Inert Gas System — How Boiler Exhaust Blankets the Tanks

The inert gas (IG) system takes the ship's own boiler exhaust, cleans it up and pushes it into the cargo tanks as a safety blanket — gas with too little oxygen to burn. Tap a step or press "Play" to follow the gas from the boiler uptake, through the scrubber and blowers, past the two control valves and the water seal, and forward along the deck main into the tanks — then try the simulator and hold the deck pressure yourself while the cargo pumps fight you.

IG O₂ % · deck pressure: tank O₂ < 8% · blanket held

Step 1 / 9

Why inert?

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Key —

Tap a step

The 125% rule — the blowers must out-run the pumpsIG ≥ 1.25 × max discharge

Common mistakesizing the IG plant to 100% of discharge — no headroom; the first pressure dip pulls air in. Size to ≥ 125%.

While the pumps empty a tank, gas has to arrive and fill the space at least as fast as the cargo leaves — otherwise the tank pressure falls and, once it goes negative, air is drawn in. So the IG plant is sized for at least 125% of the ship's maximum cargo discharge rate. A ship pumping out at, say, 6,000 m³/h needs an IG plant good for ≥ 7,500 m³/h. The extra quarter is not waste: it covers seal losses, small leaks and the pressure swings of real cargo work, and it is what lets the two control valves hold the deck positive with something in hand. Match the plant to 100% and you have built in no margin at all — the first hiccup pulls the tanks negative.

Try it — hold the tank pressure

The cargo pumps pull the tank down; the blowers push back. The deck supply valve is simply OPEN or SHUT — it does not throttle. The pressure is set by the vent valve: in AUTO you dial a pressure setpoint and the vent modulates itself to hold it; in MANUAL you set the vent opening by hand. Keep the tank positive — let it fall to −350 mmWG and the P/V valve lifts on vacuum and air pours straight in.

Blower capacity

Cargo discharge m³/h

2000

Deck supply valve open / shut

OPEN

Mode auto / manual

AUTO

Setpoint mmWG · the vent auto-holds this

800

vent (auto)22%

Gas to tank2,960m³/h · cargo out2,000· net+960

net gas +960 m³/h → tank pressure HOLDING

the open supply valve feeds gas forward, the vent valve trims the excess up the funnel — in AUTO the controller works the vent for you to sit on the setpoint; shut the supply and the tank gets nothing

Tank press800mmWG Tank O₂5.0% IG O₂ in4.0% Flow to deck2960m³/h

Tank-pressure P/V limits, marked on the manometer — vacuum relief −350 · zero 0 · pressure relief +1400 mmWG. Cross either and the P/V valve lifts.

Tank pressure is the balance of the gas fed forward against the cargo the pumps take out. The supply valve is open or shut only — all the modulating is done by the vent valve: close it to build pressure, open it to relieve. Let the pumps out-run the gas and the pressure falls through zero to −350 mmWG, where the P/V valve lifts on vacuum and air is drawn straight in — the tank oxygen then climbs and the blanket is lost.

READY — set the controls, then press Start

Press Start to run the plant — then out-run it with the cargo pumps or switch to manual and watch the tank pull a vacuum.

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.