Content area
Full Text
Pressure rise across successive compression stages requires seals at the impeller eye and rotor shaft to prevent gas backflow from the discharge to the inlet end of the compressor. The effectiveness of these seals directly affects the compressor efficiency.
The simplest and most economical of all shaft seals is the straight labyrinth seal shown in FIG. 1 and FIG. 2. This seal is commonly utilized between compression stages and consists of a series of thin strips or fins, which are normally part of a stationary assembly mounted in the diaphragms.
Calculations and field performance have demonstrated that wiped interstage seals can decrease unit efficiency by 7% or more. Operating modes that contribute to labyrinth damage include surging, passing through the critical speed and liquid ingestion.
Typically, labyrinth seals have been manufactured from soft aluminum so that, in the event of a rub, the seal will deform and not cause excessive rotor vibration. A hard rub can cause rotor whirl, and the rotor can self-destruct from rubbing on a material that will not deform. For example, in a case where excessive corrosion destroyed aluminum seals, a user made labyrinth seals from cast iron. However, during operation, the rotor touched the labyrinths and initiated a rotor whirl condition so severe that the bearing housings dropped off the compressor body.