Chapter Four Air Gap (2) The air gap may be easily Methods and Devices defeated in the event that the Air gaps are non-mechanical “2D” requirement was purposely backflow preventers that are or inadvertently compromised. for the Prevention of very effective devices to be used Excessive splash may be encoun- where either backsiphonage or tered in the event that higher Backflow and backpressure conditions may than anticipated pressures or exist. Their use is as old as flows occur. The splash may be a Back-Siphonage piping and plumbing itself, but cosmetic or true potential only relatively recently have hazard—the simple solution standards been issued that being to reduce the “2D” wide choice of devices standardize their design. In dimension by thrusting the Aexists that can be used to general, the air gap must be supply pipe into the receiving prevent backsiphonage and twice the supply pipe diameter funnel. By so doing, the air gap backpressure from adding but never less than one inch. is defeated. contaminated fluids or gases See Figure 12. into a potable water supply (3) At an air gap, we expose the system. Generally, the selection water to the surrounding air FIGURE 12. with its inherent bacteria, dust of the proper device to use is Air gap. based upon the degree of hazard particles, and other airborne posed by the cross-connection. pollutants or contaminants. In addition, the aspiration effect of Additional considerations are Diameter based upon piping size, location, “D” the flowing water can drag down and the potential need to surrounding pollutants into the periodically test the devices to “2D” reservoir or holding tank. insure proper operation. (4) Free chlorine can come out of There are six basic types of treated water as a result of the air devices that can be used to gap and the resulting splash and correct cross-connections: air churning effect as the water gaps, barometric loops, vacuum enters the holding tanks. This breakers—both atmospheric reduces the ability of the water and pressure type, double check to withstand bacteria contamina- with intermediate atmospheric tion during long term storage. An air gap, although an vent, double check valve (5) For the above reasons, air extremely effective backflow assemblies, and reduced pressure gaps must be inspected as preventer when used to prevent principle devices. In general, all frequently as mechanical backsiphonage and backpres- manufacturers of these devices, backflow preventers. They are sure conditions, does interrupt with the exception of the not exempt from an in-depth the piping flow with corre- barometric loop, produce them cross-connection control pro- sponding loss of pressure for to one or more of three basic gram requiring periodic inspec- subsequent use. Consequently, standards, thus insuring the tion of all backflow devices. public that dependable devices air gaps are primarily used at end of the line service where Air gaps may be fabricated are being utilized and marketed. from commercially available The major standards in the reservoirs or storage tanks are desired. When contemplating plumbing components or industry are: American Society purchased as separate units and of Sanitary Engineers ASSE), the use of an air gap, some other considerations are: integrated into plumbing and American Water Works Associa- piping systems. An example of (1) In a continuous piping tion (AWWA), and the Univer- the use of an air gap is shown in system, each air gap requires sity of California Foundation for Figure 13. Cross-Connection Control and the added expense of reservoirs Hydraulic Research. and secondary pumping systems. 16 • CROSS-CONNECTION CONTROL MANUAL FIGURE 13. Atmospheric Vacuum FIGURE 15. FIGURE 16. Air gap in a piping system. Atmospheric vacuum breaker. Atmospheric vacuum breaker Breaker typical installation. These devices are among the Supply piping simplest and least expensive mechanical types of backflow Seal preventers and, when installed properly, can provide excellent protection against back- Tank or reservoir siphonage. They must not be utilized to protect against Not less than 6" Barometric Loop backpressure conditions. Construction consists usually of The barometric loop consists of a polyethylene float which is a continuous section of supply free to travel on a shaft and seal Flow condition piping that abruptly rises to a in the uppermost position height of approximately 35 feet against atmosphere with an and then returns back down to elastomeric disc. Water flow the originating level. It is a loop lifts the float, which then causes in the piping system that the disc to seal. Water pressure effectively protects against keeps the float in the upward backsiphonage. It may not be sealed position. Termination of used to protect against back- the water supply will cause the pressure. disc to drop down venting the FIGURE 17. Atmospheric vacuum breaker in Its operation, in the unit to atmosphere and thereby plumbing supply system. protection against