Centrifugal pump series and parallel operation calculation

When the centrifugal pump is in use, if the required flow rate or lift is large, if one pump cannot meet the requirements, it is often necessary to use two or more pumps in combination. The combined work of the pump can be divided into two types: series and parallel. (1) Series operation The purpose of the centrifugal pump in series is to increase the lift. The performance curves of two centrifugal pumps with the same performance are connected in series, and the lifts of the two pumps are added at a certain flow rate. As shown in Figure 2-35, curves I and II are the performance curves of two centrifugal pumps, III is the characteristic curve of the pipeline, and the performance curve obtained when the centrifugal pumps are operated in series is I+II. After the centrifugal pump is connected in series, the performance curve I+II intersects with the pipeline characteristic curve III at point M. This point is the working point when working in series. At this time, the flow rate is qVM and the head is HM. After the series operation of each centrifugal pump, the parallel line according to the M point as the ordinate is assigned to point B, which is the working point after each pump is connected in series. The flow at point B is qvI, qvII, and the head is H1. , HH. Obviously, the characteristic of series operation is that the flow through each pump is equal, ie qvI=qvII=qvM, and the total lift is the sum of the lifts of each pump, ie HM=H I+HII.
The parameters of each pump before series are compared with the parameters of each pump in series: the individual working point of each pump before series is C(qvc, Hc), and the working point of pump is B (qvB, HB) when connected in series. 2-35 can be seen that qvM=qv 1=qvII≥>qvc Hc This shows that the total head HM generated when the two centrifugal pumps are operated in series is less than twice the head of the pump when it is operated alone, which is greater than the head Hc that is operated separately before the series, and the flow after the series is larger than when the pump is operated alone. Because the pump's head is multiplied after the pump is connected in series, the resistance loss of the pipeline does not increase exponentially, so the rich head increases the flow.

(2) Parallel operation When one pump cannot meet the flow requirements, several parallel working can be used. Figure 2—3 6 is the performance curve when two centrifugal pumps are operated in parallel. In the figure, I and II are the performance curves of two centrifugal pumps of the same performance, III is the characteristic curve of the pipeline, and the performance curve of the centrifugal pump in parallel is I+II.
The performance curves of the individual centrifugal pumps are superimposed on the head under the condition of equal lift, then the I+II curve is obtained, and then their common pipeline characteristic curve III is drawn, and the parallel performance curve of the pump intersects with the M point, that is, parallel At the working point of time, the flow rate is qvM and the head is HM.
In order to determine the operating conditions of a single pump in parallel, the parallel line from the M point as the abscissa points to point B. That is, the individual working point when each pump works in parallel. At this time, point B also determines the working parameters of each pump in parallel, that is, the flow rate is qvI, qvII; the head is HI, HII. The parallel operating point is characterized by: the heads are equal to each other, ie HM = HI = HII. The total flow rate is the sum of the flow rates of each pump, ie qvM=qvI+gvII. Comparison of the parameters of each pump before parallel connection with the parameters of each pump after parallel connection: the independent working point of the pump when not connected in parallel is C(qva, Hc), parallel The operating point of the pump is B (qvB, HB), as can be seen from Figure 2 - 3 6 qvB H HB=HM>Hc
This shows that the flow rate when the two pumps are connected in parallel is equal to the sum of the flow rates of the pumps in parallel. If the flow rate of the two pumps in parallel is less than the flow rate when one pump is operated separately, the total flow qVM of the two pumps in parallel is less than when one pump is working alone. The flow rate is twice that of qvc, and is greater than the flow rate qvc when a pump is working alone. Because the flow rate qVB of the pump after paralleling is smaller than gvc. The lift in parallel is higher than when each pump is working alone. Why is the flow qvB of each pump after paralleling is smaller than the flow qvc of each pump when it is not connected in parallel, and the lift HB is greater than the lift Hc? This is because the friction loss of the pipe increases with the increase of the flow rate. This requires each pump to increase its lift to overcome this increased loss of line resistance, so HB is greater than Hc and the flow is reduced accordingly.