Backflow
The term backflow means any unwanted flow of used or non-potable water or substance
from any domestic, industrial or institutional piping system
into the pure, potable water distribution system. The direction of flow under
these conditions is in the reverse direction from that intended by the system
and normally assumed by the owner of the system. Backflow may be caused by
numerous specific conditions; but, basically the reverse pressure gradient
may be due to either a loss of pressure in the supply main called back siphonage
or by the flow from a customer's pressurized system through an unprotected cross-connection, which is called backpressure. Thus the term backflow
covers both a back siphonage condition and a backpressure condition. A reversal of flow
in a distribution main--or in the customer's system--can be created by any
change of system pressure wherein the pressure at the supply point becomes
lower than the pressure at the point of use. When this happens in an unprotected
situation the water at the point of use will be siphoned back into the system;
thus, potentially polluting or contaminating the remainder of the customer's
system. It is also possible that the contaminated or polluted water could
continue to backflow into the public distribution system. The point at which it is possible for a non-potable substance to come in contact with the potable
drinking water system is called a cross-connection. To prevent backflow from
occurring at the point of a cross-connection a backflow prevention assembly
must be installed. However, it is important the backflow prevention assembly
match the particular hydraulic conditions at that location and is suitable
to protect against the degree of hazard present. The particular type of backflow
preventer appropriate for specific needs will be discussed later. First, it
is necessary to understand some basic hydraulics which govern the principles of backflow and cross-connection control.
Pure water at sea level weighs 62.4 pounds per cubic foot. Thus, if the base
of a cubic foot is divided into 144 one-inch squares, the weight of the column
of water one foot high covering a square inch area would be 0.433 pounds.
Or 0.433 pounds per square inch of pressure exists at the base of this cubic
foot of water. Since, pressure is measured in pounds per square inch, the
area covered at the base of a water column is not relative to the pressure.
As a rule of thumb, it is said that a column of water one foot high creates
a pressure of approximately 1/2 pound per square inch, or psi.
Another term used for measuring pressure aside from psi is inches of mercury, or
Hg. This is because mercury manometers are often used to measure pressure differentials. However, mercury weighs 13.5 times the weight of water.
Therefore, one Hg is approximately 1/2 psi.
Technically, if one siphons a fluid out of a container or a pipeline, one
causes that fluid to flow up over the rim of the container or top of the pipe
and then down into a lower elevation through a piece of tubing or, in this
case a piece of pipe that is part of the distribution system. In the vernacular,
the unwanted fluid is "sucked" into the potable water line. It is important
to understand that it is not necessary for the system main to be under a true
vacuum (i.e., zero psi) for back siphonage to occur. All that is required
is a negative difference in pressure and a piece of tubing or pipe that's completely full of fluid.
One of the causes of back siphonage is a situation that arises when a temporary shutdown of
a water main--or an in-plant pipe--becomes necessary for repairs.
If the repair is at some point other than that of the highest point in the
system, then there will be a potential for a reverse flow if one of the lower
points of service is opened while the main valve is closed. Under this condition
the water in the internal piping system will drain to the open valve or point
of water use, siphoning anything it may be in contact with at the time.
Due to the length of a main distribution pipe, the several pipe fittings
and the normal elevation of the service above the distribution main, the pressure
gradient within a service decreases as the point of discharge becomes further
and further from the main. Consequently, in many processing plants or high-rise
buildings there is need to use a booster pump; or, in many instances, a
recirculating pump. The use of an in-plant pump can easily increase the in-plant
pressure above that of the supply main. Thus, if there is any pathway whereby
this pressurized industrial water may enter the potable distribution system,
there will be a hazard as the result of this backpressure.
