BC for closed circuit pipe
 

Hi, Could anyone suggest me how to give BC for a fluid moving in closed circuit.
I want to do a transient analysis but could not find a suitable link/site from where i can get more info about this problem. My problem is as follows. I have a tank through which a pipe is extracting water. this water is passed through some heat source and is again fed to the same tank. Now I want to know how much time is taken by the water in tank to reach a certain level.