Page 184 - Water Engineering Hydraulics, Distribution and Treatment

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Chapter 5
Water Hydraulics, Transmission, and Appurtenances
(a) air displaced while the line is being filled and (b) air
L c
Air inlet and outlet
released from the flowing water when the pressure decreases
appreciably or summits lie close to the HGL. A manually
operated cock or gate can be substituted if the pressure at the
Float
summit is high. Little, if any, air will then accumulate, and
valve
air needs to escape only while the line is being filled.
Water line
Steel and other flexible conduits are equipped with auto-
matic air valves that will also admit air to the line and prevent
its collapse under negative pressure (see Fig. 5.26). Pressure
differences are generated when a line is being drained on
purpose or when water escapes accidentally through a break
at a low point. Locations of choice are both sides of gates
Attach to pipe at summits, the downstream side of other gates, and changes
in grade to steeper slopes in sections of line not otherwise
Figure 5.26 Air inlet and release valve.
protected by air valves.
The required valve size is related to the size of the con-
duit, and to the velocities at which the line is emptied. The
reducers and increasers. Gates 8 in. (200 mm) in diameter
following ratios of air valve size to conduit diameter provide
or larger commonly include a 4 or 6 in. (100 or 150 mm)
common but rough estimates of needed sizes:
gated bypass. When the larger gate is seated under pressure,
water admitted through the bypass can equalize the pressure
For release of air only: 1:12 or 1 in./ft (83 mm/m)
on both sides and make it easier to lift the main gate.
For admission as well as release of air: 1:8 or 1.5 in./ft
Gravity conduits are commonly provided with gate
(125 mm/m).
chambers (a) at points strategic for the operation of the sup-
ply conduit, (b) at the two ends of sag pipes and pressure
An approximate calculation will show that under a
tunnels, and (c) wherever it is convenient to drain given sec-
vacuum of 48 in. (1,220 mm) of water, an automatic
tions. Sluice gates are normally installed in grade conduits,
air valve, acting as an injection orifice with a coefficient
particularly in large ones. In special situations, needle valves ( −3 )
of discharge of 0.5 under a head of 4∕ 1.3 × 10 =
are preferred for fine control of flow, butterfly valves for ease
3,080 ft (939 m) of air of specific gravity 1.3 × 10, is
of operation, and cone valves for regulating time of closure √ 3
expected to admit about 0.5 2g × 3,080 = 220 ft ∕sof
and controlling water hammer. 2 ( 3 2 )
air/ft 67.2m ∕sof air∕m of valve. If the diameter ratio
is 1:8, the displacement velocity in the conduit can be as high
5.9.2 Blowoffs as 220∕64 = 3.5ft∕s(1 m∕s) without exceeding a vacuum
of 48 in. (1,220 mm) of water. A similar calculation will
In pressure conduits, small, gated takeoffs, known as blowoff show the rate of release of air. The amounts of air that can be
or scour valves, are provided at low points in the line. They dissolved by water at atmospheric pressure are about 2.9%
discharge into natural drainage channels or empty into a by volume at 32 F(0 C) and 1.9% at 77 F(25 C), changing
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sump from which the water can be pumped to waste. There in direct proportion to the pressure. Accordingly, they are
should be no direct connection to sewers or polluted water doubled at 2 atm or 14.7 psig (102 kPa gauge).
courses. For safety, two blowoff valves are placed in series.
The chance of both failing to close is thus reduced greatly.
Their size depends on local circumstances, especially on the 5.9.4 Check Valves
time in which a given section of line is to be emptied and Check valves are used to maintain flow in one direction only
on the resulting velocities of flow. Calculations are based by closing when the flow begins to reverse. They are placed
on orifice discharge under a falling head, equal to the dif- on force mains to prevent backflow when pumps shut down.
ference in elevation of the water surface in the conduit and When the flow is in the same direction as the specified direc-
the blowoff, minus the friction head. Frequency of operation tion of the check valve, the valve is considered to be fully
depends on the quality of the water carried, especially on silt open.
loads. The drainage gates of gravity conduits are placed in
gate chambers.
5.9.5 Pressure-Reducing Valves
Pressure-reducing valves are used to keep pressures at safe
5.9.3 Air Valves
levels in low-lying areas. These valves are often used to
Rigid pipes and pressure conduits are equipped with air separate pressure zones in water distribution networks. These
valves at all high points. The valves automatically remove valves prevent the pressure downstream from exceeding a