Centrifugal Compressor

Overview

A centrifugal compressor raises the pressure of a continuous gas stream by spinning it through one or more impellers and then converting that velocity into pressure in stationary diffusers. It is the dominant machine for high-flow, moderate-ratio compression duties, air separation, refrigeration, refinery and petrochemical process gas, and large plant-air systems, where a positive-displacement machine would be too bulky or too pulsating. The integrally geared layout used here mounts impellers on the ends of high-speed pinions driven from a single bull gear, so each stage can run at its own optimum speed.

Compression is dynamic rather than volumetric: the Centrifugal Impeller accelerates gas radially outward, and the Diaphragm passages decelerate it to recover pressure before the next stage. Because the machine has no rubbing compression elements, it runs oil-free on the process side and tolerates very high speeds, but it must be kept clear of surge, the unstable flow reversal that occurs when discharge demand exceeds what the impellers can deliver.

How it works

A Driver Motor turns the Integral Gearbox, whose bull gear and pinion step the speed up to the tens of thousands of rpm the impellers need. The Rotor Assembly carries the impeller stack and a Balance Piston that offsets the axial thrust the aerodynamic loading produces; whatever thrust remains is taken by the thrust bearing in the Bearing Set, while tilting-pad journal bearings hold the rotor radially. Gas enters through the suction side of the Process Nozzles, passes the adjustable inlet guide vanes, and is compressed stage by stage inside the horizontally split Compressor Casing.

Keeping the high-speed shaft sealed and lubricated is the job of two support systems. The Seal System uses non-contacting dry-gas seals at each shaft penetration, fed clean buffer gas so process gas cannot leak out along the shaft. The Lube Oil System supplies cooled, filtered oil to the bearings and gear mesh from a dedicated console with a standby pump.

Protection against surge is continuous. The Anti-Surge System system measures flow and the pressure rise across the machine and, as the operating point approaches the surge line, opens a fast recycle valve to route discharge gas back to suction, raising throughput and pulling the machine back to a stable point. The whole train sits on a fabricated Baseplate / Skid that aligns the motor, gearbox and casing and carries the piping loads.