The Discussion of this exercise covers the following points: Centrifugal pumps in series Centrifugal pumps in parallel. Centrifugal pumps in series

Transcription

1 Exercise 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) EXERCISE OBJECTIVE In this exercise, you will observe the effects that connecting two centrifugal pumps in series or parallel have on the maximum head and flow rate developed in a system, as compared to when a single pump is used. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Centrifugal pumps in series Centrifugal pumps in parallel DISCUSSION Centrifugal pumps in series As seen in Ex. 2-3, centrifugal pumps are part of the dynamic pump category. This type of pump does not deliver a constant flow rate. Instead, the flow rate varies as a function of the pump output head. Thus, the flow rate is greatest at low pump heads and it decreases as the pump head increases. A centrifugal pump, when used in a given system, must be able to produce a head higher than the sum of the pressure losses caused by all the components downstream of the pump in order for liquid flow to occur. The maximum head and flow rate that a centrifugal pump can produce are determined mainly by the pump impeller diameter. It sometimes happens, in an already existing system, that a centrifugal pump no longer meets the requirements in regard to maximum head. We will say that at the current operating point, the pump delivers a head h0 at a flow rate Q0. To allow higher heads to develop in the system, the pump can be connected in series with another centrifugal pump. This is performed by connecting the discharge line of one of the pumps to the suction line of the other pump, as Figure 2-47 shows. Q 1 Discharge Q 2 h F = h 1 + h 2 Output flow to the system Q F = Q 1 = Q 2 Input flow h 1 Upstream pump Suction h 2 Downstream pump Figure Centrifugal pumps connected in series. Festo Didactic

2 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Discussion When pumps are connected in series, you can obtain the overall performance curve by adding pump heads at the same flow rate. If both pumps are identical (and driven at the same speed, of course), the following occurs: Both pumps deliver the same flow rate (QF = Q1 = Q2). Both pumps produce the same discharge head (h1 = h2) so that they both contribute to the head developed in the system (hf = h1 + h2 = 2h1). Figure 2-48 shows an example of the resulting head-versus-flow curve for two identical pumps driven at the same speed in series, compared to that of a single pump. Output head (h) 2h 0 h F < 2h 0 h 0 Possible operating point (with modified system curve) Operating point (one pump) New operating point PressureA One pump (kpa) Two pumps Colonne2 in series PressureB System curve (kpa) System curve PressureC modified to have (kpa) h F = 2h 0 Q 0 Q F Output flow rate (Q) Figure Pump performance curve (series). With two identical pumps in series, the flow rate and the head delivered to the fluid increases, but neither are doubled if the system curve remains constant. That is: QF > Q0 hf < 2h0 If you want the operating point to be at the initial flow rate (QF = Q0), then you have to modify the system curve, as shown in Figure This can be done, for example, by further closing a valve in the system. In that case, the head delivered by the two pumps could be doubled the initial head (hf = 2h0). Centrifugal pumps in parallel It may also happen, in an already existing system, that a centrifugal pump no longer meets the system requirements in regard to volumetric flow rate. We will say, again, that at the current operating point, the pump delivers a head h0 at a flow rate Q0. To increase the flow rate, the pump can be connected in parallel with another centrifugal pump. This is performed by connecting the discharge lines of each pump together and the suction lines of each pump together, as Figure 2-49 shows. 82 Festo Didactic

3 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Discussion Q 2 Discharge Input flow Suction Q 1 h 1 Discharge h 1 = h 2 Output flow to the system Q F = Q 1 + Q 2 Suction h 2 Figure Centrifugal pumps connected in parallel. Since the inlets of the pumps are interconnected, and since the outlets of the pumps are interconnected, the same head increase occurs across each pump. Consequently, both pumps produce the same discharge head (h1 = h2). On the other hand, the output flow rate to the system is the sum of the flow rates delivered by each pump (Q1 + Q2). Figure 2-50 shows an example of the resulting head-versus-flow curve for two identical pumps driven at the same speed in parallel, compared to that for a single pump. When pumps are connected in parallel, you can obtain the overall performance curve by adding flow rates at the same head. Output head (h) h F h 0 New operating point Operating point (one pump) Possible operating point (with modified system curve) PressureA One pump (kpa) parallele Two pumps in parallel PressureB System curve (kpa) System curve Series4 modified to have Q F = 2Q 0 Q 0 Q F < 2Q 0 Q 1+Q 2 Output flow rate (Q) Figure Pump performance curve (parallel). With two identical pumps in parallel, the flow rate and the head delivered to the fluid increases, but neither are doubled if the system curve remains constant: QF < 2Q0 hf > h0 If you want the operating point to be at the initial output head (hf = h0), then you have to modify the system curve, as shown in Figure This may be done, for example, by further opening a valve in the system (but may not be possible if all the valves are fully open). Festo Didactic

