PURPOSE: To improve the pumping performance and to miniaturize the size by connecting a pre-load pump and a high-pressure pump in series, by feeding pre-load medium-pressure fluid to a high-pressure impeller by a return channel, and driving the pre-load pump by boosted high-pressure fluid.
CONSTITUTION: A main spindle 101 driven by a turbine 103 and a second shaft 104 coaxial thereto is provided in a casing 102. A low-pressure inducer 109 and a low-pressure impeller 110 are provided to the second shaft 104. A low- pressure baffle 111 for straightening the medium-pressure fluid is provided in the downstream part of the low-pressure impeller 110. The medium-pressure fluid is converted into the direction of the high-pressure impeller 114 by a return passage 112. The high-pressure impeller 114 discharges high-pressure entire centrifugal flow toward a boost turbine 132 and the boost turbine 132 discharges the discharged fluid to a discharge channel 106 via respective orifices 163, 154 and axial channel 142. Then, the high-pressure impeller 114 constitutes the high-pressure pump.