Overview
The safety factor (C) is a crucial component in the design of pressure pipes, included in Barlow's formula: PN = C x (MRS / (2 * SDR - 1)). It acts as an overall design coefficient to ensure reliable long-term performance under pressure.
Factors Influencing the Safety Factor
The choice of safety factor is influenced by several factors related to the material properties and application conditions:
- Material Toughness and Ductility: Materials that are tougher and more ductile can absorb more energy before failure and are less prone to brittle fracture.
- Failure Mode: The material's tendency towards brittle or ductile failure is assessed through various standard tests, including the C-ring test for fracture toughness and crack propagation tests (slow and rapid).
- Load Application: The way the load is applied can influence the failure mode.
- Pipe Laying Conditions and Backfill: The quality of the material surrounding a buried pipe and the risk of point loads (e.g., in rocky areas with thin backfill) can necessitate a higher safety factor. Protective outer layers on pipes can also mitigate risks in challenging conditions.
Typical Safety Factors for PVC
- Un-plasticised PVC (UPVC): In most European countries, the safety factor for UPVC water pipe is C=2. However, in Germany, a safety factor of 2.5 is used, resulting in a lower pressure rating for the same SDR pipe (e.g., SDR26 is PN10 with C=2, but PN8 with C=2.5). The UPVC safety factor typically ranges from 2 to 2.5.
- Modified PVC (MPVC): By adding impact modifiers to make PVC more ductile, MPVC pipes can have a lower safety factor, typically ranging from 1.4 to 1.6. However, MPVC generally has a lower MRS compared to standard UPVC.
- Oriented PVC (PVCO): Biaxial oriented PVC pipes also have a lower safety factor of 1.6 compared to UPVC. Combined with a significantly higher MRS (40-50 MPa), this allows for substantial wall thickness reductions. PVCO standards like ISO 12162 cover aspects like safety factors.
In contrast, PE materials, known for good performance in crack tests (like S4 stability test for PE100), can have safety factors as low as 1.25 for water pipes, although they are higher for gas pipes due to risks like earthquakes. Brittle plastics are generally not applied in pressure systems.
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