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Gas Hazards in Furnaces

Industrial furnaces produce, consume, and leak a range of hazardous gases. Understanding each gas’s source, hazard type, and exposure limits is the foundation of any monitoring strategy.

Key principle: Many furnace gases are both toxic AND flammable. Carbon monoxide (CO) is the single greatest killer in heat treatment environments – it is odourless, colourless, and lethal at concentrations well below its LEL.
Gas Formula Source Process Hazard Type WEL TWA (ppm) WEL STEL (ppm) LEL (%) UEL (%) IDLH (ppm)
Carbon monoxide CO Endothermic gas, carburizing, incomplete combustion Toxic + Flammable 20 100 12.5 74 1,200
Carbon dioxide CO2 Combustion products, exothermic gas Asphyxiant (simple) 5,000 15,000 — — 40,000
Hydrogen H2 Endothermic/exothermic gas, dissociated ammonia, bright annealing Flammable (explosive) — — 4.0 75 —
Nitrogen N2 Protective atmospheres, purging, vacuum backfill Asphyxiant (simple) — — — — —
Ammonia NH3 Nitriding, carbonitriding, dissociated ammonia Toxic + Corrosive 25 35 15 28 300
Hydrogen cyanide HCN Carbonitriding (NH3 + CO at temp), cyanide salt baths Toxic (highly lethal) 0.9 4.5 5.6 40 50
Methane CH4 Natural gas supply, enrichment gas, endothermic generator Flammable + Asphyxiant — — 5.0 15 —
Propane C3H8 Fuel gas, endothermic generator feed, enrichment Flammable + Asphyxiant — — 2.1 9.5 —
Hydrogen sulphide H2S Sulphur-bearing steels at temp, quench oil decomposition Toxic + Flammable 5 10 4.3 46 100
Methanol CH3OH Nitrogen/methanol atmosphere systems Toxic + Flammable 200 250 6.0 36 6,000
Quench oil mist CxHy (mixed) Oil quench baths, sealed quench furnaces Flammable + Irritant 5 mg/m³ 10 mg/m³ ~1.0 ~6.0 —
Argon Ar Vacuum furnace backfill, inert atmosphere Asphyxiant (simple) — — — — —
Helium He High-pressure gas quench (vacuum furnaces) Asphyxiant (simple) — — — — —
Simple asphyxiants (N2, Ar, He, CO2) displace oxygen without direct toxicity. There is no WEL – the hazard is oxygen depletion. Monitor O2 levels: alarm at <19.5%.
HCN warning: Hydrogen cyanide can form during carbonitriding when ammonia reacts with CO above 700°C. It is lethal at very low concentrations and has a faint “bitter almond” odour that not everyone can detect. Always use fixed HCN monitoring on carbonitriding lines.

Workplace Exposure Limits (EH40)

UK Workplace Exposure Limits are set in HSE document EH40/2005 (4th edition, 2020). These are legally binding under COSHH Regulations 2002.

TWA vs STEL

Measure Meaning Period
TWA Time-Weighted Average – maximum average concentration over a normal 8-hour working day and 40-hour week 8 hours
STEL Short-Term Exposure Limit – maximum 15-minute average; not to exceed 4 times per shift with at least 60 min between 15 minutes

Monitoring Requirements

  • Personal sampling: Badge or pump-mounted sampler worn in the breathing zone of the worker. Required where exposure is likely to exceed the WEL.
  • Area monitoring: Fixed or portable instruments measuring ambient concentrations. Useful for leak detection and alarm triggering but does NOT replace personal monitoring for COSHH compliance.
  • Frequency: COSHH Reg 10 – at intervals not exceeding 12 months, or when there is a change to process/controls.

