Page 292 - Water Engineering Hydraulics, Distribution and Treatment

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Cross-Connection Control
Chapter 9
Chemical bulk storage and holding tanks
SODIUM
HYDROXIDE
Hose with bottom fill
Water service

Water main
break and
repair

Figure 9.2 Sodium hydroxide
“Burned in the shower” backsiphoned into the water main.

This cross-connection, which could have been avoided and in another home a washing machine explosion blew a
by the installation of a backflow preventer, created a need- woman against a wall. Residents throughout the area reported
less and costly event that fortunately did not result in serious hissing and bubbling noises, coming from washing machines,
illness or loss of life. Door-to-door public notification, exten- sinks, and toilets. Faucets sputtered out small streams of
sive flushing, water sample analysis, and emergency arrange- water mixed with gas, and residents in the area were asked
ments to provide temporary potable water from tanker trucks to evacuate their homes.
all incurred expenses on top of the original problem, all of This near-disaster occurred in one 30,000-gal (113,550-
which would have been unnecessary had the proper precau- L)-capacity liquid propane tank when the gas company initi-
tions been taken. ated immediate repair procedures. To start the repair, the tank
was “purged” of residual propane by using water from one of
two private fire hydrants located on the property. Water purg-
9.2.6 Propane Gas in the Water Mains
ing is the preferred method of purging over the use of carbon
Hundreds of people were evacuated from their homes and dioxide since it is more positive and will float out any sludge
businesses in a town in Connecticut as a result of propane as well as any gas vapors. The “purging” consisted of hook-
entering the city water supply system. Fires were reported ing up a hose to one of the private fire hydrants located on
in two homes and the town water supply was contaminated. the property and initiating flushing procedures (see Fig. 9.6).
One five-room residence was gutted by a blaze resulting from Because the vapor pressure of the propane residual in the
propane gas “bubbling and hissing” from a bathroom toilet tank was 85–90 psi (590–625 kPa), and the water pressure