4 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections centrifugal pumps in series Set up and connections centrifugal pumps in parallel End of the exercise PROCEDURE Set up and connections centrifugal pumps in series In this system, the upstream pumping unit will first be operated alone. The output flow rate from this unit will drive the pump of the downstream pumping unit. Then, the downstream pumping unit will also be placed in operation. 1. Connect two pumping units in series as shown in Figure As this figure shows, the return hose of the upstream pumping unit must be connected to an auxiliary return port of this unit. The discharge hose of the upstream pumping unit must be connected to the return hose of the downstream pumping unit via an extension joint. a For PI1, use the optional pressure gauge (Model 6553-C0) or the optional industrial DP transmitter (Model 46929) as they can read higher pressures than 100 kpa (15 psi). Upstream pumping unit (A) Joint Downstream pumping unit (B) Figure Pumping units in series. 2. Make sure the reservoir of each pumping unit is filled with about 12 L (3.2 gal) of water. Make sure the baffle plate is properly installed at the bottom of each reservoir. 84 Festo Didactic

5 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Procedure 3. On the upstream pumping unit, adjust valves as follows: Close valves HV1A and HV2A completely. Set valve HV3A for directing the full reservoir flow to the pump inlet. 4. On the downstream pumping unit, adjust valves as follows: Close valves HV1B and HV2B completely. Set valve HV3B to allow flow from the upstream pumping unit to go to the downstream pumping unit inlet. 5. Open valve HV4 completely by turning its handle fully counterclockwise. 6. Turn on the pumping units. Upstream pumping unit operating alone 7. Make the upstream pumping unit run at 100%. a Water is now pumped by the upstream pumping unit. Since the pumped flow is directed to the inlet of the pump of the downstream pumping unit, it acts upon the impeller of this pump and causes it to turn, even if the downstream pump drive is set to 0%. A stream of water passes through the rotameter and returns to the reservoir of the upstream pumping unit. Since HV4 is open completely, the rotameter indicates the maximum system flow rate. 8. Close HV4. The pumped flow is now blocked at this valve, causing pressure gauge PI1 to read the maximum pressure that can build in the system when a single pump is on. In the One Pump column of Table 2-3, record the pressure gauge reading for a flow rate of 0 L/min (0 gal/min). Table 2-3. Pressure versus flow for one and two pumps. Flow rate L/min (gal/min) 0 (0) System pressure at maximum speed kpa, gauge (psig) One pump Two pumps Festo Didactic

6 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Procedure 9. By varying the opening of valve HV4, increase the system flow rate by steps of 2 L/min (or 0.5 gal/min) until you reach the maximum flow on the rotameter. After each new flow setting, note the pressure gauge reading and record it in Table Open valve HV4 completely. Upstream and downstream pumping units both operating 11. Make the downstream pumping unit also run at 100%. Water is now pumped by both the upstream and downstream pumping units. Since valve HV4 is open completely, the rotameter indicates the maximum system flow rate. 12. Close valve HV4 completely. The pumped flow rate is now blocked at this valve, causing pressure gauge PI1 to read the maximum pressure that can build in the system when both pumps are on. In the Two Pumps column of Table 2-3, record the pressure reading for a flow rate of 0 L/min (0 gal/min). 13. By varying the opening of valve HV4, increase the system flow rate by steps of 2 L/min (or 0.5 gal/min) until you reach the maximum flow on the rotameter. After each new flow setting, note the pressure gauge reading and record it in Table Stop the drives of the downstream and upstream pumping units. 15. From the data recorded in Table 2-3, plot the pressure-versus-flow curves of the system when a single pump is on and when both pumps are on. 16. From these curves, explain how the pressure developed by two pumps in series varies with flow rate as compared to when a single pump is used. 86 Festo Didactic

7 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Procedure Set up and connections centrifugal pumps in parallel In this system, pumping unit A will be operated alone first, then pumping unit B will also be placed in operation. In each case, the system will be placed in the water recirculating mode. 17. Connect two pumping units in parallel, as shown in Figure Connect the unused port at the top of the column to either of the auxiliary return ports of pumping unit A or B, using an extra-long hose. This hose serves as an overflow if the column gets full, and causes the column to be open to the atmosphere. a Use two-way, manually operated valves (Model 6520) for valves HV4 and HV5. For PI1, use the optional pressure gauge Model 6553-C0 or the optional industrial DP transmitter (Model 46929), as they can read higher pressures than 100 kpa (15 psi). Pumping unit A Overflow hose 3-way coupling Pumping unit B Figure Pumping units in parallel. Festo Didactic