EH40 Table – Furnace-Relevant Gases

Substance CAS No. TWA (ppm) TWA (mg/m³) STEL (ppm) STEL (mg/m³) Notes
Carbon monoxide630-08-02023100117Toxic. Reduces O2 carrying capacity of blood
Carbon dioxide124-38-95,0009,15015,00027,400Simple asphyxiant at high conc.
Ammonia7664-41-725183525Corrosive to eyes/respiratory tract
Hydrogen cyanide74-90-80.914.55Skin notation – absorbed through skin
Hydrogen sulphide7783-06-4571014Olfactory fatigue above 100 ppm
Methanol67-56-1200266250333Skin notation
Oil mist (mineral)—5 mg/m³510 mg/m³10Inhalable fraction. Includes quench oil
Formaldehyde50-00-022.522.5Carcinogen Cat 1B. Can form from oil decomposition
Nitrogen dioxide10102-44-00.50.9611.91Burner combustion product
COSHH monitoring requirement: Under Regulation 10, employers must ensure exposure does not exceed the WEL. Where controls are not adequate, personal exposure monitoring must be carried out and records kept for at least 5 years (40 years if the substance may cause cancer or asthma).

Portable Gas Detectors

Portable detectors are essential for personal protection, confined space entry, and spot-checking around furnace equipment. The “standard 4-gas” configuration (O2/LEL/CO/H2S) covers most scenarios, but furnace environments often require additional channels.

Detector Categories

Single-Gas

Dedicated to one gas. Compact, low cost. Common for CO personal alarm or O2 monitor in nitrogen-purged areas. Typical cost £80–250.

4-Gas (Standard)

O2, LEL (catalytic bead), CO, H2S. The industry workhorse for general entry and personal monitoring. Typical cost £400–900.

5/6-Gas & PID

Adds channels for NH3, CO2, or PID (photoionisation for VOCs/oil mist). Needed for nitriding bays, quench areas, or carbonitriding lines. Cost £1,200–3,500.

Portable Detector Comparison

Model Manufacturer Gases Sensor Life Battery IP Rating Weight Pump/Diffusion Approx. Price
MicroClip XL BW (Honeywell) 4-gas: O2/LEL/CO/H2S 2 yrs (CO/H2S), 3 yrs (O2) 18 hr Li-ion IP66/IP67 198 g Diffusion £450–550
X-am 2500 Dräger 4-gas: O2/LEL/CO/H2S 2 yrs (EC), 3+ yrs (cat bead) 12 hr alkaline or rechargeable IP67 220 g Diffusion (pump option) £500–650
X-am 5600 Dräger 6-gas: O2/LEL/CO/H2S + 2 (e.g. NH3, CO2) 2 yrs (EC), IR sensor 5+ yrs 20 hr Li-ion IP67 350 g Diffusion or pump £1,800–2,500
Altair 4XR MSA 4-gas: O2/LEL/CO/H2S 2 yrs (XCell sensors) 24 hr Li-polymer IP68 213 g Diffusion £550–700
Altair 5X MSA 5-gas: O2/LEL/CO/H2S + PID or IR 2 yrs (EC), 5 yrs (IR) 17 hr Li-ion IP67 397 g (diffusion), 539 g (pump) Diffusion or integral pump £2,200–2,800
BW Ultra Honeywell 5-gas: O2/LEL/CO/H2S + PID 2 yrs (EC), 3 yrs (PID lamp) 24 hr Li-ion IP66/IP68 348 g Integral pump £2,500–3,200
T4 Crowcon 4-gas: O2/LEL/CO/H2S 2 yrs (EC), 3 yrs (cat bead) 18 hr rechargeable IP65 182 g Diffusion £400–500
Furnace-specific tip: Standard catalytic bead LEL sensors are poisoned by silicones (silicone sealants, greases, moulds). If your furnace environment has silicone contamination, specify an IR LEL sensor instead (available on X-am 5600, Altair 5X).
Hydrogen cross-sensitivity: Electrochemical CO sensors have significant H2 cross-sensitivity (typically 10–25%). In endothermic atmosphere furnace rooms with high H2 background, the CO reading may be inflated. Use H2-compensated CO sensors (available from Dräger and MSA) or separate H2-specific sensors.

Fixed Gas Detection Systems

Fixed systems provide continuous, 24/7 monitoring of furnace areas. They connect to building alarm systems, ventilation controls, and emergency shutdowns. System design must consider gas density, ventilation patterns, and SIL requirements.

