Lower Explosive Limit ≠ Alarm Value! 3 Major Pitfalls in Combustible Gas Detection!

Gas Detection Safety Essentials: From Concentration Units to 4-in-1 Detection

During confined space operations, a worker entered without testing the oxygen concentration and fainted due to hypoxia. In a chemical plant, a combustible gas alarm falsely triggered because the unit was mistakenly set to mg/m³ instead of ppm... These real cases highlight a core issue: a lack of knowledge in gas detection can directly lead to safety accidents.

As a safety manager or frontline professional, do you understand the difference between %LEL and %VOL? Why must a 4-in-1 Gas Detector measure these four specific gases? How is the explosion risk assessed for different combustible gases?

I. Gas Concentration Units: Don't "Mismatch Them" – These 4 Units Must Be Distinguished**

The first step in gas detection is understanding "concentration units." Different units correspond to different scenarios. Confusing them leads to errors in setting alarm values and assessing risks. The four common units each have a specific role:

 1. %LEL: The "Explosion Early Warning" for Combustible Gases

*   Full Name: Percentage of Lower Explosive Limit

*   Function: Specifically used to warn of the explosion risk of combustible gases.

*   For example, the Lower Explosive Limit (LEL value) of methane is 5%VOL. Therefore, 100%LEL equals 5%VOL (at this concentration, an explosion will occur upon contact with an ignition source).

*   Practical Application: During on-site detection, the first-level alarm is typically set at ≤25%LEL (a cautionary warning), and the second-level alarm at ≤50%LEL (requires immediate gas shutoff and activation of ventilation fans).

2. %VOL: The "Intuitive Proportion" of Gas Volume**

*   Full Name: Volume Percentage

*   Function: Directly reflects the volume percentage of a gas in the air, suitable for detecting high-concentration gases.

*   For example, the normal oxygen content in air is 21%VOL. A level below 19.5%VOL indicates oxygen deficiency, while a level above 23.5%VOL may support combustion.

*   Typical Gases: Oxygen (O₂), Carbon Dioxide (CO₂), Nitrogen (N₂), etc.

3. PPM: The "Magnifying Glass" for Trace Toxic Gases

*   Full Name: Parts Per Million

*   Function: Used to detect very low concentrations of toxic/harmful gases; it's equivalent to "finding 1 gram of salt in 1 ton of water."

*   Gases like Hydrogen Sulfide (H₂S) and Carbon Monoxide (CO) can be fatal even at concentrations as low as a few dozen PPM.

*   Conversion Relationship: 1%VOL = 10,000 PPM. (Remember this formula: To convert %VOL to PPM, move the decimal point four places to the right; to convert PPM to %VOL, move it four places to the left. E.g., 2%VOL = 20,000 PPM; 500 PPM = 0.05%VOL).

4. mg/m³: The "Mass Unit" for Environmental Monitoring

*   Full Name: Milligrams per Cubic Meter

*   Function: Commonly used by environmental protection agencies to measure the mass concentration of pollutants, such as PM₂.₅ or formaldehyde in industrial exhaust.

*   Important Note on Conversion: The conversion between mg/m³ and PPM is affected by temperature and pressure. Under standard conditions (25°C, 1 atm), it can be simplified as: mg/m³ ≈ (Molecular Weight of Gas × PPM) / 24.45.

*   For example, the molecular weight of CO is 28. Therefore, 50 PPM of CO ≈ (28 × 50) / 24.45 ≈ 57.2 mg/m³.

Key Point: Unit confusion is the biggest hidden danger! For instance, the occupational exposure limit for CO is 20 mg/m³, which is approximately 17 PPM. If a detector's unit is PPM but the alarm is set at 20 mg/m³, it's equivalent to the "alarm being disabled," with potentially unimaginable consequences.

II. Four-Gas Detection: The Essential First Line of Defense

Confined spaces (such as sewage wells, storage tanks, and fermentation pits) are high-risk areas for gas poisoning and explosions. The four-gas detector serves as the indispensable "first line of defense," simultaneously monitoring four critical gases:

1. Targets: Why These Four Gases?

*   Oxygen (O₂): Essential for life! The safe range is 19.5%VOL to 23.5%VOL. Levels below 19.5%VOL can cause suffocation (dizziness, coma), while levels above 23.5%VOL easily trigger fires (in an oxygen-rich environment, even static electricity can ignite clothing).

