depending on the pipe material, could potentially damage the pipe and joints. Plastic pipe manufacturers argue that because their pipes are generally smoother, then a lower jetting pressure can be used to either clean the pipeline or remove a blockage to reinstate hydraulic performance. Nevertheless, the risk remains that a jetting contractor could use a higher pressure to clear a stubborn blockage, which could lead to a greater risk of damage of thin- walled plastic pipes in particular. It is generally accepted that smaller diameter pipes are most effectively cleaned using high pressure water as the source of energy within the pipe. In fact, in the past some water companies have specified a jetting resilience of 4000psi for sewers. It would help eliminate confusion if this pressure limit was to be applied consistently for all smaller sewers by all water companies. Q: How has the transfer of domestic foul sewers and lateral drains to water companies in England and Wales impacted material selection? A: Water utilities have been keen to ensure that pipelines do not become damaged by pressure jetting or other mechanisms and that adopted drains and sewer assets transferred from new private developments attain a minimum level of performance. Q: Why are some pipes more at risk of jetting damage than others? A: High pressure water jetting resilience is a function of the hardness of the pipe material and the thickness of the pipe wall. The reason plastic pipes are considered to be more vulnerable to jetting damage than concrete or clay, for example, is not directly attributable to the hardness of the pipe material but to the thickness of the pipe wall. Plastic pipes generally have very thin walls and any loss of material arising from jetting can result in a significant reduction in overall wall thickness or even penetration through the pipe wall. In contrast, rigid pipes such as concrete have thick walls so that any loss of material that may arise from high pressure water jetting is, generally, inconsequential to the overall performance of the pipe. Q: Should jetting pressure be standardised? A: Most reputable jetting contractors will be aware of the need to identify the material from which the sewer is manufactured and limit the jetting pressure accordingly. If, however, a blockage fails to clear or if a jetting contractor has not established the material from which a pipe is manufactured, it is not unknown or, indeed, unexpected for a contractor under such circumstances to clear an obstruction by increasing the The Knowledge Getting to Grips with... sewer jetting Thames Water, for example, in its addendum to Sewers for Adoption 7th Edition, calls for additional testing of plastic pipelines and the selection of non-polymeric components where air testing would be hard to achieve. "Thames Water requires any plastic pipes to be subjected to an air test as part of the final inspection to verify that they have not been damaged while under the developer's control, irrespective of any earlier testing that may have been done. Developers shall carry out the air testing during the final inspection with TWUL as witness. Developers are recommended to consider the selection of non-polymeric components in locations where air testing requirements will be hard to achieve during final inspection, or in areas where the rectification of jetting damage will be costly to assess and repair. Adoptions Engineers may stipulate the use of rigid materials where no suitable method of air-testing is available for review during the adoptions application". Anglian Water has a similar requirement for taking ownership of domestic lateral drains. Its guidance document states that: "The use of plastic pipes will not be permitted unless the pipe can withstand 4000psi jetting pressure". It's a convincing argument for the use of rigid pipe materials such as concrete. Q: Are there any alternatives to high pressure jetting? A: For larger diameter pipelines there is an argument that flushing the blockage through using a large volume of water at lower pressure, through the use of larger bore nozzles, is an alternative to high pressure jetting. This option does have the advantage that the higher volume of water should carry sufficient energy to flush debris to a manhole for removal, while posing a lower risk of damage to the pipe. To generate a high water flow rate, a jetting contractor will o en need access to a drinking water hydrant or, where no hydrant is available, large capacity water tankers may be required to supply the necessary volume of cleansing water. This is obviously a more expensive, wasteful, less environmentally friendly option than using a lower volume of water jetted at a higher pressure. But, if a sewer pipe is fabricated from a material that is vulnerable to damage from high jetting pressures, such as plastic, then lower-pressure high-flow flushing may be the only option available to those charged with clearing the blockage. For more information on sewer jetting go www.concretepipes.co.uk

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