What Is Lel In Gas Detector?

Lower explosive limit in gas detection

%LEL stands for percent of the lower explosive limit (LEL). The lower explosive limit is the lowest concentration of a gas or vapor that can ignite in air. LEL value of a gas can vary depending on a region. The %LEL indicates how close the gas concentration is to this flammable threshold. For instance, a reading of 50 %LEL means the gas concentration is at half the minimum concentration required for combustion.

• The lower explosive limit (LEL) is defined as the lowest concentration (by percentage) of a gas or vapor in the air that is capable of producing a flash of fire when it meets an ignition source.
• Any concentration below the LEL is considered a “too lean” mixture.
• In this state, there simply isn’t enough fuel present relative to the amount of oxygen to support and propagate a flame.
• For industrial safety and gas detection applications, the LEL is the most critical threshold to monitor, as it represents the point where a non-flammable atmosphere transitions into a potentially explosive one.

Explosive limits and the flammable range

Explosive limits, also known as flammable limits, define the range of concentrations for a flammable gas or vapor when mixed with an oxidizer (usually air) that is required for ignition and self-sustaining combustion to occur. This range is bounded by two key values: the lower explosive limit (LEL) and the upper explosive limit (UEL). Understanding these two thresholds is the cornerstone of effective LEL monitoring and gas safety.

• The upper explosive limit (UEL) is the highest concentration (by percent volume) of a gas or vapor in the air that can produce a flash of fire when an ignition source is present.
• Concentrations above the UEL are considered a “too rich” mixture.
• In this state, there is too much fuel relative to the amount of available oxygen.
• The oxygen is displaced to the point that it cannot support sustained combustion.
• While a rich mixture won’t explode, it remains extremely dangerous, as the introduction of fresh air (e.g., through ventilation or opening a door) could quickly dilute the mixture back down into the flammable range.

The flammable range (or explosive range) is the entire spectrum of gas concentrations between the lower explosive limit (LEL) and the upper explosive limit (UEL). It is within this hazardous zone that a combustible gas can ignite, burn, and explode if the other two elements of the explosion triangle (oxidizer and ignition source) are present. The wider the flammable range, the more likely it is that a leak will result in a flammable mixture, making gases with a wide range particularly dangerous.

Explosion triangle and fuel concentration

To comprehend why explosive limits matter, we must first understand what is required for an explosion to occur. The conditions for combustion are famously illustrated by the explosion triangle, which states that three essential elements must be present simultaneously:

• Fuel: In the context of gas safety, the fuel is typically a combustible gas, flammable gas, vapor, or even fine combustible dust.
• Oxidizer: The most common oxidizer is the oxygen present in the air.
• Ignition Source (Energy): The final piece of the triangle is the energy required to initiate the reaction.

While the explosion triangle outlines the necessary ingredients, their mere presence is not enough to guarantee a fire or explosion. A critical fourth factor comes into play: gas concentration. A mixture of fuel and air can fail to ignite if there is too little fuel or, counterintuitively, if there is too much fuel. The fuel-to-air mixture must fall within a specific, flammable range to sustain combustion. A mixture with insufficient fuel is described as being “too lean,” while a mixture with too much fuel and not enough oxygen is considered “too rich.” This precise window of flammability is defined by the explosive limits of the gas.

Why LEL monitoring is used

While both LEL and UEL are important concepts, safety applications almost exclusively focus on LEL monitoring. The primary goal of a gas detection system for flammable gases is to provide an early warning before conditions become hazardous.

• By monitoring for the presence of a gas and alarming at low percentages of its LEL, personnel can take corrective action (such as stopping a leak or increasing ventilation) long before the atmosphere reaches the minimum concentration required for an explosion.
• The objective is to prevent the concentration from ever reaching 100% of the LEL.
• While rich atmospheres above the UEL can occur in specific process industries (e.g., inside a fuel tank), LEL monitoring addresses the most common and immediate threat to workplace safety.

LEL (lower explosive limit) gas detectors are designed to monitor the concentration of flammable gases or vapors in the air and provide warnings when the concentration reaches or exceeds a potentially dangerous level. These monitors are important for maintaining personnel and plant safety in environments where flammable gases could lead to explosions or fires.

• LEL gas monitoring plays a crucial role in industrial settings for several reasons such as: safety of personnel, prevention of explosions, early warning, process safety, compliance with regulations, environmental protection, risk assessment, and insurance requirements
• In compliance with OSHA regulations, monitoring of the lower explosive limit (LEL) is mandatory in specific industrial applications.
• This task is achieved through a range of effective gas detection technologies.

How gas detectors measure percent LEL

Modern gas detectors designed for combustible gases do not typically display the gas concentration in percent by volume (e.g., 2% methane). Instead, they measure and display the concentration as a percentage of the lower explosive limit, or percent LEL (%LEL). This scaling system, typically from 0-100% LEL, provides a direct and intuitive indication of the immediate explosion risk.

• 0% LEL: no detectable flammable gas is present.
• 50% LEL: the gas concentration has reached half of what is needed to support combustion.
• 100% LEL: this is the critical point.
• At 100% LEL the monitored gas has reached the minimum concentration that can sustain combustion.
• The atmosphere is now explosive if an ignition source is present.

Because an LEL gas monitor is monitoring combustible gas or solvent vapor in the LEL range as described, the LEL monitor is operating and monitoring levels in a safe range, before the LEL level reaches 100% LEL, which is the minimum level that is required to support ignition or combustion.

An LEL analyzer is a safety instrument and as a fixed gas detection instrument, monitors, indicates and alarms when levels of LEL are above 0%, indicating the presence of combustible gas or solvent vapor, well before a fuel/air mixture that can combust exists where LEL monitors are located.

