Cover Blow-Off Resistance of Reinforced Thermoplastic Pipes for Gas Service

Session 6B
3:00 pm

Ernst van der Stok, Kiwa Gas Technology

There are many developments aiming at increasing the allowable gas pressure in plastic pipe systems, such as the use of fibre-reinforced thermoplastic pipeline systems (RTP). RTP systems are suitable for pressures well above 10 bars, which is not possible with conventional PE100 pipes.

The maximum operating pressure (MOP) for RTP systems is not only determined by the long-term strength, but also by the ‘cover blow-off resistance’. Cover blow-off is caused by gas permeation. Gas permeating through the liner pipe can accumulate in the empty space between the reinforcement fibres or cords, which are embedded in the plastic matrix of the pipe wall. When the gas pressure in this ring-shaped annular space rises above a certain threshold value, blister formation in the cover layer can occur. This can eventually lead to rupture of the outside cover layer.

A mathematical model was set up to assess the equilibrium gas pressure in the annular space of an RTP pipe. This model also allows calculating the influence for different liner pipe materials and different temperature gradients inside the multilayer pipe. By using the theory of gas permeation, ruled by Fick’s Law, and the theory for a temperature gradient over the pipe wall, ruled by Fourier’s law, the equilibrium gas pressure in the annulus was calculated and compared to the cover blow-off threshold pressure.

Because the ratio of permeability of the liner pipe and of the cover pipe is one of the parameters which influences the equilibrium gas pressure, a change to a liner pipe with a lower permeation coefficient can increase the cover blow-off resistance. The MOP of a commercial RTP pipe must be limited to 42 bars at room temperature due to the risk of cover blow-off. It is calculated that choosing an alternative liner pipe material with much lower methane permeability requires an internal pressure of more than 150 bars before cover blow-off will occur. With this new liner pipe the limiting factor is therefore the strength of the RTP system itself and not the cover blow-off resistance.

Ernst van der Stok
Consultant Materials, Kiwa Gas Technology

Bert Dalmolen
Technical manager, Pipelife Nederland