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CompressorFlow Surge Explained
This is a cross-section of a back-to-back two-section compressor for an FCCU wetgas service. The gas enters the large nozzle on the right, flowing into the volute area on the right before entering the first impeller. The gas is accelerated in the first impeller, then decelerates in the diffuser section to recover the energy as pressure. The return bend redirects the gas into the second impeller where the acceleration and deceleration of the gas is repeated. As the pressure rises after each impeller, the volume of the gas decreases. After the third impeller, the gas exists into an outer casing volute which carries the gas out through the center nozzle on the side of the casing.
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.
The impeller is made up of the hub, the blades and the cover. The hub attaches to the shaft at the bore and provides the mechanism for driving the blades. The blade impart the energy to the gas by scooping the gas from the eye of the impeller and flinging it out the impeller exit. The cover provides a mating surface for the impeller eye seal.
To prevent the gas from circulating from the exit of the impeller to the eye, a seal is provide to restrict the gas flow past the cover. The seal is usually a labyrinth ring pressed in to the casing near the eye of the cover.

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.
This slide shows the separation of the suction pressure and the discharge pressure. Effectively, the eye of the impeller is the only area of the impeller over which the suction pressure is applied. The back of the impeller and the cover of the impeller are exposed the the discharge pressure.
Under normal conditions when the flow is moving forward, in other word, when the compressor is not surging, the area of the cover is exposed to discharge pressure as is the back of the hub. Since the area of the eye is exposed to suction pressure, there is a net force on the impeller that acts toward to the suction that is approximately equal to the difference between the impeller suction pressure and the impeller discharge pressure applied over an area equal to the impeller eye.
During a surge condition, the gas flows backwards through the impeller from the discharge to the suction. This cause the discharge pressure to drop and the suction pressure to rise. This reversal in the pressures causes the net thrust that is normally acting on the rotor to reverse directions.

On rotors that have all their impellers facing in the same direction, a device called a balance piston is used to balance the thrust load across the rotor. This feature reduces the load on the thrust bearing, which reduces the bearing parasitic losses.
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.

During a surge cycle, the pressure change across the impeller is proportional to the density of the gas and the impeller tip speed squared. This means that for a given compressor, the surge event is more violent with a gas of higher density and if the speed is elevated. The more significant issue here is the effect of speed. When a compressor is at very low speeds, the effect of surge is not severe.
Incipient surge occurs when the flow through the impeller is not sufficient to fill the impeller flow passage. Boundary layer separation occurs on the trailing side of the vanes. Vortices (or stall cells) form in the separation area and work their way out to the exit of the impeller. When a vortex reaches the impeller exit, it moves in a counter-rotation direction to another flow passage. When this “jump” occurs, a radial pulsation is produced that acts normal to the shaft.
As with surge, the pressure pulsations that occur during incipient surge are also a function of the gas density and impeller tip speed squared. However the magnitude of the pursations is about 1/20 those experience during surge. Also, the pulsations that occur during incipient surge act in the radial direction, rather than in the axial direction.
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.

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