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. 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.