Non-Metallic Materials | Minimise Pressure Swings

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  9. Non-Metallic Materials- Minimize Pressure Swings

Fibre Reinforced Plastic (FRP) pipes offer a solution for otherwise aggressive process fluids that could corrode carbon steels. However, FRP has unique challenges involving handling and cyclic pressure. An FRP pipeline handling disposal water failed in service after 6 years. The upstream pumps had been recently replaced with a model that had higher pressure swings; the pipeline failed within 1 km of the pumps. What happened?

The top left image is a close-up of the outside of the pipe at the through wall failure. The bottom side of the pipe has regular and generally evenly spaced gouges following a wandering line. The through wall fracture is aligned with these gouges. A cross section of the failed region is shown in the top right image showing the multi-layer construction: an inner plastic layer and an outer fibreglass and resin layer.

Multiple cross sections of the FRP were examined, both near and far away from the failure. Additionally, a piece of never-used FRP was supplied for comparison. A close-up photograph of the new FRP is shown in the bottom left image. The glass fibres are bright white and have good adhesion with the off-white resin. The black regions are voids, where glass has separated from resin. The bottom right photograph shows the FRP condition of the failed piece at a region a few meters away from the failure. The FRP has extensive voids, separations, and cracking between layers. This appearance was like the appearance found near the failure.

The FRP failed at this location specifically because it had been damaged during installation. The regular and consistent damage along the OD suggested that the FRP pipe was dragged over a sharp object as it was being unspooled. However, the FRP failed after it was subjected to cyclic pressure swings greater than 20% of its pressure rating. The new pump design with significant pressure swings was likely responsible for the widespread damage.

Careful installation and minimizing cyclic effects are critical for keeping FRP in service.  See FRP requirements for CSA Z662 “Oil and gas pipeline systems.”