Theory of Backflow and Backsiphonage

Chapter Three

Pabsolute = Pgage + 14.7psi Water Pressure Theory of Backflow or For an understanding of the Pgage = Pabsolute – 14.7 psi nature of pressure and its and Backsiphonage In essence then, absolute relationship to water depth, pressure is the total pressure. consider the pressure exerted on Gage pressure is simply the the base of a cubic foot of water pressure read on a gage. If there at sea level. (See Fig. 1) The is no pressure on the gage other average weight of a cubic foot than atmospheric, the gage cross-connection1 is the of water is 62.4 pounds per would read zero. Then the link or channel connecting square foot gage. The base may A absolute pressure would be a source of pollution with a be subdivided into 144-square equal to 14.7 psi which is the potable water supply. The inches with each subdivision atmospheric pressure. polluting substance, in most being subjected to a pressure of The term vacuum indicates cases a liquid, tends to enter the 0.433 psig. that the absolute pressure is less potable supply if the net force Suppose another cubic foot than the atmospheric pressure acting upon the liquid acts in of water were placed directly and that the gage pressure is the direction of the potable on top of the first (See Fig. 2). negative. A complete or total supply. Two factors are therefore The pressure on the top surface vacuum would mean a pressure essential for backflow. First, of the first cube which was of 0 psia or -14.7 psig. Since it there must be a link between originally atmospheric, or is impossible to produce a total the two systems. Second, the 0 psig, would now be 0.433 vacuum, the term vacuum, as resultant force must be toward psig as a result of the super- used in the text, will mean all the potable supply. imposed cubic foot of water. degrees of partial vacuum. In a An understanding of the The pressure of the base of partial vacuum, the pressure principles of backflow and the first cube would also be would range from slightly less backsiphonage requires an increased by the same amount than 14.7 psia (0 psig) to understanding of the terms of 0.866 psig, or two times the slightly greater than 0 psia frequently used in their original pressure. (-14.7 psig). discussion. Force, unless com- Backsiphonage1 results in pletely resisted, will produce fluid flow in an undesirable or motion. Weight is a type of reverse direction. It is caused by FIGURE 1. force resulting from the earth’s atmospheric pressure exerted on Pressure exerted by 1 foot of gravitational attraction. water at sea level. a pollutant liquid forcing it Pressure (P) is a force-per-unit toward a potable water supply area, such as pounds per square system that is under a vacuum. inch (psi). Atmospheric pressure is 62.4#/ft3 Backflow, although literally the pressure exerted by the 12" meaning any type of reversed 12" weight of the atmosphere above flow, refers to the flow produced 12" the earth. by the differential pressure Pressure may be referred to existing between two systems using an absolute scale, pounds both of which are at pressures per square inch absolute (psia), greater than atmospheric. Sea level or gage scale, pounds per square inch gage (psig). 0.433 psig Absolute pressure and gage pressure are related. Absolute pressure is equal to the gage pressure plus the atmospheric pressure. At sea level the atmospheric pressure is 14.7 psia. Thus, 1See formal definition in the glossary of the appendix

