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High Pressure Instrument Tubing Failure

Background:

An instrument tubing in a non-corrosive propylene service experienced failure. The tube is 0.5 and operates at ~ 1500 psi and less than 80 °F. Since the tube is in a non-corrosive service and there appears to be very little evidence of any sort of corrosion/environmental cracking.

Figure 1: Macroscopic and Microscopic View of Fracture Surface of Failed Tube

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Figure 2: Tube Material Microstructure.

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Summary: 

The high pressure instrument tubing failed due to fatigue. Evidence of fatigue includes the presence of crack initiation, propagation and final ductile overload regions, rubbing, and striations on the fracture surface. The final overload regions clearly show dimples which are associated with ductile failures. Fatigue striations were also observed at micro- cracks formed at the OD in this region. The presence of inclusions/impurities dispersed throughout the microstructure and at the crack initiation region suggests these might have facilitated the failure. Finally, since crack initiation started at the intrados of the 90° bend where compressive residual stresses are present after forming the bend, large amplitude fatigue loads are most likely associated with this failure.


Take Away: 

High pressure lines can fail quickly fatigue during dynamic upset conditions.



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