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Does the outlet valve have to be closed when starting a centrifugal pump?

Time of Release:

2022-06-08

When the centrifugal water 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 large (according to the pump performance curve), which is easy to overload, and it will cause damage to the pump motor and circuit. Therefore, the outlet valve must be closed when starting to ensure the normal operation of the pump

  When the centrifugal water pump is started, there is still 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 large (according to the pump performance curve), which is easy to overload, and it will cause damage to the pump motor and circuit. Therefore, the outlet valve must 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 power of the motor is minimized. In a short period of time, due to the lack of resistance, it will run at a high flow rate, often causing pump vibration, noise, and even overload operation of the motor, which will burn out the motor. Closing the outlet valve is equivalent to manually setting the pipe resistance pressure. After the pump operates 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 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 form a vacuum and lift water. Therefore, when starting a centrifugal pump, the outlet valve must be closed first and water must be filled. When the water level exceeds the impeller position, the air in the centrifugal pump can 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.

  About centrifugal pumps

  A centrifugal pump is a type of vane pump that relies on a rotating impeller. During the rotation process, due to the interaction between the blades and the liquid, the blades transfer mechanical energy to the liquid, increasing the pressure of the liquid and achieving the purpose of conveying the liquid. The centrifugal pump has the following characteristics in operation:

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

  ② Stable work, continuous conveying, 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 with the discharge pipeline valve closed, while the vortex pump and axial flow pump start with the valve fully open to reduce the starting power.

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

  In a snail shell, the liquid slows down due to the gradual expansion of the flow channel, and then converts some kinetic energy into static pressure energy. Finally, it flows into the discharge pipeline at a higher pressure and is sent 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.
 


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