Assessment for Performance
Plastic Pipes Conference Association # 1988 Bath
Greig
The catastrophic phenomenon of rapid crack propagation in 250 mm diameter polyethylene pipes has been investigated at a full scale test facility. Tests at +3°C were conducted with pipes, either completely fully of water or containing a small percentage of air. The safe, maximum pressures were measured.
Rapid crack propagation (RCP) occurs when a crack, once initiated, runs down the plastic pipeline at high speed. If the pressure in the pipeline is reduced sufficiently then the crack will arrest after only a few metres. The crack may be initiated in any manner which produces a high speed crack but for polyethylene (PE) pipes this is most likely by high speed impact from an excavator or by the failure of a defective butt weld.
British gas have already investigated the critical conditions for buried PE pipes (125 mm to 315 mm) pressurised with nitrogen (1, 2). It was found that the pipeline behaviour would change from the crack arrest condition to fracture of the total test length (25 m) above a critical pressure. The critical propagation pressure was very sensitive to pipe temperature; increasing rapidly as the temperature was raised.
Tne Water Industry was concerned whether the same phenomenon could occur in PE pipes full of water or containing a small percentage of air for it is very difficult to remove all the air in the pipeline during high pressure commissioning tests.
Consequently, as part of the sponsored research programme “EVALUATION OF THE LONG-TERM SERVICE PERFORMANCE OF PLASTIC PIPES FOR WATER SUPPLY", the behaviour of hydrostaically pressurised blue 250 mm x 24 mm (SDR11) and yellow 250 mm x 15 mn (SDR17) at +3°C was investigated at the British Gas Engineering Reserch Station's Test Site at Spadeadam.
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