Pressure pipes are primarily designed to withstand internal tensile stress. This stress is dominant under internal pressure. The main ways to increase resistance against higher pressures are increasing wall thickness, using a stronger material, or reinforcement by winding fibers. All these solutions result in a simple uniform shape for the pressure pipe. Plastic pipes offer advantages over traditional materials like lead or copper due to their weight, corrosion resistance, flexibility, and ease of connection. Pressure applications include pipes for gas and water.
Key Design Considerations
The pressure rating (PN) for a pipe is calculated based on Barlow's formula: PN = C x (MRS / (2 * SDR - 1)). Here, MRS is the Minimum Required Strength (50-year strength) of the material under internal pressure, determined by long-term testing. SDR is the Standard Diameter Ratio (Outer Diameter / wall thickness). C is the Safety Factor or Overall design coefficient.
Different materials and pipe types have varying MRS and typical safety factors. For example, un-plasticised PVC (UPVC) typically has an MRS of 25 MPa (250 bar) or higher and a safety factor of 2-2.5 in many European countries, though Germany uses 2.5. Modified PVC (MPVC) has a lower safety factor (1.4-1.6) but also a lower MRS (approx. 22 MPa). Biaxial oriented PVC (PVCO) has a lower safety factor of 1.6 but a significantly higher MRS (40-50 MPa), allowing for less than half the wall thickness compared to UPVC for the same pressure class.
The safety factor is influenced by material properties like toughness and ductility, failure modes (brittle vs. ductile), and tests such as the C-ring test or crack propagation tests. PE materials often have lower safety factors (e.g., 1.25 for water pipes) due to good performance in crack tests, but are higher for gas pipes (earthquakes!). Pipe laying conditions and backfill material can also affect the required safety factor. Standards like ASTM F1438, ISO16422, and AS/NZS 4441 apply to PVCO pipes.
Technical Specifications Reference
Parameter
Description
PN (Pressure Nominal)
Pressure rating (bar)
MRS
Minimum Required Strength (50 yrs strength, MPa or bar)
SDR
Standard Diameter Ratio (OD/wall thickness)
C (Safety Factor)
Overall design coefficient
Application
Water, Gas
FAQ Section
What is the primary stress considered when designing PVC pressure pipes?
The dominant stress situation for pressure pipes is internal tensile stress in the cross-section of the pipe wall.
How does Biaxial Oriented PVC (PVCO) affect pressure pipe design compared to UPVC?
PVCO has a much higher MRS (40-50 MPa vs 25 MPa+) and a lower safety factor (1.6 vs 2-2.5). This allows for a significantly thinner pipe wall for the same pressure rating, potentially less than half the thickness of a UPVC pipe.
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Gerrit Jan Wilpshaar
