What is Cavitation
Cavitation, problems with pumps, is a phenomenon that bubbles form in the liquid due to pressure drop.
When the pressure of the liquid drops below the saturated vapor pressure, bubbles will be generated. For example, if a liquid with a saturated vapor pressure of -0.07 MPaG becomes lower than -0.07 MPaG, air bubbles will be generated.
When this bubble comes into a high pressure area, it will collapse and impact will occur at that time.
When cavitation occurs, the performance of the pump decreases, the vibration and noise increase, further, causing destruction phenomenon called erosion.
In order to avoid cavitation problem, it is necessary to ensure that the lowest pressure inside the pump shall not be lower than the saturated vapor.
What is NPSH
It is a value that can judge if pump will have cavitation or not, and it is used to judge if pump can suck up the liquid or not. There are two kinds of NPSH as below:

NPSHr (required net positive suction head)
NPSHa (available net positive suction head)

NPSHa must be larger than the NPSHr which pump require.
Cavitation occurs if it is small.
NPSHr is a value determined by the pump side, and NPSHa is a value determined by piping conditions.
How to control discharge amount of gear pump?
The centrifugal pump shall discharge the liquid using centrifugal force of the liquid generated by the rotation of the impeller which becomes high pressure.
The gear pump is displacement type pump, and a certain amount is discharged to the outside by utilizing the change in the volume.
To make it easier to understand, the centrifugal pump generates pressure, but, as for gear pump, the pressure varies depending on the piping etc of the discharge side.
For this reason, in the case of the centrifugal pump, by open / close the valve on the discharge side, the resistance changes and the discharge amount also changes. As for gear pump, even if the valve on the discharge side is gradually closed, only the pressure of the gear pump is increased, and finally it may damage the pump if continue to close the valve.
It is common case to use gear reducer or inverter drive to change rotation speed to control discharge amount of the gear pump. Also connecting piping of discharge side and suction side with bypass piping with valve sending back some amount of liquid back to suction side is another method to control.