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                    188  Chapter 5  Water Hydraulics, Transmission, and Appurtenances
                                             Steel and other flexible conduits are equipped with automatic air valves that will also
                                         admit air to the line and prevent its collapse under negative pressure (see Fig. 5.13).
                                         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 at summits, the downstream side of other gates, and changes in grade to steeper
                                         slopes in sections of line not otherwise protected by air valves.
                                             The required valve size is related to the size of the conduit, and to the velocities at
                                         which the line is emptied. The following ratios of air valve size to conduit diameter pro-
                                         vide common but rough estimates of needed sizes:
                                             For release of air only:           1:12 or 1 in./ft (83 mm/m)
                                             For admission as well as release of air:  1:8 or 1.5 in./ft (125 mm/m)
                                             An approximate calculation will show that under a vacuum of 48 in. (1,220 mm) of
                                         water, an automatic air valve, acting as an injection orifice with a coefficient of discharge
                                                                     3
                                         of 0.5 under a head of 4 (1.3   l0 )   3,080 ft (939 m) of air of specific gravity 1.3   l0,
                                                                                     3
                                                                                                              2
                                                                                                     3
                                                                                             2
                                         is expected to admit about 0.512g * 3,080 = 220 ft /s  of air/ft (67.2 m /s of air/m ) of
                                         valve. If the diameter ratio is 1:8, the displacement velocity in the conduit can be as high as
                                         220 64   3.5 ft/s (1 m/s) without exceeding a vacuum of 48 in. (1,220 mm) of water. A
                                         similar calculation will show the rate of release of air. The amounts of air that can be dis-
                                         solved by water at atmospheric pressure are about 2.9% by volume at 32 F (0 C) and 1.9%
                                         at 77 F (25 C), changing in direct proportion to the pressure. Accordingly, they are dou-
                                         bled at 2 atm or 14.7 psig (102 kPa gauge).
                    5.9.4  Check Valves
                                         Check valves are used to maintain flow in one direction only by closing when the flow be-
                                         gins to reverse. They are placed on force mains to prevent backflow when pumps shut
                                         down. When the flow is in the same direction as the specified direction of the check valve,
                                         the valve is considered to be fully open.

                    5.9.5  Pressure-Reducing Valves
                                         Pressure-reducing valves are used to keep pressures at safe levels in low-lying areas. These
                                         valves are often used to separate pressure zones in water distribution networks. These
                                         valves prevent the pressure downstream from exceeding a specified level, in order to avoid
                                         pressures and flows that could otherwise have undesirable effects on the system. A pres-
                                         sure or a hydraulic grade is used to control the operation of the valve.

                    5.9.6  Pressure-Sustaining Valves

                                         Pressure-sustaining valves maintain a specified pressure upstream of the valve. Similar
                                         to the other regulating valves, they are often used to ensure that pressures in the system
                                         (upstream, in this case) will not drop to unacceptable levels. A pressure or a hydraulic
                                         grade is used to control the operation of a pressure-sustaining valve.

                    5.9.7  Pressure Breaker Valves
                                         Pressure breaker valves create a specified head loss across the valve and are often used to
                                         model components that cannot be easily modeled using standard minor loss elements.

                    5.9.8  Flow Control Valves
                                         A flow control valve limits the flow rate through the valve to a specified value in a speci-
                                         fied direction. A flow rate is used to control the operation of a flow control valve. These
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