Circumferential flow type liquid pump

A circumferential flow liquid pump comprising:

• an impeller having an outer circumferential portion in which a vane portion is provided;
• a pump casing assembly rotatably supporting said impeller, said pump casing defining (1) a pump flow path having an arcuate band-like shape extending along said outer circumferential portion of said impeller and (2) a suction port and a discharge port that respectively open at opposite end portions of said pump flow path;
• and
• an air vent hole defined in said pump casing assembly, said air vent hole having (1) a first end that opens to an inner circumferential portion of said pump flow path at a position radially inwardly spaced from a bottom portion of said pump flow path, and (2) a second end that opens to the exterior of said pump casing assembly at a position radially inward of said first end;
• wherein said air vent hole has a cross-sectional configuration that (1) extends along said pump flow path, (2) defines a longitudinal axis that is parallel to a longitudinal axis of said pump flow path along an entire length of said cross-sectional configuration, and (3) is spaced apart from said suction port and said discharge port.