Flammable liquid storage tanks that operate at gauge pressures above 1 psig (6.9 kPa) are considered either low pressure or pressure vessels. (See Coffee Break Training FP-2013-1 for a definition.) In those cases where liquids are stored above ambient pressure, additional attention must be paid to the emergency venting requirements. The vent designer has three options — depending upon tank size — when venting is provided by combined normal and emergency vents:
Aboveground Flammable and Combustible Liquid Tank Emergency Venting — Part 8: Venting for Pressure Tanks