1. A multi-stage centrifugal pump unit with at least two impellers (2, 6), characterized in that it has two wheel groups (4, 8) following each other in the direction of flow, with at least one working each wheel (2, 6), and in the first wheel group (4) a return channel (24) is made connecting the output side of the first wheel group (4) with its input side. 2. The unit according to claim 1, characterized in that the return channel (24) enters the suction mouth (28) of the first stage of the first wheel group (4). 3. An assembly according to claim 1 or 2, characterized in that at least one valve (38) is located in the return channel (24) to block the return channel (24). 4. Unit according to claim 3, characterized in that the valve (38) is configured to shut off the return channel (24) upon reaching a predetermined liquid pressure in the return channel (24) .5. The unit according to claim 1, characterized in that the first wheel group (4) is made of at least two-stage, with two impellers (2) located one after the other in the direction of flow. An assembly according to claim 1, characterized in that the first wheel group (4) comprises on its output side a separator (20) made to separate air and liquid. A unit according to claim 6, characterized in that the separator (20) is located relative to the return channel (24) with the possibility of liquid entering the return channel (24) flowing from the separator (20) .8. An assembly according to claim 1, characterized in that a check valve is located on the inlet side of the first wheel group (4). The unit according to claim 1, characterized in that between the first (4) and second (8) wheel groups there is at least one fluid accumulator (48). The unit according to claim 9, excellent
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