Page 351 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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9.3 Theory of Backflow and Backsiphonage 311
It is evident that pressure varies with depth below a free water surface; in general, each foot
(0.3048 m) of elevation change, within a liquid, changes the pressure by an amount equal
to the weight-per-unit area of 1 ft (0.3048 m) of the liquid. The rate of increase for water is
0.433 psi/ft (9.84 kPa/m) of depth.
Frequently water pressure is referred to using the terms pressure head or just head,
and is expressed in units of feet of water. One foot (0.3048 m) of head would be equivalent
to the pressure produced at the base of a column of water 1 ft (0.3048 m) in depth. One foot
(0.3048 m) of head or 1 ft (0.3048 m) of water is equal to 0.433 psig (P gauge 3 kPa). One
hundred feet (30.48 m) of head is equal to 43.3 psig (P gauge 300 kPa).

9.3.2 Siphon Theory
Figure 9.12a depicts the atmospheric pressure on a water surface at sea level. An open tube is
inserted vertically into the water; atmospheric pressure, which is 14.7 psia (P absolute 102 kPa),
acts equally on the surface of the water within the tube and on the outside of the tube.
If, as shown in Fig. 9.12b, the tube is capped and a vacuum pump is used to evacuate
all the air from the sealed tube, a vacuum with a pressure of 0 psia (P absolute 0 kPa) is created

“Zero” absolute
pressure

0.0
psia
or Vacuum pump
14.7
psig

39.9´
9.7
psia or 5.0 psig

11.5´
14.7 14.7 14.7 psia or
14.7 psia psia psia 0.0 psig
Sea level Sea level

(a) (b)
Figure 9.12 (a) Pressure on the Free Surface of a Liquid at Sea Level; (b) Effect of
Evacuating Air from a Column. Conversion factors: 1 1 ft 0.3048 m; 1 psia 6.94 kPa
absolute pressure; 1 psig 6.94 kPa gauge pressure