12 • CROSS-CONNECTION CONTROL MANUAL If this process were Siphon Theory level exactly balances the because of the partial vacuum repeated with a third cubic foot weight of a column of water created by the drop in pressure. of water, the pressures at the Figure 3 depicts the atmo- 33.9 feet in height. The If the faucet were opened, base of each cube would be spheric pressure on a water absolute pressure within the however, the vacuum would be 1,299 psig, 0.866 psig, and surface at sea level. An open column of water in Figure 4 at broken and the water level 0.433 psig, respectively. It is tube is inserted vertically into a height of 11.5 feet is equal to would drop to a height of 77 evident that pressure varies the water; atmospheric pres- 9.7 psia. This is a partial feet above the ground. Thus, with depth below a free water sure, which is 14.7 psia, acts vacuum with an equivalent the atmosphere was supporting surface; in general each foot of equally on the surface of the gage pressure of -5.0 psig. a column of water 23 feet high. elevation change, within a water within the tube and on As a practical example, Figure 5 is a diagram of an liquid, changes the pressure by the outside of the tube. assume the water pressure at a inverted U-tube that has been an amount equal to the weight- closed faucet on the top of a filled with water and placed in per-unit area of 1 foot of the FIGURE 3. 100-foot high building to be 20 two open containers at sea level. Pressure on the free surface of a liquid. The rate of increase for liquid at sea level. psig; the pressure on the If the open containers are water is 0.433 psi per foot of ground floor would then be placed so that the liquid levels depth. 63.3 psig. If the pressure at the in each container are at the Frequently water pressure ground were to drop suddenly same height, a static state will is referred to using the terms due to a heavy fire demand in exist; and the pressure at any “pressure head” or just “head,” the area to 33.3 psig, the specified level in either leg of and is expressed in units of feet pressure at the top would be the U-tube will be the same. of water. One foot of head reduced to -10 psig. If the The equilibrium condition would be equivalent to the building water system were is altered by raising one of the pressure produced at the base airtight, the water would containers so that the liquid of a column of water 1 foot in remain at the level of the faucet level in one container is 5 feet depth. One foot of head or 14.7 14.7 psia psia 1 foot of water is equal to 0.433 FIGURE 5. FIGURE 4. sea level Pressure relationships in a psig. One hundred feet of head Effect of evacuating air from a continuous fluid system at the column. is equal to 43.3 psig. same elevation.

FIGURE 2. “Zero” Absolute 4.7 psia Pressure exerted by 2 feet of Pressure water at sea level. If, as shown in Figure 4, the tube is slightly capped and a vacuum pump is used to 0.0 evacuate all the air from the psia sealed tube, a vacuum with a or Vacuum pump

-14.7 '

pressure of 0 psia is created psig 10.3 psia 23 within the tube. Because the 24" pressure at any point in a static 0.433 psig

fluid is dependent upon the '

height of that point above a 10 ' reference line, such as sea level, 39.9 it follows that the pressure 9.7 within the tube at sea level psia or -5.0 psig 14.7 14.7 must still be 14.7 psia. This is psia psia 0.866 psig equivalent to the pressure at the ' Sea Level base of a column of water 33.9 11.5 feet high and with the column 14.7 14.7 psia or open at the base, water would psia 0.0 psig rise to fill the column to a depth Sea level of 33.9 feet. In other words, the 1See formal definition in the glossary of the appendix weight of the atmosphere at sea

CHAPTER THREE • 13 above the level of the other. (See level, since atmosphere cannot shown that as a fluid acceler- FIGURE 9. Dynamically reduced pipe Fig. 6.) Since both containers support a column of water ates, as shown in Figure 8, the pressures. are open to the atmosphere, the greater in height than 33.9 feet. pressure is reduced. As water pressure on the liquid surfaces Figure 7 illustrates how flows through a constriction From pollution To fixture in each container will remain at this siphon principle can be such as a converging section of source 14.7 psia. hazardous in a plumbing pipe, the velocity of the water If it is assumed that a static system. If the supply valve is increases; as a result, the state exists, momentarily, closed, the pressure in the line pressure is reduced. Under such within the system shown in supplying the faucet is less than conditions, negative pressures Figure 6, the pressure in the left the pressure in the supply line may be developed in a pipe. +50 psig tube at any height above the to the bathtub. Flow will occur, The simple aspirator is based free surface in the left container therefore, through siphonage, upon this principle. If this can be calculated. The pressure from the bathtub to the open point of reduced pressure is -10 psig at the corresponding level in the faucet. linked to a source of pollution, right tube above the free surface backsiphonage of the pollutant in the right container may also can occur. FIGURE 7. be calculated. Backsiphonage in a plumbing As shown in Figure 6, the Booster pump system. FIGURE 8. pressure at all levels in the left Negative pressure created by tube would be less than at Valve open constricted flow. corresponding levels in the right flow from the source of pollu- tube. In this case, a static Submerged inlet tion would occur when pressure condition cannot exist because on the suction side of the pump fluid will flow from the higher -10 psig is less than pressure of the pressure to the lower pressure; +30 psig +30 psig pollution source; but this is the flow would be from the backflow, which will be discussed right tank to the left tank. This below. Valve open One of the common arrangement will be recognized The preceding discussion occurrences of dynamically as a siphon. The crest of a Closed supply has described some of the reduced pipe pressures is found siphon cannot be higher than means by which negative on the suction side of a pump. 33.9 feet above the upper liquid pressures may be created and In many cases similar to the one which frequently occur to illustrated in Figure 9, the line produce backsiphonage. In FIGURE 6. The siphon actions cited supplying the booster pump is addition to the negative Pressure relationships in a have been produced by reduced continuous fluid system at undersized or does not have pressure or reversed force different elevations. pressures resulting from a sufficient pressure to deliver necessary to cause difference in the water levels at water at the rate at which the backsiphonage and backflow, two separated points within a pump normally operates. The there must also be the cross- continuous fluid system. rate of flow in the pipe may be connection or connecting link 8.2 psia 10.3 psia Reduced pressure may also increased by a further reduction between the potable water be created within a fluid system in pressure at the pump intake. supply and the source of as a result of fluid motion. One This often results in the creation pollution. Two basic types of of the basic principles of fluid ' of negative pressure at the connections may be created in 10 mechanics is the principle of pump intake. This often results piping systems. These are the ' conservation of energy. Based in the creation of negative 15 solid pipe with valved connec- upon this principle, it may be pressure. This negative pressure tion and the submerged inlet. may become low enough in 14.7 '

