A certain pump (call it pump A) that has the below head, efficiency and NPSH cha

Question

A certain pump (call it pump A) that has the below head, efficiency and NPSH characteristics at the operating speed of 1200 rpm, will be used to deliver water from a closed lower reservoir to a closed upper reservoir 30 m above the lower reservoir. The pump will be placed 0.5 m above the suction reservoir water level and the head loss in the suction piping can be taken as 2 m. The diameter of all pipes in the system is 20 cm. The loss coefficient for all major and minor losses in the system can be taken as 200. A geometrically similar, 2/3rd scale model of this pump (call it pump B) is tested in the laboratory at a speed of 1500 rpm. The model pump operates at a flow rate of 0.01 m 3 /s ? at the similar operating point of pump A when that pump will operate in the pipe system. Reynolds number effects can be neglected.

a) Determine the operating head and the power consumption of the model pump.

b) Determine the pressure difference between the two reservoirs and state which reservoir has the higher pressure.

c) Cut and paste the given pump characteristics graph in your assignment. Then, on the same graph, plot by hand (approximately) the characteristics of the pipe system in which pump A will be installed. Make sure your plot is clear with the necessary values (necessary for understanding your plot) marked on the graph. Explain/show how you plot the curve.

d) Determine the minimum pressure in the lower reservoir so that the pump (pump A) does not cavitate. The vapor pressure of water at the working temperature is 3.9 kPa.

Explanation / Answer

a)    operating head = 3.14 * 0.2 *0.2/30

                                = 0.4186 cm

        power consumption   = 1000 * 30/10

                                           = 3000 W

b)    pressure difference between the two reservoirs   = 1000 * 0.2 * 9.8

                                                                                     = 1960 Pa

    B reservoir has the higher pressure .

c)   This pipe system is to be treated as a one-dimensional flow, liquid flowing in a pipe has kinetic energy that can be evaluated as c2/2g .   A graph of energy flow through a pipe of a given diameter in order to put a magnitude to this potential loss.

d)    the minimum pressure in the lower reservoir = 3.9 +   1000 * 0.2 * 9.8/1000

                                                                                 = 5.86 kPa

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