Chapter Four

A wide choice of devicesexists that can be used toprevent backsiphonage andbackpressure from addingcontaminated fluids or gasesinto a potable water supplysystem. Generally, the selectionof the proper device to use isbased upon the degree of hazardposed by the cross-connection.Additional considerations arebased upon piping size, location,and the potential need toperiodically test the devices toinsure proper operation.

There are six basic types ofdevices that can be used tocorrect cross-connections: airgaps, barometric loops, vacuumbreakersboth atmosphericand pressure type, double checkwith intermediate atmosphericvent, double check valveassemblies, and reduced pressureprinciple devices. In general, allmanufacturers of these devices,with the exception of thebarometric loop, produce themto one or more of three basicstandards, thus insuring thepublic that dependable devicesare being utilized and marketed.The major standards in theindustry are: American Societyof Sanitary Engineers ASSE),American Water Works Associa-tion (AWWA), and the Univer-sity of California Foundation forCross-Connection Control andHydraulic Research.

Air Gap

Air gaps are non-mechanicalbackflow preventers that arevery effective devices to be usedwhere either backsiphonage orbackpressure conditions mayexist. Their use is as old aspiping and plumbing itself, butonly relatively recently havestandards been issued thatstandardize their design. Ingeneral, the air gap must betwice the supply pipe diameterbut never less than one inch.See Figure 12.

(2) The air gap may be easilydefeated in the event that the2D requirement was purposelyor inadvertently compromised.Excessive splash may be encoun-tered in the event that higherthan anticipated pressures orflows occur. The splash may be acosmetic or true potentialhazardthe simple solutionbeing to reduce the 2Ddimension by thrusting thesupply pipe into the receivingfunnel. By so doing, the air gapis defeated.(3) At an air gap, we expose thewater to the surrounding airwith its inherent bacteria, dustparticles, and other airbornepollutants or contaminants. Inaddition, the aspiration effect ofthe flowing water can drag downsurrounding pollutants into thereservoir or holding tank.(4) Free chlorine can come out oftreated water as a result of the airgap and the resulting splash andchurning effect as the waterenters the holding tanks. Thisreduces the ability of the waterto withstand bacteria contamina-tion during long term storage.(5) For the above reasons, airgaps must be inspected asfrequently as mechanicalbackflow preventers. They arenot exempt from an in-depthcross-connection control pro-gram requiring periodic inspec-tion of all backflow devices.

Air gaps may be fabricatedfrom commercially availableplumbing components orpurchased as separate units andintegrated into plumbing andpiping systems. An example ofthe use of an air gap is shown inFigure 13.

16 CROSS-CONNECTION CONTROL MANUAL

Methods and Devicesfor the Prevention ofBackflow andBack-Siphonage

An air gap, although anextremely effective backflowpreventer when used to preventbacksiphonage and backpres-sure conditions, does interruptthe piping flow with corre-sponding loss of pressure forsubsequent use. Consequently,air gaps are primarily used atend of the line service wherereservoirs or storage tanks aredesired. When contemplatingthe use of an air gap, someother considerations are:(1) In a continuous pipingsystem, each air gap requiresthe added expense of reservoirsand secondary pumpingsystems.

FIGURE 12.Air gap.

DiameterD

2D

Barometric Loop

The barometric loop consists ofa continuous section of supplypiping that abruptly rises to aheight of approximately 35 feetand then returns back down tothe originating level. It is a loopin the piping system thateffectively protects againstbacksiphonage. It may not beused to protect against back-pressure.

Its operation, in theprotection against back-siphonage, is based upon theprinciple that a water column,at sea level pressure, will notrise above 33.9 feet (Ref.Chapter 3, Fig. 4 Page 13).

In general, barometricloops are locally fabricated, andare 35 feet high.

Atmospheric VacuumBreaker

These devices are among thesimplest and least expensivemechanical types of backflowpreventers and, when installedproperly, can provide excellentprotection against back-siphonage. They must not beutilized to protect againstbackpressure conditions.Construction consists usually ofa polyethylene float which isfree to travel on a shaft and sealin the uppermost positionagainst atmosphere with anelastomeric disc. Water flowlifts the float, which then causesthe disc to seal. Water pressurekeeps the float in the upwardsealed position. Termination ofthe water supply will cause thedisc to drop down venting theunit to atmosphere and therebyopening downstream piping toatmospheric pressure, thuspreventing backsiphonage.Figure 15 shows a typicalatmospheric breaker.

