In a centrifugal pump, what happens to velocity when throttling the discharge valve?

In a centrifugal pump, throttling the discharge valve reduces the flow area for the fluid exiting the pump. This action increases the resistance against which the pump must work, leading to a change in the velocity of the fluid. As the discharge valve is throttled, the pressure at the discharge side of the pump rises, causing the fluid's velocity to decrease.

The principle behind this involves the relationship between flow rate, pressure, and velocity. As the discharge valve restricts flow, the energy imparted by the pump is transformed more into pressure than into kinetic energy (velocity). Therefore, with a smaller or restricted exit point, the fluid’s velocity must decrease to maintain the flow consistency dictated by the pump's operating characteristics and the laws of fluid dynamics.

This is a fundamental concept in pump operation, and it illustrates how controlling the discharge affects the performance of the pump and the system it serves. In practical terms, it also demonstrates why careful management of valves and flow rates is crucial in maintaining efficient system performance.