A pressure transfer device for transferring pressure energy from a high pressure first liquid to a lower pressure second liquid to provide a pressurized second liquid, which device comprises: a rotatably mounted cylindrical rotor having a pair of opposite planar end faces with a plurality of channels extending axially therethrough between openings located in said pair of opposite planar end faces; and a pair of opposite end covers (69,71) having axially inward and outward end surfaces, with said axially inward end surfaces of each having flat regions that interface with and slidingly and sealingly engage said respective planar end faces of said rotor, characterised in that each said end cover (69,71) having two or more inlet passageways (103,109) and two or more discharge passageways (105,107) which at least four passageways open into said axially inward end surfaces, said pair of opposite end covers (69,71) being aligned so that when one said channel in said rotor is aligned with an inlet passageway opening in one said end cover, it is also aligned with a discharge passageway opening in said opposite end cover, said inlet passageway openings and each said discharge passageway openings in said same end cover being constantly sealed from one another during the operation by a sealing region at the interface between said rotor planar end face and said end cover inward end surface, whereby during a single revolution of said rotor said plurality of channels are, in alternating sequence, brought into partial or full alignment with both an inlet passageway opening in one said end cover and a discharge passageway opening in the opposite said end cover and then into partial or full alignment with both a discharge passageway opening in said one end cover and an inlet passageway opening in said opposite end cover, at least twice during each revolution of said rotor, so that each said channel is at least twice supplied with the high pressure first liquid and at least twice discharges pressurized second liquid. a rotatably mounted cylindrical rotor having a pair of opposite planar end faces with a plurality of channels extending axially therethrough between openings located in said pair of opposite planar end faces; and a pair of opposite end covers (69,71) having axially inward and outward end surfaces, with said axially inward end surfaces of each having flat regions that interface with and slidingly and sealingly engage said respective planar end faces of said rotor, characterised in that each said end cover (69,71) having two or more inlet passageways (103,109) and two or more discharge passageways (105,107) which at least four passageways open into said axially inward end surfaces, said pair of opposite end covers (69,71) being aligned so that when one said channel in said rotor is aligned with an inlet passageway opening in one said end cover, it is also aligned with a discharge passageway opening in said opposite end cover, said inlet passageway openings and each said discharge passageway openings in said same end cover being constantly sealed from one another during the operation by a sealing region at the interface between said rotor planar end face and said end cover inward end surface, whereby during a single revolution of said rotor said plurality of channels are, in alternating sequence, brought into partial or full alignment with both an inlet passageway opening in one said end cover and a discharge passageway opening in the opposite said end cover and then into partial or full alignment with both a discharge passageway opening in said one end cover and an inlet passageway opening in said opposite end cover, at least twice during each revolution of said rotor, so that each said channel is at least twice supplied with the high pressure first liquid and at least twice discharges pressurized second liquid.