Fluid flows through a pipe because of the pressure difference between the two ends of a pipeline. The fluid will flow from the high-pressure end to the low-pressure end. Consider two systems as shown in the following figure. The flow in each case is caused by a pressure difference between P1 and P2. Pipe 2 has some obstructions (welding remains, a reducer, changes in area, sharp bends, etc.) which will create a pressure drop resulting in less discharge and a reduced rate of flow.