The durability of PVC pipes is related, as it is for all other thermoplastics materials, to the chemical degradation of the polymer used in the pipes. However unlike other thermoplastic pipes PVC pipes do not oxidise.
Stabilisers are used in PVC pipes to prevent degradation of the polymer during the extrusion process and storage of the pipes before they are buried in the ground. However, when the pipes are buried in the ground, no chemical degradation is expected to take place. For this reason the durability of the PVC material in buried pipes is expected to be very good (maybe even be more than 1000 years[1].
In standardised pipes for potable water (EN 1452) the expected lifetime of PVC pipes under pressure is extrapolated based on hoop stress testing of pipes for up to 20000 hours. This allows an estimation of the durability by extrapolation to a life expectancy under pressure of 50 to 100 years[2]. Real experience in Germany[3] has shown that buried PVC pressure pipes dug up after 70 years of active use were proven to be fit for purpose when analysed and likely to have a further life expectancy of 50 years.
Studies in the Netherlands have examined several potential degradation processes for PVC pipes and carried out tests on pipes up to 45 years old. These studies also concluded that the life of PVC drinking water systems could exceed 100 years[4].
A joint position paper by TEPPFA and PVC4Pipes demonstrates 100-year lifetime for PVC-U and PVC-Hi pressure pipe systems buried in the ground for water and natural gas supply.
Prolonged exposure of PVC pipes to direct sunlight may cause a thin film of degradation on the exposed surface of the pipe over time. This microscopic layer, with a thickness of about 0.05 mm, will gradually become visible as discolouration (so called bleaching) and it stops once the surface exposure ceases[1].
Experience has shown that this microscopic layer protects the underlying material from ultra-violet light ensuring that the rest of the pipe wall is unaffected by the sunlight[2]. If you make a gentle scratch on the outer pipe surface you will see that the normal colour is visible just under this very thin layer.
Extensive tests carried out on pipes exposed to sunlight over a period of up to 4 years demonstrate that there is a slight increase in tensile strength and modulus of elasticity and a minor decrease in impact strength. In practical terms the overall pipe properties are virtually unchanged, and pipes affected by this phenomenon may be used for normal installation.
PVC-U piping systems belong to the category of so called "flexible designed pipes". This flexibility provides a great advantage compared to pipes made of traditional materials such as concrete or clay.
For flexible designed pipes: the soil supports all the stresses on the pipe (including soil weight) and the pipes deform slightly but do not break. For pipes made of traditional materials, the soil concentrates the stresses directly on to the crown of the pipe; these pipes do not deform but a failure mode results in a break in the pipe.
For most of the "good quality soils" (e.g. granular types of soil) the soil supports all the stresses and, as this type of soil can be easily compacted, the deformation of the PVC pipes is only 1 or 2% which does not affect the functional properties nor the tightness of the systems at all. In weak soils ("plastic soils") the PVC piping systems deform slightly more (in the range of 5 to 10%) but they still perfom perfectly well.
For all piping materials very difficult soil conditions will need a thorough examination or calculation by qualified civil engineers and certain European or national standards ask for static calculation for the piping systems[1].
The performance of a pipe in service is not affected by low temperature as long as the fluid being conveyed is flowing freely. Several national organisations recommend best installation practices for PVC pipes[1],[2]. These manuals generally recommend installation at temperatures >0°C.
The minimum Impact resistance of a pipe is specified in the product standards (see EN 1401, EN 1452, etc.). For example, a sewage pipe with a diameter of 110 mm should withstand the impact of a striker of 1 kg mass falling from a height of 1.60 m at 0°C. In practice, the real resistance is generally much better than the minimum required value.
To highlight the strength of PVC pipes in cold conditions please look at the following video which shows how well PVC pipes perform.
All plastic materials submitted to a constant load undergo a progressive deformation over time. This phenomenon, caused by the displacement of molecular chains among themselves, is commonly called creep. This phenomenon depends principally on the type of plastic, its molecular structure, the operating temperature and time (it can for example take several hundred years for PVC pressure pipes to fail as a result of creep). For non-pressure pipes, standards describe the relationship between short-term and long-term creep: this is called the Creep Ratio[1]. This ratio is also used in designing plastic pipes.