The flow characteristics of a pump outlet pipe generally do not affect pump performance and reliability, with only a few exceptions. In some cases, sudden valve closure can cause excessively high pressure spikes (surges or water hammer) to reflect back to the pump, potentially damaging the pump, system piping, or other components. When liquid flow suddenly stops, the liquid attempts to continue flowing in the same direction. In the area where the flow velocity changes, the liquid pressure increases sharply due to the dynamic forces. When it rebounds, it increases the pressure in nearby pipes and creates a sonic pressure wave. This is similar to the waves formed when a stone is dropped into a pond; the waves radiate outwards in all directions. As the wave travels further from the center, its energy spreads to a larger area and gradually dissipates until it eventually disappears. In piping, sonic pressure waves can only propagate along the pipe. The potential intensity of the pressure wave can be significant, so transient flow analysis may be necessary in systems where check valves are suddenly closed or pumps are suddenly stopped.
Additionally, the pump system outlet pipe is a major source of friction. This will affect the pump system profile, which represents the head required to deliver a specific flow rate through the system (i.e., the pump's required head). The system curve has a static head (hs) component, which consists of the elevation difference or pressure difference between the source and target fluid. The static component of the system curve is generally independent of the flow velocity in the piping system. However, the friction head (hf) component depends on the flow velocity in the piping system. Since the size of the friction head depends on the flow velocity in the pipe, the size of the outlet pipe, along with fittings and valves in the system, are major influencing factors of the friction head. Figure 1 shows a typical pump system curve with a static friction component. Note that the friction component increases with the square of the flow rate (i.e., velocity), and the curve becomes steeper (i.e., greater friction) when throttled by valves. This is important because the pump will operate at the intersection of the pump curve and the system curve; therefore, if the system requires a higher or lower head-pressure than the selected pump, the pump may operate at low or high flow rates, which can affect the overall reliability of the pump.