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Introduction to Hazardous Gases.mp300:00 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.
Typical hazards with gases are:
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.
The lower range is called LFL, Lower Flammable Limit.
The upper range is called UFL, Upper Flammable Limit.
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.
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 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.
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.
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