NPSH Calculation

Example 1 NPSH Calculation Suppose a selected pump requires a minimum NPSH of 16 ft (4.9 m) when pumping cold water What will he the NPSH limitation to pump propane at 55 F (12.8 C) with a vapor pressure of 100 psi Using the chart in Fig. 10-25, NPSH reduction for propane gives 9.5 ft (2.9 m). This is greater than one-half of cold water NPSH of 16 ft (4.9 m). The corrected NPSH is therefore 8 ft (2.2 m) or one-half of cold water NPSH. 

Typical NPSH calculations keep the pump s lowest pressure below the liquid s vapor pressure as illustrated by the following three examples ... 

Figure 2. NPSH calculation for liquid at boiling point.

NPSH calculations might have to be modified if there are significant amounts of dissolved gas in the pump suction liquid. See Suction System NPSH Available" in this handbook for calculations when dissolved gas does not need to be considered. In that case the suction liquid s vapor pressure is a term in the equation. With dissolved gases, the gas saturation pressure is often much higher than the liquid s vapor pressure. 

Total Suction Lift (as water at 70°F) = NPSH (calculated for fluid system) — 33 feet. The vapor pressure of water at 70°F is 0.36 psia. 

Net positive suction, 188-192 NPSH calculations, 190-194 Performance, 197 Pressure, 183... 

Vapor pressure Is used in NPSH calculations when actually saturation pressure might be more appropriate. A better definition of "vapor pressure" is needed. Be wary of dissolved gases. 

Example 18.8 illustrates how to do NPSH calculations and one of the methods listed above for increasing NPSHj . The other methods are illustrated in a problem at the end of the chapter. 

LAL (low alarm level) = 5 ft from ground and pump center line is 2 ft from ground NOL (normal operating level) = 10 ft above ground level NPSH Calculations (at LAL)... 

---