System Components

Detection Side

  • Sensor heads / transmitters: Field-mounted devices that detect gas and transmit a signal. May be point detectors (measure at one location) or open-path (IR beam across a zone).
  • Controllers / panels: Receive signals from sensor heads, display readings, manage alarm logic. Typically 4–32 channel units in a control room or locally on a furnace panel.
  • Communications: 4–20 mA analogue loop (most common, one pair per sensor), HART digital overlay, Modbus RTU/TCP for integration with SCADA/BMS systems.

Response Side

  • Local alarms: Beacon + sounder at the detection point. Audible ≥85 dB(A) at 1 m.
  • Relay outputs: Volt-free contacts to start extraction fans, close gas supply valves, trip furnace power.
  • Integration: 4–20 mA / Modbus to BMS, SCADA, or fire alarm panel.
  • SIL rating: Safety Integrity Level per IEC 61508/61511. SIL 1 typical for alarm-only; SIL 2 for automatic gas isolation; SIL 3 for high-hazard zones (rare in standard furnace rooms).

Point vs Open-Path Detection

Point Detectors

Measure gas at one specific location. Best for localised leak detection near valves, seals, furnace doors. Typical detection range: 0–100% LEL or 0–toxic range. Response time: 10–30 seconds (T90).

Open-Path (Line-of-Sight)

IR beam between transmitter and receiver covering 5–200 m path. Measures gas in ppm·m (concentration × path length). Best for perimeter monitoring of large areas. Not suitable for pinpointing leak location.

Fixed System Manufacturer Comparison

Manufacturer Key Products Sensor Types Comms SIL Rating Strengths
Honeywell Analytics Searchline Excel (open-path), Sensepoint XCL (point), Touchpoint Plus (controller) EC, Cat bead, IR, Open-path IR 4–20 mA, HART, Modbus SIL 2 (Sensepoint XCL) Wide range, global support, SIL-certified sensors
MSA Safety Ultima X5000 (point), Ultima XIR (IR point), Ultima OPIR-5 (open-path), SUPREMATouch (controller) EC, Cat bead, Dual IR (Xtinguish), NDIR 4–20 mA, HART, Modbus TCP SIL 2 (Ultima X5000) XCell sensor platform, fast T90, poison-resistant cat bead option
Dräger Polytron 8000 (point), Polytron 7000 (LEL), PIR 7000 (IR), REGARD 3900 (controller) EC, Cat bead, IR, Dual IR 4–20 mA, HART, Modbus, FOUNDATION Fieldbus SIL 2 (Polytron 8000) Premium quality, excellent EC sensors, Fieldbus support
Crowcon Xgard IQ (point, smart), Xgard Bright (fixed point), IRmax (open-path), Gasmaster (controller) EC, Cat bead, IR, Open-path IR, PID 4–20 mA, HART, Modbus, RS-485 SIL 2 (Xgard IQ) UK-based, good support, cost-effective for multi-point
Det-Tronics GT3000 (toxic point), FlexSight LS2000 (open-path), Eagle Quantum Premier (controller) EC, MOS, IR, UV/IR flame 4–20 mA, HART, Modbus, FOUNDATION Fieldbus SIL 2 (GT3000), SIL 3 (EQP controller) Oil & gas heritage, robust controllers, SIL 3 capable
Furnace room specification tip: For sealed quench furnace bays, a typical fixed system might use 3–6 Crowcon Xgard IQ heads (CO + LEL near furnace doors, H2 at ceiling height) connected to a Gasmaster 4-channel controller with relay outputs to the building ventilation fan starter and furnace gas supply solenoid valve.

Sensor Technologies

Understanding sensor operating principles is critical for specifying the right detection for each furnace environment. No single sensor type covers all gases.

Electrochemical (EC) Sensors

How it works

Target gas diffuses through a membrane and reacts at a sensing electrode, generating a small current proportional to gas concentration. A reference electrode provides a stable potential.

