I. work point
The characteristic curve of a centrifugal pump is the inherent characteristic of the pump itself. It has nothing to do with the external use. However, once the pump is arranged to work in a certain pipeline system, its actual operation is not only related to the characteristics of the centrifugal pump itself, but also depends on the working characteristics of the pipeline. Therefore, to select and make good use of the centrifugal pump, we must also take into account the characteristics of the pipeline.
When the liquid is transported in a specific pipe, the pressure head He of the pipeline varies with the square of the flow Qe. Drawing the relationship on the coordinate paper is the corresponding pipeline characteristic curve.
If the characteristic curve of the centrifugal pump and the characteristic curve of its pipeline are drawn on the same coordinate paper, as shown above, the intersection point M of the two lines is called the working point of the pump. When selecting the pump, the flow and pressure head corresponding to the working point are required to meet the requirements of the pipeline system, but also provided by the centrifugal pump, namely Q = Qe, H = He.
Two, flow regulation
(1) change valve opening.
Changing the valve switch on the outlet pipe of centrifugal pump is essentially changing the pipeline characteristic curve. As shown in the following figure, when the valve closes, the local resistance of the pipeline increases, the pipeline characteristic curve becomes steeper, the working point moves from M to M1, and the flow rate decreases from QM to QM1. When the valve is opened, the resistance decreases, the characteristic curve becomes flat, the working point moves to M2, and the flow rate increases to QM2.
It is quick and convenient to adjust the flow rate with the valve, and the flow rate can be continuously changed, which is suitable for the continuous production of chemical industry. So the application is very extensive. The disadvantage is that the valve closes, the resistance loss increases, and the energy consumption increases, which is very uneconomical.
(2) changing the speed of pump.
Changing the speed of the pump is essentially changing the characteristic curve of the pump. The original speed of the pump is n and the working point is M. As shown in the following figure, if the pump speed is increased to n1, the pump characteristic curve H-Q moves up, the working point moves from M to M1, and the flow rate increases from QM to QM1. If the pump speed is reduced to N2, the working point is moved to M2, and the flow rate is reduced to QM2.
This regulation method requires a transmission device or expensive variable speed prime mover, and it is difficult to continuously adjust the flow rate, so it is seldom used in chemical production.
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