CompressorFlow Surge Explained
The gas passes through a cooler and knockout drum before re-entering the casing through the nozzle on the left. This is the high-pressure section which has four impellers. After being compressed by the four impellers in the high pressure section of the compressor, the gas is collected in the high pressure volute and exist through the nozzle in the middle of the bottom of the casing.
This cross-section shows the components that make up the rotor assembly. The shaft is supported at each end by oil film bearings. The area of the shaft under the bearing is called the bearing journal. A thrust disk is attached at the end of the shaft to limit the axial movement of the rotor.The low pressure section of the compressor is comprised of three high flow impellers. The impellers of the high pressure section are sized for much lower flows. A coupling hub is attached to the end opposite the thrust bearing for attaching the driver to the compressor.
A radial bearing at each end of the shaft provides the support for the rotor. The thrust bearing locates the rotor axially and offsets the aerodynamic forces the tend to pull the rotor towards the suction end of the compressor.
The inlet area upstream of the impeller guides the gas into the eye of the impeller where it is accelerated radially outward. The lowest pressure is generally just past the eye of the impeller where the leading edge of the blades. The gas reaches its maximum velocity at the exit of the impeller. As the gas moved radially outward in the diffuser passage, the volume increases, causing the gas to slow down. The lowest velocity (and highest pressure) is achieved in the outermost region of the diffuser.
The balance piston is mounted on the end of the shaft, just after to last impeller. One side of the piston is exposed to the final discharge pressure while the other side of the piston is piped back to the compressor suction. The compressor designers size the piston to produce a balancing force that is equal to approximately 90% of the net aerodynamic force.
The biggest problem with incipient surge is the frequency at which the pulsations occur. The frequency of incipient surge often is close to that of the rotor’s natural frequency. If the compressor operates for very long in incipient surge, the opportunity exists for exciting the rotor, provoking high radial vibration.