A 5-gallon propane tank stored on the bottom rack of a barbeque grill vented propane from the safety relief valve. The flammable vapor was ignited by the grill burners and the ensuing fireball injured three adults seated at a patio table, approximately eight feet away. Exponent determined that the tank had been overfilled and the safety relief valve functioned as designed by shutting off propane flow after the over-pressure had vented. Exponent also conducted a full scale accident reconstruction with a deliberately overfilled tank with a similar safety relief valve.