What is an inert gas system on a tanker and why is it needed?

<p>An inert gas system is a tanker installation that fills the cargo tanks with gas too low in oxygen to support combustion, and it is needed because hydrocarbon vapour in air can otherwise be ignited by a spark.</p>An inert gas system, or IGS, is a safety installation fitted to tankers that fills the cargo tanks with a gas so low in oxygen that the atmosphere inside cannot support combustion, which removes the risk of an explosion in and around the cargo space regardless of what vapor concentration happens to be present. The hazard it addresses is straightforward: petroleum and many other liquid cargoes give off flammable vapor, and any mixture of that vapor with air sitting between the lower and upper flammable limits can ignite from a spark, static discharge or hot surface, so a cargo tank is potentially explosive for a wide range of conditions unless something is done to remove one side of the fire triangle.

Inert gas works by removing oxygen rather than by removing vapor. Keeping the oxygen content of the tank atmosphere below roughly eight percent by volume, well below the level needed to sustain combustion, makes the mixture too oxygen starved to burn no matter how much flammable vapor is present, which is why maintaining and continuously monitoring that oxygen level, both centrally and inside individual tanks, is the core operational discipline around IGS.

The gas itself is produced either by cleaning, cooling and drying flue gas drawn from the ship's boilers, which is naturally low in oxygen after combustion, or by a dedicated inert gas generator that burns fuel specifically to produce inert gas when boiler gas is unavailable, for example in port. The system delivers this gas to the cargo tanks through a scrubbing tower that cools and washes the gas, blowers that provide the pressure to distribute it, a deck water seal that acts as a one way barrier preventing any flammable cargo vapor from ever finding a path back into the machinery space, and non return and pressure or vacuum breaking arrangements that protect against both overpressure and a dangerous vacuum forming in the tanks.

Operationally, the system maintains a slight positive pressure inside the cargo tanks at all times so that air cannot leak in through small deck fittings or fatigue cracks, and it is used continuously through cargo discharge, when tanks would otherwise draw in air as the liquid level drops, through tank cleaning including crude oil washing, through gas freeing before tank entry or repair work, and on ballast passages to keep any residual vapor space safe. SOLAS chapter II-2 requires inert gas systems on crude oil tankers and product tankers above defined tonnage thresholds, reflecting how central the system is considered to be to tanker safety, and a properly maintained, correctly monitored IGS, backed by trained crew who understand what the oxygen readings actually mean, remains one of the most important single pieces of safety equipment on a tanker.