R PiY o

+ 0.086 - 0 + 20 + 30 - 50.086 m o - 1,200(2n)/60 - 125.67 radians/s Ad - cross-section of discharge pipe = - "-V0.225 - - 0.040 m nD_2 - n(0.2252- - ...2 4 4

This falls outside the range of specific speeds in Figure 4.9; however, the pump should not be an axial flow pump as recommended by the designer. Ans From the figure, for a Q of 0.052 m /s and an Ns of 0.27, the pump would have to be of radial-vane type. Ans

4.6 net positive suction head and deep-well pumps

In order for a fluid to enter the pump, it must have sufficient energy to force itself toward the inlet. This means that a positive head (not negative head) must exist at the pump inlet. This head that must exist for pumping to be possible is termed the net positive suction head (NPSH). It is an absolute, not gage, positive head acting on the fluid.

Refer to the portion of Figure 4.6 where the source tank fluid level is below the center line of the impeller in either the system connected in parallel or in the system connected in series. At the surface of the wet well (point 1), the pressure acting on the liquid is equal to the atmospheric pressure Patm minus the vapor pressure of the liquid Pv. This pressure is, thus, the atmospheric pressure corrected for the vapor pressure and is the pressure pushing on the liquid surface. Imagine the suction pipe devoid of liquid; if this is the case, then this pressure will push the fluid up the suction pipe. This is actually what happens as soon as the impeller starts moving and pulling the liquid up. As soon as a space is evacuated by the impeller in the suction pipe, liquid rushes up to fill the void; this is not possible, however, without a positive NPSH to push the liquid. Note that before the impeller can do its job, the fluid must, first, reach it. Thus, the need for a driving force at the inlet side.

The pushing pressure converts to available head or available energy at the suction side of the pump system. The surface of the well is below the pump, so this available energy must be subtracted by h(. The other substractions are the friction losses hfs.

For the source tank fluid level above the center line of the pump impeller, hs will be added increasing the available energy. The losses will, again, be subtracted. In symbols,

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