Research on Pump Selection Method

Pump selection is an important issue in pumping station engineering design, selection is reasonable or not, directly related to the pumping station project investment, after the completion of the operating costs and water safety [1]. Improper pump election, the key is the lift is not allowed . Domestic low-lift axial pump station deviated from the high-performance long-term operation, in addition to high-lift design, the pump lift is higher than the actual needs of one of the important reasons is 〔2 ~ 4〕, the paper analyzes the pump and pump characteristics of the device , Pointed out the disadvantages of the traditional method of pump selection, proposed a new selection method. A traditional pump method The traditional method of pump selection is based on the net pumping head and device form to determine the need to head curve, and then determine the selected pump according to design flow loss lift, and then determine the total lift, the selected pump in the design of the highest efficiency. Out, the traditional pump method is to increase the headache way to choose the equivalent flow, did not consider the pump head in the design of the highest efficiency. It is assumed that the design point of the device is A (QZ, HZ). When the pump is selected according to the traditional method, the pump head is calculated according to the following formula: HP = HZ + SQ2, (1) To increase the headache to choose the flow of the pump, to determine the selected pump design conditions point B, as shown in Figure 1, and then select the pump according to point B, if the point B is the pump maximum efficiency point, then that election Pump to achieve the best condition.However, the highest efficiency point in the pump B, the highest efficiency of its pump device at point C, we can see that the highest efficiency point C deviation from the so-called design point A, the pump device is actually running at low head high flow This is in line with a large number of model tests and field observations.The efficiency curve of the axial flow pump device is sharply decreased with increasing flow rate on the large flow rate side, which can easily result in low pump efficiency and poor cavitation performance. High-lift pump station, due to the small proportion of pipe hydraulic losses, C and A little offset, coupled with high-lift centrifugal pump, wide area high efficiency mixed flow pump, therefore, For the low-lift pump station, due to a large proportion of pipeline loss, C and A large offset, the traditional pumping method inevitably lead to inefficient pumping station for the blade angle adjustable pump , The highest efficiency will be Moves large angle small angle, offset angle reaches 6 ~ 10 °, the offset of the point C and the point A more serious. Figure 1 traditional pump method 2 pump and pump characteristics of the relationship Define pump head lift HZ upstream and downstream water level difference, pump head is HP, a pump flow and lift can be expressed as: HP (Q) = AQ2 + BQ + C, (2) The relationship between shaft power and flow for a particular device can be expressed as: P (Q) = DQ2 + EQ + F, (3) For a certain pump, A, B, C, D, E, F are constant, the pump efficiency can be found: ηP = ρgQHP (Q) / P (Q) (4) (4) Find the extreme value of the maximum efficiency point flow, the equation is: ADQ4 + 2AEQ3 + [3AF + BE-CD] Q2 + 2BFQ + CF = 0, (5) For pump units, it is assumed that the pump performance characteristics in the unit are constant and that the hydraulic losses in the line are consistent h = SQ2, (6) Where S is the pipe resistance coefficient, the device head can be expressed as: HZ = AQ2 + BQ + C-SQ2. (7) The device efficiency ηZ can be expressed as: ηZ = ρgQHZ (Q) / P (Q) (8) The highest efficiency point of the pump device can be obtained by (8) equation is: (AS) DQ4 + 2 (AS) EQ3 + [3 (AS) F + BE-CD] Q2 + 2BFQ + CF. It can be shown that the root QZ obtained from Eq. (9) near the design flow is smaller than that obtained from Eq. (5). In order to analyze the effect of different pump devices on the efficient point flow and lift, this paper refers to three typical pump characteristic data were analyzed. Model Pump 1 (ns = 1129, Blade Angle 2 °) The performance curve can be expressed as: H (Q) = 6.6287 + 0.389273.10-1Q-0.121915.10-3Q2, (10) P (Q) = 22.2773 + 0.119194-0.345933Q.10-3Q2, (11) The maximum performance point of the basic