Tank outage

Tank outage: Tank outage is the empty space deliberately left at the top of a cargo tank or compartment so the liquid has room to expand as it warms. Federal rules require at least 1% outage for most hazardous liquids, and 5% for materials poisonous by inhalation, at a set reference temperature. Source

The federal minimum

Liquids and liquefied gases must be loaded so that outage is at least 1% of the tank or compartment's capacity, or 5% for poison inhalation hazards, measured at a reference temperature: 115°F for a noninsulated tank and 105°F for an insulated one. Source In practice, loading racks and drivers use the tank's capacity chart and the product's temperature to set the fill.

Outage and weight

Outage sets the most a tank can hold by volume; the weight limit often sets less. On the Interstate, the truck may not gross more than 80,000 lb unless the bridge formula says less. Source A dense product reaches that limit long before the tank reaches its outage line, so many tankers run with far more empty space than the minimum. That extra space adds to surge.

In practice EXAMPLE

A fuel driver loads a compartment rated 3,000 gallons on a cold morning. At least 1% has to stay empty at the reference temperature, so the rack's meter stops well short of the brim, and the driver checks the gauge stick against the capacity chart. Across all compartments, the scale ticket then shows whether the load is under 80,000 lb. Check it with the weight limit checker, and see tank truck equipment and the tanker truck guide.

Temperature and expansion

Liquids expand as they warm, and fuels expand noticeably over a hot day. A tank loaded cold in the morning and parked in the sun in the afternoon holds a larger volume of the same product. That is why the outage rule uses a reference temperature and why loading racks measure product temperature. Drivers hauling across hot regions or parking loaded tanks for long periods should never top off past what the rack sets.

Racks and drivers record the fill for each compartment on the loading paperwork, which also shows the product and quantity for the shipping papers.