There are five distinct types of piping or mechanical assemblies
which are considered to be backflow prevention assemblies; but, it must
be stressed that these are not all equally acceptable as protection against
all types of hazards. The degree of hazard must be assessed along with the
type of cross-connection present to determine which type of backflow prevention
assembly is most suitable to the situation. |
Note: Testable backflow assemblies require to have a certified inspection
and operational test at least once a year.
An Air Gap is a physical separation of the supply pipe by at least two pipe
diameters (never less than one inch) vertically above the overflow rim of
the receiving vessel. In this case line pressure is lost. Therefore, a booster
pump is usually needed downstream, unless the flow of the water by gravity
is sufficient for the water use. With an air gap there is no direct connection
between the supply main and the equipment. An air gap may be used to protect
against a contaminant or a pollutant, and will protect against both backsiphonage
and back pressure. An air gap is the only acceptable means of protecting
against lethal hazards.
The AVB is always placed downstream from all shutoff valves. Its air inlet
valve closes when the water flows in the normal direction. But, as water
ceases to flow the air inlet valve opens, thus interrupting the possible
backsiphonage effect. If piping or a hose is attached to this assembly and
run to a point of higher elevation, the back pressure will keep the air
inlet valve closed because of the pressure created by the elevation of water.
Hence, it would not provide the intended protection. 1.This type of assembly
must always be installed at least six (6) inches above all downstream piping
and outlets. 2. This assembly may not have shutoff
valves or obstructions downstream. A shutoff valve would keep the
assembly under pressure and allow the air inlet valve (or float check) to
seal against the air inlet port, thus causing the assembly to act as an
elbow, not a backflow preventer. 3.The AVB may not be under continuous pressure
for this same reason. An AVB must not be used for more than twelve (12)
out of any twenty-four (24) hour period. It may be used to protect against
either a pollutant or a contaminant, but may only be used to protect against
a backsiphonage condition.
The PVB includes a check valve which is designed to close with the aid of
a spring when flow stops. It also has an air inlet valve which is designed
to open when the internal pressure is one psi above atmospheric pressure
so that no non-potable liquid may be siphoned back into the potable water
system. Being spring loaded it does not rely upon gravity as does the atmospheric
vacuum breaker. This assembly includes resilient seated shutoff valves and
testcocks. The PVB is designed to operate under constant pressures for long
periods of time.
1. The PVB must be installed at least twelve (12) inches above all downstream
piping and the highest fixture flood level rim, outlet, or highest point
of water use.
2. The PVB may be used to protect against a pollutant or contaminant, however,
it may only be used to protect against backsiphonage. It is not acceptable
protection against back pressure.
3. The PVB shall be installed in a vertical position with adequate space
to facilitate maintenance and testing.
The Double Check Valve Assembly consists of two internally loaded, independently
operating check valves together with tightly closing resilient seated shutoff
valves upstream and downstream of the check valves. Additionally, there
are resilient seated testcocks for testing of the assembly. The DC may be
used to protect against a pollutant only. However, this assembly is suitable
for protection against either backsiphonage or back pressure.
This assembly consists of two internally loaded independently operating
check valves and a mechanically independent, hydraulically dependent relief
valve located between the check valves. This relief valve is designed to
maintain a zone of reduced pressure between the two check valves at all
times. The RP also contains tightly closing, resilient seated shutoff valves
upstream and downstream of the check valves along with resilient seated
testcocks. This assembly is used for the protection of the potable water
supply from either pollutants or contaminants and may be used to protect
against either backsiphonage or back pressure.
1. This assembly shall not be installed in a pit below ground level. Semiburied
pits are acceptable if the RP is installed above the ground or the maximum
flood level with an approved air gap between the relief valve port and the
2. The relief valve port shall not be submerged in groundwater. That would
create a cross connection is that may be more serious than the hazard that
the assembly isolates.