Combustion equipment capable of burning both gaseous and/or liquid fuels allows facilities to transition between various fuels based on the relative availability and price of the fuels. Explosion hazards, however, exist when transitioning from one fuel to another fuel in this equipment during normal operation. This paper describes a recent explosion in a single burner, positive pressure, thermal fluid heater. The explosion occurred while transitioning the combustion fuel from natural gas to #6 fuel oil. During the transition the amount of combustion air was not sufficient to accommodate the addition of the #6 fuel oil resulting in an unstable, pulsating flame. The flame instability resulted in a partial or total extinction of the flame, followed shortly thereafter by a re-ignition of the unburned fuel/air mixture. The subsequent re-ignition created an overpressure that blew out the convection section of the heater. This paper reviews the several lessons that can be learned from this incident in order to prevent future incidents, including a review of guidelines and standards applicable to similar combustion equipment that outline control strategies for safely transitioning between fuels.
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