Question 14.9: Using Pump Specific Speed for Preliminary Pump Design A pump...
Using Pump Specific Speed for Preliminary Pump Design
A pump is being designed to deliver 320 gpm of gasoline at room temperature. The required net head is 23.5 ft (of gasoline). It has already been determined that the pump shaft is to rotate at 1170 rpm. Calculate the pump specific speed in both nondimensional form and customary U.S. form. Based on your result, decide which kind of dynamic pump would be most suitable for this application.
We are to calculate pump specific speed and then determine whether a centrifugal, mixed-flow, or axial pump would be the best choice for this particular application.
Assumptions The pump operates near its best efficiency point. 2 The maximum efficiency versus pump specific speed curve follows Fig. 14–73 reasonably well.
Analysis First, we calculate pump specific speed in customary U.S. units,
N_{ Sp , US }=\frac{(1170 rpm )(320 gpm )^{1 / 2}}{(23.5 ft )^{3 / 4}}= 1 9 6 0 (1)
We convert to normalized pump specific speed using the conversion factor given in Fig. 14–72,
N_{ Sp }=N_{ Sp , US }\left(\frac{N_{ Sp }}{N_{ Sp , US }}\right)=1960\left(3.658 \times 10^{-4}\right)= 0 . 7 1 7 (2)
Using either Eq. 1 or 2, Fig. 14–73 shows that a centrifugal flow pump is the most suitable choice.
Discussion Notice that the properties of the fluid never entered our calculations. The fact that we are pumping gasoline rather than some other liquid like water is irrelevant. However, the brake horsepower required to run the pump does depend on the fluid density.
