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When the pump is started, the outlet valve should not be closed
Centrifugal pump start to close the outlet valve, positive displacement pump can not close the outlet valve. Because the centrifugal pump starts, there is no medium in the outlet pipe of the pump, so there is no pipe resistance and lifting height resistance. After the pump is started, the pump head is very low and the flow rate is very big. At this time, the pump motor (shaft power) Output is very large (according to the pump performance curve), it is easy to overload and cause damage to the pump motor and wiring, so start the export valve must be closed until the motor reaches the rated speed, then open the outlet valve to make the pump run normally. The general volume pump itself has a self-suction capacity. Centrifugal pump flow changes greatly with the pressure, while the volume pump flow changes with pressure is small, and centrifugal pump shaft power decreases with lift the contrary, the volume pump shaft power increases with pressure. This requires that safety valves be installed on the displacement pump and in the pump unit to prevent accidental blockage of the discharge line and excessive pump pressure resulting in excessive power and damage to the prime mover or pump due to overload. Therefore, the positive displacement pump can not close the outlet valve when it starts up and can be directly operated to start normal operation.