Outage Requirements by Commodity
Outage is the empty space required at the top of a tank compartment to allow for thermal expansion of the liquid. Outage requirements vary significantly by commodity. For petroleum products, the standard outage is 1-2% of compartment volume for gasoline and diesel fuel at ambient temperatures, expanding to 2-4% in hot weather due to the significant thermal expansion coefficient of petroleum distillates — approximately 0.00065 per degree Fahrenheit. Gasoline expands by about 0.6% for every 10°F temperature rise. For food-grade liquids such as milk, juices, and edible oils, outage requirements are typically 1-3% to allow for product foaming during loading and thermal expansion. Some regulatory requirements for food-grade tankers specify minimum outage in gallons rather than percentages, typically leaving 2-5 inches of ullage space in each compartment. For chemical commodities, outage requirements can be substantially higher. Liquid nitrogen, for example, requires a minimum 10% outage because cryogenic liquids can expand rapidly if warming occurs unexpectedly. Hydrofluoric acid, sodium hydroxide, and other corrosive liquids typically require 2-5% outage specified in the product's Safety Data Sheet (SDS). For any commodity with a flash point below 100°F (flammable liquids), the outage serves both thermal expansion and vapor pressure management functions — maintaining sufficient ullage space prevents the vapor space from becoming over-pressurized.