In centrifugal compressor, What is difference between surging and choking?

This question is far from the the field of my specialty But from my point of view

Surging is the complete breakdown of steady flow in the compressor which occurs at low flow rate. Surging takes place when compressor is operated off the design point and it affects the whole machine and this is aerodynamically and mechanically undesirable. It can damage the rotor bearings, rotor seals, compressor driver and affect the whole cycle operation. It results in high temperature, high vibration and leads to flow reversal

Choke flow is the high flow limit of any type of dynamic compressor curve that caused by high velocity friction. In other words, choke or stonewall is defined as the maximum flow o given impeller can handle. This value will be attained when the ratio of the relative inlet gas velocity to the sonic velocity of the gas being handle is equal to 1.0. The cause of this phenomena is excessive relative gas velocity through the impeller; as the head (energy) required by the process system is reduced, the volume flow through the impeller will increase.

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Normally occurs at about 50% of design inlet capacity at design speed, is the point at which the impeller cannot add enough power to overcome the discharge pressure. This causes flow reversal (surge).

Surge results in:

• High vibration

• Abnormal noise

• Temperature increases

• Rapid changes in axial thrust that can damage compressor internals and driver

Surge may be prevented by:

• Increase flow / Kick back (recycling)

• Reduce speed

• Reduce differential head

Compressor choke is an abnormal operating condition for centrifugal compressor. Choking of centrifugal compressor occurs when the compressor is operating at low discharge pressure and very high flow rates. These high flow rates at compressor choke point are actually the maximum that the compressor can push through. Any further decrease in the outlet resistance will not lead to increase in compressor output. This operating condition is also known as stonewalling of a centrifugal compressor.

To prevent the compressor choke or stonewall from happening it is needed to maintain a certain level of flow resistance in the compressor outlet line. Anti-choke valves can be used for this purpose in the compressor outlet line. Anti-choke valves close to restrict the flow to keep compressor from stonewalling. When flow resistance in compressor outlet falls and flow begins to increase, the anti-choke valves close to develop resistance to the increasing flow.

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