5 psia some cases to cause vaporization of the water in the line. Actu- ally, in the illustration shown, 14.7 psia

14 • CROSS-CONNECTION CONTROL MANUAL Figures 10 and 11 illustrate of the installer about the difficult to control are those reversal in differential pressure solid connections. This type of possibility of reversed flow is which are not apparent until a may occur when pressure in the connection is often installed often more difficult. Upon significant change in water level potable system drops, for some where it is necessary to supply questioning, however, many occurs or where a supply may reason, to a pressure lower than an auxiliary piping system from installers will agree that the be conveniently extended below that in the system to which the the potable source. It is a direct solid connection was made the liquid surface by means of a potable water is connected. connection of one pipe to because the sewer is occasion- hose or auxiliary piping. A The most positive method another pipe or receptacle. ally subjected to backpressure. submerged inlet may be created of avoiding this type of Solid pipe connections are Submerged inlets are found in numerous ways, and its backflow is the total or com- often made to continuous or on many common plumbing detection in some of these plete separation of the two intermittent waste lines where fixtures and are sometimes subtle forms may be difficult. systems. Other methods used it is assumed that the flow will necessary features of the fixtures The illustrations included involve the installation of be in one direction only. An if they are to function properly. in part B of the appendix are mechanical devices. All meth- example of this would be used Examples of this type of design intended to describe typical ods require routine inspection cooling water from a water are siphon-jet urinals or water examples of backsiphonage, and maintenance. jacket or condenser as shown in closets, flushing rim slop sinks, showing in each case the nature Dual piping systems are Figure 11. This type of connec- and dental cuspidors. Oldstyle of the link or cross-connection, often installed for extra protec- tion is usually detectable but bathtubs and lavatories had and the cause of the negative tion in the event of an emer- creating a concern on the part supply inlets below the flood pressure. gency or possible mechanical level rims, but modern sanitary failure of one of the systems. FIGURE 10. design has minimized or Fire protection systems are an Valved connections between eliminated this hazard in new Backflow example. Another example is potable water and nonpotable fixtures. Chemical and indus- 1 the use of dual water connec- fluid. trial process vats sometimes Backflow , as described in this tions to boilers. These installa- have submerged inlets where manual, refers to reversed flow tions are sometimes inter- the water pressure is used as an due to backpressure other than connected, thus creating a aid in diffusion, dispersion and siphonic action. Any intercon- health hazard. agitation of the vat contents. nected fluid systems in which The illustrations in part C Even though the supply pipe the pressure of one exceeds the of the appendix depict installa- may come from the floor above pressure of the other may have tions where backflow under the vat, backsiphonage can flow from one to the other as a pressure can occur, describing occur as it has been shown that result of the pressure differen- the cross-connection and the the siphon action can raise a tial. The flow will occur from cause of the reversed flow. liquid such as water almost 34 the zone of higher pressure to Non potable Potable feet. Some submerged inlets the zone of lower pressure. This type of backflow is of concern in buildings where two or more FIGURE 11 piping systems are maintained. Valved connection between The potable water supply is potable water and sanitary sewer. usually under pressure directly from the city water main. Occasionally, a booster pump is City supply Condenser used. The auxiliary system is often pressurized by a centrifical pump, although backpressure may be caused by gas or steam pressure from a boiler. A

