Why must the outlet valve be closed first when shutting down a centrifugal pump? It doesn’t matter; what harm is there?

Stop the pump; closing the outlet valve of the centrifugal pump first prevents liquid from flowing back, which could cause the impeller to rotate in reverse and allow high pressure at the outlet to flow back to the low pressure side at the inlet.

During the process of starting the pump, the current flowing through the motor is at its highest at the moment of startup. If the outlet of the pump is opened wide at this time, it is easy for the motor to be damaged due to high current and heavy load. Therefore, starting the pump first and then opening the valve can reduce the motor’s starting current and lessen the impact of startup on the pipeline system. During the process of stopping the pump, power is cut off first; if the outlet check valve does not function properly, it can cause the pump to rotate in reverse, which is not allowed in the operation of centrifugal pumps. It is acceptable to close the outlet valve for a short period of time while the pump is still running. Therefore, the correct approach is to close the outlet first and then cut off the power. For pumps with high flow rates and pressures, power should be cut off just before the outlet is completely closed, thereby protecting the pump.

Reversing causes damage to the seal； When the pump was started again, the current was extremely high.

Can someone explain why the starting current is the highest?

I’ve learned it.* . . . . . . . .

If the outlet valve is not closed, and if the outlet pipe is too long, the backflow of the fluid within the pipe can cause the pump to rotate in reverse, which may lead to the loosening of the impeller nut.

It can cause shocks to the pump chamber, impeller, and other components, leading to equipment damage. If there is a check valve at the outlet, the risk of backflow in the outlet pipe can be eliminated; however, check valves can sometimes leak as well ； The liquid in the inlet pipe, due to inertia and the negative pressure inside the pump chamber, also impacts the pump chamber and the impeller. A simple example is water pipes: when the water outlet valve is suddenly closed, the pipes vibrate for this very reason.

I’ve learned something from this, thank you all, heroes everywhere: lol