Openings: Water storage tanks must have openings for the
entry and exit of water. The opening used to facilitate water
entry is commonly located at the top of the tank to ensure the
existence of an air gap that does not allow water to be siphoned
back down the well. Water that is put into the tanks whenever
it is available, and removed when it is needed can cause
changes in shape (deformities) on the base and the sidewalls of
the tanks. Seasonal changes in ambient temperature can cause
expansion and contractions of storage tanks. For this reason
special bulkhead fittings are used that are both strong and yet
flexible enough to allow for the expansion and contraction or
slight movements of the tanks. The seal around the inspection
hatch is typically the hardest area to seal tightly, especially the
tanks made of welded thin steel. Fiberglass and poly tanks
typically have pre-manufactured coarse thread lids or bolted
hatches incorporated into their walls forming tighter seals.
Above- or below-ground water storage tanks must also
have a ventilation port that allows the air inside the tank
to escape as the tank is filled with water, and to re-enter
the tank as it is emptied. This ventilation port can double
as the safety overflow relief port in case the tank is being
overfilled with water. Screens over the ventilation port can
keep out small animals, rodents, and most insects, but not
fine dust and microorganisms. One of the most probable
paths for dust and microorganisms to get inside any potable
water storage tank is through one of the many openings.