Posted by on 17 December 2009 | 0 Comments

During this summer’s exceptional rainfall the capabilities of the UK’s ageing water infrastructure to handle higher flow rates became an important factor determining the severity of the flooding which followed. Hydraulics - a subject often disregarded in the industry - plays a key role in ensuring effective water management. Research now demonstrates how plastic is hydraulically more efficient than concrete or clay piping.

These traditional materials are also more susceptible to blockages in extreme weather conditions, leading to requirements for high jetting pressures which subsequently incurs high operating and maintenance costs. Here leading plastic pipe expert Dr Vasillios Samaras outlines the importance of the hydraulics factor in ongoing plastic versus concrete and clay debate. Even by conservative estimates, flow capacity is 30% greater in a high density polyethylene (HDPE) pipe than in a comparably sized concrete pipe.

The reason is that HDPE pipes have a roughness coefficient (ks) of 0.03 in comparison with concrete which is 0.15. A simple comparison (see figure 1)_illustrates the significance of this difference. The above example demonstrates that the HDPE pipe has a 33 % greater capacity than the same diameter concrete pipe. An alternative way to look at this is that, in order to achieve the same flow rates as that of a concrete pipe, a smaller diameter HDPE pipe could be installed, at a shallower gradient. This in turn reduces installation time since smaller excavations are needed, providing subsequent environmental benefits, such as a reduction in the carbon footprint and less disruption to the local community.

This theory of HDPE pipes’ superior behaviour over those made of conventional materials has been supported by extensive independent testing in Poland and is clarified further by Dr William B Rauen of the world renowned Hydro-environmental Research Centre at Cardiff University. “The roughness coefficient (ks) characterises the vertical size, orientation, geometric arrangement and spacing of the roughness elements. Drainage pipes with rougher surfaces (i.e. with higher ks values) will typically have a lower flow capacity due to increased drag caused by the wall. This occurs as a consequence of the pattern of dynamic pressure distribution formed over the roughness elements, with energy-consuming local accelerations and decelerations of the flow. The flow capacity of drainage pipes can thus be maximised by using pipes with the smoothest possible surface finish.” In the so-called Darmstadt abrasion test (DIN v. 19534, part 2), samples of commonly used pipe materials were filled with a mixture of sand and water, and then subjected to a specified number of rocking cycles. The amount of abraded material was measured at regular intervals. The results as illustrated in the graph (Figure 2), clearly indicate that high density polyethylene has the highest abrasion resistance properties of all the common pipe materials.

A further look to the properties of HDPE reveals its high abrasion resistance Moreover, it is important to emphasise that few materials offer better overall chemical resistance to corrosive acids, bases and salts. In addition polyethylene is unaffected by bacteria, fungi or even aggressive naturally occurring soils. Even hydrogen sulphide, the scourge of concrete pipes has no affect on HDPE. Kamila Gornas, an environmental engineer from the University of Technology in Wroclaw, Poland stresses that another distinct advantage of PE is that it provides the lowest bio film formation potential of all the common water pipe materials in use today. Bio film is the natural habitat for bacteria in water systems and forms on any surface in contact with water.

You cannot necessarily see it, but surfaces feel slimy to the touch. The bacteria migrate from the bulk water to a surface in a low flow or stagnant areas of the system and attach by producing a slime layer. Dangerous pathogens like legionella, salmonella, camphyllobacter and even viruses, present as bacteriophage, can inhibit biofilms. One of the main disadvantages of bio film is that bacteria produce acids that will cause extensive corrosion to tanks and pipes. Biofilm growth can cause flow restrictions in pipes, increasing pumping costs and reducing system efficiency. Moreover the effluent produced can cause taint, taste and odour problems. Once installed, polyethylene pipe will not be affected by micro-organisms, such as those found in normal sewer and water systems since polyethylene is not a nutrient medium for bacteria. Furthermore, siltation does not occur in the way it does in other materials such as concrete and long-term flow characteristics therefore remain constant.

This in turn results in greater hydraulic efficiency, far less risk of blockages and ultimately far lower maintenance costs. In summary, designers specifying pipe materials, should balance any relatively small initial cost saving in using traditional materials such as concrete and clay against the superior performance of the plastic option and the rapid pay-back on operating and maintenance expenditure. Added to that is the greater peace of mind during the asset life, not only to the water company but also to the general public. Dr Vasilios Samaras PhD is Technical Engineer at water management specialist company, Asset International Limited, based in Newport, south east Wales.