Stability Considerations – a Simplified Approach

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.