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Back siphonage |
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| Back-siphonage is the reversal of normal flow
in a system caused by a negative pressure (vacuum or partial
vacuum) in the supply piping. |
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Back- siphonage can be created when there is
stoppage of the water supply due to nearby fire-fighting, repairs
or breaks in the water main etc. The effect is similar to the
sipping a drink through a straw, which induces a flow in the
opposite direction. |
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| If you have a combination boiler fitted in your
house you may notice that you can only run one tap effectively
at a time. This is because combination boilers with a two-gallon
per minute flow rate cannot provide enough water to satisfy
the flow rates of two taps. Try this experiment:
Turn on your bathroom sink, hot water tap; now turn on your
bath hot water tap. The chances are that the sink tap has
stopped flowing but is still open. This means that the water
within the tap has been drawn back to the level of the bath
tap. If your sink tap had been connected to a temporary shower
hose, (e.g. when dying or bleaching hair) and this was submerged
in water, natural siphonage would occur sucking the contents
of your sink back into the mains water supply. |
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A garden hose or sprinkler system can be extremely hazardous
because they are often left submerged in fish ponds, lay in
elevated locations above the level of the out side tap, or used
with chemical sprayers used for weed-killing or fertilization. |
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| In October 1991, parasitical worms were found
in the water at two homes after a malfunctioning lawn sprinkler
system, coupled with a water main break sucked nematodes into
the water system. The nematodes first showed up in the evening
of October 1st after the backflow prevention system failed.
When the water pressure dropped, the vacuum in the system sucked
the water from the sprinkler system into the mains water supply. |
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The homeowner who found the worms swimming around in his bath
tub when he started to fill it for his child said; he was appalled
to find the critters, as well as rust and other debris in his
water. "The only reason I noticed it was because I have
children and was giving them a bath". If there was a screen
on the tap the chances are he would not have noticed it |
