Effect of Flow Rate and Speed on Centrifugal Pump Performance
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Date: October 8, 2012Summary In this experiment the investigation of a centrifugal pump was done to determine the effect of flow rate and speed on the head, efficiency and brake horsepower requirement of a centrifugal pump. Three separate experiments were done and data were measure at different speeds with different flow rates. Further calculations were made by utilizing numerous equations in order to obtain values of pump head (hp), shaft work (BHP), net positive section head (NPSH) and the efficiency. It is observed that the brake horsepower at different pump speeds decreases as flow rate increases. Moreover, it is concluded that the efficiency increases with the flow rate, however, at certain point that is 80%, and the efficiency starts to decline. Furthermore, final results exhibit that the shaft work increases gradually as the flow rate increases. Therefore, they are correlated directly to each other due to this behavior. Surprisingly, at 1700 rpm, the net positive suction head drops rapidly with increasing the flow rate.Background and MethodsPump System DescriptionThe main objective of this experiment was to determine the impact of flow rate and speed on the head, efficiency and brake horsepower requirement of a centrifugal pump. The centrifugal pump as seen in figure 11 was used for this experiment. The pump receives water from the tank, labeled as 9 in figure 1, which is connected to the suction line going into the pump. The discharge line consists of a valve to allow for user variance of the volumetric flow rate. Then, the water is returned to the tank. In order to measure the suction and discharge pressures, two gauges connected to respective lines are used. Additionally, Torque reading is obtained from the torque gauge that is fitted on the pump.Pump Equipment The system in figure 1 consists of:
1A Suction Pressure gauge, P1 [in.H2O gauge].1B Discharge Pressure gauge, P2 [psig].2A Inlet (Suction) line.2B Outlet (Discharge) line.3 Shaft Torque meter ().4 Flow rate meter.5Rheostat for pump speed control. 6 Stroboscope to set pump rpm.7Pump Impeller8Pump Motor.Measurement ProcedureThe centrifugal pump was run at fourspeeds; 1100, 1300, 1500 and 1700 revolution per minute (rpm). The motor rheostat, labeled as5 in figure 1, was used to adjust these readings and made accurate by using the stroboscope (6). At each different speed, measurements of the suction pressure using gauge 1A from figure 1, discharge pressure from gauge 1B and the torque from 3 were measured. Those measurements were taken at flow rates of 0, 2.5,5,10,15 and 20 gallons per minute. After that, the motor was stopped slowly by decreasing the rheostat to zero. Finally, the pump was operated in reverse rotation at different speeds, and the flow rate with discharge valve wide open was observed. Centrifugal Pump CalculationTo calculate head of the pump (hp), McCabe, Smith, and Harriott, 5thedition 2 gives Equation.1 tobe applied. hp=Pb−Paρ (1)Where:pa = pump suction pressure [lbf/ft2]pb = pump discharge pressure [lbf/ft2] = Density of fluid [lbm/ft3]Figure 11. The pump system that has been used for the experiment.
According to McCabe, Smith, and Harriott, 5th edition 2, the shaft work or horsepower for the pump can be calculated from the shaft torque and rpm readings in Equation.2: BHP=2πτn (2)Where:τ= shaft torque [lbfft]n = shaft speed [min1]BHP= shaft work [lbfft/min]Based on McCabe, Smith, and Harriott, 5th edition 2, the ideal pump work per unit mass flow rate[Ibf ft/Ibm} can be calculated from Equation.3: Wp= BHP/m (3) Where:m=mass flowrate (Ibm/min)McCabe, Smith, and Harriott, 5th edition 2 gave Equation.4, where NPSH can be calculated infeet: NPSHa=gcg(pa'−pvρ−hfs)−Za (4) Where,Pa’ = absolute pressure at surface of reservoir [lbf/ft2]Pv = vapor pressure of fluid [lbf/ft2]hfs = friction loss in suction line [lbf ft/lbm]Za = height of pump above surface of suction reservoirResult and DiscussionThe centrifugal pump was operated at three different times. Each time, the pump was operated at four different speeds, 1100, 1300, 1500 and 1700 revolutions per minute. Speeds were operated at six different flow rates. On each time, the inlet and outlet pressures were
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