Target gases

CO, H2S, NH3, O2, HCN, NO2, Cl2

Key characteristics

  • Typical life: 2–3 years (O2 cells 1–2 years)
  • T90 response: 15–30 seconds
  • Temperature range: −20°C to +50°C
  • Cross-sensitivity: CO sensors respond to H2 (10–25%)
  • Humidity affects accuracy below 15% RH
  • Low power consumption – ideal for portable instruments

Catalytic Bead / Pellistor (LEL)

How it works

Two matched platinum coils (active + reference) in a Wheatstone bridge. Flammable gas oxidises on the active bead, raising its temperature and resistance. The bridge imbalance is proportional to gas concentration.

Target gases

All flammable gases measured as %LEL. Typically calibrated on methane or propane.

Key characteristics

  • Typical life: 3–5 years
  • T90 response: 10–20 seconds
  • Requires oxygen to function (min ~10% O2)
  • Poisoning risk: Silicones, lead, sulphur compounds permanently damage the catalyst
  • Cannot detect gas above UEL (output drops to zero)
  • Needs regular bump testing to verify catalyst integrity
Catalytic bead poisoning in furnace environments: Silicone-based sealants, vacuum pump oils, mould release agents, and some heat treatment compounds can poison pellistor sensors without any visible change. The sensor appears to work but no longer responds to gas. This is a leading cause of gas detector failure. Use IR LEL sensors in areas with known silicone contamination, and bump test catalytic sensors before every shift.

Infrared / NDIR Sensors

How it works

Gas absorbs infrared light at specific wavelengths. A dual-beam design compares absorption at the target wavelength with a reference wavelength, making it immune to contamination and drift. Non-contact – no chemical reaction.

Target gases

CO2, CH4, C3H8, other hydrocarbons. Cannot detect H2 or CO (no IR absorption).

Key characteristics

  • Typical life: 5–10+ years (no consumable elements)
  • T90 response: 5–15 seconds
  • Not poisoned by silicones or other contaminants
  • Works in inert (zero O2) atmospheres
  • Fail-safe – reports fault if beam blocked
  • Higher cost than catalytic bead

Photoionisation Detector (PID)

How it works

UV lamp ionises gas molecules; ions are collected at an electrode. Current is proportional to gas concentration. Different lamp energies (9.8, 10.6, 11.7 eV) detect different compounds.

Target gases

VOCs, benzene, toluene, quench oil vapour, solvents. Broad-spectrum – does not identify specific compounds.

Key characteristics

  • UV lamp life: 2,000–6,000 hours depending on type
  • Very fast response: <5 seconds
  • Sensitive: sub-ppm detection
  • Affected by humidity (water vapour absorbs UV)
  • Cannot detect methane, CO, CO2 (ionisation energy too high)

Other Sensor Types

Paramagnetic (O2)

Exploits the paramagnetic properties of oxygen. Two nitrogen-filled glass spheres in a magnetic field; O2 displaces them proportionally. Very accurate (±0.1% O2), long life, no consumables. Used in process analysers and high-accuracy fixed systems.

Thermal Conductivity (TC)

Measures the thermal conductivity of a gas mixture relative to a reference. Used for H2 and He detection (both have TC ~6× air). Limited selectivity – responds to any gas with different TC. Best for binary mixtures (e.g. H2 in N2).

Metal Oxide Semiconductor (MOS)

Heated tin-oxide film changes resistance when gas adsorbs. Very sensitive but poor selectivity. Useful for general flammable gas “sniffer” applications. Low cost, long life, but high cross-sensitivity.

Cross-Sensitivity Reference

Sensors designed for one gas may also respond to others. This table shows typical cross-sensitivity as a percentage of reading for common furnace gases.

