Centrifugal Pump Torque

A centrifugal pump functions by casting fluid outward from the center of a spinning impeller. Fluid enters through the eye (center) of the impeller and, due to the spinning motion, is ejected outward into the volute chamber , the space outside the impeller and contained within the pump casing. The fluid thus gains kinetic energy from the impeller, much of which is converted to pressure energy upon slowing down on its way out of the pump. If the inlet and outlet piping are of the same diameter, the hydraulic power added to the system is $Q\times \Delta p$ , where $Q$ is the volumetric flow rate and $\Delta p$ is the difference in pressure immediately before and after the pump. Fluid density is assumed constant at all locations within and outside of the pump.

Source: Fantagu - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4332102

Between the power applied to the pump's shaft to cause it to rotate and the power ultimately gained in the system fluid, some energy is lost. One major contributor to energy loss is recirculation of fluid within the volute, which is when some of the fluid fails to exit the pump and instead circles back around the volute. The overall efficiency of converting shaft power to hydraulic power depends on the amount of resistance in the system. For example, if flow is completely blocked , the pump efficiency is zero because no fluid is flowing regardless of any rotation of the impeller. In contrast, pump best efficiency points are typically in the range of $65-90\text{\%}$ efficiency.

Consider a traditional centrifugal pump, which always operates at a single rotational frequency , pumping water through a dirty strainer. Today, a worker cleans the strainer, which reduces the system resistance. Compared to before, the torque experienced by the impeller will _ _ _ _ .