Sanitary sewer 1See formal definition in the glossary of the appendix

CHAPTER THREE • 15 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. medium hazard installations Typical agricultural and industrial application of such as food processing steam pressure vacuum breaker. kettles and apartment projects. They may be used under continuous pressure and protect against both backsiphonage and backpressure conditions.

12" minimum above FIGURE 24. the highest outlet Double check valve.

Hose bibb

At least 6" Process tanks

CHAPTER FOUR • 19 Double Check Detector to insure proper operation of Residential Dual Check It is sized for ½-, ¾-, and Check both the primary checks and 1-inch service lines and is the bypass check valve. In the The need to furnish reliable and installed immediately down- This device is an outgrowth of event of very low fire line water inexpensive backsiphonage and stream of the water meter. The the double check valve and is usage, (theft of water) the low backpressure protection for use of plastic check modules primarily utilized in fire line pressure drop inherent in the individual residences resulted in and elimination of test cocks installations. Its purpose is to bypass system permits the low the debut of the residential dual and gate valves keeps the cost protect the potable supply line flow of water to be metered check. Protection of the main reasonable while providing from possible contamination or through the bypass system. In a potable supply from household good, dependable protection. pollution from fire line chemical high flow demand, associated hazards such as home photo- Typical installations are shown additives, booster pump fire with deluge fire capability, the graph chemicals, toxic insect in Figures 27 and 28. line backpressure, stagnant main check valves open, and garden sprays, termite “black water” that sits in fire permitting high volume, low control pesticides used by lines over extended periods of restricted flow, through the two exterminators, etc., reinforced, time, the addition of “raw” large spring loaded check a true need for such a device. water through outside fire valves. Figure 26 shows a cutaway of pumper connections (Siamese the device. outlets), and the detection of any water movement in the fire FIGURE 26. line water due to fire line Residential dual check. leakage or deliberate water theft. It consists of two, spring loaded check valves, a bypass assembly with water meter and double check valve, and two tightly closing gate valves. See Figure 25. The addition of test cocks makes the device testable

FIGURE 25. Double check detector check.

FIGURE 27. FIGURE 28. Residential installation. Copper horn.

Water meter Residential dual check

1¼" meter thread female inlet with 1" NPT thread female union outlet

Water meter

20 • CROSS-CONNECTION CONTROL MANUAL Reduced Pressure FIGURE 29A. Reduced pressure zone backflow Principle Backflow preventer (¾-inch thru 2-inches). Preventer Maximum protection is achieved against backsiphonage and backpressure conditions utilizing reduced pressure 100 psi 95 psi 94 psi principle backflow preventers. These devices are essentially modified double check valves with an atmospheric vent capability placed between the two checks and designed such that this “zone” between the two checks is always kept at least two pounds less than the supply pressure. With this Supply 60 psi Out 47 psi design criteria, the reduced pressure principle backflow 50 psi preventer can provide protec- tion against backsiphonage and backpressure when both the first and second checks become fouled. They can be used under constant pressure and at high hazard installations. They are FIGURE 29B. furnished with test cocks and Reduced pressure zone backflow preventer (2½-inches thru 10- gate valves to enable testing inches). and are available in sizes ¾-inch through 10 inch. Figure 29A shows typical devices representative of ¾-inch through 2-inch size and Figure 29B shows typical devices representative of 2½-inch through 10-inch sizes.

