When the centrifugal water pump is started, there is still no water in the outlet pipe of the pump, so there is no pipe resistance and resistance to lifting height. 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), it is easy to overload, it will damage the pump motor and circuit, 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 for a short period of time because there is no resistance, and it will run at a high flow rate. 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, start the valve slowly to allow the pump to gradually work normally along its performance curve.
Before starting the centrifugal pump, you must ensure two points:
1. Fill the pump casing with water, the purpose is to form a vacuum;
2. The gate valve on the outlet pipe must be closed, so that the water pump does not form a flow rate, which can reduce the motor starting current and facilitate the smooth start of the pump. The gate valve should be opened slowly in time as the pump starts smoothly.
The centrifugal pump relies on the centrifugal force of the impeller to form the vacuum suction to lift the water. Therefore, when the centrifugal pump is started, the outlet valve must be closed and water is filled. The water level is above the impeller and the air in the centrifugal pump can be discharged before it can be started. After starting, a vacuum is formed around the impeller to suck water upwards, and it can automatically open and lift the water. Therefore, the outlet valve must be closed first.
About centrifugal pumps
The centrifugal pump is a kind of vane pump, relying on the rotating impeller in the process of rotation, due to the interaction of the vane and the liquid, the vane transfers mechanical energy to the liquid, so that the pressure energy of the liquid can be increased to achieve the purpose of transporting the liquid. Centrifugal pump work has the following characteristics:
① The head of the centrifugal pump at a certain speed has a limit value. The operating point flow and shaft power depend on the conditions of the device system connected to the pump (position difference, pressure difference and pipeline loss). The head changes with the flow.
② Stable work, continuous delivery, no pulsation of flow and pressure.
③Generally, there is no self-priming ability, and the pump needs to be filled with liquid or the pipeline can be vacuumed before it can start working.
④The centrifugal pump is started when the discharge pipe valve is closed, and the vortex pump and 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 transported; After starting, the impeller is driven by the shaft to rotate at 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 gained energy, leaving the outer edge of the impeller at high speed and entering the volute pump casing.
In the volute, the liquid slows down due to the gradual expansion of the flow channel, and then converts part of kinetic energy 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 impeller center to the outer edge, a certain vacuum is formed in the impeller center. Because the pressure above the liquid level of the storage tank is 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 rotates continuously, the liquid will be sucked and discharged continuously.
Startup of other centrifugal pumps
All the above are centrifugal pumps. For other types of pumps, the situation is as follows:
1. Large-flow start-up characteristics of axial flow pump-full-open valve start-up
The axial power of axial-flow pump is maximum at zero flow condition, which is 140%~200% of rated axial power, and the power is minimum at maximum flow. Therefore, in order to minimize the starting current, the starting characteristic of axial power should be large flow start (i.e. full open valve start).
2. Start-up characteristics of mixed-flow pump-Start-up of full-open valve
When the mixed-flow pump is in zero flow condition, the shaft power is between the above two pumps, which is 100%~130% of the rated power. Therefore, the starting characteristics of the mixed-flow pump should also be between the above two pumps, and it is best to start with the full open valve.
3. Start-up characteristics of vortex pump-Start-up of full-open valve
The maximum shaft power of the vortex pump is 130%~190% of the rated shaft power under zero flow condition, so similar to the axial flow pump, the starting characteristic of the vortex pump should be large flow start (i.e. full open valve start).
