The explosive range of gases and vapors is a key indicator of worker safety in ATEX zones. Indeed, managers of industrial sites with ATEX atmospheres often lack knowledge of the explosive limits of these substances. Knowing LELs and UELs is essential, but detecting their approach in real time is even more so. That’s why ATEX alarm systems use these values to trigger alarms before critical thresholds are reached.
LIE LSE: what do they consist of?
The explosive limit or flammable limit is the concentration of gas in air at which a combustible gas or vapor can ignite and possibly explode. The explosive range is characterized by the lower explosive limit (LEL) and the upper explosive limit (UEL ) . It is important to detect these limits before they occur. They can be detected by ATEX equipment such as horns or warning lights.
Lower explosive limit (LEL)
Also known as the Lower Flammable Limit (LFL), the LEL of a combustible gas or vapor is the minimum concentration above which the substance can be ignited. It is expressed as a % by volume in air. Below the LFL, the mixture is too fuel-poor to initiate any reaction.
Upper explosive limit (UEL)
Also known as the Upper Flammable Limit (UFL), the UEL of a combustible gas or vapor is the maximum concentration below which the substance can be ignited. Like the LEL, it is expressed as a % by volume in air. Above the UEL, an oxidizer is needed to cause ignition or explosion.
How to prevent the risk of explosion in ATEX atmospheres?
In ATEX atmospheres, it is essential to control gas and vapour concentrations outside the LEL/LSE.
As the risk of explosion is more frequent, risk prevention is all the more important.
In accordance with the requirements of ATEX Directive 1999/92/EC on health and safety protection for workers exposed to ATEX risks, as an employer you are obliged to take all necessary measures to protect workers.
Here are some examples of security strategies to adopt:
- Use a neutral gas such as nitrogen or argon to reduce the proportion of oxidizer. Absorbent resins can also be used.
- Reduce the percentage of combustible vapors or gases in the atmosphere by ensuring optimum ventilation.
- Monitor gas levels in the air using a calibrated explosimeter in the ATEX zone to be monitored. This monitoring requires ATEX sound and light alarms warning devices programmed to go off well before the LEL is reached. Our alarm systems allow you to configure customized thresholds according to the gases present on your site.
As a reminder, the implementation of a risk prevention plan is non-negotiable, and must be the subject of regular ATEX audits.
Practical application: How BE ATEX uses LEL/LSE values
BE ATEX, a gas detection specialist, supports its customers in the precise identification of the gases to be monitored (CO, H₂S, CH₄, NH₃…) and configures warning systems according to their specific LEL/LSE.
What are the LEL/LSE values for gases and vapors in ATEX zones?
The following is a non-exhaustive list of the most common gases/vapors and their LEL/LSE*. Concentrations are given in % of volume in air.
Hydrogen
- Hydrogen LEL: 4%.
- Hydrogen LSE: 75%.
Methane
- LEL for methane: 5%.
- Methane LSE: 15%.
Propane
- Propane LEL: 2.1
- LSE for propane: 9.5
Butane
- Butane LEL: 1.8%.
- Butane LSE: 8.4
Acetylene
- LEL for acetylene: 2.5
- LSE of acetylene: 82
Choosing the right alert system for your gases
Thanks to our 25 years of expertise, we can help you select the right audible and/or optical warning device.
Hexane
- LEL of hexane: 1.1 %.
- Hexane LSE: 7.5%.
Hydrogen sulfide
- LEL of H2S: 4
- LSE of H2S: 46
Carbon monoxide :
- LEL of CO: 12.5
- LSE of CO: 74.2
*Source: Orme-conseil.com
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