Page 352 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch09_297-332.qxd 8/2/10 9:54 PM Page 312

312 Chapter 9 Cross-Connection Control
within the tube. Because the pressure at any point in a static fluid is dependent on the
height of that point above a reference line, such as sea level, it follows that the pressure
within the tube at sea level must still be 14.7 psia (P absolute 102 kPa). This is equivalent
to the pressure at the base of a column of water 33.9 ft (10.3 m) high. With the column
open at the base, water would rise to fill the column to a depth of 33.9 ft (10.3 m). In other
words, the weight of the atmosphere at sea level exactly balances the weight of a column of
water 33.9 ft (10.3 m) in height. The absolute pressure within the column of water in
Fig. 9.12b at a height of 11.5 ft (3.5 m) is equal to 9.7 psia (P absolute 67.3 kPa). This is a
partial vacuum with an equivalent gauge pressure of 5.0 psig (P gauge 34.7 kPa)
As a practical example, assume the water pressure at a closed faucet on the top of a
100-ft–(30.48-m) high building to be 20 psig (P gauge 138.8 kPa)); the pressure on the
ground floor would then be 63.3 psig (P gauge 439.3 kPa). If the pressure at the ground were
to drop suddenly to 33.3 psig (P gauge 231.1 kPa) due to a heavy fire demand in the area, the
pressure at the top would be reduced to 10 psig (P gauge 69.4 kPa). If the building water
system were airtight, the water would remain at the level of the faucet because of the partial
vacuum created by the drop in pressure. If the faucet were opened, however, the vacuum
would be broken and the water level would drop to a height of 77 ft (23.47 m) above the
ground. Thus, the atmosphere was supporting a column of water 23 ft (7 m) high.
Figure 9.13a is a diagram of an inverted U-tube that has been filled with water and placed
in two open containers at sea level. If the open containers are placed so that the liquid levels in
each container are at the same height, a static state will exist; and the pressure at any specified
level in either leg of the U-tube will be the same. The equilibrium condition is altered by rais-
ing one of the containers so that the liquid level in one container is 5 ft (1.52 m) above the level
of the other (see Fig.9.13b). Because both containers are open to the atmosphere, the pressure
on the liquid surfaces in each container will remain at 14.7 psia (P absolute 102 kPa).
If it is assumed that a static state exists, momentarily, within the system shown in
Fig. 9.13b, the pressure in the left tube at any height above the free surface in the left container
4.7 psia

8.2 psia 10.3 psia

23´
10.3 psia
10´
15´
10´
14.7
5´ psia

14.7 14.7 14.7
psia psia psia

(a) (b)
Figure 9.13 Pressure Relationships in a Continuous Fluid System at the
Same Elevation; (b) at Different Elevations. Conversion factors: 1
1 ft 0.3048 m; 1 psia 6.94 kPa absolute pressure