Page 359 - 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 319

9.4 Methods and Devices for the Prevention of Backflow and Backsiphonage 319

Seal

Flow condition

Not less than 6˝

Nonflow condition
(a) (b)
Figure 9.20 Atmospheric Vacuum Breaker (a) Flow and Nonflow Conditions
(b) Typical Installation. Conversion factor: 1 1 in. 25.4 mm

they are installed in the plumbing system, but are, for the most part, dependable, trouble-
free devices for backsiphonage protection.
Figure 9.20b shows the generally accepted installation requirements. note that no
shutoff valve is downstream of the device that would otherwise keep the atmospheric vac-
uum breaker under constant pressure.

9.4.4 Hose Bib Vacuum Breakers

Hose bib vacuum breakers are small devices that are a specialized application of the
atmospheric vacuum breaker. They are generally attached to still cocks and in turn are
connected to hose-supplied outlets such as garden hoses, slop sink hoses, and spray outlets.
They consist of a spring-loaded check valve that seals against an atmospheric outlet when
water supply pressure is turned on. Typical construction is shown in Fig. 9.21.
When the water supply is turned off, the device vents to the atmosphere, thus protect-
ing against backsiphonage conditions. They should not be used as backpressure devices.
Manual drain options are available, together with tamper-proof versions.