| dc.description.abstract | A Root Cause Failure Analysis (RCFA) for repeated
impeller blade failures in a five stage centrifugal propane
compressor is described. The initial failure occurred in June
2007 with a large crack found in one blade on the third impeller
and two large pieces released from adjacent blades on the
fourth impeller. An RCFA was performed to determine the
cause of the failures. The failure mechanism was identified to
be high cycle fatigue. Several potential causes related to the
design, manufacture, and operation of the compressor were
examined. The RCFA concluded that the design and
manufacture were sound and there were no conclusive issues
with respect to operation. A specific root cause was never
identified. In June 2009, a second case of blade cracking
occurred with a piece once again released from a single blade
on the fourth impeller. Due to the commonality with the
previous instance this was identified as a repeat failure.
Specifically, both cases had occurred in the same compressor
whereas, two compressors operating in identical service in
Copyright © 2011 by Turbomachinery Laboratory, Texas A&M University
adjacent Liquefied natural Gas (LNG) trains had not
encountered the problem. A second RCFA was accordingly
launched with the ultimate objective of preventing further
repeated failures.
Both RCFA teams were established comprising of engineers
from the End User (RasGas), the OEM (Elliott Group) and an
independent consultancy (Southwest Research Institute). The
scope of the current investigation included a detailed
metallurgical assessment, impeller modal frequency
assessment, steady and unsteady computational fluid dynamics
(CFD) assessment, finite element analyses (FEA), fluid
structure interaction (FSI) assessment, operating history
assessment and a comparison change analysis. By the process
of elimination, the most probable causes were found to be
associated with:
· vane wake excitation of either the impeller 1-diameter
cover/blades modal frequency or the blade leading edge
modal frequency from mistuning
· mist carry over from third side load upstream scrubber
· end of curve operation in the compressor rear section | en |
