Root Cause Analysis of the Catastrophic Failure of a Propylene Recycle Compressor

Abstract A 2-section, 6-stage propylene recycle compressor experienced a catastrophic failure that resulted in extensive damage to its internals. The compressor was restarted after tripping on a low-low seal oil differential pressure without draining the compressor casing, resulting in a subsequent tripping of the compressor with audible noise. The initial convergence of opinions, before the current investigation, was that the compressor failed due to surge. This appears a plausible conclusion since the anti-surge system had been disabled long before the disastrous failure. Also, the nature of the failure — broken rotor blades and diaphragms, bent rotor, among others — are hallmarks of compressor failure due to surge. However, a detailed review of the process data from the database and thermodynamic performance calculations using in-house developed software by this investigator revealed that both compressor sections have been operating across all sections of the characteristic curves, from extreme left to extreme right of the curves, before and after the catastrophic failure. Radial and axial vibrations are below alarm limits when the compressor was operating at these two extremities of the characteristic curve. Significant, rapid pressure/flow and high vibration fluctuations, which characterizes flow instabilities, were not recorded before the compressor tripped, suggesting that the compressor did not experience surge before the failure. Seal oil ingresses into the compressor in large quantities due to failed mechanical seal is suspected as the root cause of the catastrophic failure because the compressor was restarted after checking all permissives without draining the compressor casing.