Each tank needs an atmospheric vent and an emergency vent. Dual wall tanks require two emergency vents; one for the primary tank and one for the secondary tank. The secondary emergency vent is sized to match the primary emergency vent.
Definitions
Atmospheric Tank — A storage tank that has been designed to operate at pressures from athospheric through 1.0 PSIG (760 mm Hg through 812 mm Hg) measured at the top of the tank (NFPA30 Pg. 30-8). Pressure not to exceed 1.0 PSIG under normal operation, and 2.5 PSIG under emergency conditions (PEI RP-200).
Emergency Venting — Emergency Vent (pressure relief only) used on Aboveground Storage Tanks, as a code requirement, to help prevent the tank from becoming over-pressurized and rupturing if exposed to fire. UL Listed.
Pressure Relieving Devices — Defined in NFPA 30 2-3.6.4, where entire dependence for emergency relief is placed upon pressure relieving devices, the total venting capacity of both normal and emergency vents shall be enough to prevent rupture of the shell or bottom of the tank if vertical, or of the shell or heads if horizontal.
Atmospheric Vents - Two types of atmospheric vents are used. Diesel fuel and oils use an open atmospheric vent. Gasoline uses a pressure/vacuum vent. Pressure/vacuum vents serve multiple purposes including reducing vapor loss and enabling vapor recovery systems.
