Do I Have To Close The Outlet Valve When Starting A Centrifugal Pump?

Mar 02, 2025

When the centrifugal pump is started, there is no water in the outlet pipeline of the pump, so there is no pipeline resistance or lifting height resistance. After the centrifugal pump is started, the head of the centrifugal pump is very low and the flow rate is large. At this time, the pump motor (shaft power) output is very high (according to the pump performance curve), which is easy to overload and can damage the pump motor and circuit. Therefore, the outlet valve should be closed when starting to ensure the normal operation of the pump.

If an axial flow water pump is used, the situation is the opposite. It must be started by opening the valve. At this time, the motor's power is minimized. In a short period of time, due to the lack of resistance, it will operate at a higher flow rate, often causing pump vibration, noise, and even motor overload operation, which will burn out the motor. Closing the outlet valve is equivalent to manually setting the pipe resistance pressure. After the pump is running normally, slowly start the valve and let the pump gradually work normally along its performance curve.

Before starting the centrifugal pump, two points must be ensured:

1. Fill the pump casing with water in order to create a vacuum;

2. The gate valve on the outlet pipe must be closed so that the water pump does not generate flow, which can reduce the starting current of the motor and facilitate the smooth start of the water pump. As the water pump starts smoothly, the gate valve should be slowly opened in a timely manner.

A centrifugal pump relies on the centrifugal force of the impeller to create a vacuum suction and lift water. Therefore, when starting the centrifugal pump, the outlet valve must be closed first and water must be filled. When the water level exceeds the impeller, the air in the centrifugal pump must be discharged before starting. After starting, a vacuum is formed around the impeller to suck water upwards, which can automatically open and lift the water. Therefore, the outlet valve must be closed first.

Centrifugal pump is a type of vane pump that relies on the rotating impeller to transfer mechanical energy to the liquid during the rotation process. Due to the interaction between the blades and the liquid, the pressure energy of the liquid increases, achieving the purpose of transporting the liquid. Centrifugal pumps have the following characteristics of operation:

① The head generated by a centrifugal pump at a certain speed has a limited value. The working point flow rate and shaft power depend on the condition of the device system connected to the pump (such as position difference, pressure difference, and pipeline loss). The head changes with the flow rate.

② Stable work, continuous conveying, and no pulsation in flow and pressure.

③ Generally, there is no self-priming ability, and the pump needs to be filled with liquid or the pipeline needs to be evacuated before it can start working.

④ The centrifugal pump starts when the discharge pipeline valve is closed, while the vortex pump and axial flow pump start when the valve is fully open to reduce the starting power.

Do I have to close the outlet valve when starting a centrifugal pump?

Before starting the pump, fill the pump casing with the liquid being transported; After starting, the impeller is driven by the shaft to rotate at high speed, and the liquid between the blades must also rotate along with it. Under the action of centrifugal force, the liquid is thrown from the center of the impeller to the outer edge and gains energy, leaving the outer edge of the impeller at high speed and entering the volute pump casing.

In the snail shell, the liquid slows down due to the gradual expansion of the flow channel, and converts some of its kinetic energy into static pressure energy. Finally, it flows into the discharge pipeline at a higher pressure and is delivered to the required location. When the liquid flows from the center of the impeller to the outer edge, a certain vacuum is formed at the center of the impeller. Due to the pressure above the liquid level in the storage tank being greater than the pressure at the pump inlet, the liquid is continuously pressed into the impeller. It can be seen that as long as the impeller continues to rotate, the liquid will be continuously sucked in and discharged.

Startup of other centrifugal pumps

The above are all centrifugal pumps. For other types of pumps, the situation is as follows:

1. The high flow starting characteristics of axial flow pumps - fully open valve starting

The axial flow pump has the maximum shaft power at zero flow condition, which is 140%~200% of the rated shaft power, and the minimum power at maximum flow. Therefore, in order to minimize the starting current, the starting characteristic of the shaft power should be high flow starting (i.e. fully open valve starting).

2. The start-up characteristics of mixed flow pump - fully open valve start-up

When the mixed flow pump is in zero flow condition, the shaft power is between the two pumps mentioned above, ranging from 100% to 130% of the rated power. Therefore, the starting characteristics of the mixed flow pump should also be between the two pumps mentioned above, preferably starting with the valve fully open.

3. The start-up characteristics of vortex pump - fully open valve start-up

The maximum shaft power of the vortex pump is 130% to 190% of the rated shaft power under zero flow conditions. Therefore, similar to axial flow pumps, the starting characteristics of vortex pumps should be high flow starting (i.e. fully open valve starting).