Calculating NPSHA

In the following two examples, we demonstrate how to calculate NPSHA for two real-world situations: (1) determining NPSHA for an open-top water tank or a municipal water storage tank with a roof and correctly sized vent, and (2) determining the NPSHA for a suction lift from an open reservoir.

10.2.12.1.1 NPSHA: Atmospheric Tank

The following calculation may be used for an open-top water tank or a municipal water storage tank with a roof and correctly sized vent, as shown in Figure 10.4 and Figure 10.5. The formula for calculating NPSHA is:

where:

Pa = atmospheric pressure in absolute or pressure of gases against the surface of the water.

h = weight of the liquid column from the surface of the water to the center of the pump suction nozzle in feet absolute.

pv = vapor pressure in absolute of water at a given temperature.

he = entrance losses in feet absolute.

hf = friction losses in the suction line in feet absolute.

Atmospheric pressure 1

Figure 10.4 Open atmospheric tank. ■ Example 10.10

Problem: Given the following, find the NPSHA:

Liquid = water Temperature (t) = 60°F Specific gravity = 1.0 Pa = 14.7 psia (34 ft) h = 15 ft

Figure 10.5 Roofed water-storage tank.

Solution:

NPSHA = 34 ft + 15 ft - 0.6 ft - 0.4 ft - 2 ft = 46 ft

10.2.12.1.2 NPSHA: Suction Lift from Open Reservoir ■ Example 10.11

Problem: Find the NPSHA in Figure 10.6, where:

Liquid = water Temperature (t) = 60°F Specific gravity = 1.0 Pa = 14.7 psia (34 ft) h = -20 ft

Pv = 0.256 psia (0.6 ft) Q = 120 gpm he = 0.4 ft hf = 2 ft

Solution:

NPSHA = 34 ft + (-20 ft) - 0.6 ft - 0.4 ft - 2 ft = 11 ft

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