Page 26 - Gi September2020

Plastic pipelines, beyond 50 years

pressure, at very long timescales, the This is because chlorine ultimately GRAPH 1 HOOP STRESS/EXPOSURE TIME (WITH WHITE ARROWS)
likely reliable lifetime limit will be as a initiates an oxidation degradation
result of polymer oxidation. mechanism. Research by water utilities
and manufacturers² shows that for
Predicting performance earlier generation materials the Rupture
Ductile
Each of these failure modes occurs at a transition to wear-out mode will likely
point in time that can be predicted, and start at around 235 years. For other
is a function of the stress applied to the applications, like natural gas or Stress Crack
Rupture
material by the internal pressure in the hydrogen, oxidation onset should take HOOP STRESS
pipeline and the operating temperature. considerably longer.
With a clever bit of science from a
1
learned chap called Arrhenius , we can Oxidation
construct an envelope of performance Many variables affecting Breakdown
describing when these failure modes can what will happen need to be EXPOSURE TIME
occur in time. measured on a job-by-job
That science, as an aside, has been
validated via more than 50 years of basis, which makes predicting
continuous testing and with real failure in corroding pipes GRAPH 2 FAILURE RATE/EXPOSURE TIME (DESIGN POINT, 50 YEARS)
installations also more than 50 years old. very difficult. And predicting
This brings us to the 50-year design failure is important when you
point. We have to pick a point in time
to classify polyethylene pipes. In the are managing critical long-
UK, in common with much of the term investments
world, we chose 50 years. Some
countries use different times. What we
do is work out the predicted strength at All this means real lifetimes for RELIABLE LIFETIME WEAR OUT
50 years for a pipe working at 20°C. plastic pipes can be achieved that are FAILURE RATE
The first thing we do is round the measured in hundreds of years, not
strength down to a convenient value; if tens of years. And the great news is that
the true strength is 10.8MPa, we round the industry continues to invest in
it down to 10.0MPa and call that a innovation to extend these lifetimes EXPOSURE TIME
PE100 pipe. That’s a safety factor built further into the future by improving
in straight away. resistance to stress crack and oxidation
Then we apply an industry safety effects in new materials. That means if
factor. In the UK, the minimum safety you specify wisely, and use welded GRAPH 3 HOOP STRESS (DESIGN LIFE)
factor for gas pipes is 2.9 and for water connections of compatible materials,
pipes it is 1.25, which means our you should be able to construct pipe
operating conditions are a long way assets with genuine lifetimes measured
from the known point of ductile rupture in hundreds of years – surely a great Ductile
Rupture
of a pipeline. So in our envelope of benefit when specifying plastic pipes
performance, the 50-year classification for critical long-term assets.
system means we are operating within Stress Crack
Rupture
the envelope a long way from the Part of the British Plastics Federation,
predicted failure points. This means it the BPF Pipes Group is a trade association HOOP STRESS
is knowledge of the probable stress representing manufacturers and material
crack and oxidation onset failure modes suppliers of plastic piping systems DESIGN LIFE
Oxidation
that prescribe the transition to a across the UK. Committed to sustainable Breakdown
wear-out mode for pipeline owners. construction, its aims are to provide
a forum for the exchange of technical EXPOSURE TIME
Designing a reliable product expertise between member companies
The bathtub reliability engineering and to promote the importance of plastic
model is one way to talk about the as a pipework material, for the full GRAPH 4 RELIABLE LIFETIME
design lifetime of a polyethylene, or spectrum of above and below ground,
other plastic pipe system. From the pressure and non-pressure applications.
predicted failure modes, it is possible to Read the group’s latest technical guidance
see where the transition from the at www.bpfpipesgroup.com/support- Ductile
reliable lifetime morphs into the downloads/technical-guidance. TEMPERATURE Rupture
20˚C
wear-out mode. As any pipeline INDUSTRY
SPECIFIC SP LCL
engineer will advise, this is usually a MRS Stress Crack
Rupture
different management phase, not References
necessarily the point of replacement. 1. Swedish chemist Svante Arrhenius (1859- HOOP STRESS PN
1927), who led the development of equations for
So, plastic pipelines continue to operate the temperature dependence of reaction rates, a
into the wear-out mode. fundamental of accelerated ageing techniques in the
field of reliability engineering.
Oxidation
In the experience of the UK, it is 2. Hoang EM, Lowe D, ‘Lifetime prediction of a blue Breakdown
likely drinking water pipes will exhibit PE100 water pipe’, Polymer Degradation & Stability, DESIGN EXPOSURE
POINT
the late life failure modes soonest. Issue 8 (2008), pp 1496-1503. 50 YEARS TIME
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