for a plastic storage tank should be sized to keep the tank at atmospheric pressure internally.
Undersizing a vent can create a vaccuum or a over-pressurized situation
that can deform or damage the tank.
The ASTM D
1998 standards for vertical polyethylene tanks (over 500
gallons) specify the following. "The vents must comply
with OSHA 1910.106 (F) (iii) (2) (IV) (9) normal venting
for tanks, or other accepted standard, or shall be at
least as large as the filling or withdrawal connection,
whichever is larger but in no case less than 25.4 mm [1
in.] nominal inside diameter."
general rule of thumb, we suggest that the vent be one
inch larger than the largest fill or drain connection to
the tank. If the tank is to be filled from a
tanker using pneumatic pressure, the vent should be
enlarged to accomodate the extra volume of air that will
be pushed into the tank.
common type of vent that we install on plastic storage
tanks is a screened PVC U-vent. The vent is
normally installed at the top of the tank dome.
Other types of vents are available when required to
accomodate specific storage applications.