Jul 07, 2019
First, the work point
The characteristic curve of the centrifugal pump is an inherent characteristic of the pump itself, and it is independent of the external use. However, once the pump is scheduled to operate in a certain piping system, its actual operation is not only related to the characteristics of the centrifugal pump itself, but also depends on the operating characteristics of the piping. Therefore, to choose and use the centrifugal pump, we must also consider the characteristics of the pipeline.
When liquid is delivered in a particular line, the required head He of the line varies with the square of the flow rate Qe. Drawing this relationship on the coordinate paper is the corresponding pipeline characteristic curve.
If the characteristic curve of the centrifugal pump and its pipeline characteristic curve are plotted on the same coordinate paper, as shown in the above figure, the intersection point M of the two lines is called the working point of the pump. When the pump is selected, the flow rate and pressure head corresponding to the working point are required to meet the requirements of the piping system, and it is exactly provided by the centrifugal pump, that is, Q = Qe, H = He.
Second, flow regulation
(1) Change the opening of the valve
Changing the valve switch on the outlet line of the centrifugal pump essentially changes the line characteristic curve. As shown in the figure below, when the valve is closed, the local resistance of the pipeline is increased, the pipeline characteristic curve becomes steep, the working point is moved from M to M1, and the flow rate is reduced from QM to QM1. When the valve is opened, the resistance of the pipeline is reduced, the characteristic curve of the pipeline becomes flat, the working point is moved to M2, and the flow rate is increased to QM2.
The valve is used to adjust the flow rate quickly and conveniently, and the flow rate can be continuously changed, which is suitable for the continuous production of chemicals. So the application is very extensive. The disadvantage is that the valve is closed, the resistance loss is increased, the energy consumption is increased, and it is not economical.
(2) Change the speed of the pump
Changing the speed of the pump essentially changes the characteristic curve of the pump. The original speed of the pump is n, and the working point is M. As shown in the figure below, if the pump speed is increased to n1, the pump characteristic curve H——Q moves up, the working point is moved from M to M1, and the flow rate is increased by QM. To QM1. If the pump speed is reduced to n2, the operating point is moved to M2 and the flow rate is reduced to QM2.
This type of adjustment requires a shifting device or an expensive shifting prime mover, and it is difficult to continuously adjust the flow rate, so it is rarely used in chemical production.
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