This study investigates the freezing phenomena and subsequent failures in PVC and CPVC pipe systems, common in residential and commercial plumbing. The research replicates pipe freezing conditions in a laboratory setting using 1/2 inch (12.5mm) PVC and CPVC pipes, monitoring pressure and temperature during freeze events, and examining fracture modes.
Plastic pipes, including PVC and CPVC, are widely used as an alternative to steel due to their lower cost, easier installation, and ability to reduce noise. They also have fewer issues with condensation and corrosion compared to metal pipes. A key advantage of plastic pipes is their ability to expand during high-pressure events, such as freezing, absorbing pressure increases. They also possess thicker walls and greater thermal resistance than metal pipes; for instance, PVC’s thermal conductivity is significantly lower than copper’s, leading to copper cooling faster and freezing earlier.
Despite these advantages, plastic pipe freezing failures are common, primarily due to improper insulation and uncontrolled indoor temperatures. This issue is particularly prevalent in southern states where milder winters lead to inadequate pipe protection.
Freezing of water causes a net expansion in volume of approximately 9% as it transforms from liquid to solid, ultimately leading to pipe bursting. While PVC and CPVC pipes can expand to absorb this volumetric change (a 9% expansion results in a 3% diameter expansion), failure often occurs indirectly when localized freezing creates an ice plug, sealing the system. Any subsequent freezing downstream of this plug causes a significant pressure rise, leading to pipe failure.
The freezing process involves stages: initial cooling, supercooling below 0 degrees C without ice formation, ice nucleation in dendritic form, and inward growth of a solid ice structure. Dendritic ice formation on pipe walls can create strong plugs that withstand high pressures. In a closed system, further ice formation after a plug has formed leads to extreme pressure increases and potential failure.
The experimental setup involved exposing a metal pipe to subfreezing temperatures to induce failure in an adjacent plastic pipe, simulating a closed system without requiring an ice plug to form the initial pressure rise. Pressure and temperature were monitored. Results showed that both 1/2 inch CPVC and PVC pipes experienced burst failures when the metal pipe was exposed to subfreezing temperatures. A sudden rise in temperature coinciding with a pressure increase was observed, attributed to the latent heat of fusion and dendritic ice formation.
When comparing fracture modes, PVC pipes generally showed greater expansion and significant bulging before failure than CPVC pipes. Pipes exposed to lower temperatures exhibited less ductility, greater stiffness, higher maximum pressures, and less bulging with more crack propagations. Fracture surfaces revealed rougher initiation regions for PVC and multiple initiation sites for CPVC. However, the failure modes observed in the laboratory often did not accurately replicate those seen in actual residential settings, which typically show larger crack propagations, more bifurcations, and less bulging. This discrepancy suggests that variables like material formulation, pipe length, temperature variations along the pipe, and aging effects may influence real-world failures. Further research is needed to better understand and replicate these factors to more accurately study residential and commercial system failures.
