Overview
ISO 13760:1998 provides an empirically based method (Miner's rule) to calculate the cumulative damage that plastics pressure pipes can accumulate under varying internal pressures and temperatures over their service life. The standard is used to determine the maximum allowable hoop stress for a given temperature/pressure profile and expected lifetime. It emphasizes that Miner's rule is a first approximation and that each failure mechanism must be assessed separately (e.g., mechanical rupture vs. oxidative degradation).
Key Topics
- Miner's rule principle: damage is proportional to exposure time at a given set of conditions and damages from successive conditions are additive.
- Total Yearly Damage (TYD): sum of the fractional yearly damage contributions from each condition in the profile.
- Maximum permissible time: calculated from TYD to establish the allowable service life under the combined profile.
- Separate failure mechanisms: the method is applied independently for each distinct failure mode (mechanical, oxidative, etc.).
- Informative annexes: examples and guidance are provided, including temperature profiles and oxidative-extrapolation considerations.
Key practical terms and formulas from the standard are presented conceptually: for each condition i, the contribution is the fraction of the year exposed multiplied by the inverse of the lifetime at that condition. The TYD is the sum of these contributions; the allowable service time is derived from the TYD.
Applications
ISO 13760:1998 is intended for engineers, material scientists, and designers who need to:
- Predict service life of thermoplastic pressure pipes subject to variable operating regimes.
- Derive design hoop stress and determine required wall thickness or SDR for pipeline systems.
- Evaluate combined profiles that include normal operation, short-term excursions, and malfunction temperatures.
Practical example (from the standard): a hot-water class profile comprising 49 years at 70 °C, 1 year at 80 °C and 100 h at 95 °C is used to illustrate how to compute TYD and adjust design stress. The standard also discusses oxidative lifetime extrapolation (Annex B) and recommends an activation energy around 110 kJ/mol for conservative extrapolation between temperatures.
Benefits:
- Enables consistent lifetime prediction and conservative design margins.
- Facilitates comparison of materials and schedules by using a common cumulative-damage framework.
- Can be implemented efficiently in spreadsheets or design software using life–stress–temperature models.
Related Standards
ISO 13760:1998 references and aligns with several standards useful for implementation:
- ISO 10508 (thermoplastic pipes for hot/cold water systems)
- ISO/TR 9080 (extrapolation of hydrostatic stress rupture data)
- ISO 1167 (resistance to internal pressure - test method)
- ISO 12230 (polybutene time/temperature strength effects)
For design work, apply Miner's rule separately to each failure mechanism and consult the referenced standards and annexes for procedures and example calculations.
