Introduction to Hazardous Gases

Log In to Seably
You need to log in to save your progress and earn a certificate.
Log in to Seably to save your progress.
Log In

Step 1 of 513 minutes read

Introduction to Hazardous Gases

Introduction to Hazardous Gases.mp3

00:00 00:00

This course is intended for onboard personnel looking to gain knowledge about dangerous gases on ships and the systems used to detect them. The most common types of gases are explained, why they are dangerous, how they are detected and how the detection systems operate and are maintained. After completing this course you will have gained a basic understanding of how to detect gas and how to avoid incidents involving gas on board.

The purpose of this training is to give an understanding of why gas detection is necessary on ships, where it's installed and what gases are detected. The difference between toxic and flammable gases will be explained as well as which concentrations are monitored and considered dangerous for life on board.

Gas Hazards

Typical hazards with gases are:

Explosive gases
The risk of explosion or fire.
Toxic gases
They poison personnel, some already at very low concentrations.
Low oxygen content due to gases displacing the breathing air or due to chemical reactions that consume oxygen.

Flammable Gases

Various mixes of hydrocarbons are present in cargo or fuels on ships. A mixture of hydrocarbon gas and air cannot be ignited and burned unless it's within the flammable range.

  1. The lower range is called LFL, Lower Flammable Limit.

  2. The upper range is called UFL, Upper Flammable Limit.

Lower Flammable Limit (LFL)

A term known as LFL, Lowest Flammable Limit, is used to describe the concentration levels where a combustible gas mixture is becoming dangerous. The term LEL, Lower Explosive Limit, is previously known and is usually considered interchangeable with LFL.

The lower flammable range of hydrocarbons is relatively low and normally the gases cannot be seen or sensed by smell at these concentrations. A proper gas detection system is hence required to detect the gases at these low levels.

In practise as described by ISGOTT (International Safety Guide for Oil Tankers and Terminals), the Lower Flammable Limit can be considered around 1% for cargo carried by oil tankers. The concentrations below the LFL limit are described as a percentage of LFL.

These are the concentrations where gas detection systems normally operate.

Upper Flammable Limit (UFL)

The UFL, Upper Flammable Limit or UEL, Upper Explosive Limit, is where the concentration is too rich to ignite, something likely to be the case inside cargo tanks and fuel tanks and hence not in areas where gas detection is typically present.

UFL levels are not explosive in theory, but can be toxic due to the high levels and instead present other hazards.

In practise as described by ISGOTT, the UFL of hydrocarbon gases can be considered about 10% of gas in the air but varies between flammable gases.

Toxic Gases

Toxic gases or poisonous gases. On tankers mainly H2S, hydrogen sulphide is measured. H2S can be a part of crude or fuel oils on board ships.

Benzen, mercaptans, ammonia, refrigerant gases and CO/CO2 are also gases to detect on various ships. They are measured in PPM, Parts Per Million.

Methane gas CH4 is a gas that may be present on LNG carriers, and also on ships with LNG propulsion systems. The 100% LFL level where the gas is potentially explosive/flammable equates to about 5% gas by volume in the air. Below 5% gas in the air, the mixture is too lean and will not ignite. Above 17% gas in the air, it's too rich and will also not ignite, but of course, present a large hazard in many other ways. 100% of LFL for methane = 5% UFL of methane = 17% Methane gas is light and rises, so on LNG carriers and in engine rooms with LNG propulsion, methane CH4 is detected. Gas detectors and measuring points will be fitted high up in these areas. Propane C3H8, butane C4H10, and benzene C6H6 are heavy gases and will sink. Also H2S toxic gas is slightly heavier than air. Hence on regular tankers and LPG carriers, the detection points are usually located low down.
H2S, hydrogen sulfide gas, is highly toxic to humans and can be found on ships in crude oil and other cargoes. It is also common in deep wells, offshore environments etc. H2S is dangerous already in very low concentrations. It is measured in PPM levels, Parts Per Million. The concentration deemed safe for long-term exposure is usually 10 PPM, which can vary locally. This level is referred to as OEL, Occupational Exposure Limit. Some rules state that no more than 50% of OEL, 5 ppm of H2S, is to be present in an area deemed safe for entry. Some crude oil cargo or products can contain very high levels of H2S, requiring special actions during loading and discharging operations! At a 5 PPM volume of H2S gas in cargoes, taking precautions is necessary. Unlike hydrocarbon gases in low concentrations, the H2S gas smells distinctly of rotten eggs so it is easy to detect in low concentrations. However, be aware that in higher concentrations, the H2S will void your breathing organs and the smell will not be detected anymore. Therefore NEVER use your smell as a personal measuring device. A portable H2S measuring device is required in many areas on board. At the first sign of an H2S smell, take precautionary measures in enclosed spaces! If any uncertainty arises: leave the area immediately. There are no second chances with toxic gases in high concentrations.


Oxygen levels are measured on board to ensure that air is breathable, especially important for enclosed spaces and tank areas.

O2 is measured in percent volume, 20.9% O2 in the air is normal. At 19% percent O2 the air is considered oxygen-depleted, and anything below that is considered dangerous and lethal.

Oxygen levels can drop in enclosed spaces due to corrosion taking place.

Various gases emitted from cargo on board can displace oxygen in enclosed areas making entry very hazardous.

An area where low O2 levels are suspected must never be entered without proper breathing apparatus! This is particularly important when rescue operations take place, or the rescuer will also need to be rescued – or worse!

Accidents, sometimes with fatal results, happen on annual basis in the marine world but could, to a great extent, be avoided if proper devices are carried.