*   Combustible Gases (LEL): Detects the explosion risk from gases like methane and propane, using the %LEL unit. The first-level alarm is set at ≤25%LEL, and the second-level alarm at ≤50%LEL (reaching 100%LEL means the concentration has hit the lower explosive limit, where ignition will cause an explosion).

*   Hydrogen Sulfide (H₂S): A highly toxic gas with a rotten egg smell, commonly found in sewage pools and septic tanks. Even concentrations around 100 PPM can be instantly fatal ("lightning strike" poisoning).

*   Carbon Monoxide (CO): A colorless, odorless "invisible killer" produced by incomplete combustion (e.g., gas leaks, internal combustion engine exhaust). Exposure to levels above 200 PPM can lead to unconsciousness and death.

2. Detection Procedure: The Three Crucial Steps - "Ventilate → Test → Work"

*   Ventilate First: Before entering a confined space, forced ventilation is mandatory (using explosion-proof fans; pure oxygen ventilation is strictly prohibited! Pure oxygen can turn the environment into a "powder keg").

*   Then Detect: Detection should follow the sequence of "Oxygen → Combustible Gas → H₂S →CO," with results available within 30 seconds. Monitoring points should be close to gas release sources (Open areas: combustible gases ≤10 meters from the source, toxic gases ≤4 meters; Enclosed spaces: combustible gases ≤5 meters, toxic gases ≤2 meters).

*   Then Work: Entry is permitted only after passing the detection. Continuous real-time monitoring is required during work (the detector should be worn on the chest, near the mouth and nose). Evacuate immediately upon any alarm.

3. Alarms & Interlocks: "Automatic Lifesaving" in Critical Moments

*   Combustible Gas Alarm:

*   First-level alarm (≤25%LEL): Personnel on-site must immediately investigate.

*   Second-level alarm (≤50%LEL): Must automatically activate exhaust fans and shut off the gas supply valve (e.g., the quick-closing gas valve in a boiler room).

*   Oxygen Alarm: If levels fall below 19.5%VOL or exceed 23.5%VOL, stop work immediately and initiate forced ventilation.

*   Toxic Gas Alarms (H₂S, CO): Set based on the "Occupational Exposure Limit" (OEL).

*   First-level alarm: ≤100% OEL

*   Second-level alarm: ≤200% OEL

*   Example: The OEL for CO is 20 mg/m³ (approximately 17 PPM). Thus, the first-level alarm is 17 PPM, and the second-level is 34 PPM.

According to the Chinese National Standard GB/T 50493-2019 (Design Standard for Combustible and Toxic Gas Detection and Alarm in Petroleum and Chemical Industries), the first-level alarm setpoint for toxic gases should meet the following requirements:

⚠️ 1. Standard First-Level Alarm Setpoint

*   Value: ≤100% OEL (Occupational Exposure Limit)

*   Purpose: Triggers when the toxic gas concentration reaches the OEL, prompting personnel to take emergency measures like ventilation and personal protection to avoid cumulative health damage from prolonged exposure.

⚠️ 2. Alternative Standard under Special Circumstances

*   If the detector's range cannot accommodate the conventional 0~300% OEL measurement range, the first-level alarm can be adjusted to ≤5% IDLH (Immediately Dangerous to Life and Health Concentration).

*   Example: The IDLH for Hydrogen Sulfide is 300ppm, so the first-level alarm would need to be ≤15ppm.

📖 3. Definition and Classification of OEL

*   OEL (Occupational Exposure Limit) includes three types:

*   MAC (Maximum Allowable Concentration): An instantaneous limit that must never be exceeded.

*   PC-TWA (Permissible Concentration-Time Weighted Average): The average exposure limit over an 8-hour workday.

*   PC-STEL (Permissible Concentration-Short Term Exposure Limit): The short-term exposure limit allowed for a 15-minute period.

*   Priority: MAC > PC-TWA > PC-STEL. If multiple limits exist for a gas, the highest priority standard should be used for alarm setting. 

