The relationship between flow rate, velocity, and pressure

The flow rate is determined by the process; once the pump is designed, the impeller diameter, the inlet and outlet diameters, and the rotational speed are all fixed. The flow rate and head of this pump are thus determined (ignoring viscosity). For pipelines for which the flow rate is already known, increasing the pipe diameter reduces the flow velocity, decreases pipeline losses, and increases the net positive suction head available to the system. Head is calculated based on vertical height, the pressure at the end of the pipeline, as well as frictional and local losses. This pipe diameter was chosen based on the target value for pressure drop.

As the flow rate decreases, the pressure should increase relatively, and the temperature as well. Passing through an eccentric reducer also constitutes a throttling process； Will the flow rate decrease just like the velocity?

Throughout the entire pipeline, the flow rate is constant everywhere, because according to the law of conservation of mass, the amount that enters is equal to the amount that exits! ! ! What changes is the flow rate! ! ! At the same flow rate, different pipe diameters result in different flow velocities! When we calculate pressure loss, it is related to flow velocity, rather than directly to flow rate. Please distinguish this concept clearly! !

Reducing the flow velocity at the pump inlet is done to increase the net positive suction head… To boost the net positive suction head, we generally increase the pressure in the inlet pipeline… or reduce the pressure loss, that is, the resistance, in the inlet section… This is why the diameter of the pump inlet pipeline is larger than that of the outlet. To explain further: when the conditions along the pipeline remain unchanged, the higher the flow velocity, the greater the resistance, and thus the greater the pressure loss. As a result, the pressure near the impeller at the pump inlet becomes lower, making cavitation more likely to occur. This is not the same as an increase in pressure or flow velocity; in the first case, the pressure can remain constant while the pressure loss increases, while in the second case, both pressure and flow velocity increase, with the increase in flow velocity being greater…

In other words… when the source of pressure and the condition of the pipes remain unchanged, the reason why higher resistance leads to a lower flow rate is due to greater pressure loss; there is less dynamic pressure available to drive the fluid flow, resulting in a reduced flow rate. This in turn causes low pressure at the pump inlet, increasing the risk of cavitation, as most of the pressure loss occurs in the pipes. To increase the pressure at the pump inlet and prevent cavitation, we reduce the flow rate by enlarging the diameter of the suction pipeline. At the same flow rate, the flow velocity naturally decreases, which in turn reduces pressure loss and increases the pressure reaching the pump inlet………