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Total head, N.P.S.H. and other calculations…8Velocity head difference between the outlet and inlet of the systemv 1 and v 2 are respectively the velocities of the fluid particles at the inlet of the systemand the outlet. The inlet of the system is at the position of the surface of the liquid in thesuction tank. The velocity (v 1 ) of the fluid particles at the surface is quite low and smallenough to be considered nil. The outlet of the system is at the position of the surface ofthe liquid in the discharge tank. The velocity (v 2 ) of the fluid particles at the surface isquite low and small enough to be considered nil.Notice that the discharge end of the pipe is submerged, the fluid particles will travel fromthe discharge pipe end to the liquid surface in the discharge tank. If the pipe were notsubmerged then the outlet of the system would be located at the discharge pipe endand the velocity v 2 would be the velocity at the end of the pipe.v 1 v 2 v 2 1 /2g v 2 2/2g v 2 2/2g – v 2 1 /2g (ft fluid)0 0 0 0 0Tank pressure head difference between the outlet and inlet of the systemIf the suction tank were pressurized with pressure p 1 , there would be a correspondingpressure head H 1 . Since the tank is not pressurized and is open to atmosphere then thepressure p 1 is zero and therefore H 1 is zero. The same applies to the discharge tank.H 1 H 2 H 2 – H 1 (ft fluid)0 0 0Calculation results (total head)Table 5 brings together all the previous calculations and the result is the total headrequired of the pump.Component Sign (ft fluid) ResultsPipe friction head loss + ∆H FP1-2 16.71Fittings friction head loss + ∆H FF1-2 12.12Equipment friction head loss + ∆H EQ1-2 28.86Check valve head loss + ∆H CHECK 1.65Total static head + z 2 – z 1 38Velocity head difference + v 2 2/2g – v 2 1 /2g 0Tank pressure head difference + H 1 – H 2 0Total head (ft fluid) = ∆H P 97.34Table 5. Summary of the calculation results of the total head.
Total head, N.P.S.H. and other calculations…9Brake horsepower(determine the pump efficiency from the pump performance curve prior to doing thiscalculation)The power absorbed by the pump is given by equation[7] (see reference 1):Ppump( hp)=SG×∆HP( ftfluid)×3960 ×ηq(US gal / min)pump[7]For example, we have selected an impeller size of 10.7 inches (see Figure 12). Thepump efficiency at 500 gpm is given by the pump’s characteristic curve and is 0.71. Thetotal head is 97.3 feet as calculated.P pump0.98 × 97.3 × 500( hp)== 16.93960 × 0.71SPECIFICGRAVITYSGTOTALHEAD∆H P (ft fluid)FLOWq(Usgal/min)PUMPEFFICIENCYη0.98 97.3 500 .713 16.9Table 6 Power to the pump shaft calculation results.BRAKEHORSEPOWERP(hp)
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Page 37: 18 The Moody diagram, friction fact

calc_example

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