⚙️ 4. Practical Application Notes

*   Alarm Grading: Typically used with a second-level alarm (≤200% OEL), which indicates concentrations approaching acute hazard levels.

*   Detector Selection: Must be matched to the gas characteristics (e.g., electrochemical detectors for H₂S, infrared detectors for benzene).

*   Calibration Requirements: Alarm error must be controlled within ±3% F.S., and regular calibration is essential to ensure accuracy.

Reminder: A four-gas detector is not a "disposable item"! It requires regular calibration (to check alarm accuracy) and sensor replacement (typically every 1-2 years). Failure to do so can result in false alarms or failure to alarm when needed.

III. Combustible Gas Classification: Identifying the True Nature of the "Invisible Killer"

Not all combustible gases are equally dangerous! Accurate prevention and control require understanding their classifications.

The core risk of combustible gases is the "**Explosive Limit**"—the concentration range in air where exposure to an ignition source will cause an explosion (below the Lower Explosive Limit, the mixture is 'too lean to burn'; above the Upper Explosive Limit, it's 'too rich to burn').

1. Classification by Hazard Level: Category I is More "Lethal" than Category II

✅ Category I Combustible Gases (Class A): Lower Explosive Limit (LEL) ≤10%. These gases have a wide explosive range and are extremely hazardous.

✅ Representative Gases: Methane (natural gas, explosive range 5%-15%), Hydrogen (4%-75%, exceptionally wide range), Acetylene (1.5%-82%, highly dangerous—even a small amount can cause an explosion).

✅ Category II Combustible Gases (Class B): Lower Explosive Limit (LEL) >10%. Relatively safer but still require caution.

✅ Representative Gases: Ammonia (15%-28%), Carbon Monoxide (12.5%-74%).

2. Classification by "Weight": Gases Can "Sink" or "Rise"

*   Heavier than Air (Density >1): e.g., Propane (1.52), Liquefied Petroleum Gas (LPG). These accumulate in low-lying areas (sewers, basements) upon leakage. Detectors should be placed near the ground.

*   Lighter than Air (Density <1): e.g., Hydrogen (0.07), Methane (0.55). These rise and disperse upwards upon leakage. Detectors should be placed near ceilings or high points.

3. Detection Methods: "Selecting the Right Sensor" for Different Gases

*   Catalytic Combustion (CAT) Sensors: Detect hydrocarbon gases like methane and propane. (Requires oxygen; inaccurate in oxygen-deficient environments).

*   Infrared Sensors (NDIR): Detect methane, CO₂. (Strong anti-interference, suitable for oxygen-deficient environments like sealed tanks).

*   Electrochemical Sensors: Detect toxic gases like CO and H₂S. (Fast response, high accuracy, but susceptible to cross-interference; e.g., an H₂S sensor should not be used to measure CO).

4. Safety Protection: Comprehensive Control from "Source" to "Emergency Response"

*   Early Leak Detection:

*   Odorants (e.g., Tetrahydrothiophene, giving a rotten egg smell) are added to natural gas for timely leak detection.

*   Check valves and hoses for aging in LPG systems.

*   Preventing Explosions:

*   Use explosion-proof electrical equipment (e.g., IP68 rating, resistant to water, dust, and sparks).

*   Prohibit hot work in combustible gas areas. (When hot work is essential, a "hot work permit" is required, and gas concentration must be confirmed to be <25%LEL).

*   Emergency Measures:

*   Install combustible gas alarms + emergency shut-off valves.

*   Calibrate alarms regularly using standard test gas (e.g., test with 50%LEL methane gas to verify the alarm triggers).

Final Note: Safety is No Small Matter; Detection is the Bottom Line

Gas detection is not a mere formality—it is the "red line" that protects lives. As a safety manager, you must:

✅ Distinguish between %LEL, %VOL, PPM, and mg/m³ to avoid unit confusion.

✅ Strictly implement the four-gas detection procedure: "Ventilate → Detect → Work," ensuring no step is missed.

✅ Understand the properties of combustible gases and develop prevention measures according to their hazard level.

Remember: Every standardized detection procedure is like taking out an "insurance policy" for life.

Share this "Gas Detection Safety Guide" with safety professionals around you to help more people master these essential practices!