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The Investigation of a Centrifugal Pump

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Date: October 8, 2012SummaryIn this experiment the investigation of a centrifugal pump was done to determine theeffect of flow rate and speed on the head, efficiency and brake horsepower requirement of acentrifugal pump. Three separate experiments were done and data were measure at differentspeeds with different flow rates. Further calculations were made by utilizing numerousequations in order to obtain values of pump head (hp), shaft work (BHP), net positive sectionhead (NPSH) and the efficiency. It is observed that the brake horsepower at different pumpspeeds decreases as flow rate increases. Moreover, it is concluded that the efficiency increaseswith 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 rateincreases. 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 flowrate.Background and MethodsPump System DescriptionThe main objective of this experiment was to determine the impact of flow rate andspeed on the head, efficiency and brake horsepower requirement of a centrifugal pump. Thecentrifugal pump as seen in figure 11was used for this experiment. The pump receives waterfrom the tank, labeled as 9 in figure 1, which is connected to the suction line going into thepump.The discharge line consists of a valve to allow for user variance of the volumetric flowrate. Then, the water is returned to the tank. In order to measure the suction and dischargepressures, two gauges connected to respective lines are used.Additionally, Torque reading isobtained from the torque gauge that is fitted on the pump.Pump EquipmentThe 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.5-Rheostat for pump speed control.6- Stroboscope to set pump rpm.7-Pump Impeller8-Pump Motor.Measurement ProcedureThecentrifugal 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 figure1, discharge pressure from gauge 1B and the torque from 3 were measured. Thosemeasurements 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 wasoperated in reverse rotation at different speeds, and the flow rate with discharge valve wideopen was observed.Centrifugal Pump CalculationTo calculate head of the pump (hp), McCabe, Smith, and Harriott, 5thedition2gives Equation.1 tobe applied.hp=PbPaρ(1)Where:pa= pump suction pressure [lbf/ft2]pb= pump discharge pressure [lbf/ft2]= Density of fluid [lbm/ft3]Figure11. The pump system that has been used for theexperiment.
According to McCabe, Smith, and Harriott, 5thedition2, the shaft work or horsepower for thepump can be calculated from the shaft torque and rpm readings in Equation.2:BHP=2πτn(2)Where:τ= shaft torque [lbf-ft]n = shaft speed [min-1]BHP= shaft work [lbf-ft/min]Based on McCabe, Smith, and Harriott, 5thedition2, 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, 5thedition2gave Equation.4, where NPSH can be calculated infeet:NPSHa=gcg(pa'pvρhfs)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 wasoperated at four different speeds, 1100, 1300, 1500 and 1700 revolutions per minute. Speedswere operated at six different flow rates. On each time, the inlet and outlet pressures were
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