Why should the outlet valve be closed when the centrifugal pump is started?
When the centrifugal pump is started, there is no water in the pump outlet pipeline, so there is no pipeline resistance and lifting height resistance. After the centrifugal pump is started, the centrifugal pump has a very low lift and a large flow. At this time, the pump motor (shaft Power) output is very large (according to the pump performance curve), it is easy to overload, which will damage the motor and circuit of the pump, so the outlet valve must be closed when starting to make the pump run normally.
No matter what kind of pump it is, the vanes must be immersed in water to start to establish the lift. Centrifugal and mixed-flow pumps have a suction head, so the suction pipe must be filled with water before starting. At this time, a water inlet valve needs to be set at the water inlet to ensure that there is water in the pipeline. The axial flow pump has no suction head, and the blades must be immersed in water at any time. So there is no problem with the water inlet valve.
Two points must be ensured before the centrifugal pump is started
1. Fill the pump casing with water to form a vacuum;
2. The gate valve on the outlet pipe must be closed so that the pump does not form flow, which can reduce the starting current of the motor, which is conducive to the smooth start of the pump. With the smooth start of the pump, the gate valve should be opened slowly in time.
The centrifugal pump relies on the centrifugal force of the impeller to form a vacuum to lift the water. Therefore, when the centrifugal pump is started, the outlet valve must be closed first to fill with water. When the water level exceeds the impeller, the air in the centrifugal pump can be discharged before starting. After starting, a vacuum is formed around the impeller, which sucks the water upwards, and it can be automatically opened to lift the water. Therefore, the outlet valve must be closed first.
A Centrifugal pump is a kind of vane pump. During the rotation process of the rotating impeller, due to the interaction between the vane and the liquid, the vane transmits the mechanical energy to the liquid, which increases the pressure energy of the liquid and achieves the purpose of conveying the liquid. The centrifugal pump has the following characteristics:
①The lift generated by the centrifugal pump at a certain speed has a limited value. The operating point flow and shaft power depend on the conditions of the plant system (head difference, pressure difference, and line losses) to which the pump is connected. Head varies with the flow.
② Stable work, continuous delivery, 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 can be evacuated before starting to work.
④The centrifugal pump is started when the valve of the discharge pipeline is closed, and the vortex pump and the axial flow pump are started when the valve is fully opened to reduce the starting power.
Before the pump starts, the pump casing is filled with the liquid to be conveyed; after starting, the impeller is driven by the shaft to rotate at a high speed, and the liquid between the blades must also rotate with it. Under the action of centrifugal force, the liquid is thrown from the center of the impeller to the outer edge and obtains energy, leaving the outer edge of the impeller at a high speed and entering the volute pump casing.
In the volute, the liquid is decelerated due to the gradual expansion of the flow channel, and part of the kinetic energy is converted into static pressure energy, and finally flows into the discharge pipeline at a higher pressure and is sent to the required place. When the liquid flows from the center of the impeller to the outer edge, a certain vacuum is formed in the center of the impeller. Since the pressure above the liquid level of the storage tank is greater than the pressure at the inlet of the pump, the liquid is continuously pressed into the impeller. It can be seen that as long as the impeller rotates continuously, the liquid will be continuously sucked and discharged.
Centrifugal pumps are generally driven by electric motors. Before starting the pump, the pump body and suction pipeline are filled with liquid. When the impeller rotates at high speed, the impeller drives the liquid between the blades to rotate together. Due to the centrifugal force, the liquid is thrown from the center of the impeller to the outer edge of the impeller (the flow rate can be increased to 15-25m/s), and the kinetic energy also increases.
When the liquid enters the pump casing, due to the gradual expansion of the flow channel in the volute-shaped pump casing, the liquid flow rate gradually decreases, and a part of the kinetic energy is converted into static pressure energy, so the liquid flows out along the discharge port at a higher pressure.
At the same time, a certain vacuum is formed at the center of the impeller because the liquid is thrown out, and the pressure Pa at the liquid level is higher than that at the center of the impeller, so the liquid sucked into the pipeline enters the pump under the action of the pressure difference.
The impeller rotates continuously, and the liquid is continuously sucked in and pressed out. Because the centrifugal pump can transport liquid mainly by the action of centrifugal force, it is called a centrifugal pump.