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Design of Flow Systems 449

Cavitation will not occur as long as the pressure at the suction side of the
pump is sufficiently high. The suction pressure required to avoid cavitation de-
pends on the pump design and is specified by the pump manufacturer. The term
manufacturers use to describe the pressure required is NPSH (net-positive-suction-
head). NPSH is defined as the difference between the pressure head and the head
corresponding to the liquid vapor pressure at the pump inlet, i.e.,
(8.36)

The required NPSH, (NPSH)a, is specified by the pump manufacturer, and
the available NPSH, (NPSH)A, is determined by the design of the pump suction
piping. To prevent cavitation the available NPSH must be equal to or greater than
the required NPSH.

(NPSH) A>(NPSH) R (8.37)

The vapor-pressure head of the liquid in Equation 8.36 is calculated by con-
verting the vapor pressure into head in feet. The pressure head at the pump inlet is
calculated by applying Bernoulli's Equation between the surface of the liquid at

Table 8.5 Friction-Loss Factors for Flow Meters

a
Meter Type Friction-Loss Factor , K

Disk 7.0
Piston 15.0
Rotary (star-shaped Disk) 10.0
Turbine 6.0

Pressure Drop

Orifice 50inH 2O
Flow Totalizer 4.0 psi
Rotameter 3.0 psi
Flow Tube psi

a) Source: Reference 1
b) Source: Reference 25

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