Large flow pumps are widely used in electronics, electricity, chemical industry, metallurgy, medicine, food, electroplating, environmental protection, fire protection, municipal administration, water purification, textile printing and dyeing, mining and mineral processing, civil construction and other industries due to their good performance, high efficiency, long life and large flow. In actual operation, the flow of large flow pumps can be infinitely adjusted by various methods. Under normal circumstances, the pump works reasonably under the rated point condition, but sometimes due to some reasons, the pump runs under the small flow condition point, which will cause the following negative effects.