Unstable operation of slurry pump
For some low specific speed centrifugal pumps, the H-Q performance curve is often a hump shape, as shown in figure 1-56. This kind of performance curve and pipeline characteristic curve may have two intersections, m and M1, which are working points in theory. If the pump is set to work at point m, and the working condition deviates from the direction of large flow due to some reasons (such as voltage fluctuation in the circuit, frequency change resulting in speed change, liquid level position fluctuation, equipment vibration, etc.), the pump lift is greater than the energy head required by the pipeline system. The surplus of the energy head increases the flow rate and flow rate of the liquid in the pipeline, and the working point moves along the pump performance curve to the direction of large flow until M1 point, when the energy head given is equal to the energy head required by the pipeline device again, the flow rate stops increasing. The pump works stably at M1, so M1 is the stable working point. On the contrary, if the flow is reduced due to some reason when the pump is working at point m, the head of the pump is less than the energy head required by the pipeline system, which reduces the wave body flow rate in the pipeline and the flow until the flow is equal to zero. If no check valve is installed on the pipeline, the liquid will be poured back into the pump. It can be seen that the working point m is temporarily balanced. Once the actual working condition deviates slightly from the HM and QM corresponding to the m point, the working point will not return to the m point, so the m point is called the unstable working point of the centrifugal pump. The judgment of the stability and instability of the working point is: when the slope of the pipeline characteristic curve at the intersection is greater than the slope of the pump performance curve, it is the stable working point; otherwise, if the slope of the pipeline characteristic curve at the intersection is less than the slope of the pump performance curve, it is the unstable working point. In this way, any H-Q performance curve is humped, and each point on the left line segment of the highest point t of the curve may become the unstable working point of the centrifugal pump.
There are two conditions that may produce unstable working conditions in the operation of slurry pump: first, the H-Q performance curve of the pump is in the shape of a hump; second, the pipeline device should have a liquid level that can rise and fall freely or other parts that can store and release energy (Figure 1-57)。 Therefore, when encountering a management device with this feature, in order to prevent unstable working conditions, the H-Q performance curve with the hump shape should not be selected Centrifugal pump。