Understanding PVC Pipe Applications: Drinking Water vs. Sewer Systems

Properties and Applications

Plastic pipes, including those made from PVC (Un-plasticised Poly Vinyl Chloride), differ significantly from traditional materials. PVC is a high-volume commodity polymer used in both pressure and non-pressure applications.

For pressure applications, such as gas and water pipes, the primary load is tensile stress. Key properties for drinking water pipes include low diffusion through the pipe wall, which is crucial in contaminated soil (brown fields). PVC pipe has a lower risk of biofilm in the bore and is not as vulnerable to chlorinated water as PE pipe can be. Traditional UPVC has a Minimum Required Strength (MRS) of 25 MPa (250 bar) or higher. Modified PVC (MPVC) can have a lower safety factor (1.4-1.6) but also a lower MRS (approx. 22 MPa). CPVC (Chlorinated PVC) is used in the USA and India for Hot and Cold applications and needs no barrier layer.

For non-pressure applications, such as drain, waste, venting (DWV), and sewers, the bending load or flattening load is dominant. Stiffness is a main characteristic. Flexible plastic pipe attracts soil loads less than rigid pipe, preventing cracks. Structured Wall pipes and Foam core pipes are examples of light-weight, stiff designs for non-pressure use. Standards for Structured Wall Pipes prescribe a minimum pipe stiffness (SN) rather than wall thickness, allowing formulations with higher filler levels, such as CaCO3, for cost and stiffness gains. Foam core pipes can contain significant amounts of CaCO3 (up to 15-20 phr).

Standards and Specifications

Standards ensure good practices for production and installation. ASTM (American Society for Testing and Materials) sizes like IPS, CTS, C900/C905 are common in the USA, while ISO (International Standard Organisation) and EN (European Norm) standards are used in Europe. ISO standards are gaining international acceptance.

Pressure pipe design is often based on Barlow's formula, which relates the pressure rating (PN) to the minimum required strength (MRS) and the Standard Dimension Ratio (SDR) with a safety factor (C). The safety factor for UPVC water pipe is typically C=2 in most European countries, but 2.5 in Germany.

For non-pressure pipes, Pipe Stiffness (PS) or Ring Stiffness (STIS/SN) is a key metric. ASTM uses PS in psi, while ISO/EN use STIS in kPa. For instance, a 400 kPa pipe has a STIS of 7.45 kPa is SN 7.45.

Technical Specifications Overview

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