Frans Scholten and Mannes Wolters
Papers # 2010 Vancouver
PA11, PA12 and plasticized PA6.12 pipes are interesting candidates for gas transport and distribution pipelines at 16 bars pressure, which is beyond the maximum pressure for PE100 pipeline systems (10 bars). However, PA pipelines are not meant as an alternative to PE pipelines, but to steel pipelines. Therefore, permeability is important. Permeability coefficients have been measured at 16 bars methane gas pressure on 110 mm SDR11 PA pipes and compared with the values for PE100 pipes. In comparison to PE100 pipes, PA pipes show a reduction in the methane permeability coefficient of 16 to 90, depending on the PA type. Plasticized PA6.12 shows the largest reduction. Hydrogen permeability through three brands of PA12 pipe was also assessed. Special attention was paid to butt fusion and electro fusion jointing. PA materials will absorb some moisture from the surrounding air or from rain and - after installation - from groundwater. During subsequent fusion steam bubbles may be formed which lead to voids at the joint plane. The worst-case scenario was chosen, meaning butt fusion and electro fusion jointing of water-saturated PA pipes. The maximum water content after immersion ranged from 1.5 to 2.4 weight percent, depending on the PA type. Despite this, electro fused joints in water-saturated PA12 and plasticized PA6.12 pipes meet the requirements of the Peel Test (ISO 13954), which is required for PA pipe systems (ISO 22621). Butt fused joints in water-saturated PA pipes were tested at constant load in nitrogen gas at stresses up to 15 MPa and at temperatures up to 100 °C. Failure modes were ductile and no signs of slow crack growth were noted. Results at lower stresses and temperatures were converted to 15 MPa and 100 °C using the Miners Rule. Times to failure converted in this manner range from 153 to more than 2625 hours under these conditions, which is encouraging. Steam bubbles formed in the joint may be pushed into the weld beads. Joint quality depends on the number and size of the voids at the fusion plane and much less on the presence of voids in the weld beads. Therefore, the process of pushing the steam bubbles into the weld beads is important. It seems possible to optimise joint quality further by changing the fusion temperature, pressure and procedure.
