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
- As Cargo pump are running, there will be vacuum in cargo tanks
- If O2 is allowed to enter into Cargo tank, there will be formation of flammable range and which can lead to explosion
- So Inert Gas is used as a substitute for O2.
Inert Gas
- It is oxygen deficient gas
- It can be generated from Boiler exhaust or Main engine exhaust ( exhaust from Main Engine generally has High oxygen concentration.
- It is Dirty (Soot, carbon monoxide etc) and Hot ( temperature above 200 degree c)
- Inert gas system should able to deliver IG having O2 content less than 5 %
- It is used for Inerting, Purging or Topping Up of Cargo tanks.
- 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
- Water sprays :Cooling and Cleaning of Inert gas
- Venturi slots : Cleaning of inert gas
- Elephant foot
- Impingement Plate
- Baffle plate
- 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.
- If oxygen concentration is less than 5 %, then it allow to inert gas to towards Cargo tank.
- 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.
- Gauge glass of Deck seal should be clean and water level should be visible.
There are 3 types of Deck Seal
- Dry type
- Semi wet type
- 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.
- There is a individual PV valve for each tank
- 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.
- It is filled with water and anti freezing solution is also added.
- Gauge glass of PV Breaker should be clean and water level should be visible.
Reason due to which PV valve will not get activated
- If it is properly maintained
- If it is not properly calibrated
- If cargo is frozen near the opening of Pressure Vacuum Valve and not allowing it to open.
- 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
- High Casing Temperature
- Low Scrubber Water
- No Flow Scrubber Water
- Low Flow Deck Seal Water
- No Flow Deck Seal Water
- Soot Blowing Interlock
Alarms
- Scrubber Low Level
- Scrubber High Level
- Deck Seal Low Level
- Deck Seal High Level
- High O2 Content
- High Blower Casing Temperature
- Inert gas Failure
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