STABILITY CONSIDERATIONS – A SIMPLIFIED APPROACH
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
Turbomachinery Laboratory, Texas A&M Engineering Experiment Station
Abstract
In many cases it is necessary to judge the stability behavior
of a specific compressor. This might be the OEM checking the
feasibility of a proposed compressor layout or it might be the
user judging one of his machines which causes troubles in
operation. API 617 suggests a stability level I or level II
analysis. The shortcoming of the level I analysis is that it
considers only the global cross-coupling stiffness of the entire
stages neglecting the actual seal design of a compressor. Thus
the damping of a machine is always decreasing with increasing
load which is not reflecting the true stability behavior of
modern turbo compressors. On the other hand a level II analysis
requires detailed insight into the machine design which might
not be defined at that time or which is not known to the user.
To overcome these problems this tutorial proposes a
different kind of stability check which only requires a minimum
of operational and design data and delivers a comprehensible
and (relatively) reliable picture of the stability behavior of the
investigated compressor.
This tutorial derives the used stability criterion and
explains the effect of the circumferential speed of the gas in the
seal gap on the rotordynamic behavior. Finally it presents
example calculations for different seal configurations and
compares the resulting stability judgments to the results of the
classical API level II analysis and to the results of
experimentally determined rotor damping.
Description
Tutorial