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渣漿泵的不穩(wěn)定工作
有些低比轉速的離心泵,其H-Q性能曲線常常是一種駝峰形的，如圖1-56所示。這種性能曲線和管路特性曲線可能有兩個交點M和M1,理論上講都是工作點。設泵在M點工作,由于某種原因(如電路中電壓波動、頻率變化導致轉速改變、液面位置波動以及設備振動等)使工況向大流量方向偏離，則泵的揚程大于管路系統(tǒng)所需的能頭。能頭有富余,使管路中液體流速加大，流量增加，工作點沿泵性能曲線維續(xù)向大流量方向移動，直至M1點時來給出的能頭重新與管路裝置所需的能頭相等，流量停止增加。泵在M1點穩(wěn)定工作，所以M1點是穩(wěn)定工作點。相反，如果泵在M點工作時，由于某種原因使流量減小了，則泵的揚程小于管路系統(tǒng)所需的能頭，使管路中波體流速減小，流量減小，直到流量等于零為止。若管路上不裝止回閥，液體將倒灌進泵內(nèi)。由此可見，工作點M是暫時平衡，一且實際工況與M點對應的Hm和Qm略有偏離時，工作點便不再回到M點，所以稱M點為離心泵的不穩(wěn)定工作點。工作點的穩(wěn)定與不穩(wěn)定的判別是:當交點處管路特性曲線的斜率大于泵性能曲線的斜率時是穩(wěn)定工作點;反之，如交點處管路特性曲線的斜率小于泵性能曲線的斜率，則是不穩(wěn)定工作點。這樣，凡是H-Q性能曲線呈駝峰形的，曲線最高點T的左側線段上的各點都有可能成為離心泵的不穩(wěn)定工作點。
渣漿泵工作中可能產(chǎn)生不穩(wěn)定工況的兩個條件:一是泵的H-Q性能曲線呈駝峰形，二是管路裝置中要有能自由升降的液面或其他能儲存和放出能量的部分(圖1-57).因此，當遇到具有這種特點的管理裝置時，為防止發(fā)生不穩(wěn)定工況，則不要選用具有駝峰形H-Q性能曲線的離心泵。
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.
