What is a larger ignition risk, a partially full fuel truck or a mostly full fuel truck?

• A. Mostly full, due to higher fuel volume
• B. Depends on weather
• C. Neither; ignition risk is the same
• D. Partially full, due to the greater mixture of fuel fumes and oxygen

Answer: (D)

Ignition risk rises when enough fuel vapor mixes with air to form a combustible vapor–air mixture. A partially filled fuel truck has more headspace, so gasoline can evaporate into that space and accumulate as vapors. With more vapor available and plenty of oxygen around, the concentration can reach the flammable range, making ignition more likely if a source is present. In a mostly full tank, there’s less headspace for vapors to collect, so the vapor–air mixture is smaller even though there’s lots of liquid; the ignition risk is reduced because there isn’t as much vapor in the air to ignite. Weather can influence vaporization, but the main factor is the amount of vapor that can mix with air in the available space—more headspace means greater risk.