Root Cause Investigation of High Frequency Structural Vibration on a High Pressure Hydrogen Compressor

Abstract

During the commissioning of a diesel hydrotreating unit, high frequency vibration was detected on the Hydrogen makeup compressor, causing machine shutdown and alarming levels of vibration on the piping system. The high frequency components – concentrated between 11× and 15× the compressor speed – were present even during no-load operation and reached values around 3 to 4 times the fundamental frequency amplitude. The high frequency made impossible the improvement of the piping by inclusion of supports. Pressure pulsations, initially believed to be the most likely root cause, were measured and did not show any abnormal values. The measurements ultimately showed that the compressor itself seemed to be the main source of the high frequency. The crankshaft’s second torsional frequency and flywheel’s first bending mode were changed by redesign of these elements, which also showed no positive result. Several further tests were carried out in order to assess the main cause of the vibration. Spectrum analysis during startup and shutdown, PxV diagram evaluation and bump tests finally revealed that a complex interaction between two crankshaft torsional natural frequencies and two structural natural frequencies were the cause of the problem.