Analysis of cracks in polyvinylidene fluoride lined reinforced thermoplastic pipe used in acidic gas fields

Cracking of polyvinylidene fluoride (PVDF) pipe as lining layer used in reinforced thermoplastic pipe (RTP) was occurred after running for half a year operation in gathering and transportation in acidic gas fields. For investigating the cause of cracking, the characteristics of the PVDF pipe with crack, such as composition, thermal mechanical and structural, have been compared with the new one and analyzed. Results showed that the cracks have been probably caused by the weld lines inside the liner PVDF pipe, which were caused by un-proper merging of PVDF melt during extrusion after passing through the six legged spider mandrel. It is suggested to assess the extrusion conditions of PVDF. Moreover, it is recommended to use a different mandrel geometry, such as spiral mandrel, in order to avoid weld lines in the final item.