Which hazard involves the burning of a vapor cloud over a pool of flammable liquid that vaporizes as it comes into vapor-liquid equilibrium?

• A. Flash
• B. Ignition Source (Heat)
• C. Pool
• D. Jet

Answer: (C)

The main idea is a pool fire: a flame that sits on and is fed by vapors coming off a pool of flammable liquid. When heat makes the liquid evaporate, these vapors form above the surface and, at vapor–liquid equilibrium, continue to feed the flame as more liquid keeps vaporizing. So the hazard is the pool itself acting as the fuel source for burning vapors, not just a one-time ignition or a dispersed vapor cloud. This differs from a flash, which is a quick ignition of a vapor cloud away from a liquid surface; it’s also not a jet, which would involve a spray or directed flame, or simply an ignition source initiating a fire.