6 December, 2016: The recent introduction of 560 mm and 630 mm polyvinylchloride (PVC) sewer pipe is challenging the dominance of concrete as the material of choice in this market segment, according to DPI Plastics Technical and Product Manager Renier Snyman.
While PVC constitutes 95% of the total sewer-pipe market in the US, it is only predominant in small to medium pipe sizes in South Africa. However, the myriad advantages of PVC over traditional materials such as concrete means it is ideal for stormwater drains and large sewers.
Like most thermoplastics, PVC has a smooth surface, which translates into low friction or resistance. “This presents quite an advantage for pressure pipes and gravity sewers,” Snyman comments. PVC is also classed as a semi-flexible material, meaning it has good resistance to ground movement. Traditional materials such as clay and concrete, on the other hand, have high stiffness and are hard.
“The moment there is a little bit of movement, they can crack and spring a leak. This poses a major health risk with sewer-pipe systems in particular,” Snyman stresses. Another benefit is that PVC can be painted easily with either an exterior acrylic or alkyd enamel paint.
“In South Africa, it is quite popular to paint soil and vent pipes, as we tend to plumb on the outside of a wall, and these external areas are highly visible as a result,” Snyman adds. In addition, PVC has a high resistance to damage and scratches, which means that sewer lines made from this material can be rodded or cleaned by means of water-jetting.
The smooth internal bore is not only non-corrosive, but also resists algae and scale build-up. This may not seem to be an issue affecting plastic pipes, but Snyman explains that a biofilm does form inevitably, which affects the flow and volume. However, this is negligible compared to other materials such as concrete.
“The end result is a pipe that still performs reasonably well, compared to a new pipe. That is quite important, especially as any pipeline is a long-term investment, with an average lifespan of 50 to 100 years,” Snyman points out. PVC also has double the tensile strength of high-density polyethylene (HDPE), meaning a thinner wall thickness. This not only reduces costs, but means that PVC has a lower carbon footprint as it uses less raw materials.
“A major advantage of PVC for sewer pipes is it has a high modulus of stiffness. A sewer pipe is empty for most of its lifespan, with only the ground pressure bearing down upon it. Trenching plays a role here, but the pipe has to have a certain stiffness in order to maintain its round shape. “If a sewer pipe deforms excessively or collapses, it can result in blockages in the line. Hence it is important that the pipe retains its circular shape, and here PVC plays a critical role,” Snyman elaborates.
Extra benefits include the fact that PVC is formulated specifically to be highly UV-resistant, as well as having excellent fire properties. The high Limiting Oxygen Index (LOI) means it is difficult to ignite. In addition to normal rubber ring joints, solvent welding can be used to join PVC pipes, which is easy-to-use and cost-effective. The normal temperature range of a sewer and drainage system fluctuates from zero to 60°C. “PVC performs well within that range,” Snyman asserts. It also has a low co-efficient of linear expansion, about half that of polyolefins.
PVC came under the spotlight at the Southern African Plastic Pipe Manufacturers’ Association (SAPPMA) Pipes X conference, held at the Byte Conference Centre in Midrand on 6 September, where Snyman gave a presentation on the potential of this material for large-diameter sewer pipes.