Understanding Outage and Fill Limits
Outage is the empty space intentionally left in a cargo tank to account for thermal expansion of the liquid load. Liquids expand when temperature increases, and if a tank is filled completely, thermal expansion can cause the tank to rupture, valves to fail, or pressure to build to dangerous levels. The general industry standard is an 80% fill rule — tanks are loaded to no more than 80% of their rated capacity. However, the precise outage required varies by product type and the temperature range anticipated during transport.
For petroleum products such as gasoline and diesel, the coefficient of thermal expansion is approximately 0.0006 per degree Fahrenheit. This means for every degree Fahrenheit increase in temperature, a gallon of gasoline expands by about 0.06%. This seems small, but in a 9,000-gallon tanker, a 50°F temperature swing (common in summer conditions) could add roughly 270 gallons of apparent volume. If the tank had no outage, that expansion would have nowhere to go.
Drivers loading petroleum products must check the cargo temperature at the terminal and calculate the expected temperature the product will reach during delivery. The difference between load temperature and maximum expected ambient temperature determines how much outage to leave. Terminal loading racks often display product temperature, and the driver records this on the bill of lading. Understanding this calculation protects the driver from overload violations, cargo damage, and spills.
Cryogenic liquids such as liquid nitrogen or liquid oxygen require much larger outage because they are loaded at extremely cold temperatures and will warm during transport. These products may be transported at temperatures of −300°F or colder, and any significant warming causes rapid expansion. Cryogenic tankers use vacuum-jacketed tanks with pressure relief valves calibrated to vent safely before catastrophic overpressure. The driver must verify that relief valves are properly set and unobstructed before departure.