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CVEN 3323 Updated November 17, 2004 Single, Series, and Parallel Pumps Theory Introduction: Whereas turbines convert fluid energy into mechanical energy, pumps convert mechanical energy into fluid energy, increasing the energy possessed by the fluid. There are two main pump types: positive displacement pumps—pistons, plungers, diaphragms, vanes, screws, lobes, which have a fixed flow rate per stroke or revolution; and turbo-hydraulic or kinetic pumps— centrifugal pumps, which convert fluid kinetic energy into static pressure energy. Method of Operation In 1730, Demour demonstrated that when rotating a T formation pipe with bottom end submerged in a liquid and primed—loaded with the liquid, the centrifugal force lifts the liquid and discharges it through the arms when the centrifugal force is greater than the gravitational force. Today, the design of a centrifugal pump has the water entering the low-pressure center of the impeller. The vanes then lead the water to the higher-pressure region to the casing. The casing is designed with a gradually expanding spiral shape so that minimum loss occurs in the transformation of kinetic energy to pressure energy. The pump receives the water at a low velocity on the interior edge of the set of moving impeller vanes and discharges it from the outer edge with kinetic energy sufficient to raise it to a desired height; and through the gradually expanding spiral passage transforms the kinetic energy into pressure energy. The "Performance Characteristics" of a pump at a fixed speed are represented by the following graphical relationships: Total Head (HP) versus Discharge (Q) Power Input (P) versus Discharge (Q) Efficiency ( %) versus Discharge (Q) This lab examines the head/flow rate and efficiency/flow rate relationships. Pump characteristics or performance curves are created when the head delivered by a pump (or pumps) is plotted against the flow rate. These curves represent the behavior of a given size pump (or pumps) operating at a given speed and are important tools in pump selection. Generally, the higher the flow rate, the lower the head that the pump can contribute. Additionally, pumps are used to lift -1- CVEN 3323 Updated November 17, 2004 water up or to increase the energy so that the water can travel farther. This lab determines the head/flow characteristics of centrifugal pumps operating at a single speed: a single centrifugal pump, two similar centrifugal pumps operating in parallel and in series. Recall the total head is the difference between the total energy head at the outlet and the total energy head at the inlet (neglecting the small differences in velocity heads). As shown be the following equation: p 2 V22 p1 V12 HP HL z z 2 1 g 2 g g 2 g where subscripts 1 and 2 refer to inlet and outlet sections. H p is the pressure head produced by the pump and H L is the energy loss due to friction and pipe fittings. By conservation of mass, V1 = V2 if the pipe diameters are equal at the inlet and outlet sections. The incoming pressure is read on the compound gauge at the pump inlet. After the water flows through the pump, it travels up 0.8 meters (assuming the pump is placed on the floor) to the manifold gauge. The pressure at the manifold is read on the manifold gauge. However, the pressure head at the manifold is 0.8 meters less than the pressure head at the exit of the pump. Therefore, the total head is determined by adding the manifold pressure head to the datum correction and subtracting the inlet pressure head. This should be approximately the same as subtracting the inlet pressure head from the outlet pressure head (verify that this is the case for the single pump). Total Head: HP = (pressure head increased by the pump) Total Head Outlet: HP = (pressure head at pump outlet - pressure head at pump inlet) = (outlet pressure/) – (inlet pressure/) Total Head Manifold: HP = ({manifold pressure head + datum correction} - inlet pressure head}) = [{(manifold pressure/+ datum correction} - (inlet pressure/)] Single pump: Qout Qin Qin =Qout A hpump = hwater Pumps in Parallel: When two or more similar pumps are connected in parallel, the head across each pump is the same and the total flow rate is shared equally between the pumps, QP/n, where n is the number of pumps in parallel. For identical