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11.1 Centrifugal and Axial Flow Pumps

3 min readLast Updated on July 19, 2024

Centrifugal and axial flow pumps are crucial in fluid mechanics, converting mechanical energy into fluid energy. These pumps differ in design and application, with centrifugal pumps using impellers for high head, and axial pumps using propellers for high flow rates.

Understanding pump performance is key to selecting the right pump for a system. Head, flow rate, and efficiency are vital parameters. Proper sizing and cavitation prevention ensure optimal pump operation and longevity in various applications.

Centrifugal and Axial Flow Pumps

Components of centrifugal vs axial pumps

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  • Centrifugal pumps convert rotational kinetic energy into hydrodynamic energy of the fluid flow using an impeller (rotating component) that imparts energy to the fluid, a casing (stationary component) that guides the fluid to the outlet, a shaft that transmits mechanical energy from the motor to the impeller, and seals that prevent fluid leakage and maintain system pressure (mechanical seals, packing glands)
  • Axial flow pumps develop head by the propelling action of the impeller blades on the fluid using a propeller (rotating component) that imparts energy to the fluid, a casing that guides the fluid and houses the propeller, a shaft that connects the propeller to the motor, and guide vanes that improve efficiency by directing the fluid flow (inlet guide vanes, outlet guide vanes)

Performance analysis of pumps

  • Head (HH) represents the energy per unit weight of fluid, expressed in units of length, and is the sum of static head (HsH_s), velocity head (HvH_v), and pressure head (HpH_p) to give total head (HtH_t)
  • Flow rate (QQ) represents the volume of fluid discharged per unit time, expressed in units such as m3/sm^3/s (cubic meters per second) or gal/mingal/min (gallons per minute)
  • Efficiency (η\eta) represents the ratio of the power output to the power input, calculated as η=PoutPin=ρgQHτω\eta = \frac{P_{out}}{P_{in}} = \frac{\rho g Q H}{\tau \omega}, where ρ\rho is fluid density, gg is acceleration due to gravity, τ\tau is torque, and ω\omega is angular velocity
  • Pump performance curves provide a graphical representation of the relationship between head, flow rate, and efficiency and are used to determine the optimal operating point for a given system (best efficiency point, operating range)

Pump selection and sizing

  • Determine the required flow rate and head for the application based on system requirements and design constraints
  • Select a pump type (centrifugal or axial flow) based on the specific requirements, with centrifugal pumps suitable for high head, low to medium flow rate applications (water supply, boiler feed) and axial flow pumps suitable for low head, high flow rate applications (irrigation, flood control)
  • Use pump performance curves to select a pump that meets the required head and flow rate while considering factors such as efficiency, operating costs, and maintenance requirements
  • Size the pump based on the system's piping layout and pressure drop calculations, ensuring adequate net positive suction head (NPSH) and minimizing friction losses

Cavitation prevention in pumps

  • Cavitation is the formation and collapse of vapor bubbles in a liquid due to localized low-pressure regions, occurring when the local pressure falls below the vapor pressure of the liquid (vaporization, bubble implosion)
  • Causes of cavitation include insufficient net positive suction head (NPSH), high fluid temperature, and excessive suction lift, leading to reduced pump performance and efficiency, damage to pump components due to the implosion of vapor bubbles, and increased noise and vibration
  • Methods to prevent cavitation include:
    1. Ensuring sufficient NPSH by increasing suction head, reducing fluid temperature, and minimizing suction line losses
    2. Using inducer stages or pre-rotation vanes to improve suction performance
    3. Proper pump sizing and selection based on the system requirements (operating range, safety margin)
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© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.

© 2025 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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