What is pump surging?

surging is not occur in pump but  Cavitation  occur  in witch  pump 

and surging is  occur in centrifugal compressor 

The pump suction is below atmospheric pressure (vacuum) and air is coming into the packing, Stop it by converting to a mechanical seal.

The answers given below are incorrect.

Surging is a phenomema when a pump or fan sees two or more duty points at the same time on their H-Q Curves plotted against system curves.

Alhough surging can be more of a problem with Fans and a great deal of study has been done using such evaluations known as the "Hagan Rule", it can also occur on Centrifugal Pumps on Water Systems. I have had personal experience of this.

This can result in mechanical damage and reptutable pump and fan manufacturers will advise accordingly. Cavitation has nothing to do with Surging.

Pump cavitation is when the pump is not drawing enough liguid to keep it running smoothly, therefor the pump operation temperature will increase causing the pump to cavitate.

Surge can be defined as the capacity below 55 to 70%, which the centrifugal compressor performance becomes unstable. When surging occurs, the high-pressure gas at the discharge of the machine flows back through the compressor in a complete reversal of the normal direction of flow causing the collapse of pressure producing ability will repeat and cycle through the same sequence. Surging controlled by;

Increase the flow from process to compressor suction

By-pass some discharge through coolers, back to suction of the unit

Decrease the pressure ratio.

Pump surging is something similar to Pump Cavitation. i.e.  When the suction side of the pump doesnot have enough liquid to suck then the temperature of the pump will increase and results in vapor burst of the liquid present in the pump.