Why should the outlet valve be closed when the centrifugal pump is started?
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 is large. At this time, the pump motor (shaft Power) output is very large (according to the pump performance curve), and it is easy to overload, which will damage the pump motor and circuit. Therefore, 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 will be at a minimum because there is no resistance in a short period of time, and it will run at a high flow rate. Pump vibration, noise, and even overload operation of the motor will 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.
Two points must be guaranteed before the centrifugal pump is started:
1. Fill the pump casing with water in order to form a vacuum;
2. The gate valve on the outlet pipe must be closed, so that the pump does not form a flow, which can reduce the motor starting current, which is beneficial to 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 started. After starting, a vacuum is formed around the impeller to suck up the water. 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 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 first or the pipeline can be evacuated before it can start to work.
④ The centrifugal pump is started when the discharge pipeline 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 is started, the pump casing is filled with the liquid to be transported; 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 gains energy. It leaves the outer edge of the impeller at a high speed and enters the volute pump casing.
In the volute, the liquid decelerates due to the gradual expansion of the flow path, and converts part of the kinetic energy into static pressure energy, and finally flows into the discharge pipe at a higher pressure and is sent to the place where it is needed. 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. Because 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.
Start of other centrifugal pumps
The above are all centrifugal pumps. For other types of pumps, the situation is as follows:
1. The large flow starting characteristics of the axial flow pump-fully open valve start
The shaft power of the axial flow pump is the largest under the zero-flow condition, which is 140%~200% of the rated shaft power, and the power is the smallest at the maximum flow. Therefore, in order to minimize the starting current, the starting characteristic of the shaft power should be large flow starting (that is, full open The valve is activated).
2. The starting characteristics of mixed flow pump-fully open valve start
The shaft power of the mixed-flow pump in the zero-flow condition is between the above two pumps, which is 100%~130% of the rated power, so the starting characteristics of the mixed-flow pump should also be between the above two pumps, preferably fully open the valve start up.
3. The starting characteristics of the vortex pump-fully open valve start
The maximum shaft power of the vortex pump in the zero flow condition is 130%~190% of the rated shaft power, so similar to the axial flow pump, the start characteristic of the vortex pump should be large flow start (that is, full open valve start).