FAQ - Physical properties

What is the life expectancy of PVC pipe?

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 years1).
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 to100 years2.

Real experience in Germany3 has shown that buried PVC pressure pipes dug up after 60 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

References
1.    Janson, Lars Eric 1996 "Plastics Pipes - How long can they last? KP Council Nov. 1996
2.    EN-ISO 9080
3.    60 Jahre Erfahrungen mit Rohrleitungen aus Weichmachfreiem PVC, 1995, KRV
4.    'Long Term Performance of Existing PVC Water Distribution Systems' by A. Boersma and J Breen, 9th International PVC Conference, Brighton, 26-28th April 2005, pp 307-315

What effect does exposure to ultraviolet radiation have on PVC pipe?

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 ceases1.

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.

References

1. BPF Plastics Pipes Group 2002 "PVC Pipe Systems for Water Supply" Version 01/02 (Revised)

2. www.uni-bell.org/resources/New2Tech Brief -UV2.pdf

What is a flexible pipe?

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 perform 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 systems1.

Reference
1. EN 1295 being developed tries to make a compromise between the two more widely used methods in Europe:

  •  ATV 127 ( Germany )
  •  Fascicule 70 ( France )

How do PVC pipes behave at sub-zero temperatures?

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 pipes1,2.
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.



The video shows an impact test carried out on a solid wall PVC pipe of 110mm diameter,  with a striker of 8 kg mass being dropped from a height of 2 m at 0°C.

Impact resistance of modified PVC (PVC-M) or molecularly oriented PVC (PVC-O) is even higher than the resistance described above.
References

(1) UK: Plastics Pipes Group "PVC Pipes Systems", France: STR-PVC : livret Syndotec, Italy: 'le condotte in PVC", Spain: AseTUB: Manual Tecnico conduccionnes de PVC, etc.

(2) Uni-Bell PVC Pipes Association Technical Blog, March 13th 2013.

How do PVC pipes behave under constant stress?

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 Ratio1. This ratio is also used in designing plastic pipes.

Among plastic pipes, PVC pipes have the lowest creep ratio.

As an example, in the European project for structured wall pipe standards2, the ratios demanded for different materials are:

PVC -U < 2.5 and PP, PE < 4

A lower creep ratio indicates that in the long term, the material maintains similar properties to those it initially had.
References

1. ISO EN 9967
2. prEN 13476

UK : Plastic Pipes Group "PVC Pipe Systems", France : STR-PVC : livret Syndotec, Italy : "le condotte in PVC", Spain : Asetub : Manual Tecnico conducciones de PVC, etc…

Permeability of PVC pipes

Permeability is the ability of chemical substances to enter the pipe system through the pipe walls or joints.

The occurrence of this kind of event has been reviewed by various water distribution companies and no major problem has occurred with PVC piping systems.
As an example the intrinsic permeability of PVC is in the order of 10 times lower than for polyolefins.

There have been several studies concerning the permeability of plastic pipes and pipes of other materials1-3. The most recent study of plastics pipes was carried out by the Awwra Research Foundation on the 'Impact of Hydrocarbons on PE/PVC pipes and Pipe Gaskets [Project #2946]'4.

This concluded that either PVC or ductile iron (DI) water mains can be safely used in any level of gasoline contamination, even free product, as long as there is a minimal average water flow in the mains. Although benzene, toluene, ethylbenzene, and xylenes (BTEX) will permeate the gaskets, US EPA MCLs will not be exceeded. Similarly, PVC and DI pipes can be used with periods of stagnation (i.e., service connections) for any level of groundwater contamination by gasoline. PVC itself is impervious to gasoline, BTEX, and trichloroethylene (TCE) in groundwater at commonly encountered levels of contamination.

Berens3 concluded that rigid PVC is an effective barrier against permeation of environmental pollutants.

In PVC pipe systems, the joint zone may be a weak point, but the exposed area of the elastomeric seal compared to the total area of the pipe surface is very small. Several of the studies mentioned below have also looked at the effect of hydrocarbons on the sealing rings; two of them concluded that NBR seals are more resistant than SBR seals.
References

1. Vonk, 'Permeation of Organic Compounds Through Pipe Materials', Pub. # 85, KIWA, Neuwegein, Netherlands, 1985.
2. Cassaday, Cole, Bishop & Pfau, 'Evaluation of the Permeation of Organic Solvents Through Gasketed Jointed Unjointed Poly (Vinyl Chloride), Asbestos Cement and Ductile Iron Water Pipes - Phase 1 Report'; Battelle Columbus Laboratories, Columbus, OH, for the Vinyl Institute, Div. of Soc. of Plastics Indus, Inc. 1983.
3. Berens, 'Prediction of Organic Chemical Permeation through PVC Pipe', JAWWA 77 (11), 57-64 (1985).
4. Ong, Gaunt, Mao, Cheng, Esteve-Agelet, Hurburgh, ' Impact of Hydrocarbons on PE/PVC Pipes and Pipes Gaskets [Project #2946] , Awwra Research Foundation, 2007.