8 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Procedure 18. Open HV4 completely. 19. Close HV5 completely. 20. On pumping unit A, adjust valves as indicated below: Close valves HV1A and HV2A completely. Set valve HV3A for directing the full reservoir flow to the pump inlet. 21. On pumping unit B, adjust valves as indicated below: Close valves HV1B and 2B completely. Set valve HV3B for directing the pumping unit input flow directly to the pump inlet. Upstream pumping unit operating alone Since pumping unit A will be operated alone first, the pumped flow will go towards the rotameter and also toward pumping unit B, where it will make this unit s pump rotate in the reverse direction and compress the air entrapped in the hoses connected to valves HV5 and HV1B. After a short period of time, though, the water flow throughout pumping unit B will cease and all the pumped flow will go toward the column. 22. Make sure pumping unit B is not running. 23. Make pumping unit A run at 50%. 24. Let the water level in the column rise to about 40 cm (16 in), then set valve HV3 for directing this unit s input flow directly to the pump inlet (turn handle fully counterclockwise). a The pumped water is now being recirculated in the loop formed by pumping unit 1, the rotameter, and the column. 25. Make pumping unit A run at 100%. 26. Since HV4 is open completely, the rotameter indicates the maximum system flow rate. Close HV4 completely. Pressure gauge PI1 now reads the maximum pressure that can build in the system when a single pump is on. In the One Pump column of Table 2-4, record the PI1 pressure reading for a flow rate of 0 L/min (0 gal/min). 88 Festo Didactic

9 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Procedure 27. By varying the opening of valve HV4, increase the system flow rate by steps of 2 L/min (or 0.5 gal/min) until you reach the maximum flow on the rotameter. After each new flow setting, note the pressure gauge reading and record it in Table 2-4. Table 2-4. Pressure versus flow for one and two pumps. Flow rate L/min (gal/min) 0 (0.0) System pressure at maximum speed kpa, gauge (psig) One pump Two pumps 28. Open valve HV4 completely. Two pumping units operating at the same time 29. Open valve HV5 very slowly. This will cause the air entrapped in the hoses between the bottom of the column and the input of pumping unit B to escape through the column water, thereby creating a momentary stream of air bubbles in the water. 30. Make the pumping unit B also run at 100%. The water pumped by pumping units A and B is now recirculated through both units. Since valve HV4 is open completely, the rotameter indicates the maximum system flow rate. 31. Close valve HV4 completely. Pressure gauge PI1 now reads the maximum pressure that can build in the system when both pumps are on. In the Two Pumps column of Table 2-4, record the pressure reading for a flow rate of 0 L/min (0 gal/min). 32. By varying the opening of valve HV4, increase the system flow rate by steps of 2 L/min (or 0.5 gal/min) until you reach the maximum flow on the rotameter. After each new flow setting, note the pressure gauge reading and record it in Table Stop the drives and turn off the pumping units. 34. From the data recorded in Table 2-4, plot the pressure-versus-flow curves of the system when a single pump is on and when both pumps are on. Festo Didactic

10 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Conclusion 35. From these curves, explain how the flow rate created by two pumps in parallel varies with pressure as compared to when a single pump is used. End of the exercise 36. Disconnect the circuit. Return the components and hoses to their storage location. 37. Wipe off any water from the floor and the training system. CONCLUSION In this exercise, you learned the effects of connecting two centrifugal pumps in series or parallel on the maximum head and flow rate developed in a system, as opposed to when a single pump is used. You observed that, when two pumps are connected in series, higher heads can develop in the system than when a single pump is used. This occurs because both pumps contribute to the head developed in the system. You also observed that, when two pumps are connected in parallel, higher flow rates can occur in the system than when a single pump is used. This occurs because the system output flow rate is the sum of the flow rates delivered by each pump. REVIEW QUESTIONS 1. To which category of pumps do centrifugal pumps belong? What is the main characteristic of this category of pumps? 2. When two identical pumps driven at the same speed in series are used, does the system flow rate increase compared to when a single pump is used? Why? 90 Festo Didactic

11 Ex. 2-4 Centrifugal Pumps in Series and in Parallel (Optional Exercise) Review Questions 3. When two identical pumps driven at the same speed in series are used, how does the system head increase as compared to when a single pump is used? Why? 4. When two identical pumps driven at the same speed in parallel are used, how does the system head increase compared to when a single pump is used? Why? 5. When two identical pumps driven at the same speed in parallel are used, how does the system flow rate increase as compared to when a single pump is used? Why? Festo Didactic