A cross-connection is any actual or potential connection or structural arrangement
between a public or private potable water system and
any other source or system through which it is possible to introduce into
any part of the potable system any used water, industrial fluids, gas, or
substance other than the intended potable water with which the potable system
is supplied. By-pass arrangements, jumper connections, removable sections,
swivel or change-over arrangements or other "temporary" arrangements through
which backflow could occur are considered to be cross-connections
General progress in the elimination of cross-connections has been very slow. It required
over a score of years for various water, health and fire insurance representatives
to understand that no direct connection should be
made between the potable water supply system and polluted auxiliary water
supplies. It was not until the Chicago amoebic dysentery outbreak of 1933
that a general move toward correction of plumbing hazards was given impetus.
And, since that time the recognition of a backflow prevention program and
its value has been very slow, until about 1970.
There are two basic types of cross-connections: a direct cross-connection
and an indirect cross-connection. The difference between these two types of
cross-connections is very simple. A direct cross-connection is subject to
backpressure (as shown above); an indirect cross-connection is not subject
to backpressure. An example of a direct cross-connection would be the make-up
water line feeding a recirculating system. An over-the-rim inlet used to fill
an open receiving vessel would be an example of an indirect cross-connection.
Backpressure could not be introduced into the supply line with this type of
connection.
The type of backflow preventer used to prevent backflow from occurring at
the point of cross-connection depends on the type of substance which may
flow into the potable water supply. A pollutant is considered to be any substance
which would affect the color or odor of the water, but would not pose a health
hazard. This is also considered anon-health hazard. A substance is considered
a health hazard if it causes illness or death if ingested. This health hazard
is called a contaminant.
Sewage and radioactive materials are considered Lethal Hazards. This is because
of the epidemic possibilities associated with sewage and the tremendous dangers
associated with radioactive material.
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 they type of
cross-connection present to determine which type of backflow prevention assembly
is most suitable to the situation.
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 back
siphonage
and backpressure. An air gap is the only acceptable means of protecting against lethal hazards.
The AVB is always placed downstream from all shut-off 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 back siphonage
effect. If piping or a hose is attached to this assembly and run to a point
of higher elevation, the backpressure 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. Therefore, this type of assembly must
always be installed at least six (6) inches above all downstream piping and outlets. Additionally, this assembly may not have shut-off valves or
obstructions downstream. A shut-off 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.
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 back siphonage 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 shut-off valves and test cocks. The PVB must be installed at least twelve (12) inches
above all downstream piping and outlets. The PVB may be used to protect against a pollutant
or contaminant, however, it may only be used to protect against back siphonage.
It is not acceptable protection against backpressure.
The Double Check Valve Assembly consists of two internally loaded, independently operating
check valves together with tightly closing resilient
seated shut-off valves upstream and downstream of the check valves. Additionally,
there are resilient seated test cocks 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 back siphonage or backpressure.
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 shut-off valves upstream
and downstream of the check valves along with resilient seated test cocks.
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 back
siphonage
backpressure.
The DCDA is composed of a line-sized double check valve assembly with a specific
bypass meter and meter-sized double check valve assembly. The meter registers
accurately for very low flow rates to detect any unauthorized use of water.
This assembly is used when the protection of a double check valve assembly
is required, yet where the added requirement of detecting any leakage or
unauthorized use of water exists. Normally these assemblies are reserved for
use on fire sprinkler lines.
The RPDA is very similar to the double check detector assembly except that
the RPDA is designed for situations requiring the protection of a reduced
pressure principle assembly and detection of unauthorized use of water or
leaks. As with the DCDA, the bypass meter must register accurately at low
flows. This assembly is normally used on fire lines which may contain
contaminants, such as anti-freeze additives or foamite.
The chart above shows which type of assembly is acceptable for use under certain hydraulic
conditions and degrees of hazard.
For more information please contact us
at (281) 340-9206
or go to our online request form. [Home]
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Pressure
Back siphonage
Backpressure
Types of Cross-Connection
Degree of Hazard
Types of Backflow Preventer
Air Gap
Atmospheric (non-pressure) Type Vacuum Breaker {AVB}
Pressure Vacuum Breaker {PVB}
Double Check Valve Assembly {DC}
Reduced Pressure Principle Assembly {RP}
Double Check Detector Assembly {DCDA}
Reduced Pressure Principle Detector Assembly {RPDA}
