Backflow Testing for Austin by HHCC
Servicing the entire Metro Area including Cedar Park, Georgetown, and Round Rock
Methods and Devices for the Prevention of Backflow and Back-Siphonage
A wide choice of devices exists that can be used to prevent backsiphonage and backpressure from adding contaminated fluids or gases into a potable water supply system. Generally, the selection of the proper device to use is based upon the degree of hazard posed by the cross-connection. Additional considerations are based upon piping size, location, and the potential need to periodically test the devices to insure proper operation.
There are six basic types of devices that can be used to correct cross-connections: air gaps, barometric loops, vacuum breakers—both atmospheric and pressure type, double check with intermediate atmospheric vent, double check valve assemblies, and reduced pressure principle devices. In general, all manufacturers of these devices, with the exception of the barometric loop, produce them to one or more of three basic standards, thus insuring the public that dependable devices are being utilized and marketed. The major standards in the industry are: American Society of Sanitary Engineers ASSE), American Water Works Association (AWWA), and the University of California Foundation for Cross-Connection Control and Hydraulic Research.
Air Gap
Air gaps are non-mechanical
backflow preventers that are
very effective devices to be used
where either backsiphonage or
backpressure conditions may
exist. Their use is as old as
piping and plumbing itself, but
only relatively recently have
standards been issued that
standardize their design. In
general, the air gap must be
twice the supply pipe diameter
but never less than one inch.
An air gap, although an
extremely effective backflow
preventer when used to prevent
backsiphonage and backpressure
conditions, does interrupt
the piping flow with corresponding
loss of pressure for
subsequent use. Consequently,
air gaps are primarily used at
end of the line service where
reservoirs or storage tanks are
desired. When contemplating
the use of an air gap, some
other considerations are:
(1) In a continuous piping
system, each air gap requires
the added expense of reservoirs
and secondary pumping
systems. (2) The air gap may be easily
defeated in the event that the
“2D” requirement was purposely
or inadvertently compromised.
Excessive splash may be encountered
in the event that higher
than anticipated pressures or
flows occur. The splash may be a
cosmetic or true potential
hazard—the simple solution
being to reduce the “2D”
dimension by thrusting the
supply pipe into the receiving
funnel. By so doing, the air gap
is defeated.
(3) At an air gap, we expose the
water to the surrounding air
with its inherent bacteria, dust
particles, and other airborne
pollutants or contaminants. In
addition, the aspiration effect of
the flowing water can drag down
surrounding pollutants into the
reservoir or holding tank.
(4) Free chlorine can come out of
treated water as a result of the air
gap and the resulting splash and
churning effect as the water
enters the holding tanks. This
reduces the ability of the water
to withstand bacteria contamination
during long term storage.
(5) For the above reasons, air
gaps must be inspected as
frequently as mechanical
backflow preventers. They are
not exempt from an in-depth
cross connection control program
requiring periodic inspection
of all backflow devices.
Air gaps may be fabricated
from commercially available
plumbing components or
purchased as separate units and
integrated into plumbing and
piping systems.
Barometric Loop
The barometric loop consists of
a continuous section of supply
piping that abruptly rises to a
height of approximately 35 feet
and then returns back down to
the originating level. It is a loop
in the piping system that
effectively protects against
backsiphonage. It may not be
used to protect against backpressure.
Its operation, in the
protection against backsiphonage,
is based upon the
principle that a water column,
at sea level pressure, will not
rise above 33.9 feet.
In general, barometric
loops are locally fabricated, and
are 35 feet high.
Atmospheric Vacuum Breaker
These devices are among the
simplest and least expensive
mechanical types of backflow
preventers and, when installed
properly, can provide excellent
protection against backsiphonage.
They must not be
utilized to protect against
backpressure conditions.
Construction consists usually of
a polyethylene float which is
free to travel on a shaft and seal
in the uppermost position
against atmosphere with an
elastomeric disc. Water flow
lifts the float, which then causes
the disc to seal. Water pressure
keeps the float in the upward
sealed position. Termination of
the water supply will cause the
disc to drop down venting the
unit to atmosphere and thereby
opening downstream piping to
atmospheric pressure, thus
preventing backsiphonage.
Figure 15 shows a typical
atmospheric breaker.
In general, these devices
are available in ½-inch through
3-inch size and must be
installed vertically, must not
have shutoffs downstream,
and must be installed at least
6-inches higher than the final
outlet. They cannot be tested
once they are installed in the
plumbing system, but are, for
the most part, dependable,
trouble-free devices for
backsiphonage protection.