In general, these devicesare available in -inch through3-inch size and must beinstalled vertically, must nothave shutoffs downstream,and must be installed at least6-inches higher than the finaloutlet. They cannot be testedonce they are installed in theplumbing system, but are, forthe most part, dependable,trouble-free devices forbacksiphonage protection.

Figure 16 shows thegenerally accepted installationrequirementsnote that noshutoff valve is downstreamof the device that wouldotherwise keep the atmosphericvacuum breaker under constantpressure.

Figure 17 shows a typicalinstallation of an atmosphericvacuum breaker in a plumbingsupply system.

CHAPTER FOUR 17

FIGURE 13.Air gap in a piping system.

Supply piping

Tank or reservoir

FIGURE 14.Barometric loop.

FIGURE 15.Atmospheric vacuum breaker.

35'

FIGURE 16.Atmospheric vacuum breakertypical installation.

FIGURE 17.Atmospheric vacuum breaker inplumbing supply system.

Flow condition

Seal

Non flow condition

Not less than 6"

Hose BibbVacuum Breakers

These small devices are aspecialized application of theatmospheric vacuum breaker.They are generally attached tosill cocks and in turn areconnected to hose suppliedoutlets such as garden hoses,slop sink hoses, spray outlets,etc. They consist of a springloaded check valve that sealsagainst an atmospheric outletwhen water supply pressure isturned on. Typical constructionis shown in Figure 18.

When the water supply isturned off, the device vents toatmosphere, thus protectingagainst backsiphonage condi-tions. They should not be usedas backpressure devices. Manualdrain options are available,together with tamper-proofversions. A typical installation isshown in Figure 19.

PressureVacuum Breakers

This device is an outgrowth ofthe atmospheric vacuumbreaker and evolved in responseto a need to have an atmospher-ic vacuum breaker that could beutilized under constant pressureand that could be tested in line.A spring on top of the disc andfloat assembly, two added gatevalves, test cocks, and anadditional first check, providedthe answer to achieve thisdevice. See Figure 20.

These units are available inthe general configurations asshown in Figure 20 in sizes-inch through 10-inch andhave broad usage in theagriculture and irrigationmarket. Typical agricultural and

industrial applications areshown in Figure 21.

Again, these devices maybe used under constant pressurebut do not protect againstbackpressure conditions. As aresult, installation must be atleast 6- to 12-inches higherthan the existing outlet.

A spill resistant pressurevacuum breaker (SVB) isavailable that is a modificationto the standard pressurevacuum breaker but specificallydesigned to minimize waterspillage. Installation andhydraulic requirements aresimilar to the standard pressurevacuum breaker and thedevices are recommended forinternal use.

18 CROSS-CONNECTION CONTROL MANUAL

Hose bibb vacuum breaker

inch thru 2 inches

2 inches thru 10 inches

Spring

Gate Valve

Gate Valve

Test cock

Test cock

First check valve

FIGURE 18.Hose bibb vacuum breaker.

FIGURE 19.Typical installation of hose bibbvacuum breaker.

FIGURE 20.Pressure vacuum breaker

Double Check withIntermediateAtmospheric Vent

The need to provide a compactdevice in -inch and -inchpipe sizes that protects againstmoderate hazards, is capable ofbeing used under constantpressure and that protectsagainst backpressure, resultedin this unique backflowpreventer. Construction isbasically a double check valvehaving an atmospheric ventlocated between the two checks(See Figure 22).

Line pressure keeps thevent closed, but zero supplypressure or backsiphonage willopen the inner chamber toatmosphere. With this device,extra protection is obtainedthrough the atmospheric ventcapability. Figure 23 shows atypical use of the device on aresidential boiler supply line.

Double Check Valve

A double check valve isessentially two single checkvalves coupled within one bodyand furnished with test cocksand two tightly closing gatevalves (See Figure 24).

