Should the outlet valve be closed when the centrifugal pump is started?
Should the outlet valve be closed when the centrifugal pump is started?
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.
If an axial-flow 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 the smallest in a short period of time because there is no resistance, and the flow rate will be too large. Pump vibration, noise, and even motor overload operation often occur. Burn the motor. Closing the outlet valve is equivalent to artificially setting the pipe resistance pressure. After the pump runs normally, slowly start the valve to allow the pump to work normally along its performance curve.
Before starting the centrifugal pump, two points must be ensured:
1 Fill the pump casing with water, the purpose is to form a vacuum;
2. The gate valve on the water outlet pipe must be closed, so that the pump does not form a 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.
About centrifugal pumps
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 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 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.
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