What happens to the motor's amperage draw when the discharge valve on an electric motor driven centrifugal pump is closed during pumping?

When the discharge valve on an electric motor-driven centrifugal pump is closed during operation, the system experiences a change in flow conditions. Specifically, as the discharge valve closes, the resistance to flow increases, leading to a significant reduction in the pump's ability to move water.

Centrifugal pumps operate under the principle that they convert mechanical energy into hydraulic energy. As the load on the pump increases (which occurs when the discharge valve is opened) and the flow rate decreases (when the valve is closed), the impeller does not have to do as much work against the flow of liquid. Consequently, the pump can reach its shut-off head, a condition where no flow output is possible; this situation occurs at maximum pressure but zero flow rate.

In this scenario, since the flow is significantly restricted or ceases altogether, the pump motor corresponds by drawing less current, which results in a decrease in amperage. Thus, a closed discharge valve leads to reduced resistance and less power required from the motor, causing the amperage draw to decrease.