The test capability featuregives this device a big advan-tage over the use of twoindependent check valves inthat it can be readily tested todetermine if either or bothcheck valves are inoperativeor fouled by debris. Each checkis spring loaded closed andrequires approximately a poundof pressure to open.

This spring loadingprovides the ability to bitethrough small debris and stillseala protection feature notprevalent in unloaded swingcheck valves. Figure 24 shows across section of double checkvalve complete with test cocks.Double checks are commonlyused to protect against low tomedium hazard installationssuch as food processing steamkettles and apartment projects.They may be used undercontinuous pressure and protectagainst both backsiphonage andbackpressure conditions.

CHAPTER FOUR 19

Vent

2nd check1st check

Drain

Air gap

Automatic feed valveSupply

Return

Boiler

FIGURE 21.Typical agricultural andindustrial application ofpressure vacuum breaker.

FIGURE 22.Double check valve withatmospheric vent.

FIGURE 23.Typical residential use of doublecheck with atmospheric vent.

FIGURE 24.Double check valve.

At least 6"Process tanks

12" minimum abovethe highest outlet

Hose bibb

Double Check DetectorCheck

This device is an outgrowth ofthe double check valve and isprimarily utilized in fire lineinstallations. Its purpose is toprotect the potable supply linefrom possible contamination orpollution from fire line chemicaladditives, booster pump fireline backpressure, stagnantblack water that sits in firelines over extended periods oftime, the addition of rawwater through outside firepumper connections (Siameseoutlets), and the detection ofany water movement in the fireline water due to fire lineleakage or deliberate watertheft. It consists of two, springloaded check valves, a bypassassembly with water meter anddouble check valve, and twotightly closing gate valves. SeeFigure 25. The addition of testcocks makes the device testable

to insure proper operation ofboth the primary checks andthe bypass check valve. In theevent of very low fire line waterusage, (theft of water) the lowpressure drop inherent in thebypass system permits the lowflow of water to be meteredthrough the bypass system. In ahigh flow demand, associatedwith deluge fire capability, themain check valves open,permitting high volume, lowrestricted flow, through the twolarge spring loaded checkvalves.

Residential Dual Check

The need to furnish reliable andinexpensive backsiphonage andbackpressure protection forindividual residences resulted inthe debut of the residential dualcheck. Protection of the mainpotable supply from householdhazards such as home photo-graph chemicals, toxic insectand garden sprays, termitecontrol pesticides used byexterminators, etc., reinforced,a true need for such a device.Figure 26 shows a cutaway ofthe device.

It is sized for -, -, and1-inch service lines and isinstalled immediately down-stream of the water meter. Theuse of plastic check modulesand elimination of test cocksand gate valves keeps the costreasonable while providinggood, dependable protection.Typical installations are shownin Figures 27 and 28.

20 CROSS-CONNECTION CONTROL MANUAL

Residentialdual check

Water meter

Water meter

1" meter thread female inlet with1" NPT thread female union outlet

FIGURE 25.Double check detector check.

FIGURE 26.Residential dual check.

FIGURE 27.Residential installation.

FIGURE 28.Copper horn.

100 psi 95 psi

Out 47 psi

50 psi

Supply 60 psi

94 psi

Reduced PressurePrinciple BackflowPreventer

Maximum protection isachieved against backsiphonageand backpressure conditionsutilizing reduced pressureprinciple backflow preventers.These devices are essentiallymodified double check valveswith an atmospheric ventcapability placed between thetwo checks and designed suchthat this zone between thetwo checks is always kept atleast two pounds less than thesupply pressure. With thisdesign criteria, the reducedpressure principle backflowpreventer can provide protec-tion against backsiphonage andbackpressure when both thefirst and second checks becomefouled. They can be used underconstant pressure and at highhazard installations. They arefurnished with test cocks andgate valves to enable testingand are available in sizes -inchthrough 10 inch.

Figure 29A shows typicaldevices representative of -inchthrough 2-inch size and Figure29B shows typical devicesrepresentative of 2-inchthrough 10-inch sizes.

CHAPTER FOUR 21

FIGURE 29A.Reduced pressure zone backflowpreventer (-inch thru 2-inches).

FIGURE 29B.Reduced pressure zone backflowpreventer (2-inches thru 10-inches).

