The operating principles of a regenerative turbine pump are rather subtle compared to other pumps. From a mechanical viewpoint, the regenerative turbine pump is a centrifugal pump. However, the performance characteristics of a regenerative turbine pump correspond to a positive displacement pump. A regenerative turbine pump pressurizes fluid the same way as a centrifugal pump. It accelerates the fluid to convert kinetic energy to potential energy. The regenerative turbine pump differs from the centrifugal pump because it breaks the acceleration/pressurization process into dozens of separate steps. The fluid is slightly accelerated and slightly pressurized with each step.
As fluid enters the pump, it is picked up by the impeller and begins to make a spiraling motion around the circumference of each side of the impeller (see figure 2.5A). Each spiral represents an acceleration/pressurization cycle. The impeller has teeth cut on both sides of its edge. The impeller spins at high speed in a very close fitting casing.
Because the fluid is only accelerated to a speed slightly above the speed at the pump suction, the pressure drop and vapor formation in the pump suction is very low. The close tolerances between the impeller and casing are so tight that very little slippage can take place past the teeth. The tight fit makes the pump perform much like a positive displacement pump which allows it to move vapor.
The mechanical design of the regenerative pump closely resembles a typical centrifugal pump. The lightweight impeller allows an overhung impeller design to be used. This design requires only one mechanical seal to seal off the pumping chamber. The impeller is designated to float on the shaft so hydraulic forces in the pump may perfectly center the impeller to prevent metal to metal contact between the impeller and casing. A free floating impeller is much easier to install than a centrifugal pump impeller.