Reduced pressure zone 1st check valve 2nd check valve

100 psi 94 psi 93 psi

Relief valve (rotated 90˚ for clarity)

CHAPTER FOUR • 21 The principles of operation the pressure increases down- relief valve 3 should remain hazard installations such as of a reduced pressure principle stream from the device, tending fully open to the atmosphere to plating plants, where they backflow preventer are as to reverse the direction of flow, discharge any water which may would protect against primarily follows: check valve 2 closes, preventing be caused to backflow as a backsiphonage potential, car Flow from the left enters backflow. Because all valves result of backpressure and washes where they would the central chamber against the may leak as a result of wear or leakage of check valve 2. protect against backpressure pressure exerted by the loaded obstruction, the protection Malfunctioning of one or conditions, and funeral parlors, check valve 1. The supply provided by the check valves is both of the check valves or relief hospital autopsy rooms, etc. pressure is reduced thereupon not considered sufficient. If valve should always be indi- The reduced pressure principle by a predetermined amount. some obstruction prevents cated by a discharge of water backflow preventer forms the The pressure in the central check valve 2 from closing from the relief port. Under no backbone of cross-connection chamber is maintained lower tightly, the leakage back into circumstances should plugging control programs. Since it is than the incoming supply the central chamber would of the relief port be permitted utilized to protect against high pressure through the operation increase the pressure in this because the device depends hazard installations, and since of the relief valve 3, which zone, the relief valve would upon an open port for safe high hazard installations are the discharges to the atmosphere open, and flow would be operation. The pressure loss first consideration in protecting whenever the central chamber discharged to the atmosphere. through the device may be public health and safety, these pressure approaches within a When the supply pressure expected to average between devices are installed in large few pounds of the inlet pres- drops to the minimum differen- 10 and 20 psi within the quantities over a broad range of sure. Check valve 2 is lightly tial required to operate the normal range of operation, plumbing and water works loaded to open with a pressure relief valve, the pressure in the depending upon the size and installations. Figures 31 and 32 drop of 1 psi in the direction of central chamber should be flow rate of the device. show typical installations of flow and is independent of the atmospheric. If the inlet Reduced pressure principle these devices on high hazard pressure required to open the pressure should become less backflow preventers are installations. relief valve. In the event that than atmospheric pressure, commonly installed on high

FIGURE 30. FIGURE 31. Reduced pressure zone backflow Plating plant installation. preventer — principle of operation.

1 2

3 Direction Meter of flow Reversed direction of flow Reduced pressure principle backflow preventer er main Wat

FIGURE 32. Car wash installation.

Reduced pressure principle backflow preventer

Main

22 • CROSS-CONNECTION CONTROL MANUAL FIGURE 33. FIGURE 34. Typical bypass configuration Typical installation reduced reduced pressure principle pressure principle device devices horizontal illustration.

Reduced pressure Reduced pressure Air gap principle device principle device

Drain

Water meter Air gap

Drain 12" min. 30" max.

Note: Device to be set 12" minimum from wall. Reduced pressure principle device

Air gap

FIGURE 35. Drain Typical installation reduced pressure principle device vertical Note: Devices to be set a min. of 12" and a max. of 30" from the floor and 12" from any wall. illustration.

Typical fire line installation double check valve vertical installation.

Reduced pressure principle device

Elbow Siamese Air gap check Siamese Alarm check fitting Drain

Grade

Double check Water meter valve

Note: (1) Refer to manufacturers installation data for vertical mount. (2) Unit to be set at a height to permit ready access for testing and service. (3) Vertical installation only to be used if horizontal installation cannot be achieved. OS&Y gate valve

Fire pipe

CHAPTER ONE • 23 FIGURE 36. FIGURE 37. Typical installation double check Typical installation residential dual valve horizontal and vertical check with straight set and installation. copperhorn.

Double check valve

Water meter Water meter Residential dual check valve 12" min. 30" max.

¾" ball valve

Double check valve

Residential dual check (unit to be set at a height that permits ready access for testing and service)

Copperhorn with Copperhorn with water meter water meter

¾" ball valve

¾" K-copper

Note: Vertical installation only to be used if horizontal installation cannot be achieved.

24 • CROSS-CONNECTION CONTROL MANUAL