Understanding Flammable/Combustible Gas Detector Ranges, Alarm, and Trip Values in Confined Spaces – A Practical Guide for Instrumentation Engineers

One common challenge for budding instrumentation engineers is selecting the right flammable or combustible gas detector range and setting appropriate alarm and trip values, particularly in confined spaces. The key lies in understanding the concepts of Lower Explosive Limit (LEL) and Upper Explosive Limit (UEL) and their relevance to gas detection strategies.

The Fire Triangle explains gas flammability using three essential elements:

Fuel (Flammable Gas) – The combustible gas (e.g., methane, propane) that can burn.

Oxygen (Oxidizer) – Typically from the air, it supports combustion.

Heat (Ignition Source) – A spark, flame, or high temperature that initiates combustion.

For a fire or explosion or combustion to occur, all three elements must be present within the flammable range (between LEL and UEL). Removing any one element—fuel, oxygen, or heat—prevents ignition, which is why gas detection, ventilation, and ignition control are key safety measures.

e.g. A simple chemical equation representing the Fire Triangle for a flammable gas can be shown using methane (CH?) as fuel, oxygen (O?) as the oxidizer, and heat as the ignition source:

This reaction illustrates that when methane (fuel) mixes with oxygen and an ignition source (heat) is present, combustion occurs, producing carbon dioxide (CO?), water (H?O), and heat energy.

Similarly, for propane (C?H?):

These equations reinforce the Fire Triangle concept, where removing any of the three elements—fuel, oxygen, or heat—prevents combustion.

Let’s break it down.

LEL and UEL – The Basics Every flammable gas has a range within which it can ignite when mixed with air in the presence of an ignition source. This range is defined by: LEL (Lower Explosive Limit) – The minimum concentration of gas in air required for combustion. Below this level, the mixture is too "lean" to ignite. UEL (Upper Explosive Limit) – The maximum concentration at which the gas can ignite. Beyond this, the mixture is too "rich" to burn.

LFL (Lower Flammability Limit) and UFL (Upper Flammability Limit) are essentially the same as LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit), respectively.

LEL/UEL terms are commonly used in industrial safety and gas detection, while LFL/UFL are more common in fire protection and combustion science.

Both terms describe the flammable range of gases, and their values are identical for a given gas. The choice of terminology depends on the industry or standard being followed.

We follow LEL-based detection for consistency with gas detection equipment.

For example, Methane has an LEL of ~5% and a UEL of ~17% by volume in air. If the concentration is below 5%, there’s not enough fuel for ignition; above 17%, there’s too much fuel and insufficient oxygen to sustain combustion.

LEL vs. UEL in Confined Spaces Flammable gas detectors in confined spaces are typically set up to monitor LEL% rather than UEL% for safety reasons. Why? Because detecting gas buildup at low concentrations (LEL-based monitoring) allows early intervention before the mixture reaches a dangerous ignitable range.

Where LEL Monitoring is Used

• Confined spaces like tanks, sumps, or enclosed process areas
• Hydrogen, methane, propane, and other flammable gas detection
• Pre-entry checks in vessels during maintenance

When UEL Becomes Relevant While UEL is not commonly monitored in typical scenarios, there are specific cases where it is critical:

Inerted Systems – Some processes deliberately maintain gas concentrations above UEL to prevent ignition, such as in crude oil storage tanks where nitrogen blanketing keeps hydrocarbon vapors above UEL, making combustion impossible due to insufficient oxygen.

Confined Spaces with Rich Gas Vapors – Consider a welder or maintenance worker needing to enter an empty oil or gas storage tank that is full of hydrocarbon vapor above the UEL limit. While these vapors are too "rich" to ignite, entry is hazardous due to oxygen deficiency and the risk of the mixture falling into the flammable range if disturbed or mixed with air. Before entry, the tank must be purged to reduce the gas concentration below LEL, ensuring a safe working environment. In such cases, a flammable gas detector capable of measuring across the entire range—from below LEL to UEL—is essential for safe monitoring during purging and entry operations.

Instrumentation Takeaway For flammable gas detectors, monitoring in %LEL is the primary focus, with alarms typically set at 5-10% LEL (low alarm) and 10–15% LEL (high alarm/trip) depending on safety standards. However, for special cases like purging operations, detectors with full-range capability (from below LEL to UEL) are crucial.

Understanding these principles ensures safer instrumentation design and effective hazard mitigation in industrial environments.