Explain why positive displacement pumps are generally preferred over centrifugal pumps for high-viscosity fluid applications.

Positive displacement pumps are generally preferred over centrifugal pumps for high-viscosity fluid applications because their flow rate is less affected by changes in fluid viscosity. Viscosity is a fluid's resistance to flow. Centrifugal pumps operate by imparting kinetic energy to the fluid, which is then converted to pressure. As viscosity increases, the internal friction within the pump also increases, reducing the pump's efficiency and its ability to generate head (pressure). The pump's flow rate decreases significantly with increasing viscosity. In contrast, positive displacement pumps deliver a relatively constant volume of fluid per revolution or stroke, regardless of the fluid's viscosity. These pumps work by trapping a fixed volume of fluid and then forcing (displacing) that volume into the discharge line. Examples of positive displacement pumps include gear pumps, screw pumps, and diaphragm pumps. While the required power to drive the pump will increase with increasing viscosity due to higher frictional losses, the flow rate remains relatively constant. This makes positive displacement pumps suitable for applications where a consistent flow rate is required, even with viscous fluids. For example, in pumping heavy oils or polymers, a positive displacement pump is much more reliable than a centrifugal pump in delivering a predictable flow rate. The performance curve of a centrifugal pump flattens out significantly with increasing viscosity, whereas a positive displacement pump maintains a relatively stable flow rate.