Hose Bibb Vacuum Breakers
These small devices are a
specialized application of the
atmospheric vacuum breaker.
They are generally attached to
sill cocks and in turn are
connected to hose supplied
outlets such as garden hoses,
slop sink hoses, spray outlets,
etc. They consist of a spring
loaded check valve that seals
against an atmospheric outlet
when water supply pressure is
turned on. When the water supply is
turned off, the device vents to
atmosphere, thus protecting
against backsiphonage conditions.
They should not be used
as backpressure devices. Manual
drain options are available,
together with tamper-proof
versions.
Pressure Vacuum Breakers
This device is an outgrowth of
the atmospheric vacuum
breaker and evolved in response
to a need to have an atmospheric
vacuum breaker that could be
utilized under constant pressure
and that could be tested in line.
A spring on top of the disc and
float assembly, two added gate
valves, test cocks, and an
additional first check, provided
the answer to achieve this
device.
These units are available in
the general configurations in sizes
½-inch through 10-inch and
have broad usage in the
agriculture and irrigation
market.
Again, these devices may
be used under constant pressure
but do not protect against
backpressure conditions. As a
result, installation must be at
least 6- to 12-inches higher
than the existing outlet. A spill resistant pressure
vacuum breaker (SVB) is
available that is a modification
to the standard pressure
vacuum breaker but specifically
designed to minimize water
spillage. Installation and
hydraulic requirements are
similar to the standard pressure
vacuum breaker and the
devices are recommended for
internal use.
Double Check with
Intermediate
Atmospheric Vent
The need to provide a compact device in ½-inch and ¾-inch pipe sizes that protects against moderate hazards, is capable of being used under constant pressure and that protects against backpressure, resulted in this unique backflow preventer. Construction is basically a double check valve having an atmospheric vent located between the two checks. Line pressure keeps the vent closed, but zero supply pressure or backsiphonage will open the inner chamber to atmosphere. With this device, extra protection is obtained through the atmospheric vent capability. Figure 23 shows a typical use of the device on a residential boiler supply line. Double Check Valve A double check valve is essentially two single check valves coupled within one body and furnished with test cocks and two tightly closing gate valves. The test capability feature gives this device a big advantage over the use of two independent check valves in that it can be readily tested to determine if either or both check valves are inoperative or fouled by debris. Each check is spring loaded closed and requires approximately a pound of pressure to open. This spring loading provides the ability to “bite” through small debris and still seal—a protection feature not prevalent in unloaded swing check valves. Double checks are commonly used to protect against low to medium hazard installations such as food processing steam kettles and apartment projects. They may be used under continuous pressure and protect against both backsiphonage and backpressure conditions.
Double Check Detector Check
This device is an outgrowth of
the double check valve and is
primarily utilized in fire line
installations. Its purpose is to
protect the potable supply line
from possible contamination or
pollution from fire line chemical
additives, booster pump fire
line backpressure, stagnant
“black water” that sits in fire
lines over extended periods of
time, the addition of “raw”
water through outside fire
pumper connections (Siamese
outlets), and the detection of
any water movement in the fire
line water due to fire line
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. The addition of test
cocks makes the device testable
to insure proper operation of
both the primary checks and
the bypass check valve. In the
event of very low fire line water
usage, (theft of water) the low
pressure drop inherent in the
bypass system permits the low
flow of water to be metered
through the bypass system. In a
high flow demand, associated
with deluge fire capability, the
main check valves open,
permitting high volume, low
restricted flow, through the two
large spring loaded check
valves
Residential Dual Check
The need to furnish reliable and inexpensive backsiphonage and backpressure protection for individual residences resulted in the debut of the residential dual check. Protection of the main potable supply from household hazards such as home photograph chemicals, toxic insect and garden sprays, termite control pesticides used by exterminators, etc., reinforced, a true need for such a device. It is sized for ½-, ¾-, and 1-inch service lines and is installed immediately downstream of the water meter. The use of plastic check modules and elimination of test cocks and gate valves keeps the cost reasonable while providing good, dependable protection.
Reduced Pressure
Principle Backflow
Preventer
Maximum protection is
achieved against backsiphonage
and back- pressure conditions
utilizing reduced pressure
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
design criteria, the reduced
pressure principle backflow
preventer can provide protection
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
furnished with test cocks and
gate valves to enable testing
and are available in sizes ¾-inch
through 10 inch.