Deck Water Seal – Design, construction and purpose in Inert Gas (IG) system on board ships
The Deck Water Seal
The deck water seal is the main safeguard against the reverse flow of gases from the distribution system to the IG plant.
SOLAS Regulation:
SOLAS Chapter 11-2, Regulation 62, provides the requirements for the non-return devices installed in the inert gas main of a typical system.
Regulation 62 requires that the inert gas system is equipped with two non-return devices, one of which shall be a water seal, to prevent the return of hydrocarbon vapour to the gas-safe spaces.
In addition, Regulation 62 requires that the arrangement of the deck seal shall be such that it will prevent the backflow of hydrocarbon vapours and will ensure the proper functioning of the seal under operating conditions.
The IMO “Guidelines for Inert Gas Systems”, Sections 3.6 and 3.7, indicate the design considerations, and provide details on the function, of various types of deck water.
TYPES OF DECK SEAL:
- Dry type deck seal.
- Semi dry deck seal.
- Wet type deck seal.
The picture below shows the construction of a wet type deck water seal and its functioning. Basically it consists of a chamber semi-filled with water and two pipes for inlet and outlet of flue gases while another two small pipes denote inlet and outlet for sealing water. There is a demister pad to remove water droplets from gas. The operation of this device is pretty simple and the two diagrams shows conditions where the inert gas is flowing from the plant to the distribution area and the right hand side showing a condition where back pressure tends to push cargo gases into the IG system and is prevented by the water seal.
Semi Dry Type Deck Water Seal
The construction of this type of seal as well as the functioning under both conditions is shown in the diagram below. The main difference with the previous type of seal is that it uses venture action to draw water when there are chances of back flow of the gases thus reducing if not completely eliminating water carry over to the cargo tanks.
Dry Type Deck Water Seal
This seal totally eliminates any water carry over and uses automated valve control to deliver water to the seal in case there is any back flow but the only disadvantage is that if automation system fails then there is a danger of blow back of cargo gases.
Just keep in mind that only one of these seals is used, and in addition to this deck seal there is a mechanical non return valve for extra safety in the pipeline as well. We will study about the design considerations of non-return devices used in the IG system in our next article.
