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MEO Class II: MEKG MEKM MEP MET SC&S SSEP 📊 Trends
Q7 (20 Marks) Cargo & Dangerous Goods 🔥 Repeated 2x in exams
SSEP • Written Exam

(a) Draw and Explain flammability diagram. (5)

(b) What is critical dilution in flammability diagram? (5)

(c) How does inert gas reduce flammability range (5)

(d) Describe purging and gas freeing operation of cargo tank in Crude Oil Tanker. (5)

Appeared In: Mar 2025Nov 2023

✓ Verified Model Answer (Text Solution)

Structured for DG Shipping MEO Class II examination scoring criteria.

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Part (a)

Draw and explain the flammability diagram.

Part (b)

What is critical dilution in flammability diagram?

Part (c)

How does inert gas reduce flammability range?

Part (d)

Describe purging and gas freeing operations of a cargo tank in a crude oil tanker.

Part (a)

Flammability diagram: The flammability diagram is a graph plotting the concentration of oxygen (vertical axis, % O2) against the concentration of hydrocarbon vapour (horizontal axis, % volume) or the fuel-air, showing the regions within which a fuel-air mixture is capable of ignition (flammable). The diagram has:

  • The Lower Flammable Limit (LFL) - the lowest vapour concentration that will ignite in air.
  • The Upper Flammable Limit (UFL) - the highest vapour concentration that will ignite in air.
  • The critical dilution line (sometimes shown) and the "critical oxygen concentration".
  • The enclosed "flammable" region (the shaded/oval area) where an ignition source will cause combustion; a stoichiometric line near the centre.
  • For a hydrocarbon, outside this region the mixture is too lean (below LFL) or too rich (above UFL) to burn, or the oxygen is too low.

The diagram (sketch): vertical axis O2 %, horizontal axis hydrocarbon %, and the "nose" of the flammability envelope at high O2-less end; a line marking the minimum O2 below which the mixture is non-flammable (the "critical oxygen" / "minimum oxygen for combustion") and the dilution line. Inert gas works by moving the operating point down the O2 axis below the critical oxygen concentration for that fuel.

Part (b)

Critical dilution: Critical dilution is the "point" (the lowest) on the flammability diagram at which the mixture becomes non-flammable by increasing the inert gas (reducing O2) — i.e., the point where the critical oxygen concentration is reached, or the condition where, for the fuel/air/inert system, the mixture can no longer be ignited regardless of ratio, because the oxygen content is reduced below the minimum required for combustion (about 11-12% for most hydrocarbons, but lower for some, and for the "binary" critical dilution curve). In practice, below this oxygen level (the "inert condition"), adding more fuel or air cannot make the mixture flammable; the tank is considered inert when O2 is reduced to a value at which the vapour/air mixture in the tank is not flammable (commonly below 8% for crude/product as the safety margin beneath the critical value).

Part (c)

How inert gas reduces flammability range: Inert gas (mainly N2 and CO2, with low O2) replaces the oxygen in the vapour space, reducing the O2 content of the vapour/air mixture. On the flammability diagram, reducing the O2 moves the operating point downward so that it falls below the critical oxygen concentration (the "safe" lower region) where no mixture ratio is flammable. Because combustion requires both a fuel concentration within LFL-UFL and sufficient oxygen, taking oxygen out of the flammable envelope shrinks/eliminates the flammable range; the vapour space is rendered inert (non-explosive) usually when O2 is reduced to below about 8% (and maintained), despite the presence of hydrocarbon vapour. Inerting thereby prevents explosion even at a rich/lean vapour condition.

Part (d)

Purging and gas freeing operations of a crude oil tank:

  • Purging (turning to inert): used to remove hydrocarbon vapour from an inerted tank and replace it with inert gas, e.g. when the tank is to be gas-freed or before entry, or when a tank is to be brought to a lower hydrocarbon concentration. It consists of forcing inert gas into the tank through the IG main, venting the hydrocarbon/inert mixture to atmosphere through the tank's vent/manifold (using a suitable inert gas flow and keeping the O2 content low). In a crude tanker, "purging" is normally done with inert gas to keep the tank non-flammable while the hydrocarbon level is reduced; the tank is kept at low O2 so no flammable mixture forms.
  • Gas freeing: the removal of the hydrocarbon vapour (and toxic gases) from the tank so it becomes breathable (oxygen at/near 20.9%, and hydrocarbon below LFL and below toxidity), usually by aerating/ventilating with air (not inert) by opening hatches, using the tank-cleaning machines as fans or portable/ventilation fans, or using the "gas-freeing" arrangement which draws fresh air into the tank and displaces the vapour; monitoring with combustible gas indicator (LFL meter) and oxygen meter; purging with inert gas is often required first to lower the vapour to below LFL before air ventilation begins. Proper controlled ventilation (never by opening all hatches if tank is flammable), watching for a possible re-ignition if a flammable zone is retained; the crew use the enclosed space entry procedure before any personnel enter; the tank is certified gas-free (O2 21%, hydrocarbon <1% LFL) by the responsible officer before entry.
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