Oil tankers carry fuels that can release flammable vapors into the empty space above the liquid cargo. If enough oxygen is present, a spark, hot surface, or static discharge can ignite the vapor and cause an explosion. An inert gas system reduces this danger by replacing normal air in the cargo tank vapor space with gas that has very little oxygen.
This is a major safety system on many ships that carry crude oil or petroleum products.
The inert gas often comes from cleaned boiler exhaust or a dedicated inert gas generator. Before it enters the tanks, the gas is cooled, cleaned, and sent through pipes into the cargo tank vapor spaces. By keeping oxygen concentration below the level needed for combustion, the system prevents the formation of a flammable mixture.
Crew members monitor oxygen level, pressure, and gas flow to keep the tanks protected during loading, unloading, and transit.
Key Facts
- Fire needs fuel vapor, oxygen, and an ignition source.
- Normal air contains about 21% oxygen by volume.
- Inert gas systems often keep cargo tank oxygen below about 8% by volume.
- Lower oxygen concentration reduces the chance that hydrocarbon vapor can ignite.
- Tank pressure is kept slightly positive so outside air does not leak inward.
- Percent oxygen = oxygen volume / total gas volume x 100
Vocabulary
- Inert gas
- Inert gas is gas with low oxygen content that does not readily support combustion.
- Cargo tank
- A cargo tank is a sealed compartment in a tanker that holds liquid cargo such as crude oil or fuel.
- Vapor space
- Vapor space is the region above the liquid cargo where gases and fuel vapors collect.
- Flammable mixture
- A flammable mixture is a combination of fuel vapor and oxygen that can ignite if an ignition source is present.
- Oxygen concentration
- Oxygen concentration is the percentage of a gas mixture made up of oxygen molecules.
Common Mistakes to Avoid
- Thinking inert gas removes the fuel vapor, which is wrong because it mainly lowers the oxygen level so the vapor cannot burn easily.
- Assuming empty tanks are safe, which is wrong because the vapor space can contain flammable hydrocarbon gases even when little liquid remains.
- Ignoring tank pressure, which is wrong because low pressure can allow oxygen-rich outside air to leak into the tank.
- Treating inert gas as pure nitrogen, which is wrong because ship systems often use cleaned exhaust gas that contains nitrogen, carbon dioxide, water vapor, and small amounts of other gases.
Practice Questions
- 1 A cargo tank vapor space contains 5000 m3 of gas. If the oxygen concentration is 7%, what volume of oxygen is in the vapor space?
- 2 Normal air has about 21% oxygen. An inert gas system lowers a tank to 8% oxygen. By how many percentage points did the oxygen concentration decrease?
- 3 A tanker crew is unloading cargo, and the liquid level is falling. Explain why the inert gas system must keep adding gas to the vapor space during this operation.