PVC pipes are designed differently depending on whether they are intended for pressure or non-pressure applications. Standards play a critical role in ensuring good practices for production and installation.
Design Principles for Pressure PVC Pipes
For pressure pipes, the primary stress situation is tensile stress. Resistance against higher pressures can be increased by:
Increasing the wall thickness.
Using a stronger material (higher MRS).
Reinforcement, such as winding fibers around the pipe.
Pressure pipes typically have a simple, uniform shape. The pressure rating (PN) is related to the Minimum Required Strength (MRS) of the material, the Standard Diameter Ratio (SDR), and a Safety factor (C).
Design Principles for Non-Pressure PVC Pipes
In non-pressure applications like drain, waste, venting (DWV), and sewers, bending load is often dominant, particularly from soil loads for buried pipes. Shape plays a critical role in resisting these loads. Structured Wall pipes, which move material away from the center, are common examples (e.g., Double Wall Corrugated pipe, Foam core pipe). Stiffness is a main characteristic for non-pressure pipes.
International Standardization
Pipe standards and dimensions vary globally. Key standards bodies include:
ASTM (American Society for Testing and Materials) in the USA (e.g., IPS, CTS, C900/C905 sizes). ASTM standards for pressure pipe design use the Hydrostatic Design Basis (HDB) extrapolated to 11 years. ASTM D1784 sets minimum properties for pressure compounds. ASTM D891 is a standard for Foamcore pipe.
ISO (International Standard Organisation) and EN (European Norm) in Europe. ISO standards are gaining wider adoption. ISO standards extrapolate to 50 years to determine the MRS. EN standards, like EN13476 for Structured Wall Pipes, may specify composition limits rather than just properties.
JIS (Japanese Industry Standard) in Japan.
SANS (South African National Standard).
National or US State standards also apply. ISO standards use Renard or ISO figures for dimensions and material properties based on constant percentage steps.
Regression Lines for Life Expectancy
The long-term strength of pipe materials like PVC is determined by long-term testing where pipes are tested until rupture at constant pressure. The rupture points plotted on a log-log graph form a regression line. The MRS (Minimum Required Strength) is derived as the 97.5% Lower Confidence Limit (LCL) of this regression line, representing the strength after 50 years. For PVC, the regression line is typically straight without a "knee" in the lifetime graph.
Safety Factors for Pressure Pipes
The Safety factor (C) is used in pressure pipe calculations (Barlow's formula) to account for material properties, testing, and installation conditions. Typical values for C vary:
UPVC water pipe: C=2 in most European countries, C=2.5 in Germany.
MPVC (Modified PVC): C=1.4 to 1.65.
PVCO (Oriented PVC): C=1.61.
PE water pipes: As low as C=1.25, higher for gas pipes (due to factors like earthquakes).
Factors influencing the safety factor include material toughness, ductility, failure mode (brittle or ductile), crack propagation resistance, pipe laying conditions, backfill quality, and risk of point loads.
FAQ section
Why are there different safety factors for different pipe materials and applications?
Safety factors vary based on material properties like toughness and ductility, which affect how the pipe behaves under stress and its failure mode (brittle vs. ductile)8. Installation conditions, potential for point loads, and the critical nature of the application (e.g., water vs. gas) also influence the required safety factor8....
How is the long-term strength (MRS) of a material determined for standards?
The MRS is determined by conducting long-term hydrostatic pressure tests on pipes at different stress levels until they rupture23. The data points are plotted on a log-log graph, and a regression line is drawn7.... The MRS is statistically derived from this line as the 97.5% Lower Confidence Limit, typically representing the expected strength after 50 years according to ISO standards17....
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Gerrit Jan Wilpshaar
