PVC-O pipes design criteria
PVC-O pipe specifications for a certain working pressure are regulated mainly by ISO 16422:2006 and ASTM F 1483-05, although there are several national PVC-O pipe standards.
The parameters considered are:
Long-term hydrostatic strength (σLPL) for 50 years at 20ºC: Quantity with the unit of stress, MPa, which can be considered to be a property of the material under consideration. It represents the 97.5% lower confidence limit for the long-term hydrostatic strength and equals the predicted average strength at a temperature of 20ºC and for a time of 50 years with internal water pressure.
Minimum Required Strength (MRS): Required σLPL value at 20 ºC and for a period of 50 years.
Hydrostatic Design Basis (HDB): Equivalent to MRS according to the ASTM standard, only calculated for a period of 100,000 hours.
Global Service Coefficient design (C): Overall coefficient with a greater value than one that takes into account not only all service conditions, but also the components that make up a piping system, other than those represented in σLPL.
Design Stress (σs): Based on the value of the long-term hydrostatic strength (σLPL) for the resistance to internal pressure of the material according to ISO 9080 standard. Hydrostatic stress (σ) is expressed in MPa, inducted in the pipe wall when it is subjected to internal hydraulic pressure. It is related with the real PVC-O pipe diameter and wall thickness through the equation:
Hydrostatic Pressure (p): Internal pressure applied to a piping system.
Nominal External Diameter (dn): Numerical designation of size which is common to all components in a thermoplastic piping system other than flanges and components designated by screw-thread size.
Nominal wall thickness (en): Pipe wall thickness expressed in millimeters, which is identical to the minimum wall thickness at any given point ey,min.
The previous formulas are valid for any plastic pipe design. Due to the improvement on the physical PVC-O pipe, its MRS or HDB is considerably superior to other materials, which increases the Hydrostatic Stress value. Moreover, the superior resistance enables a reduction on the safety coefficient, contributing to increase once again the value.
The result, a PVC-O pipe with a reduction of wall thickness between 40-60% or even more compared to other materials.