Experimental rotordynamic coefficient results for: (a) a labyrinth seal with and without shunt injection and (b) a honeycomb seal

Abstract

Centrifugal compressors are increasingly required to operate in higher pressure, higher speed, and large fluid density. In these conditions, compressors are susceptible to rotordynamic instabilities. To avoid this situation, labyrinth seals have been modified by (a) using swirl brakes, and (b) using shunt injection. In some cases, labyrinth seals have been replaced entirely by honeycomb seals. In shunt injection, the gas is taken from the diff-user or discharge volute, and injected into an upstream cavity of the labyrinth seal. The injection can be radial or against rotation. Even though labyrinth seals with shunt injection have successfully been used since 1970s, this thesis contains the first measured rotordynamic data for labyrinth seals with shunt injection. A comparison has been made between labyrinth seals, with and without swirl brakes, labyrinth seal with shunt injection (radial and against rotation), and a honeycomb seal. The leakage comparison demonstrates that the honeycomb seal has the best flow control, and the labyrinth seals with shunt injection the worst. For low and medium running speed, and most conditions, the labyrinth seal with injection against rotation has the best rotordynamic stability followed in order by the honeycomb seal, radial shunt injection, and the labyrinth seal with and without swirl brakes. When running speed increases, the effective damping decreases for the labyrinth seal with injection against rotation. For high speed and high pressure ratio across the seal, the honeycomb seals have better rotordynamic performance. In centrifugal compressors, the selection of the seal should be cautiously analyzed. Seals with shunt injection against rotation will have a better performance due to possibility of a higher pressure injection ratio. On the other hand, honeycomb seals will have a better stability control due to the higher density of the fluid.