performance parameters (QP = 396m3.s-1, HP = 4.229m, ηP = 0.86). Suppose the device pipeline resistance coefficient were 0,1.348 × 10-6,2.697 × 10-6, 4.045 × 10-6,5.394 × 10-6,6.742 × 10-6, can find a variety of pipeline hydraulic loss of the highest performance point of the device performance parameters, shown in Table 1. As can be seen from Table 1, the low lift axial flow pump device with the pipe resistance coefficient increases, the maximum efficiency of the device to reduce the flow rate decreases, but the effective point slightly increased head. Mixed-flow pump (1000HLB-16) performance curve can be expressed as: HP (Q) = -2.1754 × 10-5Q2 + 0.13163Q-179.4852, (12) P (Q) = -3.269 × 10 -4 Q 2 + 1.99 Q -2403.76 (13) Device head - the relationship between flow: HZ (Q) = HP (Q) -SQ2 (14) Suppose the pipeline loss coefficients are 0, 7.47 × 10-8, 14.94 × 10-8, 22.41 × 10-8, 29.89 × 10-8, 37.35 × 10-8, respectively. 2. As can be seen from Table 2, the mixed flow pump device with pipe resistance coefficient increases, the highest efficiency of the device to reduce traffic flow, but the head of the fluctuations in the resistance coefficient is small, the high point of the lift increases, the drag coefficient is large, efficient The head loss is reduced, assuming that the losses of the mixed-flow pump pipe account for 15% and 20% of the total head, respectively. Within this range, the flow in this case is reduced by 6% to 7% and the lift is reduced by 1% to 5%. Centrifugal pump (12sh-6) performance curve can be expressed as: HP (Q) = - 5.923 × 10 -4 Q 2 + 8.571 × 10 -2 Q + 99.95238, (15) P (Q) = 6.694 × 10 -4Q2 + 0.4035Q + 128.803, (16) HZ (Q) = HP (Q) -SQ2 (17) Suppose the pipe resistance coefficient S were 0.3 × 10-5, 18.6 × 10-5, 27.90.3 × 10-5 respectively, and the change of device efficiency point can be obtained as shown in Table 3. As can be seen from Table 3, the centrifugal pump with the pipe resistance coefficient increases, the highest efficiency of the device to reduce the flow decreases, reduce the head, flow reduction is larger, lower lift value is small, for this example, assuming that the total loss of pipeline head Of the 10% and 15%, flow reduction of 13.5% to 17.86%, head reduced by 2% to 3.4%. Table 1 model pump 1 device maximum efficiency point changes Pipeline resistance coefficient S (× 10-6) Q / m3.s-1 H / m P / kW ηmax 0.000 396 4.229 19.093 0.86 1.348 386 4.396 20.173 0.819 2.697 376 4.511 21.205 0.784 4.045 368 4.59 21.996 0.753 5.394 359 4.703 22.849 0.724 6.742 352 4.76 23.485 0.699 Table 2 mixed-flow pump device high efficiency point changes S (× 10-8) QZ / m3.s-1 HZ / m P / kW ηmax 0.000 3359.0 16.86 593.9 86.00 7.470 3214.6 17.73 616.0 83.56 14.94 3186.0 17.20 619 79.67 22.41 3160.0 16.65 621.9 75.90 29.89 3135.0 16.08 623.0 72.23 37.35 3111.5 15.50 624.0 68.00 Table 3 Centrifugal pump device efficiency point changes S (× 10-5) QZ / m3.s-1 HZ / m P / kW ηmax 0.0 220.0 90.0 250.0 77.5 9.3 201.0 89.4 236.0 74.9 18.6 190.0 88.1 229.6 71.4 27.9 180.7 86.9 223.0 68.8 3 to increase the flow, change head selection process Through the above analysis we can see that the original increase the flow of the pump head pumping method is only suitable for basic selection of centrifugal pumps, for low lift axial flow pump, should take appropriate to reduce head, increase flow Of the pump method, the mixed flow pump can be used to increase the pump head pumping method, for the centrifugal pump should be taken at the same time increase the flow and lift election pump method, the three pump devices on the efficient point flow, lift the situation shown in Table 4. Table 4 devices efficient point flow, head changes Project Axial Flow Pump Mixed Flow Pump Centrifugal Pump Q (compared to design flow) Reduced by 8% ~ 10% Decreased by 5% ~ 10% Decreased by 10% ~ 15% H (compared with the design head) Increase 5% ~ 8% Decrease or increase 1% ~ 5% Decrease 2% ~ 4% 4 Conclusion Pumping point on the efficient flow and lift and the pump characteristics and device type factor related to the need to pass a large number of tests in order to reach the impact coefficient, this article is only through the formula derivation and numerical analysis to illustrate the disadvantages of the traditional pump method, plus Large flow variable lift way pump election, the device can make the highest efficiency.

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