Inert gas system is one of the most critical component of a tanker vessel.
Vessels using inert gas in their tanks are enjoying the benefits of safe operation against fire and explosion.
No smoking on deck, no naked lights, intrinsically safe equipment only and always following rules and regulations put tankers as one of the safest ships.
But IG system takes us to a whole new level of safety.
This takes me to my cadetship where our vessel was not equipped with an IG system.
I thought it was normal and okay until our Captain explained to me the risk of not having one. After that explanation, I got very very careful on deck.
First off, let’s see some historical record stressing the importance of IG.
Accidents and Inert Gas
Sixteen years ago, a group of investigators reviewed 35 incidents involving fire and explosions on tanker ships’ cargo deck area.
In a report that followed, it was noted that most explosions happened during tank cleaning and gas freeing operations.
A lot of them occurred inside the cargo tank where tank atmosphere was not monitored.
But the profound case they discovered was that none of the accidents happen during the operation of inert gas!
What is an Inert Gas System
Going back to our training days, an IG system is a component in an oil or product tanker that produces an inert gas. This gas is used inside the tanks to lower its oxygen content and other hydrocarbon contents.
Since this is a system, it is composed of different parts from production in the engine room to distribution, monitoring and venting on deck.
Triangle of Fire
To understand better the use case of IG, let us first visit a review on the triangle of fire.
There are three main components of fire namely fuel, oxygen and heat. Add a chemical reaction in the middle and you have a blaze.
Now the ship’s cargo itself is a massive fuel already. Surrounding it is an ever abundance of oxygen.
And since the cargo is flammable, you don’t need that much heat to start a fire. You only need a spark!
Out of the three components of fire, oxygen is most easily controlled.
We can replace oxygen with something else, something that doesn’t support combustion and at the same time, cheap, abundant and very easily produced.
Enter Inert Gas.
What is an Inert Gas?
An inert gas is a mixture of gas which contains very little amounts of oxygen, so little that it doesn’t support combustion.
Let’s take a look at the components of this gas.
Around 12% of oxygen is needed for a hydrocarbon gas to burn or continue burning.
Here lies its importance. It lowers the oxygen content inside the tanks to make combustion or explosion not possible.
Regulation states that a tanker should be inerted to 8% oxygen by volume. However, that level is reduced to 5% for port regulations.
Three Sources of Inert Gas
Inert gas can be generated in any of the following machinery on board.
- Uptake gas from the ship’s main or auxiliary
An independent Inert Gas Generator (IG Plant).
A gas turbine plant equipped with afterburner.
Before digging in further, let us discuss first a few definition involving the use of IG.
Inert Condition – an atmospheric condition in the tank to which the oxygen has been reduced to 8% or less by volume by addition of inert gas.
Inerting – means the introduction of inert gas into a tank with the object of attaining the inert condition.
Inert Gas Plant – means all equipment specially fitted to supply, cool, clean, pressurize, monitor and control delivery of IG to cargo tank systems.
Inert Gas System – means an IG plant and IG distribution system together with means for preventing backflow of cargo gases to the machinery spaces, fixed and portable measuring instruments and control devices. In essence, this is the IG plant and IG distribution system.
Inert Gas Distribution System – means all piping, valves, and associated fittings to distribute IG from the IG plant to cargo tanks, to vent gases to atmosphere and to protect tanks against excessive pressure or vacuum.
Purging – means the introduction of inert gas into a tank already in the inert condition with the object of:
- further reducing the existing oxygen content; and/or
- reducing the existing hydrocarbon gas content to a level below which combustion cannot be supported if air is subsequently introduced into the tank.
Topping up – means the introduction of IG into a tank which is already in the inert condition with the object of raising the tank pressure to prevent any ingress of air.
How IG is Delivered on Deck
- An IG generator creates an inert gas buy burning its fuel. The combustion burns the air so that the remaining oxygen is reduced to the required level.
- We now have an IG which is still a bit dirty. Before it reaches into the cargo tanks, it is first cooled, refined and filtered to remove excessive soot and water vapor.
- The gas passes into a scrubber and demisters to purify it. Two blowers pushes these gases into the distribution system until it reaches the cargo tanks.
- The OOW in the CCR monitors the gases including its rate of flow. An alarm sounds off whenever its oxygen content falls above 8%.
- Before the IG goes into the cargo tanks, it passes into a Deck Water Seal, also know as deck seal. Here, the gas is “washed”. At the same time, the deck seal prevents backflow of hydrocarbon gases from the cargo tanks into entering the engine room.
- Some safety devices are installed to prevent over pressure and under pressure inside the tanks.
Operations Requiring the Use of Inert Gas
As we discussed in the beginning of this article, inert gas plays an important role for safe operations on board.
Hydrocarbons that are present in our cargoes are very flammable. Here are deck operations where inert gas is required.
1. Discharging Operation
When a tanker is discharging her cargo, the pressure inside her tank becomes negative. This negative pressure triggers the vacuum valve of our P/V valve to suck in air to avoid implosion of our tanks.
However, introducing fresh air inside our tanks poses the risk of fire and explosion.
Instead of fresh air, inert gas is introduced at 125% of the cargo discharge rate to keep the tank in positive pressure.
2. Tank Cleaning Operation
Cargo tank should be washed in an inert condition while keeping the pressure inside the tank positive.
This is where IG is used.
There is also a build up of static electricity inside the tank as high pressure tank cleaning is utilized to take out the remaining cargo residue.
If the tank is not inerted, explosion could occur.
3. Purging Prior to Gas Freeing
Whenever a cargo tank needs to be inspected, it must attain a gas free condition.
This means that the atmosphere inside the tank should have 20.9% oxygen by volume, 0% LEL and toxic gases below the Threshold Limit Value or TLV.
To accomplish this, air is introduced. But before pumping air inside cargo tanks, its hydrocarbon gas must be reduced to avoid flammable mixture.
Purging the tank with inert gas achieve this goal.
4. Prior Entering Port
Port facilities require that the oxygen level inside the tank be less than 8%.
Thus, purging the tank or topping it up with inert gas enables the ship to avoid heavy fines or denied of port entry.
Inert Gas Introduction into the tanks
There are three main ways that inert gas is introduced into tanks especially if the tank is from a gas free condition.
Knowing which method enable us to understand the readings we receive when checking for the tank atmosphere.
Moreover, each method offers variety of choices to the seafarer as to which one works best on that particularly ship.
1. Dilution Method
This is a mixing process of inert gas and the current gas inside the tank.
IG is introduced into the tank inhigh velocity. It then mixes inside forming a homogeneous mixture.
This mixture exits outside the tank via purging pipe thereby lowering the oxygen content inside the tank over time.
2. Displacement Method
This method works in a process by which the inert gas being introduced into the tank displaces the current gases inside.
It is done slowly so as not to mix the IG with the gases inside.
The inlet of IG and outlet of the purged gases are located at opposite direction. If you introduce inert gas from the aft topside of the tank, the outlet of purged gases should be on the forward bottom part.
3. Cascading Method
The third method uses either of the two above but takes it even further.
Instead of releasing the purged gas directly into the atmosphere, it is transferred to another tank by means of cargo piping system.
This means that you are also purging that tank together with the ones where you introduced the inert gas.
Inert gas is an asphyxiant. You could die or suffer brain damage if exposed at long intervals.
During any inerting operation on deck, it is best to keep away and stay inside the accommodation. Lock all water tight doors and recirculate the air to avoid taking in IG in your blower intakes.
If for some reason you need to get out, you must call the bridge, inform OOW and bring a radio with you.
Have a safe operation on board and
May the winds be in your favor.