COMPOSITES | MATERIALS
Building a composite picture
IMAGE: STROHM
Composite pipes – ranging from pure GFRP to multi- layer TCPs – are finding increasing use in applications as diverse as infrastructure and the oil and gas sector
Pipe manufacturers typically turn to composite materials to ensure high performance in a variety of end-use applications. Composites rely on a combination of materials – such as a polymer matrix, a reinforcement and even metals such as aluminium or steel – to imbue pipe with added strength. While this is typically more expensive and complicated to produce, the combined effect helps to lift attributes such as burst strength and mechanical performance. Composite pipes are a relatively small part of
the market and are subject to a large amount of fundamental research, in order to further expand their performance.
Burst strength A recent paper in the International Journal of Pressure Vessels and Piping investigates the long-term rupture pressure in PEX-Al-PEX composite pipes. This type of pipe is widely used in a building’s
hydronic heating and cooling and plumbing systems. It comprises three main layers: outer and inner layers of PEX, and an aluminium (Al) middle layer. A polyethylene adhesive is typically used between the layers to prevent slipping of the polymer layers over the Al layer. One of the most vital parameters in making these pipes is the tolerated internal long-term hydrostatic
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pressure, say the Iran-based researchers. The rupture pressure of composite pipes depends on operating temperature and time – as well as each layer’s dimensions (inner polymer, outer polymer, and aluminium layer), the mechanical properties of the layers, and the welding types of the aluminium layer. The researchers measured the long-term life behaviour of PEX multi-layer pipes for up to 1000 hours – for different diameters and welding types. The hoop and radial stress of the pipe layers are calculated analytically, based on classical elasticity theory. The pressure that can be tolerated by each layer was defined as a function of long-term hydro- static pressure, size, and mechanical properties of both the aluminium and PEX layers in the pipe. The results showed that the pressure bearing by the aluminium layer is 84-91% of the pipe’s hydrostatic pressure. The PEX outer layer pressure bears less than 3% of the hydrostatic pressure – with the rest taken up by the PEX inner layer.
Overcoming defects Similarly, researchers at Robert Gordon University in Aberdeen, UK, have assessed manufacturing defects in thermoplastic composite pipes – and how this can affect performance in oil and gas applications. Thermoplastic composite pipes (TCPs) use fibre reinforcement to add benefits such as light weight
January/February 2023 | PIPE & PROFILE EXTRUSION 11
Main image: Strohm has won a contract to provide thermoplastic composite pipe for a green gas terminal in Germany
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