NPSH Requirements for Other Liquids

NPSH Requirements for Other Liquids NPSH values depend on the fluid being pumped. Since water is considered a standard fluid... 

NPSH Requirements for Other Liquids NPSH values depend on the fluid being pumped. Since water is considered a standard fluid for pumping, various correction methods have been developed to evaluate NPSH when pumping other fluids. The most recent of these corrective methods has been developed by the Hydraulic Institute and is shown in Fig. 10-25. 

NPSH is the pressure available at the pump suction nozzle after vapor pressure is subtracted. It is expressed in terms of liquid head. It thus reflects the amount of head loss that the pump can sustain internally before the vapor pressure is reached. The manufacturer will specify the NPSH that his pump requires for the operating range of flows when handling water. This same NPSH is normally used for other liquids. 

The chart shown in Fig. 10-25 is for pure liquids. Extrapolation of data beyond the ranges indicated in the graph may not produce accurate results. Figure 10-25 shows the variation of vapor pressure and NPSH reductions for various hydrocarbons and hot water as a function of temperature. Certain rules apply while using this chart. When using the chart for hot water, if the NPSH reduction is greater than one-half of the NPSH required for cold water, deduct one-half of cold water NPSH to obtain the corrected NPSH required. On the other... 

Figure 3-36A represents a typical manufacturer s performance curve. The values of NPSHr given are the minimum values required at the pump suction. As mentioned, good practice requires that the NPSH available be at least two feet of liquid above these values. It is important to recognize that the NPSHr and Suction Lift Values are for handling water at about 70°F. To use with other liquids it is necessary to convert to the equivalent water suction lift at 70°F and sea level. 

In order to make the correlation described in this paper, it is necessary to perform the calculation at a point where the curves become vertical, in other words, the point where the pump is fully cavitating. For this unit the difference in NPSH is 0,6 foot, Calculations using only the water data show that the unit should perform satisfacto rily with liquid oxygen requiring a NPSH which is 0,6 foot lower than that needed for water. It is noteworthy that the corresponding difference in Suction Specific Speed values is a factor of 2,... 

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