Inert gas

Inert Gas System

Nobody wants a " Fire Triangle and  Vacuum or  Excessive Pressure " to form in Cargo tanks on a Oil tanker. 

During discharging

1.    As Cargo pump are running, there will be vacuum in cargo tanks
2.    If O2 is allowed to enter into Cargo tank, there will be formation of flammable range and which can lead to explosion 
3.   So Inert Gas is used as a substitute for O2.

Inert Gas 

1. It is oxygen deficient gas 
2. It can be generated from Boiler exhaust or Main engine exhaust ( exhaust from Main Engine generally has High oxygen concentration.
3. It is Dirty  (Soot, carbon monoxide etc) and Hot ( temperature above 200 degree c)
4. Inert gas system should able to deliver IG having O2 content less than 5 %
5. It is used for Inerting, Purging or Topping Up of Cargo tanks.
6. Inert gas is used in oil and product tanker 

It is not used in Chemical Tanker , as IG is dirty and can contaminate chemical cargo.

On chemical tanker there is Nitrogen generator ( It is a  inert in nature and clean gas) 

Inert gas system can be used in gas free mode for entry into cargo tank. 

Components of Inert gas system

Scubber : Cooling and cleaning of inert gas happens in scrubber.

Various parts in scrubber are 

1. Water sprays :Cooling and Cleaning of Inert gas  
2. Venturi slots : Cleaning of inert gas 
3. Elephant foot
4. Impingement Plate
5. Baffle plate
6. Demister pads : Absorbs the moisture from Inert Gas, if it is nor there, it can lead to cargo tank corrosion.

Blowers: Blowers has a capacity of 125 % of total pumping capacity, so that cargo tanks should never go in vacuum. There are 2 blowers  

Oxygen analyser: It analysis the oxygen concentration in Inert gas.

1.  If oxygen concentration is less than 5 %, then it allow to inert gas to towards Cargo tank.
2.  If oxygen concentration is more than 5 %, then it  does not allow  inert gas to go towards Cargo tank and is vented to atmosphere.

Non Return valve : It prevent back flow of cargo vapor from hazardous area to Non hazardous area

Deck seal : It prevent back flow of cargo vapor from hazardous area to Non hazardous area.

1. Gauge glass of Deck seal should be clean and water level should be visible. 

There are 3 types of Deck Seal

1. Dry type
2. Semi wet type
3. Wet type

PV valve : Pressure vacuum valve is similar to whistle fitted on Pressure cooker used in our home. If a Pre-set value of Pressure \ vacuum reaches in a cargo tank, then Pv valve will activate automatically or manually also. 

1. There is a individual  PV valve for each tank
2. Height of Pressure Vacuum valve is low but the venting speed is high so that cargo vapour should accommodate at deck level.

PV Breaker : It is a safety device, if PV valve does not lifts up due some reason, then it order to save cargo tanks from excessive pressure\ vacuum, PV breaker will get activated.

1. It is filled with water and anti freezing solution is also added.  
2. Gauge glass of PV Breaker should be clean and water level should be visible. 

Reason due to which PV valve will not get activated 

1.  If it is properly maintained
2. If it is not properly calibrated 
3. If cargo is frozen near the opening of Pressure Vacuum Valve and not allowing it to open.
4. If it is stuck

Mastriser is also a venting device, which is well above deck level (high height)  but the venting speed is at a lower.

Main deck valve: Normally a gate valve on deck.

Tank Isolation valve : Every cargo tank has a IG isolation valve which should a "Opened and Locked"

Various Trips and Alarm in Inert gas System

Trips

1.  High Casing Temperature 
2. Low Scrubber Water 
3. No Flow Scrubber Water 
4. Low Flow Deck Seal Water 
5. No Flow Deck Seal Water 
6. Soot Blowing Interlock 

Alarms

1.  Scrubber Low Level
2. Scrubber High Level 
3. Deck Seal Low Level
   
4. Deck Seal High Level 
5. High O2 Content 
6. High Blower Casing Temperature
7. Inert gas Failure

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