[Q&A] July 5, 2017

The head, flow rate, and efficiency of the pump remain unchanged; only the shaft power of the pump varies depending on the medium being transported

Changes in the conveying medium involve two aspects: a. Change in medium density – When the density of the conveying medium differs from that of plain water at room temperature, the pump’s head, flow rate, and efficiency remain unchanged; only the shaft power of the pump varies depending on the conveying medium, as can be calculated using the following formula. b. Conveying viscous liquids – Viscous liquids have a significant impact on the performance of pumps, and the extent of this impact depends on various factors such as the viscosity of the liquid, the design of the pump, the shape of the impeller, and its surface roughness. As the viscosity of the liquid increases, the Reynolds number decreases, hydraulic friction losses increase, which results in a reduction in head and flow rate; in other words, the Q-H curve declines (although the head at the shut-off point remains almost unchanged). At the same time, the shaft power increases due to increased friction losses, causing the pump’s efficiency to drop sharply; meanwhile, the net positive suction head available also increases.

Flow rate is usually divided into volume flow rate and mass flow rate. The volume flow rate (m3/h) is not affected by changes in density, whereas the mass flow rate (t/h) is proportional to changes in density. The head (head pressure) is affected by density changes and is proportional to those changes. If other factors are not considered, changes in temperature have no effect on flow rate and head. However, temperature changes usually cause variations in parameters such as the density, viscosity, and vaporization pressure of the medium, which actually affect the performance of the pump.

Changes in the conveying medium involve two aspects: a. Change in medium density – When the density of the conveying medium differs from that of plain water at room temperature, the pump’s head, flow rate, and efficiency remain unchanged; only the shaft power of the pump varies depending on the conveying medium, as can be calculated using the following formula. b. Conveying viscous liquids – Viscous liquids have a significant impact on the performance of pumps, and the extent of this impact depends on various factors such as the viscosity of the liquid, the design of the pump, the shape of the impeller, and its surface roughness. As the viscosity of the liquid increases, the Reynolds number decreases, leading to increased hydraulic friction losses, which results in a decrease in head and flow rate – that is, the Q-H curve declines (although the head at the shut-off point remains almost unchanged). At the same time, the shaft power increases due to these friction losses, causing the pump’s efficiency to drop sharply; meanwhile, the allowable net positive suction head also increases.

The medium density affects the outlet pressure of the centrifugal pump; the higher the density, the greater the outlet pressure. It also affects the shaft power of the centrifugal pump – the higher the medium density, the greater the shaft power required.

1. The density decreases, resulting in lower pump efficiency. 2. A decrease in density can cause air locking in the impeller

The head, flow rate, and efficiency of the pump remain unchanged; only the shaft power of the pump varies with the density of the fluid being transported