When constructing a pipe network using Pipe Flow Expert, you cannot set both a pressure and a flow rate at the same point in a system, since in a real-world system the flow rate will be determined by the difference in pressure at that point and some other point downstream. i.e. the flow rate that occurs will be a result of pressure difference between two points.

In Pipe Flow Expert you can either set the pressure at a specific point (using a tank, or end pressure) or you can set the flow rate demand at a specific point (the amount of flow either going in to or leaving the system at that point).

If you set the pressure at a specific point then the software will calculate the flow rates in the pipes coming in to and out of that point.

If you set the flow rate demand at a specific point then the software will calculate the pressure required at that point to support the demanded flow rate against all the other flows that may be coming in and out of the same point.

To illustrate the above in a simple manner, consider a single pipe, where flow through the pipe will produce a pressure loss. The pressure at the end of the pipe will be dependent on the pressure at the start of the pipe minus the pressure loss through the pipe. If you set the pressure at the start of the pipe and you set a flow rate demand out of the end of the pipe, then the software will calculate the pressure loss for that flow rate and it can then use this to calculate what the pressure at the end of the pipe will be (the start pressure minus the pressure loss). Alternately, if you set the pressure at both ends of the pipe then this defines a pressure drop and the software can calculate what flow rate will need to occur to satisfy and match that exact pressure drop.

In summary, it is the difference between two pressures that causes flow and determines the actual flow rate that occurs in a pipe (this is the case in the real-world). Therefore in a mathematical model of a piping system you can only fix and set either the flow rate or the pressure at a specific point but not both at the same point.