Pump for pumping fluid without vacuum boiling

A gear pump for pumping fluid sucked from a suction space comprising:

• an annular outer rotor formed with internal gear teeth on an inner periphery thereof;
• an inner rotor received in said outer rotor and formed with external gear teeth on an outer periphery thereof for meshing engagement with said internal gear teeth, said inner rotor cooperating with said outer rotor to define between said internal gear teeth and said external gear teeth a plurality of pressure chambers;
• a casing in which said outer and inner rotors are eccentrically rotatably received and which forms side walls of said pressure chambers;
• driving means for rotating said outer and inner rotors and moving said pressure chambers while changing the volumes thereof;
• a suction port formed in said casing and communicated with the pressure chambers the volumes of which are being increased;
• a discharge port formed in said casing and communicated with the pressure chambers the volumes of which are being decreased;
• and
• a pressure relief passage through which the pressure chamber moving from said discharge port to said suction port is communicated with said suction space before being communicated with said suction port so as to reduce the fluid pressure in said pressure chamber.