Purging & Gas Freeing — From Inert Blanket to Breathable Air

After discharge a cargo tank is inert but loaded with hydrocarbon vapour — it cannot burn, yet it is one careless fan away from being able to. Getting from that atmosphere to one a person can enter is a two-stage journey: purge with inert gas until the fuel is diluted below ~2% by volume, and only then gas free with fresh air to 20.9% O₂. Tap a step or press "Play" to follow the gas through the tank — then take the simulator and walk the tank atmosphere across the flammability chart yourself without crossing the envelope.

Tank atmosphere: after discharge · O₂ ~5% · HC ~10%

Step 1 / 9

Why you can't just open up

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Key —

Tap a step

The purge-first rule — two paths to air, only one survivesIG until HC < 2% → then air

Common mistakeopening hatches and starting the fans straight after discharge "to save time" — mixing air into a rich inerted atmosphere drags it straight through the flammable envelope. Purge to < 2% HC with inert gas first.

Mixing is geometry: as air enters, the tank atmosphere travels a straight line on the HC/O₂ chart toward the air point. Start that line at 10% HC and it cuts through the flammable wedge — for a while the tank is a bomb waiting for a static spark. Start it below 2% HC and the same line passes clear underneath — no mixture anywhere along the journey can burn. Purging costs an hour or two of inert gas; it buys a gas-freeing operation that is never flammable at any moment. That is the entire logic of the two-stage procedure.

Try it — walk the atmosphere to breathable air

The dot is your tank atmosphere; the wedge is where it burns. You choose what to blow in (inert gas or fresh air) and how hard. Blowing drags the dot in a straight line toward the source point — IG pulls it down, air pulls it toward 20.9% O₂. The drill: purge with IG until HC is below 2%, then switch to air and ride under the envelope to outside-air readings. Switch to air too early and you will watch the dot sail into the wedge.

Gas source what the fan blows in

INERT GAS

Fan / blower flow m³/h

1500

Residues gassing off sludge & clingage on the bottom · STRIPPED = cleaned out

LIGHT

BlowingIGat1,500m³/h · tank volumes through0.0· elapsed0h 00m

atmosphere → PURGING

one tank volume blown through cuts the concentration to ~37% of what it was — but the meter, not the clock, decides when a stage is done

Tank HC10.00% vol Tank O₂5.0% HC as LEL>100%LEL Vol changes0.0× Flow in1,500m³/h

Chart guide — red wedge = flammable · dashed red = the mix-with-air line tangent to the wedge · dashed teal = purge gate 2% HC · the dot is your tank, the faint dashes show where the current source is dragging it.

Blowing any gas through the tank pulls the atmosphere point in a straight line toward that gas — IG toward (5% O₂, 0% HC), air toward (20.9% O₂, 0% HC). Residues on the bottom keep adding HC the whole time, which is why a "clean" tank can creep back over night. The accident lives on one path only: air admitted while HC is still above 2%.

READY — set the controls, then press Start

Press Start to blow the tank down — purge with IG first, or switch to air too early and watch the dot sail into the flammable wedge.

Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.