OIL & GAS | PROCESSING
Safe mode: designing RCP out of PE pipe
Polyethylene has become the material of choice for gas distribution, due in part to a deep understanding of its RCP properties, write Chris O’Connor and Richard Beedell of DNV GL
Polyethylene (PE) has revolutionised low pressure gas distribution pipeline design. A unique combi- nation of flexibility, ease of joining, lack of corro- sion, long-term durability and lower installation cost makes PE the preferred material for gas transporters and operators. In order to maximise these benefits, gas trans-
porters continue to push the PE design envelope, using pipe in ever more demanding applications – including larger pipe diameters, and at higher operating temperature and pressures. However, when increasing the diameter and wall thickness of PE pipe there is an increased risk of rapid crack propagation (RCP) – also referred to as a ‘fast brittle fracture’ or a ‘linear split’ – that must be accounted for.
Brittle behaviour RCP is caused by the forced introduction of a brittle crack into a material. A combination of material quality, temperature and internal pressure then determine how far the crack will propagate along the pipe. For RCP to occur, a critical temperature (Tc
) and critical pressure (Pc ) must be established.
Finally, there must be a fracture initiation point to enable the RCP event to begin.
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In PE pipes, RCP is initiated at defect points by sudden mechanical shock, such as a high velocity impact from excavation equipment or a pipeline pressure pulse. Once initiated, ruptures can travel at high speeds – up to 300 metres/second – over long distances. RCP is of most concern in piping systems that convey compressed gases. The rapid energy dissipation from a compressed gas provides the energy needed to sustain crack growth. At a critical speed, the crack will become unstable, branching in a sinusoidal pattern until it slows and stops. Alternatively, a crack may be arrested at a point where a change in cross section occurs or a flange connection breaks the continuity. RCP depends on several factors:
n Pipe diameter – with larger diameters increasing RCP risk;
n Operating pressure – with more stress in the pipe wall increasing RCP risk;
n Operating temperature – with lower temperature increasing RCP risk;
n Material properties; and, n Others, including fracture toughness, impact strength, fatigue resistance and pipe processing history (residual stress).
Main image: RCP failure of buried PE pipe can be explosive, generating significant pressure waves
� July/August 2018 | PIPE & PROFILE EXTRUSION 19
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