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Dangerous Gases on Board


Dangerous Gases on Board

🧪 Dangerous Gases on Board – What Seafarers Must Know

Gas hazards are a silent threat on ships. Most dangerous gases are odorless, colorless, and hard to detect without instruments. This blog post explains the types of gases that may be present onboard, where they come from, how they behave, and how to recognize and respond to gas risks.


⚠️ Why This Matters

Many fatal incidents at sea have occurred due to:

  • Entering enclosed spaces without gas testing

  • Unnoticed oxygen deficiency

  • Toxic gas exposure from cargo or operations

  • Explosion risks from flammable atmospheres

Understanding the properties and sources of gases helps prevent accidents.


🔍 Key Characteristics of Gases

Property

Description

Odor

Many gases are completely odorless (e.g., CO, N₂, CH₄)

Color

Most gases are invisible; no color or fog (except Cl₂, NO₂)

Density

Some gases are heavier than air (sink), others are lighter (rise)

Toxicity

Some gases are harmful even in small concentrations

Flammability

Some gases can ignite or explode under certain conditions

📦 Common Gases Onboard – Sources & Behavior

Gas

Source

Density vs. Air

Hazard

O₂ (Oxygen)

Everywhere

–

Low → asphyxiation risk

CO₂ (Carbon Dioxide)

Fire extinguishers, scrubbers

Heavier

Oxygen displacement

CO (Carbon Monoxide)

Engines, incomplete combustion

Slightly lighter

Highly toxic

H₂S (Hydrogen Sulphide)

Slops, sewage, cargo tanks

Heavier

Extremely toxic, smells like rotten eggs

CH₄ (Methane)

Organic cargo, crude oil, tanks

Lighter

Flammable/explosive

NH₃ (Ammonia)

Refrigeration leaks, cargo

Lighter

Corrosive, toxic

NO₂ (Nitrogen Dioxide)

Exhausts, tank cleaning

Heavier

Toxic, brownish gas

SO₂ (Sulphur Dioxide)

Fuel combustion, cargo

Heavier

Irritating, toxic

Cl₂ (Chlorine)

Chemical cargo

Heavier

Toxic, greenish gas

🧭 Where Dangerous Gases on Board Are Found

  • Cargo tanks – hydrocarbon vapors, inert gases

  • Ballast tanks – H₂S, low O₂ due to corrosion

  • Void spaces & double bottoms – inert gases, low O₂

  • Machinery spaces – CO, NOx

  • Refrigeration plants – Ammonia leaks

  • Scrubber systems – CO₂, SO₂

🛠️ How to Identify Gas Presence

  • Use calibrated gas detectors (personal or portable units)

  • Check atmosphere before entering any enclosed space

  • Know typical gas behavior (sink or rise?)

  • Understand cargo characteristics (MSDS)

✅ Precautions & Best Practices

  • Never enter confined space without a proper gas test

  • Always use gas detectors and bump test them

  • Ventilate tanks and spaces thoroughly

  • Maintain up-to-date MSDS for all cargos

  • Wear SCBA if entering suspected toxic environments

  • Train all crew in gas awareness and rescue procedures

📘 Experience-Based Lessons

  • Ships carrying fuel oil or slops often experience H₂S buildup in slop tanks — fatal in minutes

  • After CO₂ fire-fighting release, oxygen levels drop to zero — do not enter without SCBA

  • Refrigerated vessels may leak ammonia, which is corrosive and irritating to eyes and lungs

  • Inert gas purging (e.g., N₂) can create asphyxiation zones — no odor or warning signs










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