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A correctly sized vent keeps the tank at atmospheric pressure while it fills and drains.
Plastic tanks are built for atmospheric pressure only. An undersized vent can let pressure build while the tank fills or pull a vacuum while it drains, and either can deform or damage the tank.
ASTM D 1998 calls for vents that meet OSHA normal-venting requirements or are at least as large as the largest fill or withdrawal connection, and never smaller than 1 inch.
As a rule of thumb, we suggest a vent one inch larger than the largest fill or drain connection. If the tank is filled from a tanker using air pressure, the vent needs to be larger still to handle the extra air pushed into the tank.
Our most common vent is a screened U-vent on the top of the dome, in white PVC or gray CPVC for sizes 4″ and smaller, set on a 5″ nipple to hold it up off the tank. Other vent types are available for specific chemicals and applications.
Talk to a tank specialist