Table of Contents

Introduction

A flammable vapour mixed with air in the right proportions does not leak — it detonates. For oil and gas plants, refineries, tank farms, gas-processing plants and chemical facilities across Russia and the Middle East, combustible gas detection is therefore the single most important layer of explosion protection. A combustible gas detector continuously watches the atmosphere for methane, propane, natural gas, solvent vapours and any other fuel that has reached its explosive range.

This guide explains what LEL and UEL mean, how a catalytic-bead sensor actually works, where methane and other flammables are monitored, how to set alarms, and how to choose fixed and portable detection. Regulatory framing follows EAC (TR CU 012/2011), ATEX and IECEx — the frameworks that matter for Eurasian Union and Gulf projects.

LEL, UEL and Explosive Range: The Basics

Every flammable gas has an explosive range. Below the Lower Explosive Limit (LEL) the mixture is too lean to burn; above the Upper Explosive Limit (UEL) it is too rich. Between those two points, an ignition source — a spark, a hot surface or static discharge — will trigger combustion or deflagration.

Gas detectors do not alarm at a single ppm value for flammables; they measure the percentage of the LEL. Industry convention is to alarm at 10% LEL (low) and 20% LEL (high), long before the mixture reaches the 100% LEL ignition threshold. The table below lists the LEL/UEL of common fuels.

Gas / vapour	Formula	LEL (vol%)	UEL (vol%)
Methane (natural gas)	CH4	5.0	15.0
Propane	C3H8	2.1	9.5
Butane	C4H10	1.4	8.5
Hydrogen	H2	4.0	75.6
Gasoline vapour	mix	~1.4	~7.6

Because methane is the dominant gas in natural-gas transmission, LNG and LPG service, most combustible gas detectors are calibrated to methane, with correction factors available for other fuels.

How a Catalytic-Bead Combustible Gas Detector Works

The classic combustible gas detector uses a catalytic bead (pellistor). A platinum bead is heated to a high temperature; when a flammable gas oxidises on its surface, the reaction raises the bead’s temperature and changes its electrical resistance. That resistance change is proportional to the gas concentration, which the instrument displays as %LEL.

Two operational facts matter for users. First, catalytic-bead sensors need oxygen to function — they will under-read or read zero in an oxygen-deficient atmosphere, so a proper confined-space entry always pairs LEL with an O2 sensor. Second, the beads can be poisoned or deactivated by silicones, lead compounds, sulfur and halogenated vapours, and they consume sensor life with every high-gas exposure. Infrared (IR) sensors offer an alternative for hydrocarbon gases: they detect absorption at a specific wavelength, need no oxygen, and are immune to catalytic poisons.

Where Combustible Gas Detection Is Required

Upstream and midstream oil and gas: wellheads, separators, compressors, metering stations, pig launchers and gathering lines.
Refineries and petrochemical plants, including GCC hydrocrackers and Russian crude-refining units.
Tank farms, LPG and LNG storage, loading gantries and road-rail tanker filling points.
Offshore platforms and onshore processing facilities, where fugitive gas can accumulate in enclosed modules.
Gas-distribution substations, boiler houses and turbine packages using natural gas.
Confined spaces and sewers where biogas (methane) can accumulate.

Because methane is lighter than air, detectors in gas service are mounted above potential leak points; propane and butane are heavier, so their detectors go low. Mounting height follows the specific gas being monitored.

Fixed vs Portable Combustible Gas Detection

Permanent, continuously monitored process areas need fixed detection. The HD9300 fixed detector mounts near flanges, valves and pump seals, outputs 4–20 mA / RS485 / MBUS to the DCS or PLC, switches dual alarms, and is calibrated from the field by IR remote without opening the Ex d IIC T6 Gb enclosure. Its –40 °C to +70 °C operating range covers Siberian winter and Gulf summer alike.

See the fixed detector: HD9300 Fixed Gas Detector with LCD Display

See the controller: HD6000 Fixed Gas Alarm Controller (2–128 channels)

Workers and confined-space entry need portable detection. The HD620 four-in-one pump detector measures LEL alongside H2S, CO and oxygen — the universal oil-and-gas configuration — on a 2.4-inch colour screen. It is rated Ex ia IIC T4 Ga, IP65, runs about 20 hours on a 5,000 mAh battery, and combines audible, visual and vibration alarms.

See the portable detector: HD620 Four-In-One Pump Portable Gas Detector

For dust-bearing combustible atmospheres — grain, wood, chemical powders — a dedicated dust gas alarm such as the HD9100, on 2/3/4-wire systems with a number display, addresses the additional DUST explosion risk.

See the dust alarm: HD9100 Fixed Dust Gas Alarm

Hangwei Combustible Gas Detection Solutions

Henan Hangwei Sensing Technology Co., Ltd. manufactures fixed and portable combustible gas detectors, controllers and custom sensor packages with more than 20 years’ experience, 20 patents and 10 production lines. We support EAC, ATEX and IECEx projects, and offer OEM/ODM and global distributor recruitment.

Fixed LEL detection: HD9300, Ex d IIC T6 Gb, –40 to +70 °C, 4–20 mA/RS485/MBUS, dual relays, IR calibration.
Portable LEL detection: HD620 four-in-one (H2S/CO/O2/LEL), Ex ia IIC T4 Ga, IP65, ~20 h battery.
Controllers: HD6000 from 2 to 128 channels, 2/3/4-wire, LCD or colour; HD6100 single-point.
Custom packages: IR hydrocarbon sensors, mixed configurations and detector-plus-controller systems.

FAQ

What does %LEL mean on my combustible gas detector?

%LEL expresses the gas concentration as a percentage of the Lower Explosive Limit. 0% LEL is clean air; 100% LEL is the concentration at which the mixture can ignite. Standard alarms are set at 10% and 20% LEL.

Why do I need both LEL and oxygen measurement?

Catalytic-bead LEL sensors require oxygen to oxidise the gas. In an oxygen-deficient confined space the sensor can read low even when a fuel is present, so LEL must always be read together with an O2 measurement — exactly how the HD620 combines them.

Is 100% LEL the same as 100% gas?

No. For methane, 100% LEL equals 5 vol% methane in air. The detector scale tops out at 100% LEL by design, because any reading above roughly 25% LEL already demands an emergency response.

What can poison a catalytic-bead sensor?

Silicones, leaded fuels, sulfur compounds and halogenated vapours can coat and deactivate the bead. If your process contains these, specify an IR sensor or a periodic bead-replacement plan.

How often should a combustible gas detector be bumped and calibrated?

Bump test before each day’s use with known test gas; full calibration at least every six months, or after a failed bump test or high-gas exposure.

Where should fixed methane detectors be mounted?

Above potential leak points, because methane is lighter than air. Mounting height follows the monitored gas: heavy fuels (propane, butane) are monitored low, light gases (methane, hydrogen) high.

Get Expert Guidance

Selecting combustible gas detection involves choosing sensor technology, mounting height, alarm set-points and controller architecture — all safety-critical. Hangwei Sensors supplies fixed and portable LEL detectors and controllers for oil, gas, petrochemical and general industry across Russia, the Middle East and worldwide. Contact our engineers for placement advice, EAC/ATEX documentation and OEM or distributor terms.

Email: hw@hangweisensors.com | Phone: +86 371 6667 0886

Contact Hangwei Sensors

About Hangwei Sensors