Gas-related risks: flammability, toxicity and asphyxiation
The air we breathe contains oxygen and nitrogen, natural gas or methane is used in many homes for heating and cooking... Gas is part of our everyday environment but if we are not careful, it can quickly become dangerous.
There are three categories of gas-related risks:
- Risk of fire and/or explosion due to flammable gases such as methane, butane, propane, etc.
- Risk of poisoning due to toxic gases such as carbon monoxide (CO), hydrogen sulphide (H₂S), carbon dioxide (CO₂), chlorine (Cl₂), and so on.
- Risk of suffocation or asphyxiation due to a lack of oxygen, which may be consumed or displaced by another gas.
Definition and types of gas-related risks
By definition, a gas is a swarm of molecules that move randomly and chaotically, bumping into each other and with whatever else happens to be in their way. Gases fill all the space available and, because they move so quickly, they soon mix with everything else in the atmosphere in which they find themselves. Gases may be heavier, lighter or of the same density as air. Some gases have a smell, while others are odourless. They may be coloured. Just because you can’t see, smell or touch a gas doesn’t mean it is not there.
The word gas comes from the Greek “chaos” which refers in mythology to the chasm, void or empty space that existed before life began. The word was coined in the 17th century by the Dutch chemist Jean-Baptiste Van Helmont to describe the notion of emptiness.
Understand the risks related to flammable gases
Hydrocarbon compounds such as acetylene, ammonia, hydrogen, propane, propylene and methane are all flammable gases. They are also known as combustible gases.
Combustion is a chemical reaction in which oxygen mixes quickly with another substance, resulting in the release of energy, in the form of heat or even flames.
Combustion can be represented by the fire triangle, which shows that a fire starts when three factors are combined:
- An ignition source
- Fuel in the form of a gas or vapour
This is why the objective of all fire protection systems is to remove one of these potentially dangerous factors.
Flammability or explosive limits
The flammability or explosive limits of a gas refer to the minimum and maximum concentrations of the gas necessary to support its combustion in the air (burning or explosion):
- The lower limit is called the LEL (Lower Explosive Limit)
- The upper limit is called the UEL (Upper Explosive Limit)
If the concentration is lower than the LEL, we say that the gas/air mixture is too lean to burn, because there is not enough gas to produce an explosion. The risk of fire increases in proportion to the percentage of flammable gas present. Above the UEL, the mixture lacks oxygen. The air becomes too rich to burn, and the risk of a fire starting decreases. Generally speaking, increasing the pressure, temperature or concentration increases the flammability range.
In normal operating conditions, industrial facilities do not have gas leaks or, at most, they will contain a few traces of gas. Gas detectors do the job of monitoring the risks before combustion, in other words, they measure the concentration of gas present between 0% and the Lower Explosive Limit (LEL). If the lower limit is reached, shut-off or site evacuation procedures are triggered. In practice, these measures are usually taken when the concentration level is less than 50% of the LEL, to guarantee a sufficient safety margin.
In confined spaces (closed or poorly ventilated environments), the concentration can sometimes be greater than the UEL. Danger can occur when the doors or hatches are opened, because the arrival of air from the outside can dilute the gases and form a dangerous combustible mixture. For this reason, perfumes are added to some gases to make them easier to detect.
At a certain temperature, flammable gases will spontaneously ignite in a normal atmosphere without an external source of ignition such as a flame or spark. We call this temperature the autoignition temperature. The surface tempertaure of the equipment used in hazardous areas must not be higher than the autoignition temperature. The maximum surface temperature is therefore indicated on the equipment.
Flash point (lowest temperature in °C)
The lowest temperature at which the surface of the liquid produces enough vapours to ignite if given an ignition source.
Helps determine where the sensor will be located. Gas/vapour density is relative to that of the air: if the air = 1.0 and vapour density < 1, it will rise in the air, while if vapour density > 1, it will sink in the air.
Understand the risks related to toxic gases
Flammable gases and toxic gases are treated separately because they involve different risks, regulations and sensors. However, there are many gases that are both combustible and toxic, meaning that toxic gas detectors have to be approved for hazardous areas.
In the case of toxic substances, the main concern – aside from environmental issues - is the effect of these gases on employees. Inhaling, ingesting or having one of these gases penetrate your skin may be harmful even at very low concentrations. The number of deaths caused by exposure to toxic gases is higher than the number of deaths due to explosions involving flammable gases.
The most common way of measuring the concentration of toxic gases is in parts per million (ppm) and parts per billion (ppb). For example, 1ppm is the equivalent of a room filled with a million balls, 1 of which is red: the red ball represents 1ppm. However, it is essential to measure the exposure time, not just the concentration of gas, because the harmful effects are often caused by regular exposure over a long period of time.
An important notion is that of Occupational Exposure Limit, or OEL. The purpose of the OEL is to prevent irreversible damage to a person’s health caused by the use of one or more hazardous substances. It offers protection against adverse effects on health, but does not cover the risk of explosion. In other words, the goal is to make sure that the exposure levels in the workplace are lower than the acceptable legal limit. These limits are a valuable tool for risk evaluation and management.
To obtain a representative measurement of the degree of contamination of the air inhaled, three elements are required:
- Workplace assessment (description of the potential sources of exposure)
- Individual monitoring using detectors worn or carried by users
- Samples taken as close as possible to the breathing area.
The toxicity of the gas concerned must without fail be taken into consideration. For example, a detector that only measures a weighted average or a detector that takes a sample for analysis in the laboratory does not protect employees against short-term exposure to a lethal dose of a highly toxic substance. However, it may be normal to exceed the long-term exposure limits temporarily in certain parts of the plant or facility, and it is not necessary to trigger a warning in such cases. The optimal detector is therefore one which is capable of monitoring both the long-term and short-term exposure levels as well as the instant warning levels.
Exposure times are calculated based on eight-hour periods (8-hour OEL) and 15-minute periods (STEL). For certain substances, even a very short exposure can be so serious that they only have a short-term exposure limit, which should never be exceeded, even for a few seconds. The cancerogenic nature, the toxic effects on reproduction, irritation and degree of sensitivity are taken into consideration when putting together a proposal for an OEL based on current scientific knowledge.
Understand oxygen-related risks
Oxygen deficiency (or asphyxia)
We all need to breathe the oxygen (O₂) in the air in order to live. However, oxygen is just one of the many gases present in the air. Normal ambient air contains an oxygen concentration of 20.9% v/v. When the oxygen level falls below 19.5% v/v, the air is considered oxygen-deficient. Oxygen concentrations below 16% v/v are considered unsafe for humans.
Oxygen depletion can be caused by:
- Chemical reaction
- Bacterial action
It is often forgotten that oxygen enrichment can also cause a risk. At increased O₂ levels the flammability of materials and gases increases. At levels of 24% items such as clothing can spontaneously combust.
Oxyacetylene welding equipment combines oxygen and acetylene gas to produce an extremely high temperature. Other areas where hazards may arise from oxygen-enriched atmospheres include manufacturing areas for storing rocket propulsion systems, products used for bleaching in the pulp and paper industry and clean water treatment facilities.
Sensors have to be specially certified for use in O₂ enriched atmospheres.
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