The user is able to elect to enter a fixed flow rate for a pump. This situation may occur when the natural flow distribution to different parts of the network has to be determined, or when the system has a variable speed pump that has been set to produce a set flow rate. The Fixed Flow Rate Pump is modeled in a similar manner to the Flow Control Valve described above. Pipe Flow Expert removes the pipe on which the pump is mounted and sets an Out-flow demand at the ‘From node’ and an equal In-flow demand at the ‘To node’. The pipe and the pump are replaced by these flow demands while the system is being solved.

FixedFlowPump_Modelling02.dib

Figure 147  Fixed Flow Rate Pump replacement

Since the head pressure provided by the pump is unknown, the system balance cannot be ‘tuned’ by iterating along the pump performance curve. For this reason it is not possible to include pressure control devices such as flow control valves on all outlet paths through the system when a fixed flow rate pump has been selected (this would result in an over-controlled system).