Controlling the pump flow with a valve on the outlet is recommended. Since this is a centrifugal pump, you can completely close the valve for long periods of time with the pump running and not have any damage. If you listen to the motor, you will hear the pump motor speed up as the valve is fully closed. This indicates that the load on the pump actually decreases. This is the same thing that happens when you put your hand over the nozzle on a vacuum cleaner, the speed goes up because the pump is not really doing anything except spin. Its not moving the fluid.