1. Insufficient liquid surface of suction tank or insufficient infusion head;

2. Lower atmospheric pressure;

3. Lower pressure in the system;

4. The medium temperature is high, the saturated steam is large and the medium is easy to vaporize.

5. Higher liquid velocity and greater loss of resistance;

6. The suction pipe resistance is large, which mainly depends on the structure of the pump and whether the installation of the suction pipeline is reasonable;

7. The suction line leaks into the air.