@Er.
With regards to dP - remember you are specifying a control valve! So the dP is what you want it to be! Of course the valve has a max capacity - but that is implicit at the dP you specify!
So in order to specify you need to know your dP and your flow! This is the tricky part on sizing a CV - you have a lot of combinations of flow and dP and you wish the valve to be operating at say 30-70% open!
This was also a bit hard for me to grasp initially but consider this, you have a vessel and you wish to control level. Your upstream vessel normally has a pressure between 5-7 barg and the downstream is atmpsheric (fixed 0 barg). SO this means that the dP will be 5-7 bar. Your required flowrate will depend on your inlet rate- If this was 20-40 m3/hr then you would specify max flowrate with min dP so 40 m3/hr and 5 bar. But should this be you ideal operating point? Hardly. This should be your CV for say 70% open. And the other end would be dP=7 bar and flow 20 m3/hr and this could be you CV and 30% open.
If its the pressure you wish to control its the same way around.
Best regards, Morten
--- Best regards, Morten Andersen