Abstract: A pressure relief door for an enclosed vessel such as a combustion chamber for an incinerator. The door is pivotally attached to the combustion chamber and is movable between a closed position wherein it confronts and seals a vent duct attached to the chamber and an open position wherein said vent duct is unsealed to permit excessive pressure to be relieved from the chamber. A liquid holding trough is attached around the perimeter of the duct and is adapted to retain therein a liquid at a predetermined level. A seal member is attached around the perimeter of the door and is adapted to extend into the liquid to achieve an airtight seal between the door and duct when the door is in a closed position.