Equipment: 60in Flare Header Piping of Refinery Blow Down to Flare System

Problem: After a particular refinery-wide relieving scenario, it was found that the 60in main pipe header has failed characteristically as with the inward plastic collapse that could be attribute to 'Buckling'

Analysis:A failure analysis based on ASME Sec-VIII div-2 was done to evaluate plastic-Collapse Buckling analys using the elastic & elastic-plastic stress methods, by considering the static loads & dynamic loads in combination that act on the 60â€� pipe header of the flare system. The results were analyzed for the location of buckling failure that occurred during the 1st Failure recorded. The expansion stresses in the pipe header were twice as much higher than that those due to the normal operating temperature loads and were greatest at the time of the failure, when compared to normal operating temperatures. It was found that the failure in the pipe header characterized as the inward plastic collapse could be attribute to 'Buckling' failure mechanism and that may have been caused by multiple static loads creating the pre-stressed state combined with the dynamic load that triggers the actual failure. The overall vibration characteristics of the 60in flare header piping system, based on the first 15 modes, suggested concerns regarding the presence of maxima / minima and Point of Inflections on the 60in€� section of the main header in the vicinity of Node-3383, which is location of actual failure that occurred during the failure incident. The current configuration of the sulfur lateral branch was also found to provide restraint to the 60" header, such that it is believed to impose unfavorable modes characteristics at the location of actual failure that occurred during the failure incident. The mode characteristics thus acting on the old (0.375" thickness) header when combined with the high D/t ratio would result in header bending. Under bending, when the cylinder is very long, the flattening of the cross-section leads to large reduction of the effective section modulus of the 60" header, and the instability occurs as a single transverse wave (Buckle) on the compression side of the shell. This phenomenon was attributed to the failure of the 60€� sections of flare header piping system. Decreasing the D/t ratio is the most effective way to counter this failure mechanism.

Results Conclusion:The theoretical solution for the one & two dimensional model of the 60in€� header cylinder, estimated the critical buckling load under compressive torsional & bending stress to be much higher than what existed in the 60in€� flare header. However, the modal analysis suggested that given the situation that the flare header piping system would vibrate under a given dynamic condition, it would be much likely that it would do so with a maximum situated at the N-3383, the region of failure incident that occurred. This would be a pre-requisite for a 60â€� pipe header to buckle at this specific location.

Benefits: This resulted in a better understanding of the failure mechanisms related to the flare header. This further resulted in a series of scope of work to reinforce the weak section / nodes most susceptible to buckling related failures typical to the natural frequencies of vibrations of a 60in pipe header as in this case. Finally this led to enhanced plant integrity and confidence in the reliability of the flare header.