What purpose does a deck water seal serve on tankers?

The purpose of the deck water seal is to prevent feedback of hydrocarbon gases from the cargo tanks via the inert gas deck main to the engine room and boiler uptake. A small leakage of hydrocarbon gas can built up to a dangerous concentration over a period of time.

What is the major function of deck water seal in an inert gas system?

A water seal is fitted which 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.

How many types of deck seals are there?

Deck water seals come in three variants known as wet type seal, semi-wet seal and dry type seal.

Why are oil tankers so big?

Until 1956, tankers were designed to be able to navigate the Suez Canal. This size restriction became much less of a priority after the closing of the canal during the Suez Crisis of 1956. Forced to move oil around the Cape of Good Hope, shipowners realized that bigger tankers were the key to more efficient transport.

Why is the bunker tank not inerted?

Generally, bunkers have a flashpoint of greater than 60 degree Celsius. If the bunker tanks cannot reach 60 degree Celsius, it cannot produce cargo vapours that are rich in hydrocarbons; thus no explosion can occur. And thus, inert gas is not being used in bunker tanks.

How does a deck water seal work?

OPERATION OF SEMI-DRY DECK WATER SEALS Instead of passing through a water trap, the inert gas flowing through the venturi creates a suction effect in the holding tank which draws the sealing water from the loop seal into the holding tank. This mitigates the undesirable ‘carry over’ of moisture to the cargo tanks.

How does deck seal work?

Why cargo tank atmosphere evaluation is done on tanker ships?

It is very important to take measurement of the tank atmosphere at various points to ensure that there are no harmful gases trapped inside.

What is the purpose of deck seal?

Deck seal: The purpose of the deck seal is to stop the gases to return back which are coming from the blower to cargo tanks. Normally wet type deck seals are used. A demister is fitted to absorb the moisture carried away by the gases.

How big are the super tankers?

These shipping economies have produced the largest ships in the world, appropriately named supertankers and have two classes—very large crude carriers (VLCC) of a size range from 160,000 to 320,000 dead weight tons (dwt) and ultra large crude carriers (ULCC) of a size range from 320,000 to 550,000 dwt.

What is the maximum allowed oxygen content in a cargo tank?

Oxygen content to less than 8\% is required to be in the tanks containing flammable cargoes. And to be able to bring the cargo tank to that oxygen level, Inert gas containing less than 5\% of oxygen is introduced in the cargo tank.

What is a water seal on a tanker?

Deck Water Seal. In an inert gas system onboard Tanker ships, Deck water seal is the principal barrier. A water seal is fitted which 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.

What happens if the deck seal fails on a gas tank?

If both the deck seal as well as the non-return valve fail, the relief valve vents the gases from the tank into the atmosphere. If during the process of separation of the ‘production’ and ‘distribution’ components the oxygen content of the gas rises above 8\%, the oxygen analyzer detects it and triggers an alarm, and shuts down the plant.

What is a 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. Deck water seals come in three variants known as wet type seal, semi-wet seal and dry type seal. This distinction is based on the manner in which they achieve their purpose and a brief description of each is as follows.

How does inert gas pass through the deck seal?

The gas passes through the deck water seal, before entering the deck line, which doubles up as a non-return valve so as to stop the explosive gas from the cargo tanks. After passing the deck seal the inert gas is then mounted to create an equilibrium with the deck water seal pressure.