Relief valve (rotated 90 for clarity)

Reduced pressure zone1st check valve 2nd check valve

94 psi 93 psi100 psi

22 CROSS-CONNECTION CONTROL MANUAL

The principles of operationof a reduced pressure principlebackflow preventer are asfollows:

Flow from the left entersthe central chamber against thepressure exerted by the loadedcheck valve 1. The supplypressure is reduced thereuponby a predetermined amount.The pressure in the centralchamber is maintained lowerthan the incoming supplypressure through the operationof the relief valve 3, whichdischarges to the atmospherewhenever the central chamberpressure approaches within afew pounds of the inlet pres-sure. Check valve 2 is lightlyloaded to open with a pressuredrop of 1 psi in the direction offlow and is independent of thepressure required to open therelief valve. In the event that

the pressure increases down-stream from the device, tendingto reverse the direction of flow,check valve 2 closes, preventingbackflow. Because all valvesmay leak as a result of wear orobstruction, the protectionprovided by the check valves isnot considered sufficient. Ifsome obstruction preventscheck valve 2 from closingtightly, the leakage back intothe central chamber wouldincrease the pressure in thiszone, the relief valve wouldopen, and flow would bedischarged to the atmosphere.

When the supply pressuredrops to the minimum differen-tial required to operate therelief valve, the pressure in thecentral chamber should beatmospheric. If the inletpressure should become lessthan atmospheric pressure,

relief valve 3 should remainfully open to the atmosphere todischarge any water which maybe caused to backflow as aresult of backpressure andleakage of check valve 2.

Malfunctioning of one orboth of the check valves or reliefvalve should always be indi-cated by a discharge of waterfrom the relief port. Under nocircumstances should pluggingof the relief port be permittedbecause the device dependsupon an open port for safeoperation. The pressure lossthrough the device may beexpected to average between10 and 20 psi within thenormal range of operation,depending upon the size andflow rate of the device.

Reduced pressure principlebackflow preventers arecommonly installed on high

hazard installations such asplating plants, where theywould protect against primarilybacksiphonage potential, carwashes where they wouldprotect against backpressureconditions, and funeral parlors,hospital autopsy rooms, etc.The reduced pressure principlebackflow preventer forms thebackbone of cross-connectioncontrol programs. Since it isutilized to protect against highhazard installations, and sincehigh hazard installations are thefirst consideration in protectingpublic health and safety, thesedevices are installed in largequantities over a broad range ofplumbing and water worksinstallations. Figures 31 and 32show typical installations ofthese devices on high hazardinstallations.

Directionof flow

1 2

3

Reversed direction of flow

Reduced pressure principle backflow preventerWate

r main

Meter

Main

Reduced pressure principlebackflow preventer

FIGURE 30.Reduced pressure zone backflowpreventer principle of operation.

FIGURE 31.Plating plant installation.

FIGURE 32.Car wash installation.

CHAPTER ONE 23

FIGURE 33.Typical bypass configurationreduced pressure principledevices

FIGURE 34.Typical installation reducedpressure principle devicehorizontal illustration.

Typical fire line installation doublecheck valve vertical installation.

FIGURE 35.Typical installation reducedpressure principle device verticalillustration.

Reduced pressureprinciple device

Water meter

Note: Device to be set 12" minimum from wall.

Air gap

Drain 12" min. 30" max.

Reduced pressureprinciple device

Water meter

Air gap

Elbow

Drain

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.

Reduced pressure principle device

Note: Devices to be set a min. of 12" and a max. of 30" from the floor and 12" from any wall.

Reduced pressureprinciple device

Air gap

Drain

Air gap

Drain

Double checkvalve

Alarm check

Grade

OS&Y gate valve

Fire pipe

Siamesecheck

Siamesefitting

24 CROSS-CONNECTION CONTROL MANUAL

FIGURE 36.Typical installation double checkvalve horizontal and verticalinstallation.

FIGURE 37.Typical installation residential dualcheck with straight set andcopperhorn.

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

Double check valve

Double check valve

(unit to be set at a heightthat permits ready accessfor testing and service)

Water meter

Copperhorn with water meter

12" min. 30" max.

Residential dualcheck valve

Residentialdual check

" ball valve

" ball valve

" K-copper

Water meter

Copperhorn with water meter