Alarm thresholds for LEL detectors

To provide a sufficient safety margin, the alarm thresholds on LEL detectors are set well below 100% LEL. Common practice includes:

• Low alarm: often set at 10% or 20% LEL.
• This serves as an initial warning to investigate the source of the leak.
• High alarm: often set between 25% and 50% LEL.
• This indicates a more serious situation that may require evacuation or the activation of emergency shutdown systems.

Acting on low-level alarms is a fundamental tenet of proactive gas safety.

Units used with LEL readings

Gas detection is a crucial aspect of safety in various industrial and environmental settings. It involves measuring the concentration of potentially hazardous gases to prevent accidents, ensure compliance with safety standards, and protect human health. Three common units used in gas detection are %LEL, %LFL, % Vol and ppm. Understanding the differences between these units and knowing how to convert between them is essential for the gas detection industry.

%LEL (percent of lower explosive limit)

%LEL stands for percent of the lower explosive limit (LEL). The lower explosive limit is the lowest concentration of a gas or vapor that can ignite in air. LEL value of a gas can vary depending on a region. The %LEL indicates how close the gas concentration is to this flammable threshold.

%LFL (percent of lower flammable limit)

%LFL refers to the same minimum concentration of gas or vapor in air that can ignite. The difference lies in the context: LEL is mostly used in explosion prevention, while LFL is more relevant to fire hazards.

% Vol (percent by volume)

Percent by volume (% Vol) stands for percent by volume and represents the concentration of a gas in a mixture. It is expressed as a percentage of the total volume.

ppm (parts per million)

Parts per million (ppm) stands for parts per million and is a unit of measurement used to express the concentration of a gas in a mixture. One ppm represents one part of the gas per one million parts of the total volume.

In practical terms, LEL and LFL are the same values, and both are typically expressed as a percentage by volume in air (e.g., 4.4% of methane) or in parts per million (44 000 ppm of methane).

Sensor technologies for LEL detection

Two primary sensor technologies dominate the field of LEL monitoring:

• Catalytic bead sensor: a traditional and robust technology that detects a broad range of combustible gases.
• It works by oxidizing the gas on a heated catalyst, measuring the resulting temperature change.
• Infrared (IR) sensor: this technology uses infrared light to detect specific hydrocarbon gases.
• IR sensors are immune to sensor poisoning and do not require oxygen to operate, but they cannot detect hydrogen.

Infrared (IR) and catalytic bead (CATEx) are widely adopted methods.

Factors that change explosive limits

The LEL and UEL are not static values; they are influenced by the physical and chemical properties of the gas and its surrounding environment.

• The most significant factor determining explosive limits is the chemical composition of the gas itself.
• The LEL and UEL of gases vary greatly depending on their molecular structure and chemical bonds.
• For example, methane (CH4), the primary component of natural gas, has an LEL of 5% by volume, while hydrogen (H2) has an LEL of 4%.
• These differences require gas detection systems to be calibrated specifically for the target gas to provide accurate readings.

Both temperature and pressure can have a significant impact on explosive limits.

• Temperature Effects: Generally, an increase in temperature will widen the flammable range.
• It tends to lower the LEL (less gas is needed to form a flammable mixture) and raise the UEL.
• Pressure Effects: An increase in pressure also typically widens the flammable range.
• For many gases, the UEL increases significantly with pressure, while the LEL is less affected.

The standard explosive limits are based on a mixture with normal air (approximately 20.9% oxygen). If the oxygen concentration changes, the limits will also change.

• In an oxygen-enriched atmosphere, the flammable range will widen, making ignition easier and the resulting fire more intense.
• Conversely, in an oxygen-deficient atmosphere, the range narrows.
• The limiting oxygen concentration (LOC) (also called minimum oxygen concentration or MOC) is the minimum oxygen level required to support combustion for a given fuel.
• Below the LOC, ignition cannot occur, regardless of the fuel concentration.

Combustible gas examples and LEL values

Combustible gas is a substance that can ignite in the presence of an ignition source, whether in air or oxygen. Hydrogen, methane, propane, and iso-butane are some of the most common examples of combustible gases. These gases can cause catastrophic gas explosion accidents when mixed with a certain amount of oxygen.

The following are the lower explosive limits (LEL) of selected gases

• Acetone — 2.5% vol
• Acetylene — 2.5% vol
• Ammonia — 15.0% vol
• Benzene — 1.2% vol
• Butane — 1.9% vol
• Carbon monoxide — 12.5% vol
• Ethane — 3.0% vol
• Hydrogen — 4.0% vol
• Methane — 5.0% vol
• Propane — 2.1% vol
• Propylene — 2.0% vol
• Toluene — 1.1% vol
• Xylene — 1.1% vol

Gas detection system design

A deep understanding of explosive limits is fundamental to engineering a reliable and effective gas detection system. This knowledge informs every stage of the design process:

• Sensor selection and calibration: the choice of sensor technology and, most importantly, the gas detector calibration must be matched to the specific combustible gas being monitored.
• Using a detector calibrated for methane to monitor propane will result in inaccurate %LEL readings and a compromised state of gas safety.
• Sensor placement strategy: detectors must be placed where a leak is likely to be detected quickly.
• This involves considering potential leak sources (valves, flanges, pumps) and the vapor density of the target gas.
• Lighter-than-air gases (like methane or hydrogen) will rise, so sensors should be placed high.
• Heavier-than-air gases (like propane) will sink and pool in low areas, requiring low-level sensor placement.
• System integration: the gas detection system should be integrated with other safety layers.
• This includes audible and visual alarms to alert personnel, as well as relays that can automatically activate ventilation systems, shut down processes, or close emergency isolation valves.