When modeling closed loop systems, we need to include a reservoir node (simply to assign a fluid, pressure and temperature) to the system. The reservoir node is modeled "in-line" and upstream of the pump node. The image below gives an example of this modeling approach.
Note, closed loop systems can also be modeled using known pressure boundary nodes (see lower model in the image below). This simplified case in the screenshot shows an identical system which has been modeled using both approaches. The results developed in both cases are the same. You can therefore use either approach when modeling your closed loop systems.
