MULTIPLE STAGE PUMP

PROBLEM TO BE SOLVED: To provide a multiple stage pump capable of achieving size reduction and ensuring high performance without causing cost increase.SOLUTION: A back board vane 12a includes a protruding portion 12e provided in an outlet portion of a return flow passage 12, has a meridian plane shape of an impeller 1 near a shroud 1a in a next stage formed by a curved portion 12g concave toward the shroud 1a, and protrudes to a position corresponding to an inlet diameter Ra of the centrifugal impeller 1 in the next stage, a shroud inlet shape of the centrifugal impeller 1 in the next stage is formed along the curved portion 12g of the back board vane 12a, and the shroud inlet shape of the centrifugal impeller 1 in the next stage constitutes part of the outlet portion of the return flow passage 12.SELECTED DRAWING: Figure 2 【課題】小型化でき、しかもコスト増を招くことのない高性能な多段ポンプを提供する。【解決手段】水返し羽根１２ａは、戻り流路１２の出口部において、次段の遠心羽根車１のシュラウド１ａ側の子午面形状を当該シュラウド１ａ側に向けて凹形状となる曲線部１２ｇで形成して次段の遠心羽根車１の入口径Ｒａとなる位置まで突出する突出部１２ｅを備え、次段の遠心羽根車１は、当該次段の遠心羽根車のシュラウド入口形状が水返し羽根１２ａの曲線部１２ｇに沿って形成され、次段の遠心羽根車１のシュラウド入口形状が戻り流路１２の出口部の一部を構成している。【選択図】図２