What is Inert Gas or IG, Its Composition, Source of Generation, and Types in Detail

Introduction

Working at sea, especially onboard tankers, comes with a fair share of risks — and fire is easily one of the most dangerous threats. Ever wondered how we keep those flammable vapors in check inside cargo tanks? That’s where Inert Gas (IG) steps in. Whether you’re a cadet learning the ropes or an experienced engineer looking for a quick refresher, let’s break down this vital shipboard safety system in clear, seafarer-friendly terms.

In this blog post, we’ll explore what inert gas is, how it works, where it comes from, and the different types used on merchant vessels. We’ll also cover why it’s so critical for operational safety and compliance with international maritime standards.

What is Inert Gas?

In simple terms, Inert Gas (IG) is a type of gas that doesn’t support combustion. That means it can’t catch fire, and more importantly, it can’t help anything else catch fire either. For tanker operations, especially crude oil and product carriers, inert gas is used to replace oxygen-rich air inside cargo tanks. The target is to reduce the oxygen level to below 8%, thus creating an environment where explosions can’t occur.

From a seafarer’s perspective, think of inert gas as our unseen fire extinguisher working silently in the background.

Why is Inert Gas Important on Ships?

Imagine offloading a cargo of gasoline or crude oil. The vapours that fill the tanks can easily ignite if mixed with air. That’s a recipe for disaster. Using inert gas eliminates this possibility by replacing oxygen with a gas mixture that doesn’t support combustion.

According to IMO guidelines, all oil tankers of 20,000 DWT and above must be fitted with an IG system. And here’s why:

• Prevents explosions: Reduces oxygen concentration inside cargo tanks.
• Enhances safety during cargo operations: Especially during loading, unloading, tank cleaning, and gas freeing.
• Compliance with MARPOL and SOLAS standards: Helps meet the mandatory safety regulations.

What is the Composition of Inert Gas?

Not all gases can be classified as “inert.” What makes inert gas special is its chemical makeup. The composition typically includes:

• Carbon Dioxide (CO₂) – around 14%-17%
• Nitrogen (N₂) – Approx. 80%-85%
• Other gases – such as Argon and traces of Oxygen

The important thing to note is that oxygen levels in inert gas are kept below 5%, which is below the flammability limit. That’s the magic number when it comes to tank safety.

How is Inert Gas Generated Onboard?

Let’s talk about the engine room side of things. Depending on the vessel type and engine setup, there are different sources and methods for producing inert gas onboard:

1. Flue Gas from Boiler Uptakes (Most Common on Crude Oil Tankers)

This method is popular on large vessels like crude oil tankers. Here’s how it works:

• Flue gases are extracted from the boiler uptake.
• These gases already have low oxygen content and high CO₂.
• Before entering the cargo tanks, the gas passes through several cleaning processes – scrubbing (to cool and remove soot), demisting, and drying.

Note: A typical IG system of this type includes components like a scrubber, blower fans, deck water seal, and pressure/vacuum breakers.

2. Inert Gas Generators (Used on Product and LNG Tankers)

Where boilers might not be running all the time – for instance, on chemical tankers – we use an IG generator. These systems generate inert gas by burning fuel (usually marine diesel) in a controlled manner to create a flue gas with low oxygen content.

These systems also loop in a combustion air blower, a cooler, a scrubber, and other treatment units to ensure the gas is clean and non-corrosive.

3. Nitrogen Generators (For High-Purity Requirements)

Highly specialized tankers, like LNG or chemical carriers transporting reactive cargoes, often require high-purity inert gas. That’s where nitrogen generators come in.

Here, air is compressed and passed through filters and a membrane separator. The nitrogen is separated from oxygen to create a gas with about 95%-99.9% nitrogen content — effectively keeping the tanks safe.

Types of Inert Gas Systems

Depending on the operation and cargo, ships may use different IG systems. Here’s a quick breakdown:

• Flue Gas System: Uses gas from boiler exhaust – common on oil tankers.
• Inert Gas Generator: Burns fuel in a clean combustion process – suited for products and chemical tankers.
• Nitrogen Generator: Delivers high-purity inert gas – ideal for LNG and chemical tankers.

Operational Tip for Seafarers:

Before starting cargo operations, always check:

• Oxygen content in the cargo tank – must be below 8%
• Deck seal and scrubber system health
• Positive pressure in tanks during inerting

If you overlook these checks, you might risk unsafe tank conditions or fail a PSC (Port State Control) inspection.

When Do We Use Inert Gas?

The application of inert gas isn’t just a one-time thing during loading. It’s used across multiple operations, such as:

• During cargo discharge: Keeps tanks under positive pressure with inert atmosphere.
• After tank cleaning: Tanks are inerted before pigging or preparatory loading.
• Cargo tank purging: Replaces hydrocarbon vapors with inert gas after discharge.
• Gas freeing for repairs: Once tanks are inerted, they can then be gas-freed with fresh air.

The timing and procedure are critical—doing things out of order can lead to unsafe conditions.

Maintenance and Safety: Don’t Ignore These

Like any machinery onboard, IG systems need proper care. Failure to maintain scrubbers, water seals, or flame arrestors can lead to big problems — corrosion, backfires, or worse. Here are a few basic tips:

• Regular inspection of scrubber internals
• Check pH levels of scrubbing water
• Clean demister pads and blower filters
• Monitor oxygen analyzers for faulty sensors

Always follow your vessel’s PMS (Planned Maintenance System) for trouble-free operation.

Final Takeaway

Inert gas might be invisible, but it’s a true lifeline for tanker safety. Whether it’s generated from boilers, generators, or nitrogen plants, inert gas prevents one of the biggest dangers at sea: fire and explosion inside cargo tanks.

As modern seafarers, understanding the purpose, components, and types of inert gas systems isn’t just about passing audits — it’s about staying safe. Always respect the system, monitor critical parameters, and follow safety procedures. After all, safety doesn’t happen by accident — it’s built into systems like these, and enforced by people like us.

Safe sailing and stay gas-free—unless it’s inert!

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