The flow must go on
Hydraulics - often disregarded in the industry - can play a key role in ensuring effective water management. Dr Vasilios Samaras outlines the importance of hydraulics in the ongoing plastic versus concrete and clay debate.Research shows 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 that subsequently incur high operating and maintenance costs.
Even by conservative estimates, flow capacity is 30% greater in a high-density polyethylene (HDPE) pipe than in a comparably sized concrete pipe. This is because HDPE pipes have a roughness coefficient (ks) of 0.03 in comparison with concrete, which is 0.15.
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 more shallow gradient.
This reduces installation time since smaller excavations are needed, providing subsequent environmental benefits including a reduced 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 at Cardiff University's Hydro-environmentalResearch Centre.
Rauen says: "The roughness coefficient (ks) characterises the vertical size, orientation, geometric arrangement and spacing of the roughness elements. Drainage pipes with rougher surfaces - hence 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 (see graph below left), 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 indicated that HDPE has the highest abrasion resistance properties of all the common pipe materials.
It is important to emphasise that few materials offer better overall chemical resistance to corrosive acids, bases and salts. Also, 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 biofilm formation potential of all the common water pipe materials in use today.
Biofilm 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 area of the system and attach by producing a slime layer.
Dangerous pathogens such as legionella, salmonella, camphyllobacter and even viruses present as bacteriophage, can inhibit biofilms.
One of the main disadvantages of biofilm 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. And 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 payback 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 is technical engineer at Asset International.
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