For pressure pipe applications, the material's strength under internal pressure over time is defined by the Minimum Required Strength (MRS).
Defining MRS
The MRS is the 50-year strength of the material used for the pipe under constant internal pressure. It is determined through long-term testing of pipes at different pressure levels until rupture. The MRS is statistically derived as the 97.5% Lower Confidence Limit (LCL) of the rupture line in a log-log graph, following the Arrhenius law for polymer decay. The ASTM Hydrostatic Design Basis (HDB) is a similar concept, extrapolated to 11 years.
UPVC vs. MPVC
Un-plasticised PVC (UPVC) has an MRS of 25 MPa (=250 bar) or higher. In most European countries, a safety factor (Overall design coefficient) of C=2 is used for PVC water pipes, though in Germany, it's 2.5.
Modified PVC (MPVC) includes impact modifiers for increased ductility. MPVC pipes have a lower safety factor (1.4 to 1.6) compared to UPVC (2-2.5), and their MRS is also lower, approximately 22 MPa.
Oriented PVC (PVCO), a biaxially stretched form, has a significantly higher MRS of around40-50 MPa, depending on stretching conditions.
Technical Specifications Table
Property
UPVC Pipe
MPVC Pipe
PVCO Pipe
MRS (MPa / bar)
≥ 25 MPa / 250
≈ 22 MPa / 220
≈ 40-50 MPa / 400-500
Typical Safety Factor
2 - 2.5
1.4 - 1.6
1.6
Applications
MRS is a fundamental parameter used in Barlow's formula (or approximate variations) to determine the pressure rating (PN) for pressure pipes based on the pipe's Standard Diameter Ratio (SDR) and the material's safety factor.
FAQ Section
What does MRS stand for and what does it represent?
MRS stands for Minimum Required Strength, representing the material's estimated strength after 50 years under internal pressure, derived from long-term testing.
Why is the MRS for MPVC lower than that for UPVC?
MPVC includes impact modifiers to enhance ductility, which results in a lower MRS compared to standard un-plasticised PVC.
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Nick Belshof
