Inert gas system IG System oil tankers components

IG system is system which used mainly on oil tankers . Oil tankers load oil of different characteristics, grade and quality . These can can produce flammable hydrocarbon vapours or gases when loaded on board oil tankers . When these harmful vapours react with air with good amount of oxygen it will result in explosion, which may result in accidents, loss of life or harm to marine environment. The presence of harmful vapours is seen sometimes even if there is no cargo loaded in the tank.

To prevent the explosion an inert gas system should be fitted onboard oil tankers. IG system can either be a separate plant or flue gas produced by ship's boiler. Inert gas system (IG) is the most important system which is to be integrated on an oil tanker. A minimum amount of oxygen is required for the vapour to explode. Inert gas contains insufficient oxygen (8%) , that is it prevents the flammable hydrocarbon gases from combustion.

Once introduced to an oil tank the inert gas increases the lower explosive limit LEL( Lower concentration of a hydrocarbon gas to support combustion) and reduces the upper explosive limit UEL( Highest concentration of a hydrocarbon gas to support combustion). This occurs because of the reduction of oxygen in the tank. Introduction of the inert gas to the tank reduces the flammable range of the vapours and it subsequently creates an atmosphere in which hydrocarbons can't burn. 

Components of the inert gas system: 

  • Exhaust gases source : Source of inert gas is from exhaust uptakes of boiler or using flue gases.
  • Inert gas isolating valve : This valve acts as a supply valve which gives the uptakes of boiler to the rest of the system. This valves also serve the purpose of isolating both parts while not in use.
  • Scrubbing Tower : Source gas then passes through the scrubbing tower . Gas enters through the bottom of the tower and is subjected to water spray which causes the gas be clean and moist. As a result the SO2 level decreases significantly and the gas gets soot less.
  • Demister : It is used to absorb the moisture and water from the gas. This is made of polypropylene.
  • Gas blower: Two blowers are used . An electrically driven blower for topping up and a steam driven blower for the IG operation.
  • IG Pressure regulating valve : According to the properties of oil and atmospheric conditions the pressure inside the tank varies. Due to this variation the blower fan may overheat. To prevent this a regulator valve is attached after the blower. It sends the excess gas back to the scrub tower .
  • Deck seal : This ensures to seal and prevent the returning of the gases which comes from blower to the cargo tanks. Wet type of deck seals are used normally.
  • Mechanical non return valve: This also serve the same purpose as a deck seal and is fitted in line with the deck seal.
  • Deck isolating valve : The valves help in isolating the deck system from the engine system
  • Pressure vaccum (PV) Breaker : It helps in controlling the pressurization of the tanks. It ensures that the pressure inside the tank is not over or under. It is fitted with a flame trap to prevent fire during loading and discharging of cargo .
  • Cargo tank isolating valves : There can be many tanks on a vessel. Each tank may be fitted with an isolating valve. This is to control the flow of gas to the holds. If a hold does not require the gas the hold can be isolated with this valve. This valve is to be operated by a responsible officer.
  • Mast riser : It maintains a positive pressure of inert gas inside the hold before loading and during loading. It is kept open to avoid the pressurization of the cargo tank.
  • Safety and alarm system: The IG system is provided with safety and alarm systems to ensure the safety of tank and its own machinery.