A multistage sealed pump is provided for use in an X-ray tube cooling system which is substantially more efficient than pumps of known construction and which provides substantially higher pumping pressure at lower motor current than conventionally. The pump employs multiple impellers which are plumbed in series and which are directly coupled to an electrical motor which with the impellers is submerged and runs in the coolant liquid. The impellers and motor are sealed within a housing and the pump unit is hermetically sealed, with no rotatable shaft seals being used or required. In one embodiment, the multistage pump employs a motor having oppositely extending motor shaft ends, with one or more impellers on each end of the motor shaft. The cooling liquid can be transferred from stage to stage by interconnecting tubing external of the housing or within the housing, through a hollow motor shaft, or through the motor casing. In another embodiment, the multiple impellers can be directly mounted on a shaft extending from a single end of the motor.