Page 349 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch09_297-332.qxd 8/2/10 9:54 PM Page 309

9.3 Theory of Backflow and Backsiphonage 309
arrived to perform the weekly cleaning, and after drinking the water from the drinking
fountains, and sampling the coffee from the coffee machines, they noticed that the water
smelled rubbery and had a strong bitter taste. On notifying the Manchester Water
Company, water samples were taken and preliminary analysis disclosed that the contami-
nants found were not the typical contaminants associated with fire line disturbances.
Investigating teams suspected that either the nearby fire could have siphoned contaminants
from adjacent buildings into the water mains, or the contamination could have been caused
by a plumbing deficiency occurring within the seven-story building itself.
The pH levels of the building water indicated that an injection of chemicals had proba-
bly taken place within the seven-story building. Tracing of the water lines within the building
pinpointed a 10,000-gal (37,850-L) hot-water storage tank that was used for heat storage in
the solar heating system. It did not have any backflow protection on the makeup supply line.
As the storage tank pressure increased above the supply pressure, as a result of thermal ex-
pansion, the potential for backpressure backflow was present. Normally, this would not occur
because a boost pump in the supply line would maintain the supply pressure to the storage
tank at a greater pressure than the highest tank pressure. The addition of rust-inhibiting
chemicals to this tank greatly increased the degree of hazard of the liquid. Unfortunately, at
the same time that the fire took place, the pressure in the water mains was reduced to a dan-
gerously low pressure and the low-pressure cutoff switches simultaneously shut off the stor-
age tank booster pumps. This combination allowed the boiler water, together with its chemi-
cal contaminants, the opportunity to enter the potable water supply within the building (see
Fig. 9.10). When normal pressure was reestablished in the water mains, the booster pumps
kicked in, and the contaminated water was delivered throughout the building.

9.3 THEORY OF BACKFLOW AND BACKSIPHONAGE

A cross-connection is the link or channel connecting a source of pollution with a potable
water supply. The polluting substance, in most cases a liquid, tends to enter the potable
supply if the net force acting on the liquid acts in the direction of the potable supply. Two
factors are therefore essential for backflow. First, there must be a link between the two sys-
tems. Second, the resultant force must be toward the potable supply.
An understanding of the principles of backflow and backsiphonage requires an under-
standing of the terms frequently used in their discussion. Force, unless completely resisted,
will produce motion. Weight is a type of force resulting from gravitational attraction.
2
2
Pressure (P) is a force-per-unit area, such as lb/in. (psi) or kN/m (kPa). Atmospheric pres-
sure is the pressure exerted by the weight of the atmosphere above Earth.
2
Pressure may be referred to using an absolute scale, lb/in. absolute (psia), or kN/m 2
2
absolute (kPa absolute). Pressure may also be referred to using a gauge scale, lb/in. gauge
2
(psig) or kN/m gauge (kPa gauge). Absolute pressure and gauge pressure are related.
Absolute pressure is equal to the gauge pressure plus the atmospheric pressure. At sea level
the atmospheric pressure is 14.7 psia using the U.S. customary units. Thus,
P absolute P gauge 14.7 psi (U.S. Customary Units) (9.1)
or P gauge P absolute 14.7 psi (U.S. Customary Units) (9.2)
2
In the SI Units where P absolute and P gauge are in kPa (1 kPa 1,000 Pa 1,000 N/m
2
1 kN/m ) their relationship can be given as follows:
P abso1ute P gauge 102 kPa (SI Units) (9.3)
or P gauge P absolute 102 kPa (SI Units) (9.4)
In essence then, absolute pressure is the total pressure. Gauge pressure is simply the
pressure read on a gauge. If there is no pressure on the gauge other than atmospheric, the