pumps in parallel, the pressures at the two inlets and outlets are identical and the maximum head the two pumps can deliver is no greater than for a single pump. Theoretically, the flow rate is doubled, although in practice, this will not occur, due to losses in the piping systems. Total head (using outlet, not manifold) is determined the in the same manner as for the single pump. The theoretical curve for the parallel pump configuration is obtained from the single pump data -2- CVEN 3323 Updated November 17, 2004 by multiplying the flow rate by two. For theoretical parallel pump curve, plot: Hp(single pump) vs. 2*Q(single pump) Parallel Pumps: n= 2 pumps QA QT hpumpA = hpumpB =hwater QA + QB = QTotal QB Since the head loss across the parallel pumps is equal, the pump curve derived for each should be the same. For the experimental parallel pump curves, plot: Hp(pump i) vs. Q where i = 1 or 2 Pumps in Series: When two or more similar pumps are connected in series, the same flow rate passes through each pump and under goes a head boost of total head divided by number of pumps, HP/n. Therefore, the series configuration of two identical pipes provides a pump characteristic of twice the head as for a single pump. For series pumps, the total head can be computed as follows: Total Head: HP = (pressure head at pump 2 outlet - pressure head at pump 1 inlet) = [(outlet 2 pressure/) – (inlet 1 pressure/)] For the experimental series pump curve, plot: Hp vs. Q The theoretical curve for the series pump configuration is obtained from the single pump data by multiplying the head by two. This doubled head is plotted with the measured flow rate. To get the theoretical series pump curve, plot: 2*HP(single pump) vs. Q(single pump) Series pumps: n = 2 pumps Qout Qin A B Qin = Qout = Qtotal hpumpA + hpumpB =hwater Pump Efficiency For a pump, the efficiency is defined as = Po/Pi where Po = power out from the pump = power imparted to the fluid = *Q*Hp = [N/m3]*[m3/s]*[m] = [N-m/s] = [J/s] = [W] Pi = power input to the pump shaft = power output from the motor = [W] -3- CVEN 3323 Updated November 17, 2004 Output power is determined experimentally. Input power should be given in the manufacturer’s specifications for the pump. For the pumps used in this lab, Pi = 0.37 kW = 370 W. An important objective when selecting a pump for an engineering system is maximizing the efficiency for the desired flow conditions. Experimental Procedure Purpose: Determine the head/flow characteristics of centrifugal pumps operating at a single speed: a single centrifugal pump, two similar centrifugal pumps operating in parallel and in series. Equipment: Hydraulics bench, two auxiliary centrifugal pumps, discharge manifold, appropriate hoses and connectors, graduated cylinder, and stopwatch. NOTE: For this lab, it is important to emphasize the need to turn off the pumps before any hoses are disconnected. If other experiments are being performed at the same time, it can be difficult to hear the motor(s). Procedure: 1. Straddle the manifold block across the channel of the bench. Make sure the outlet is pouring into the stilling basin for correct measurement of flow rate. 2. The auxiliary pump should be placed on the floor, which is important for the datum correction, and connected to the bench with the appropriate hoses as follows: 3. First set up for the SINGLE PUMP: i. The single pump receives its water directly from the sump. The sump drain outlet is located next to the built-in bench pump. A hose should extend between the sump and the auxiliary pump. The hose is attached to both the sump outlet and the auxiliary pump inlet. Next, a hose continues from the auxiliary pump to the manifold. Make sure manifold outlet drains down into hydraulic bench for water collection. Figure 1 shows the proper piping connections. -4- CVEN 3323 Updated November 17, 2004 Figure 1. Single Pump Operation ii. After the connections are made open the sump drain valve. Close the discharge control valve on the manifold; turn on the auxiliary pump, then completely open the discharge control valve. Turn on only the auxiliary pump, not the hydraulic bench. Water should flow out the manifold. If water is not emitted, check that: a) the auxiliary pump is plugged in; b) the sump valve is open; c) the discharge manifold control valve is open; d) the sump has sufficient water. iii. Slowly close the flow control knob to obtain manifold pressure readings. iv. Record inlet pressure (compound gauge) and outlet pressure (pump) using gauges. Record the flow rate using the graduated cylinder and stopwatch. v. If the sump drain valve is not fully open, inlet pressure will fall low enough to where cavitation can occur. Cavitation will be marked by a loud noise and a sharp decrease in pump performance. To avoid cavitation, make sure sump drain valve is fully open. vi. Turn off pump motor after all readings have been taken. 