Sensor Target CO (cross) H2 (cross) H2S (cross) NH3 (cross) NO2 (cross) CH4 (cross)
CO sensor (EC)100%10–25%<2%<1%−5%<1%
H2S sensor (EC)<5%<3%100%<1%15–30%<1%
NH3 sensor (EC)<3%<5%−10%100%−20%<1%
O2 sensor (EC)<0.1%<0.1%<0.1%<0.1%<0.1%<0.1%
LEL sensor (Cat bead)~50%~50%Poison~30%Nil100%
CO2 sensor (NDIR)NilNilNilNilNil<2%
Practical impact: In an endothermic furnace room with ~20% CO and ~40% H2 in the atmosphere, a standard EC CO sensor reading 50 ppm ambient CO will also show an additional 5–12 ppm from H2 cross-sensitivity. This is usually conservative (over-reads) which is acceptable for safety, but can cause nuisance alarms.

Calibration & Maintenance

Gas detectors are life-safety devices. Without regular calibration and bump testing, they cannot be relied upon. UK HSE guidance and manufacturer requirements are clear: untested detectors are worse than no detectors (false confidence).

Bump Test vs Full Calibration

Bump Test (Functional Check)

  • Frequency: Before each use / start of shift (daily minimum)
  • Purpose: Confirms sensors respond to target gas and alarms trigger
  • Method: Brief exposure (30–60 seconds) to calibration gas at known concentration above alarm setpoint
  • Pass criteria: Sensor reads within ±20% of applied concentration AND alarm activates
  • Duration: ~60 seconds per instrument
  • Does NOT adjust: No changes to calibration factors

Full Calibration (Span Adjustment)

  • Frequency: Every 6 months (manufacturer recommendation), or when bump test fails, or after sensor replacement
  • Purpose: Adjusts sensor response to match a known reference gas concentration
  • Method:
    1. Zero cal: Expose to clean air or zero gas (N2) – set baseline
    2. Span cal: Apply calibration gas at target concentration – adjust reading to match
  • Duration: 5–15 minutes per sensor channel
  • Records: Date, gas batch, before/after readings, engineer signature

Calibration Gas Cylinder Specifications

Standard calibration gas mixture for 4-gas detectors in furnace environments:

Component Concentration Purpose Cylinder Type Typical Shelf Life
Carbon monoxide (CO) 100 ppm Span calibration of CO sensor (alarm at 20/100 ppm) Disposable 34L or refillable 58L 12–24 months
Hydrogen sulphide (H2S) 25 ppm Span calibration of H2S sensor (alarm at 5/10 ppm) Disposable 34L or refillable 58L 12 months (reactive gas)
Methane (CH4) 2.5% vol (50% LEL) Span calibration of LEL sensor Disposable 34L or refillable 58L 24–36 months
Oxygen (O2) 18% vol Span check of O2 sensor (alarm at 19.5%) Included in 4-gas mix 24–36 months
Balance gas Nitrogen (N2) Inert carrier gas — —
Additional calibration gases for furnace environments:
  • NH3: 50 ppm in N2 (for nitriding bay detectors)
  • CO2: 5,000 ppm in N2 (for IR CO2 sensors)
  • H2: 2% vol in N2 (50% LEL for H2-specific sensors)
  • Isobutylene: 100 ppm in air (standard PID calibrant – use correction factors for other VOCs)

Record Keeping Requirements

Legal requirement: Under COSHH Regulation 9, employers must ensure monitoring equipment is properly maintained, examined, and tested. Records must include:
  • Instrument serial number and location
  • Date and type of test (bump / full calibration)
  • Gas cylinder batch number and expiry date
  • Before-calibration readings (as-found) and after-calibration readings (as-left)
  • Name and signature of competent person
  • Any corrective actions (sensor replacement, instrument swap-out)
Records should be kept for at least 5 years. Many modern detectors (MSA Altair, Dräger X-am) have internal data logging and docking station software that automates this.

Furnace Monitoring Setups

Each furnace type presents different gas hazards depending on the atmosphere, temperature, and process. These recommended configurations are based on practical experience and HSE/IGEM guidance.

