Case 4 Braided Hose Failure

A hose assembly in fluorine service failed because of forming a leak (Fig. 10.38). The hose consisted of a PTFE tube (liner) at its core which had been reinforced with a stainless steel braiding. The failure occurred in the PTFE tube below and near the lower edge of the collar, away from the joint. The failed hose handled fluorine that had been diluted by a neutral gas. A lengthy analysis was performed to determine the cause of the failure because of the safety concerns about fluorine service. 

Braid surfaces at the failure point and a spot away from that area were analyzed. The external and internal surfaces of the braid at the failure area were mildly discolored because of oxidization, similar to the effect of overheating a metallic object. Optical microscopy and scanning electron microscopy showed no disturbance to the weave pattern, no deformation of metal wires, and no surface defects, based on the comparison of defective and normal areas. Energy dispersive x-ray (EDX) showed similar concentrations of iron (Fe), nickel (Ni), chromium (Cr), and molybdenum (Mo) in both areas, indicating stainless steel. A small amount of silicon (Si) was detected that was attributable to environmental contamination such as dust and dirt. 

Analysis of a liner sample by FTIR confirmed that the tube was made of PTFE. A peak was detected for perfluoropropyl vinyl ether (PPVE) indicating that PTFE was modified by a small amount of PPVE. No differences were found between the failure and normal areas after analysis by optical and scanning electron microscopy and EDX except for the small crack in the PTFE tube at the failure point. EDX detected similar amounts of F and C in both areas, as expected. 

EDX analysis results showed that iron, nickel, and chromium, all typical components of stainless steel, were present. The fluorine peak was weaker in this area than in the normal areas. No evidence of an external piece of metal or wire being trapped in the crack was detected. All data suggested that the source of contamination must have been mixed with the PTFE prior to or during the fabrication of the tube. 

ESCA was conducted on the internal surface of the failure area and a normal area. The theoretical result calls for 33 atomic% C and 67 atomic% F for a PTFE surface with an F/C ratio of 2. The actual data for as-is surfaces yielded an F/C ratio of 

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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. 

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