Why Should the Valve Be Closed When Starting a Centrifugal Pump?

When a centrifugal pump starts, the outlet pipeline is initially empty of water, meaning there is no pipeline resistance or lift resistance. Immediately after startup, the pump operates at a very low head and a very high flow rate. Under these conditions, the pump motor's output (shaft power) becomes exceptionally high (as indicated by the pump performance curve), which can easily lead to overload. This overload risks damaging the pump's motor and associated electrical circuits. Therefore, closing the outlet valve during startup is very necessary for the pump to begin operating correctly. Closing the valve artificially creates pipeline resistance pressure. Once the pump is running normally, the valve should be opened gradually, allowing the pump to follow its performance curve and transition smoothly into standard operation.

Two Essential Requirements Before Starting a Centrifugal Pump:

1.Fill the pump casing with water: This is crucial for creating a vacuum.

2.Close the valve on the outlet pipe: This prevents the pump from generating flow initially, reducing the motor's starting current and facilitating a smooth startup. The valve should then be slowly opened after the pump has successfully started.

Centrifugal pumps rely on the centrifugal force of the impeller to create a vacuum that lifts the water. Hence, before starting, the outlet valve must be closed, and the pump must be primed with water. The water level must exceed the height of the impeller to expel any air from the pump. Once started, the impeller creates a vacuum around it, drawing water upwards. The system can then begin to lift water effectively. This process fundamentally requires the initial closure of the outlet valve.

About Centrifugal Pumps

A centrifugal pump is a type of vane pump that operates by transferring mechanical energy to the fluid through the interaction between its rotating impeller blades and the liquid, thereby increasing the fluid's pressure energy to achieve fluid transport. Key operational characteristics of centrifugal pumps include:

1.The head generated at a fixed rotational speed has a maximum limit. The operating point's flow rate and shaft power are determined by the connected system's characteristics (e.g., static head, pressure difference, and pipeline friction losses). Head varies with the flow rate.

2.Operation is stable and continuous, with no pulsation in flow or pressure.

3.They generally lack self-priming capability. The pump must either be filled with liquid beforehand or the suction line must be evacuated before operation can begin.

4.Centrifugal pumps are started with the discharge valve closed, unlike vortex pumps or axial flow pumps, which are started with valves fully open, to minimize starting power requirements.

Before starting the pump, its casing is filled with the fluid to be moved. After activation, the impeller, driven by the shaft, rotates at high speed, forcing the liquid between the blades to rotate as well. Centrifugal force then propels the liquid from the impeller's center to its periphery, where it gains kinetic energy and is discharged at high speed into the volute casing.

Inside the volute, the liquid slows down as the flow channel expands, converting some of its kinetic energy into static pressure energy. The fluid finally exits into the discharge pipeline at a higher pressure, ready for delivery to its destination. As liquid moves from the center to the periphery of the impeller, a partial vacuum forms at the impeller's eye. Because the pressure above the liquid in the supply reservoir is greater than the pressure at the pump's inlet, liquid is continuously forced into the impeller. Thus, as long as the impeller keeps rotating, the pump will continually draw in and discharge the fluid.