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Lifetime - B

What factors can influence the design or service lifetime?The service lifetime is influenced by five factors:

• The pipe operating conditions (temperature and pressure)
• The pipe material used
• External pipe loading (traffic loading, high water table, etc.)
• The surrounding environment, including the chemical loading from, for example, contaminated soil.
• Installation conditions and methods

As an example, for design purposes the MRS is specified at a nominal temperature of 20°C. Many buried pipelines, particularly in Europe, operate at temperatures significantly below this (typically 7°C to 10°C). Hence when operating the PE pipe at pressures based on the MRS, the actual strength of the pipe is significantly greater due to the lower operating temperature. As a result the service life will theoretically be extended beyond the nominal 50 year design lifetime.

Similarly if the actual operating temperature is in excess of the nominal 20°C then the operating pressure must be reduced to achieve the lifetime of 50 years, or the theoretical service life would be reduced. This could be a factor to consider if the PE pipe is laid in close proximity to, for example, a district heating pipe, which is a source of heat especially if it leaks.

The derivation of reduction factors from the nominal 20°C is detailed in ISO 13761. Due to the high toughness of PE 100 pipes, the minimum reduction factors specified in ISO 13761 can be applied to the Maximum Operating Pressure (MOP) for higher operating temperatures.

Pressure reduction factors at operating temperatures

Chemical loading can also affect PE pipe. In addition to the risk from certain contaminated soils in the long term, care should be taken when placing PE close to tank stations where gasoline and other oil products are stored. Leakage of these into the soil in high volumes and over long periods may affect the performance of PE pipe. Typical run-off from roads is not sufficient to cause concern except in the most extreme circumstances.

References

ISO 1167:1996/EN 921:1995 Plastic piping systems - Thermoplastics pipes - Determination of resistance to internal pressure at constant temperature.
ISO 9080:2003 Plastic piping and ducting systems – Determination of the long-term hydrostatic strength of thermoplastic materials in pipe form by extrapolation.
ISO 13761:1996 Plastics pipes and fittings - Pressure reduction factors for polyethylene pipeline systems for use at temperatures above 20° C