DECK SEAL OR DECK WATER SEAL

The purpose of the deck water seal is to prevent the back flow of hydrocarbon gases from the engine room and boiler uptakes via the IG main. 

1. WET TYPE SEAL

The basic form of deck seal is the wet type, in which the gas passes through a water reservoir and a demister before exiting to the deck main.                                                                             

  As long as the gas pressure is more than the pressure in the tanks, there is no danger of a back flow. In case inert gas pressure falls, creating a danger of a back flow the water forms a plug and hydrostatic head, preventing the flow of hydrocarbon gases back to the boiler uptake. The disadvantage of a wet seal is that water carry-over may be high, resulting in increased corrosion. 

2. DRY TYPE SEAL

     In the dry type seal, a sensor release water from a holding tank if a loss of pressure is detected. The advantage is that there is no water carry over, but there is a risk of failure of the automatic control valves rendering the system ineffective. 

3. SEMI-DRY TYPE SEAL

  In a semi-dry type deck water seal, the inert gas flow passing a venture draws the sealing water into a separate holding tank by vacuum action, reducing the water carry-over. The disadvantage of this system is that a blockage of the venturi tube may allow water into U-bend. Alternatively, a blockage of the tube leading to the U-bend may prevent water from entering the seal and render the system ineffective.