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448 Chapter 8

Cavitation

If the pressure in a flowing liquid falls below its vapor pressure, the liquid will
vaporize. If vapor bubbles form on the suction side of the pump, the bubbles will
move with the stream and will subsequently collapse in a region of high pressure.
This phenomenon is called cavitation. Dissolved gases in the fluid, such as air in
water, could also form bubbles. The collapsing vapor or gas bubbles subject the
pump surfaces to tremendous shock. The energy involved in the shock is explo-
sive enough to flake off small bits of metal and in time the pump will become pit-
ted. Cavitation also results in a loss of energy. Immediate clues of cavitation are
reduced flow rate, loss of head, pumping in spurts, and excessive noise and vibra-
tion.

Table 8.4 Pipe- Roughness Factors

Pipe Material Roughness Factor, E
IxlO^ft

Riveted Steel 3 3,000 - 30,000
Concrete 3 1,000-10,000
Wood Stave" 600 - 30,000
Cast Iron" 850
Galvanized Iron 3 500
Asphalted Cast Iron" 400
Steel or Wrought Iron" 150
Tubing" 5
Hard Plastic b 0.17-0.83
Glass b 0.17-0.83
Electropolished Stainless b 0.17-0.83
Mechanically-polished Stainless b 0.33-1.3
New-unpolished Stainless b 1.3-8.3
New Copper or Brass b 1.3-8.3
Rubber b 2.7-10
Seamless Carbon Steel b 10-42
Corrugated Steel b >170
Tuberculated Iron Pipe b 42-170

a) Source: Reference 1
b) Source: Reference 24

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