Water hammer - Gradual valve closure
 

I am currently simulating gradual valve closure in Ansys fluent for a low pressure system. (i.e., 2m upstream water head). When the valve is closed slowly say around 1 sec, pressure rises and reach peak and then it goes down below -20m of water head. This situation is un favourable as it forms cavitation. How to control the negative pressure without touching the cavitation point.
Does anyone have an idea about this !

Thanks