Radial Vane or Radial Impeller
The term radial describes impellers found in low specific speed pumps, approximately Ns < 2000.
Called Radial Vane because the impeller vanes radiate at 900 angles away from the center of rotation (shaft or axle). Since fluid flow follows the vanes, fluid enters the impeller eye, and is then thrust sharply away from the center of rotation resulting in centrifugal force being the primary means of energy impartation from shaft into the fluid.
Power characteristic is linear, (power increases as flow increases).
Slang for Radial Vane is "Pancake" impeller due to the flat disc like shape.
Related Subjects on this Website:
Glossary - Axial Flow (Impeller / Propeller)
Glossary - Francis Vane (Impeller)
Glossary - Mixed Flow (Impeller)
Radial Thrust - See Thrust
Recirculation (Discharge and Suction)
Recirculation is a reversal of a portion of the fluid flow at or near the impeller or the pump intake or discharge nozzles. This reversal of fluid and consequent recirculation occurs at low flow rates.
In a centrifugal pump clearances between the impeller and the pump case control leakage of fluid back into the pump suction. This leakage is not considered to be, nor is this leakage called recirculation.
If however the flow rate through a pump drops below a critical amount, fluid reversal occurs at the intake or discharge areas of the pump. This is called recirculation. This low flow reversal is a turbulent phenomena resulting in fluid eddies and high velocities causing localized areas of low pressure. When pressure drops below the vapor pressure of the fluid, cavitation can occur in these low pressure areas resulting in cavitation damage to the pump and/or pipe connections to the pump.
Suction Recirculation - Recirculation occurring at the suction nozzle or impeller eye. Fluid enters the pump nozzle or the eye of the impeller but then a fluid reversal occurs.
Discharge Recirculation - Recirculation occurring at either the discharge nozzle or the discharge side of the impeller. Fluid leaving the impeller or the discharge nozzle reverses direction, causing high velocity fluid vortexes, resulting in pockets of localized low pressure. If pressures in these pockets drops below the vapor pressure of the fluid, cavitation occurs.
Damage to the impeller shown in the two photographs above was caused by Discharge Recirculation,
Caused by long term operation at low flow rates.
Related Subjects on this Website:
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