TECHNICAL FEATURE
drain TRADER
Plastic pipelines, beyond 50 years By DerekMuckle, BPF PipesGroup
NEW EVIDENCE TELLS US TO THINK IN CENTURIES, NOT DECADES, FOR PE PIPES’ EFFECTIVE LIFETIMES
T
here is a popularmyth that plastic pipes have a lifetime of 50 years, but they
actually have reliableworking lifetimes that aremuch longer than this. Even if they do start to fail, it is not a sudden loss of the asset, but rather a gradualwear process that can bemanaged. What does thatmean in practice? Quite simply plastic pipes are capable of operationmeasured in hundreds of years, not tens of years. So, it is useful to understandwhy people talk about 50 years. It is usually in connectionwith two phrases, depending on their level of knowledge:‘lifetime’or‘design point’. The latter is correct, the former is the misinterpretation. To keep it simple, it is a 50- year design point, and the design point is used as a classification index for different strength materials. It has nothing to dowith lifetime. One of the great things about plastic pipes is that they do not suffer fromcorrosion related defects. That’s obvious tomany but the issue with corrosion is not that it occurs but that its occurrence is very hard to predict. Many variables affectingwhatwill happen need measuring on a job by job basiswhichmakes predicting failure in corroding pipes very difficult. And predicting failure is important when you aremanaging critical long term investments.
Predicting failure
The failuremodes of a plastic pipe system, particularly awelded systemsuch as that achieved using polyethylene (PE)materials for example, are predictable. And they are predictable fromknowledge of the polymer used, the transported fluid in the application, and the pressure regime used for a pipe network. With that knowledge, awell- constructed pipeline can have predictable performancemeasured in hundreds of years. Ascience example can clearly illustrate the pipe classification system. Polyethylene pipes (in our example) have three time-dependent failuremodes. These are variously referred to as ductile rupture, stress crack rupture, or oxidation breakdown. For ductile rupture, independent of time, a very high internal pressure can exceed the strength of the pipe causing it to stretch and fail in a ductileway. With stress cracks, at lower pressures butmuch longer timescales,
the reliable lifetime limit could be brittle crack grown through thematerial. With oxidation, largely independent of pressure, at very long timescales, the likely reliable lifetime limitwill be as a result of polymer oxidation.
Predicting performance
Each of these failuremodes occurs at a point in timewhich can be predicted and is a function of the stress applied to thematerial by the internal pressure in the pipeline and the operating temperature. With a clever bit of science froma learned chap called Arrhenius1we can construct an envelope of performance describingwhen these failure modes can occur in time.
[Graph2] Failure rate/exposure time
operatingwithin the envelope a longway fromthe predicted failure points. Thismeans it is knowledge of the probable stress crack and oxidation onset failuremodes that prescribe the transition to awear-outmode for pipeline owners.
Designing a reliable product [Graph1]Hoopstress/exposure time
That science, as anaside,hasbeenvalidated withmore than50yearsof continuous testing andwithreal installations alsomore than50 yearsold.
Thisbringsus tothe50-yeardesignpoint. We have topick apoint intime toclassify polyethylenepipes. IntheUK, incommonwith muchof theworld,we chose50years. Some countriesusedifferent times. Whatwedois workout thepredictedstrengthat50years for apipeworkingat20◦C. Andthefirst thingwe dois roundthe strengthdowntoa convenient value. So, if the true strengthis10.8MPa,we rounditdownto10.0MPa andcall that aPE100 pipe. That’s a safety factorbuilt instraightaway. Thenwe apply an industry safety factor. In the UK, theminimumsafety factor for gas pipes is 2.9, forwater pipes it is 1.25; all ofwhichmean our operating conditions are a longway from the known point of ductile rupture of a pipeline. So, in our envelope of performance, the 50-year classification systemmeanswe are
34 drain TRADER | July 2020 |
www.draintraderltd.com
The bathtub reliability engineeringmodel is oneway to talk about the design lifetime of a polyethylene, or other plastic pipe system. Fromthe predicted failuremodes it is possible to seewhere the transition fromthe reliable lifetimemorphs into thewear-outmode. As any pipeline engineerwill advise, this is usually a differentmanagement phase, not necessarily the point of replacement. So, plastic pipelines continue to operate into the wear-outmode.
[Graph3]Hoopstress (Designlife)
In the experience of theUK, it is likely drinking water pipeswill exhibit the late life failure modes soonest. This is because chlorine ultimately initiates an oxidation degradation mechanism. Research bywater utilities and
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