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304 Chapter 9 Cross-Connection Control
9.2.6 Propane Gas in the Water Mains

Hundreds of people were evacuated from their homes and businesses in a town in
Connecticut as a result of propane entering the city water supply system. Fires were re-
ported in two homes and the town water supply was contaminated. One five-room resi-
dence was gutted by a blaze resulting from propane gas “bubbling and hissing” from a
bathroom toilet and in another home a washing machine explosion blew a woman against
a wall. Residents throughout the area reported hissing and bubbling noises, coming from
washing machines, sinks, and toilets. Faucets sputtered out small streams of water mixed
with gas, and residents in the area were asked to evacuate their homes.
This near-disaster occurred in one 30,000-gal (113,550-L) capacity liquid propane
tank when the gas company initiated 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 purging is the preferred method of purging
over the use of carbon dioxide since it is more positive and will float out any sludge as
well as any gas vapors. The “purging” consisted of hooking up a hose to one of the
private fire hydrants located on the property and initiating flushing procedures (see
Fig. 9.6).
Because the vapor pressure of the propane residual in the tank was 85 to 90 psi (590 to
625 kPa), and the water pressure was only 65 to 70 psi (451 to 486 kPa), propane gas back-
pressure backflowed into the water main. It was estimated that the gas flowed into the
3
3
water mains for about 20 min and that about 2000 ft (57 m ) of gas was involved. This
was approximately enough gas to fill 1 mile of an 8-in. (1.61 km of a 200-mm) water main.

Hose used for propane
tank purging cross- Fire
connected to private fire
hydrant

Recommended
backflow preventer
installation

Water main
pressure 65 psi

Explosion
Figure 9.6 Propane Gas in the Water Mains. Conversion factor: 1 psi 6.94 kPa