Corrugated PTFE Hose

A PTFE-lined, stainless steel braided hose was used to transport sulphur trioxide at a chemical plant. The hose was attached to an external tank system and was insulated and heated. One day the hose failed and sulphur trioxide leaked and caused serious environmental problems. The failed hose had one long axial crack that had split the hose open, which allowed the liquid to be released. The engineering assignment in this case was to examine the cause of the failure. 

An inspection of the failed hose clearly demonstrated that the sulphur trioxide had permeated through the PTFE liner and corroded large portions of the stainless steel liner. As a consequence, the weight of a valve that was attached to the end of the hose was partly supported by the PTFE liner itself. In addition to this axial load, the liner was also exposed to an internal pressure. A schematic representation of the applied loads acting on the liner is shown in Fig. 11.8. 

A separate calculation determined that the internal pressure was between 0.15 and 0.60 MPa, and that the weight of the valve at the end of the hose was 280 N. 

One further complication in this case was that the temperature of the hose was not known. The ambient temperature was 0°C, but the function of the heating element at the time of the failure was not known. The temperature of the hose could have been as high as 100°C. 

The strength of the hose was determined through a set of ten tension-to-failure experiments performed on miniature tension specimens cut from the accident hose. The experiments were performed both at room temperature and at 100°C (Fig. 11.9). 

---