--- title: Flammability, Explosive Limits and Gas Safety Basics url: https://www.sarveshanalytics.com/learn/flammability-explosive-limits-and-gas-safety-basics.html description: Why gases burn or explode: the fire triangle, lower and upper explosive limits for hydrogen, methane and CO, ignition energy. updated: 2026-11-02 source: html --- Learn · Safety and hazardous areas # Flammability and explosive limits: gas safety basics Every hazardous-area decision starts here. If you know why a gas burns, the rest of gas safety follows. Beginner 3 min read Lesson 1 of 8[Safety and hazardous areas](https://www.sarveshanalytics.com/learn/track-safety.html) Quick answer A flammable gas burns only when fuel, oxygen and an ignition source are present and the fuel-air mixture lies between the lower and upper explosive limits (LEL and UEL). Hydrogen is explosive from about 4 to 75 % in air, methane about 5 to 15 % and carbon monoxide about 12.5 to 74 %. Removing oxygen, keeping the mixture outside the limits, or removing ignition sources all prevent a fire. Updated 2 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - Explain the fire triangle and the explosive limits - State approximate limits for hydrogen, methane and CO - Compare ignition energies and why hydrogen is special - Distinguish process analysers from gas detectors ## The fire triangle Fire needs three things: fuel, an oxidiser (usually oxygen in air) and an ignition source. Remove any one and it cannot start. Safety design uses all three: keep fuel contained, keep oxygen out or below a limit, and control ignition sources, including electrical equipment. ## Explosive limits A mixture of flammable gas and air burns only within a range of concentration. Below the **lower explosive limit (LEL)** there is too little fuel; above the **upper explosive limit (UEL)** there is too little oxygen. In between, the mixture can burn or explode. | Gas | LEL, % in air | UEL, % in air | |---|---|---| | Hydrogen | about 4 | about 75 | | Methane | about 5 | about 15 | | Carbon monoxide | about 12.5 | about 74 | | Propane | about 2.1 | about 9.5 | | Ethylene | about 2.7 | about 36 | | Hydrogen sulphide | about 4 | about 44 | Values are approximate and depend on temperature and pressure; use the figures in your safety data sheets for design. ## Ignition energy Some gases ignite with very little energy. Hydrogen needs a tiny spark, of the order of a hundredth of a millijoule, compared with a few tenths of a millijoule for methane. That is why hydrogen service calls for the most demanding equipment group, IIC: [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html). **Why it matters** A wide explosive range plus very low ignition energy makes hydrogen easy to ignite and hard to keep outside the limits. ## Analysers versus detectors - A **gas detector** warns that a gas has reached a hazardous level, usually a percentage of the LEL, in ambient air. It is a safety device for people and plant. - A **process analyser** measures concentration in the process with defined accuracy, for control, protection or quality: [What is online gas analysis? A plain introduction](https://www.sarveshanalytics.com/learn/what-is-online-gas-analysis.html). - An analyser can be part of a protection scheme, as when oxygen in a blanketed vessel is monitored to keep it outside the flammable region: [What is limiting oxygen concentration (LOC)](https://www.sarveshanalytics.com/learn/what-is-limiting-oxygen-concentration.html). **Good to know** Process analysers are not personal gas detectors and do not replace area gas detection. ## Three ways to stay safe 1. **Stay below the LEL or above the UEL** by dilution, ventilation or enrichment, and prove it by measurement. 2. **Remove oxygen** by inerting with nitrogen or another inert gas: [Inerting and purging: how they work and how to prove them](https://www.sarveshanalytics.com/learn/inerting-and-purging-how-they-work.html). 3. **Control ignition sources** by using certified equipment in classified areas: [Protection concepts: flameproof, intrinsic safety, purging](https://www.sarveshanalytics.com/learn/protection-concepts-flameproof-intrinsic-safety-and-purging.html). **In short** - Fire needs fuel, oxygen and ignition. - Mixtures burn only between the LEL and the UEL. - Hydrogen: about 4 to 75 %; methane: about 5 to 15 %; CO: about 12.5 to 74 %. - Process analysers and gas detectors are different devices. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **What does the LEL represent?** The lowest fuel concentration in air that can burn or explode; below it there is too little fuel. **Why is hydrogen especially hazardous?** It has a very wide explosive range and needs very little energy to ignite. **Name the three ways to prevent a gas explosion.** Keep outside the explosive limits, remove oxygen, and control ignition sources. ## Key terms - [LEL](https://www.sarveshanalytics.com/learn/glossary.html#t-lel) - [UEL](https://www.sarveshanalytics.com/learn/glossary.html#t-uel) - [Inerting](https://www.sarveshanalytics.com/learn/glossary.html#t-inerting) - [Gas group IIC](https://www.sarveshanalytics.com/learn/glossary.html#t-gas-group-iic) ## Frequently asked questions **What is the explosive range of hydrogen?** About 4 to 75 % by volume in air. **Is a process analyser a safety device?** It can be part of a protection scheme, but it is not a personal gas detector and carries no SIL claim from us. **What does LEL stand for?** Lower explosive limit, also called lower flammable limit. **Does pressure change the explosive limits?** Yes; the range widens with pressure and temperature. Use design data for your conditions. ## Related products [ ### ACCURRA-Hz Process gas analyser in an enclosure certified for Gas Group IIC, Zone 1 and 2, T6. View ACCURRA-Hz ](https://www.sarveshanalytics.com/hazardous-gas-analysers.html) ## Where this applies - [Electrolyser safety and quality](https://www.sarveshanalytics.com/applications/hydrogen-electrolyser-gas-analysis.html) ## Where to go next - [Hazardous area classification: zones, gas groups, T-Classes](https://www.sarveshanalytics.com/learn/hazardous-area-classification-zones-gas-groups-temperature-classes.html)