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Pay more attention to the public account: environmental protection water treatment
Why should the outlet valve be closed when the centrifugal pump is started?
When the centrifugal water pump is started, there is no water in the outlet pipeline of the pump, so there is no pipeline resistance and lifting height resistance, after the centrifugal pump is started, the lift of the centrifugal pump is very low, the flow rate is very large, and 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 line of the pump, so the outlet valve should be closed when starting to make the pump run normally.
If the axial flow pump is used in the opposite situation, 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, it will run on the large flow rate, and the pump vibration, noise, and even the motor is overloaded, and the motor is burned out. Closing the outlet valve is equivalent to artificially setting the pipe resistance pressure, and after the normal operation of the pump, slowly start the valve, so that the pump can gradually work normally along its performance curve.
Two points must be ensured 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 rate, which can reduce the starting current of the motor, which is conducive to the smooth start of the pump.
The centrifugal pump relies on the centrifugal force of the impeller to form a vacuum to lift the water, so when the centrifugal pump starts, the outlet valve must be closed and the water is filled. The water level exceeds the impeller position, and the air in the centrifugal pump is discharged before it can be started, and after starting, a vacuum is formed around the impeller to suck the water upward, which can be automatically opened to lift the water. Therefore, the outlet valve must be closed first.
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About centrifugal pumps
Centrifugal pump is a kind of vane pump, relying on the rotating impeller in the rotation process, due to the interaction between the blades and the liquid, the blades will transfer mechanical energy to the liquid, so that the pressure of the liquid can be increased, to achieve the purpose of transporting the liquid. The centrifugal pump has the following characteristics:
(1) There is a limit value for the head generated by the centrifugal pump pump at a certain speed. The operating point flow and shaft power depend on the condition of the unit system connected to the pump (differential position, differential pressure and loss of line). The head varies with the flow rate.
(2) The work is stable, the conveying is continuous, and the flow and pressure are pulsation-free.
(3) Generally, there is no self-priming ability, and the pump needs to be filled with liquid or the pipeline pumped into a vacuum before it can start working.
(4) The centrifugal pump is started in the closed state of the discharge pipeline valve, and the vortex pump and axial flow pump are started in the fully open state of the valve to reduce the starting power.
Before the pump starts, the pump casing is filled with the liquid being conveyed; After starting, the impeller is driven by the shaft to rotate at high speed, and the liquid between the blades must also rotate. Under the action of centrifugal force, the liquid is thrown from the center of the impeller to the outer edge and gains energy, leaving the outer edge of the impeller at high speed and entering the volute pump casing.
In the volute, the liquid is slowed down due to the gradual expansion of the flow channel, which 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, and the liquid is continuously pressed into the impeller because the pressure above the liquid level of the tank is greater than the pressure at the inlet of the pump. It can be seen that as long as the impeller rotates continuously, the liquid will be continuously inhaled and discharged.
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Initiation of other centrifugal pumps
The above are centrifugal pumps, for other types of pumps, the situation is as follows:
1. The high-flow starting characteristics of the axial flow pump - fully open valve start-up
The axial flow pump has the maximum shaft power under zero flow conditions, which is 140%~200% of the rated shaft power, and the power is the smallest at the maximum flow, so in order to minimize the starting current, the starting characteristics of the shaft power should be large flow start (i.e., full open valve start).
2. The starting characteristics of the mixed-flow pump - fully open valve start
The shaft power of the mixed-flow pump is between the above two pumps under zero flow conditions, 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, and it is best to start with a fully open valve.
3. The starting characteristics of the vortex pump - full 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 starting characteristics of the vortex pump should be large flow start (i.e., full open valve start).