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 backsiphonage,
or by the flow from a customer's pressurized system through an unprotected
cross-connection, which is called back pressure. Thus the term backflow
covers both a backsiphonage condition and a back pressure 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 psia) for backsiphonage
to occur. All that is required is a negative difference in pressure and
a piece of tubing or pipe that is completely full of fluid.
 One of the
causes of backsiphonage 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 back pressure.
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. Bypass arrangements, jumper connections, removable sections,
swivel or changeover arrangements or other "temporary" arrangements
through which backflow could occur are considered to be cross-connection
General progress in the elimination of cross-connection 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-connection: a direct cross-connection and an
indirect cross-connection. The difference between these two types of cross-connection
is very simple. A direct cross-connection is subject to back pressure (as
shown above); an indirect cross-connection is not subject to back pressure.
An example of a direct cross-connection would be the makeup 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. back pressure
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 a 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 colour or odor of the water, but would
not pose a health hazard. This is also considered a non-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. |
|
|
