Deck seals are a safety device fitted on oil tankers to prevent explosive hydrocarbon vapours to travel from the Cargo area to the Engine area (where the sources of Ignition are present.)
In other words, the back draft of gas picks up the water in the deck seal up the return line and it is the weight of the water column that prevents the gas from going to the flammable area. As a result, the deck seal needs to be big enough to accommodate the quantity of water required.
Purpose of deck seal:
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. Since a mechanical non return valve will permit a very slIGht leakage, it is necessary to provide a liquid seal. The deck water seal is manufactured in mild steel and is internally coated with glass flake.
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.
Dry Type Deck Seal
The dry type deck water seal has an upper reservoir (drop tank) and a lower reservoir (sealing tank). The flow of water from the drop tank to the sealing tank and from the sealing tank to the overboard discharge is controlled automatically via level sensors and other control equipment.
The above figure indicates a typical dry type deck water seal with upper and lower reservoirs. The upper reservoir is filled with water at all times and supplies water to the lower reservoir.
The lower reservoir is empty during system operation (gas flowing to tanks) and filled with water when the inert gas blower is shut down, either because of normal stopping or because of automatic system shutdown activated by off-design conditions. The filling and drainage of the upper and lower reservoirs is triggered automatically by the sensing of levels in the sealing tank and drop tanks, by the on/off status of the blowers or by the gas-to-tank push button switch. The upper and lower reservoirs are usually equipped with float-type level sensors.
The IMO Guidelines indicate that the deck seal lower reservoir will be filled with water when the cargo tank pressure exceeds the blower discharge pressure. However, there appears to have been little action by administrations to ensure that this detail of the Guidelines is followed by, for example, requiring devices to sense the pressure differential between the inert gas main and the pressure at the blower discharge.
Semi Dry Deck Seal
The semi-dry deck water seal has a separate holding chamber which is connected by pipe to the inert gas main via an eductor. 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.
When the gas flow ceases, the venturi effect is lost and the water from the holding tank rapidly flows back into the inert gas main loop seal thus creating a water seal which serves to prevent the backflow of hydrocarbon vapour and/or inert gas from the hazardous to non-hazardous area.
Wet Type Deck 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 and thus prevents backflow. The drawback of this type of water seal is that water droplets may be carried over with the inert gas which, although it does not impair 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.