4. Next we’ll set up for the SERIES PUMPS. i. Close the sump drain valve. Disconnect the hose from the manifold. Connect this hose to the inlet of the second pump. Attach a hose from the outlet of the second pump to the manifold. ii. After the connections are made, close the discharge control valve, turn on both pumps, and then completely open the discharge control valve. Water should flow out the manifold. iii. Slowly close the flow control knob to obtain manifold pressure readings. iv. Using the appropriate gages, record the outlet pressure for pump 2 and the inlet pressure -5- CVEN 3323 Updated November 17, 2004 for pump 1. Record the flow rate using the volumetric tank and a stopwatch. v. Turn off both pumps and close the sump drain valve when all readings have been taken. 5. Finally, set up for the PARALLEL PUMPS. i. Disconnect all hoses, being careful not to spill water everywhere. Re-connect two auxiliary pumps in a parallel configuration as shown in instructed by the TA. ii. After the connections are made open the sump drain valve. Turn on only pump 1. Water should flow out the manifold. Next, turn off pump 1 and turn on pump 2. Again, water should flow out the manifold. Now the pumps should be primed. Close the discharge control valve; turn on both pumps, then completely open the discharge control valve. Check outlet pressure gages on pumps to ensure equal flow is moving through each. iii. Slowly close the flow control knob to obtain manifold pressure readings. iv. Record inlet and outlet pressures for both pumps using the gauges. Record the flow rate using the volumetric tank and a stopwatch. Turn off both pumps, close sump drain valve, and disconnect hoses. Return pumps and hoses to ITLL module room. -6- CVEN 3323 Updated November 17, 2004 Data Tables Table 1. Single Pump Manifold Pressure Outlet Pressure Inlet Pressure Height Correction Units [psi] [m] 1 6 0.8 2 9 0.8 3 12 0.8 4 15 0.8 5 18 0.8 6 21 0.8 Total Head Manifold Total Head Outlet Volume [s] Table 2. Series Pumps Manifold Pressure Units [psi] 1 9 2 15 3 21 4 27 5 33 6 39 7 45 8 51 9 57 Pump 2 Pump 1 Outlet Pressure Inlet Pressure Total Head Outlet Volume Time [s] -7- Time Flow Rate Flow Rate CVEN 3323 Updated November 17, 2004 Table 3. Parallel Pumps Manifold Pressure Units [psi] 1 14 2 16 3 18 4 20 5 22 Pump 1 Outlet Pressure Pump 1 Inlet Pressure Pump 2 Outlet Pressure Pump 2 Inlet Pressure Total Head Outlet 1 Total Head Outlet 2 Volume Time [s] Table 4. Single Pump Efficiency Pi Po Units [W] [W] [W/W] [%] 1 370 2 370 3 370 4 370 5 370 6 370 -8- Flow Rate CVEN 3323 Updated November 17, 2004 Results and Questions 1. Fill in the tables on the data sheet for each pump configuration. USE CONSISTENT UNITS. For the single pump, discuss the relationship between the total head determined by the outlet pressure gage and the total head determined by the manifold pressure + datum correction. Explain any differences. 2. Plot the Total Head Outlet vs. Flow Rate for the single pump, parallel pumps, and series pumps on the same graph. (Graph 1) 3. For the parallel pumps, plot the experimental and theoretical Total Head vs. Flow Rate curves on the same graph. (Graph 2) 4. Comment on the flow rates obtained on Graph 2. What are some reasons that flow rates do not increase in proportion to the number of pumps used? 5. For the series pumps, plot the experimental and theoretical Total Head vs. Flow Rate curves on the same graph. (Graph 3) 6. Does the head from the experimental series pump graph in Graph 3 increase exactly by a factor of two? Discuss. 7. For the single pump, plot both the Total Head Outlet vs. Flow Rate and the Efficiency (as percent) vs. Flow Rate on the same graph. (Graph 4) 8. From this graph (Graph 4), estimate the desired operating flow rate for the single pump. -9-