The deck water seal is the principal barrier; a water seal is fitted that permits inert gas to be delivered to the deck main but prevents any backflow of cargo gas, even when the inert gas plant is shut down; it is vital that a supply of water is maintained to the seal at all times, particularly when the inert gas plant is shut down; in addition, drains should lead directly overboard and not pass through the machinery spaces; one of three principal types of design may be adopted.
Types of Deck Water Seal:
This is the simplest type of water seal; when the inert gas plant is operating, the gas bubbles through the water from the submerged inert gas inlet pipe, but if the tank pressure exceeds the pressure in the inert gas inlet line, the water is pressed up into this inlet pipe, thus preventing backflow; the drawback to this type of water seal is that water droplets may be carried over with the inert gas, which, although not impairing the quality of the inert gas, could increase corrosion; a demister should, therefore, be fitted in the gas outlet from the water seal to reduce any carry-over.
Instead of bubbling through the water trap, the inert gas flow draws the sealing water into a separate holding chamber by venturi action, thus avoiding or at least reducing the amount of water being carried over; otherwise this seal is functionally the same as the wet type.
In this type, the water is drained when the inert gas plant is in operation (gas flowing to the tanks), and filled with water when the inert gas plant is either shut down or the tank pressure exceeds the inert gas blower discharge pressure; filling and drainage are performed by automatically operated valves controlled by the levels of the water seal and drop tanks and by the operation of the blowers; the advantage of this type is that it prevents water carry-over; the drawback could be the risk of failure of the automatically controlled valves that may render the water seal ineffective.