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Centrifugal pump regulating mode and energy consumption analysis

Centrifugal pump is widely used in chemical industry system general-purpose fluid machinery. It has a wide range of performance (including flow, pressure head and the conveyed medium nature adaptability), small volume, simple structure, easy operation, low operating cost and other merits. Usually, the selected centrifugal pump flow rate, pressure head may be different from pipeline requirement, or because the production tasks, technological requirements changes, requiring the pump flow adjusting, essence is changing centrifugal pump operating point. Centrifugal pump operating point is jointly determined by pump characteristic curve and pipeline system characteristic curve, therefore, change any of the characteristic curve can achieve the flow adjusting purpose. At present, the centrifugal pump flow regulating ways mainly have the regulator control, variable speed control and pump parallel, serial adjustment, etc. Because different principles of all sorts of regulating modes, except its own advantages and disadvantages, causing the loss of energy is also different, in order to find the best, minimum energy consumption, most energy-saving flow regulating mode, we must fully understand the relationship between centrifugal pump flow regulating mode and energy consumption.

1. Pump flow adjusting main ways

1.1 change pipeline characteristic curve
the easiest way to change centrifugal pump flow is using the pump outlet valve opening to control, its essence is to change the position of the pipeline characteristic curve in order to changing pump operating point.
1.2 change centrifugal pump characteristic curve
According to proportion law and cutting law, change pump rotating speed, change pump structure (such as cutting impeller diameter method) both methods can change centrifugal pump characteristic curve, thus achieved the flow adjustment (meanwhile changing the pressure head) purpose. But to worked pumps, change pump structure change is hard, and the change of the pump structure, reduce the generality of the pump, although at some time adjust flow economy is convenient [1], while it is seldom used during production. Here only analysis centrifugal pump rotation speed changing flow adjustment methods. Figure 1, when changing pump rotating speed adjust flow from Q1 dropped to Q2, pump rotating speed (or motor speed) from n1 to n2, speed is n2 pump characteristic curve Q - H and pipeline characteristic curve He=H0+G1Qe2 (pipeline characteristics curve is not change) delivered over to point A3 (Q2, H3), A3 is speed adjust flow through after new operates. This adjustment methods adjust effect is obvious, quick, safe and reliable, can prolong pump service life, saving energy, in addition to reducing speed operation is effective in reducing centrifugal pump NPSHr, making pump far away from cavitation zone, decrease centrifugal pump cavitation possibility [2]. Defect is change pump rotating speed needs the rotating speed through frequency conversion technology to change the prime mover (usually motors), complex principle, greater investment, and small flow adjusting range.
1.3 parallel, serial adjustable mode
When single centrifugal pump cannot satisfy the conveying tasks, can use centrifugal pump parallel or tandem operation. Using two same model centrifugal pump parallel, although the pressure head change is not big, but increase the overall conveying flow, the shunt pump total efficiency and single pump efficiency is the same; Centrifugal pump series total pressure head increases, flow changes a little, series pump total efficiency and single pump efficiency are the same.

2 different adjustment modes pump energy consumption analysis

To different adjustment modes pump energy consumption analysis, this article only aiming at the widespread adoption of valve adjusting and variable speed regulating two regulating mode analysis. Due to the centrifugal pump parallel, serial operation aims to raise the pressure head or flow, in chemical field is useless, the energy consumption can be combined figure 2 analysis, the method are basically the same.
2.1 power consumption when adjusting flow
Centrifugal pump operation, motor input shaft power N is:
N=vQH/η
in formula N——shaft power,w;
Q——pump effective pressure head,m;
H——actual flow,m3/s;
v——fluid proportionN/m3;
η——efficiency
When using the valve adjust flow from Q1 to Q2, in A2 the shaft power consumption is:
NA2=vQ2H2/η
vQ2H3—— actual available power,W;
vQ2(H2-H3)—— power consumption,W;
vQ2H2(1/η-1)—— pump loss power,W。
2.2 Variable speed adjust flow power consumption
The variable speed analysis should use the proportion law of centrifugal pump, according to its applied conditions, the following analysis means the variable speed scope within ±20%, and the efficiency of the centrifugal pump itself change little [3]. Electricity variable flow adjustment flow to Q2, in A3 pump shaft power consumption is:
NA3=vQ2H3/η
also can be changed as
NA3=vQ2H3+vQ2H3(1/η-1) (2)
in formula vQ2H3—— actual effective power,W;
vQ2H3(1/η-1)—— pump loss power,W。
2.3 energy consumption analysis

3、 conclusion

For the present centrifugal pump outlet valve adjusting and general variable speed adjustment two main flow regulating mode, variable speed regulating energy saving is larger than outlet valve adjusting, this can be from both power analysis and power consumption comparison analysis. Through centrifugal pump flow and head relationship diagrams, can be more intuitive reflects two adjustment modes of energy consumption relationship. Through the pump variable speed regulating to reduce flow and thus reducing centrifugal pump cavitation possibility. When flow reduction is bigger, variable speed adjustment energy saving efficiency is also bigger, namely valve adjusting power loss is bigger, but, pump variable speed too big will cause the pump efficiency decreases, beyond the pump proportion law scope, therefore, in practical application should consider from many sides, comprehensive gives optimal flow regulating methods.

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