back- opening downstream piping to siphonage, is based upon the atmospheric pressure, thus principle that a water column, preventing backsiphonage. Non flow condition at sea level pressure, will not Figure 15 shows a typical rise above 33.9 feet (Ref. atmospheric breaker. Chapter 3, Fig. 4 Page 13). In general, these devices Figure 16 shows the In general, barometric are available in ½-inch through generally accepted installation loops are locally fabricated, and 3-inch size and must be requirements—note that no are 35 feet high. installed vertically, must not shutoff valve is downstream have shutoffs downstream, of the device that would and must be installed at least FIGURE 14. otherwise keep the atmospheric Barometric loop. 6-inches higher than the final vacuum breaker under constant outlet. They cannot be tested pressure. once they are installed in the Figure 17 shows a typical plumbing system, but are, for installation of an atmospheric the most part, dependable, vacuum breaker in a plumbing trouble-free devices for supply system. backsiphonage protection. ' 35 CHAPTER FOUR • 17 Hose Bibb FIGURE 19. Pressure industrial applications are Typical installation of hose bibb Vacuum Breakers vacuum breaker. Vacuum Breakers shown in Figure 21. Again, these devices may These small devices are a This device is an outgrowth of be used under constant pressure specialized application of the the atmospheric vacuum but do not protect against atmospheric vacuum breaker. breaker and evolved in response backpressure conditions. As a They are generally attached to to a need to have an atmospher- result, installation must be at sill cocks and in turn are ic vacuum breaker that could be least 6- to 12-inches higher connected to hose supplied utilized under constant pressure than the existing outlet. outlets such as garden hoses, and that could be tested in line. A spill resistant pressure slop sink hoses, spray outlets, A spring on top of the disc and Hose bibb vacuum breaker vacuum breaker (SVB) is etc. They consist of a spring float assembly, two added gate available that is a modification loaded check valve that seals valves, test cocks, and an to the standard pressure against an atmospheric outlet additional first check, provided vacuum breaker but specifically when water supply pressure is the answer to achieve this designed to minimize water turned on. Typical construction device. See Figure 20. spillage. Installation and is shown in Figure 18. These units are available in hydraulic requirements are When the water supply is the general configurations as similar to the standard pressure turned off, the device vents to shown in Figure 20 in sizes vacuum breaker and the atmosphere, thus protecting ½-inch through 10-inch and devices are recommended for against backsiphonage condi- have broad usage in the internal use. tions. They should not be used agriculture and irrigation as backpressure devices. Manual market. Typical agricultural and drain options are available, together with tamper-proof versions. A typical installation is FIGURE 20. shown in Figure 19. Pressure vacuum breaker Spring FIGURE 18. Hose bibb vacuum breaker. Test cock First check valve Gate Valve Test cock Gate Valve ¾ inch thru 2 inches 2½ inches thru 10 inches 18 • CROSS-CONNECTION CONTROL MANUAL Double Check with FIGURE 22. Double Check Valve Double check valve with Intermediate atmospheric vent. A double check valve is Atmospheric Vent 1st check 2nd check essentially two single check The need to provide a compact valves coupled within one body device in ½-inch and ¾-inch and furnished with test cocks pipe sizes that protects against and two tightly closing gate moderate hazards, is capable of valves (See Figure 24). being used under constant The test capability feature pressure and that protects gives this device a big advan- against backpressure, resulted tage over the use of two in this unique backflow independent check valves in preventer. Construction is Vent that it can be readily tested to basically a double check valve determine if either or both having an atmospheric vent check valves are inoperative or fouled by debris. Each check located between the two checks FIGURE 23. (See Figure 22). Typical residential use of double is spring loaded closed and Line pressure keeps the check with atmospheric vent. requires approximately a pound vent closed, but zero supply of pressure to open. pressure or backsiphonage will Automatic feed valve This spring loading Supply open the inner chamber to provides the ability to “bite” atmosphere. With this device, through small debris and still extra protection is obtained seal—a protection feature not through the atmospheric vent prevalent in unloaded swing capability. Figure 23 shows a check valves. Figure 24 shows a typical use of the device on a cross section of double check residential boiler supply line. Drain Boiler valve complete with test cocks. Double checks are commonly Return Air gap used to protect against low to FIGURE 21.