Inert Gas System(I.G systems)

Inert Gas 

• Inert gas is a gas or vapour which will not support combustion and will not react with the cargo.
• Inert gas does not support life.

 Inert gas system

• Vapour spaces within cargo tanks and, in some cases, spaces surrounding cargo tank may requires controlled atmosphere.
• Inert gas system is the most important integrated system for oil tankers for safe operation of the ship
• IG is used in cargo tanks to prevent fire,to protect cargo from polymerization,to prevent oxidation,to prevent humidity.
• Inert gas system spreads the inert gas over the oil cargo  which increases the lower explosion limit LEL (lower concentration at which the vapors can be ignited), simultaneously decreasing the Higher explosion limit HEL (Higher concentration at which vapor explodes). When the concentration reaches around 10 %, an atmosphere is created inside tank in which hydrocarbon vapors cannot burn. The concentration of inert gas is kept around 5% as a safety limit.
 

 

Boiler uptake valve 
             Provides a take off point for the flue gas, A cleaning arrangement is fitted to prevent soot build up.
 

Scrubber 
             Flue gas passes to the scrubber via the uptake valve, here it enters at the bottom via a waterseal and passes up through a series of sea water sprays and baffle plates being cooled and cleaned before exiting via a demister .

The water is supplied via the scrubber pump, the sprays reduce the temperature to within 2^oC of the sea water temperature, the sulphur dioxide content is reduced 90%, and the gas is clear of soot.          

The tower is rubber lined and other parts are made of inconel or glassfibre to protect against the SO2 .The water seal at the bottom is provided by the weir arrangement fitted to the drain system. 

The following alarms are fitted;

·         High water level alarm and trip

·         Low water level alarm

·         Low water level alarm

    

   Fan

              In some ships two types of fans are fitted, a steam turbine driven one of sufficient capacity to supply I.G requirements during cargo ops, and an electrically driven unit with sufficient capacity for topping up only.

 

Shut down of the fans occurs due to;

·         High gas temperature after the blowers

·         Low scrubber water pressure, high scrubber water lvl.

                           Blower failure             

     

      A blank on a stub pipe  removed to allow the fans to blow fresh air up the I.G. main when gas freeing. 

Recirculating and regulating valves

          The pressure within the tanks is controlled by automatic or manual operation of the regulating v/v, if the demand is low and the regulating v/v nearly closed then the recirculating v/v opens thereby reducing the possibility of the fan overheating by passing the gas back to the scrubber tower. 

Oxygen monitor 

         Fitted just upstream of the regulating v/v and initiates an alarm if oxygen content above 8%. 

Deck water seal 

          The inert gas leaving the engine room to deck passes through the deck water seal whose purpose is to prevent gases from passing back to the engine room from the cargo tanks. A demister is fitted on the outlet side.

The seal is internally rubber lined and a heating coil fitted to allow use in low temperatures. The weir controlled water level has a low level alarm fitted.

As the seal is a primary safety feature it is supplied from the general salt water system when the scrubber pump is not in use. Checks are made on the pipe from the seal to the regulating v/v to ensure no HC prescience which would indicate corrosion damage

Non-return v/v- Purpose similar to the deck seal 

Deck isolating valve 

       Allows the isolation of the deck system from the engine room system

Pressure/vacuum breaker ( PV) 

        This is the safety valve for the system and prevents both under and over pressurisation of the tanks. Flame screens are fitted to the PV breaker vent. The PV breaker may be of the mechanical or liquid type. The liquid type is filled with a liquid of the correct specific gravity, such as monoethylene Glycol and water. 

Deck distribution system

                  After the PV breaker the inert gas is led down a main and valveless distributors to all the tanks, excepting the slops tanks whose lines contain isolating valves. On the main there are three PV hi velocity safety vents, the slops tanks are fitted with individual safety vents of same design.

On the for'd end of the main is the for'd pressure release which allows regulation of the inert gas pressure during loading

During loading the ballast is discharged at the same time as the cargo is loaded thereby limiting vapour release to deck, the I.G. plant is kept running in a state of readiness so in the event of sudden cargo stoppage pressure is not lost via the for'd pressure release; this may have to be throttled in to prevent overloading the fans.