I was controlling the Flow by FCV but once i introdue the FCV in my model , the pressure become negative at FCV and the rest
This could generally happen if there is no storage downstream i.e. tank or reservoir element on the downstream side (i.e. An "open" system downstream of the FCV) or the path through the FCV in question is the only one connected to the demands (in which case the flow is controlled by the demands)
Could you please describe your modeling case here?
Below wiki might help you to understand what's happening due to FCV, please go through it.
Why do I see such a large headloss through my FCV, PSV or PRV?
Regards,
Sushma Choure
Bentley Technical Suppport