Sealed Quench Furnace (Endo Atmosphere)

Atmosphere: 20% CO, 40% H2, 40% N2 (typical endothermic)

SensorLocationAlarm 1Alarm 2Notes
CO (EC, H2-compensated)1.5 m height, within 2 m of furnace front door20 ppm100 ppmCO is slightly lighter than air (SG 0.97) but released warm, so rises initially
LEL (catalytic bead or IR)1.5 m height, near quench vestibule door20% LEL40% LELDetects general flammable gas leak (mixed CO/H2/CH4)
H2 (TC or EC)Ceiling level (H2 SG 0.07 – rises rapidly)1% vol2% volCritical: H2 accumulates at ceiling and is not detected by IR sensors
O2Breathing zone (1.5 m)<19.5%<18%Atmosphere gas displaces oxygen; required for confined space assessment

Relay actions: Alarm 1 → audible/visual warning + increase ventilation. Alarm 2 → evacuate area, isolate furnace gas supply, emergency ventilation max speed.

Vacuum Furnace Room (Ar/N2 Backfill)

Hazard: Oxygen depletion from argon or nitrogen release

SensorLocationAlarm 1Alarm 2Notes
O2 (EC or paramagnetic)300 mm above floor level (Ar SG 1.38, N2 SG 0.97)<19.5%<18%Argon is denser than air and pools at floor; N2 disperses more evenly
O2 (second unit)Breathing zone 1.5 m, near furnace loading area<19.5%<18%Redundant sensor at worker height

Special considerations: Vacuum furnace rooms with bulk Ar storage can experience rapid O2 depletion if a large-bore valve or pipe fails. Calculate worst-case leak rate and ensure ventilation capacity exceeds it. BCGA CP30 provides guidance.

Nitriding Bay (NH3 Atmosphere)

Atmosphere: Undissociated NH3 or NH3/N2/H2 mix

SensorLocationAlarm 1Alarm 2Notes
NH3 (EC)1.5 m height, within 3 m of furnace25 ppm (TWA)35 ppm (STEL)NH3 is lighter than air (SG 0.59), pungent odour detectable at ~5 ppm
NH3 (second sensor)Near exhaust/vent point25 ppm35 ppmMonitors exhaust system effectiveness
H2 (TC or EC)Ceiling level1% vol2% volDissociated ammonia is 75% H2 – significant explosion risk
LEL (IR preferred)1.5 m, near furnace front20% LEL40% LELUse IR to avoid NH3 poisoning of catalytic bead

Relay actions: Alarm 1 → warning + check exhaust fans. Alarm 2 → evacuate, isolate NH3 supply, max ventilation. NH3 is corrosive to eyes at >50 ppm.

Carburizing Furnace (CO-Rich)

Atmosphere: 20–25% CO (endothermic + enrichment)

SensorLocationAlarm 1Alarm 2Notes
CO (EC, H2-compensated)1.5 m, at furnace front and rear20 ppm100 ppmMinimum 2 sensors per furnace – front and rear doors
CO (additional)Operator rest area / control desk20 ppm50 ppmLower Alarm 2 for occupied areas where response time is critical
LELNear flame curtain / door seal20% LEL40% LELFlame curtain failure allows atmosphere gas escape

Hydrogen Furnace (Bright Annealing)

Atmosphere: 75–100% H2

SensorLocationAlarm 1Alarm 2Notes
H2 (TC sensor)Ceiling – directly above furnace entry/exit0.5% vol (12.5% LEL)1.6% vol (40% LEL)H2 is extremely buoyant (SG 0.07) – ceiling mounting essential
H2 (second TC)Ceiling – at highest point in room0.5% vol1.6% volH2 migrates to highest point; check for roof voids/dead spots
LEL (IR)1.5 m near furnace entry/exit20% LEL40% LELIR cannot detect H2 directly but catches hydrocarbon leaks
O21.5 m breathing zone<19.5%<18%H2 furnace purge can displace O2
Hydrogen explosion risk: H2 has the widest flammable range of any common gas (4–75%). Its minimum ignition energy is extremely low (0.017 mJ – 10× lower than methane). Static discharge from clothing can ignite a H2/air mixture. Ensure proper earthing/bonding and eliminate ignition sources.

Quench Oil Area

Hazard: Oil vapour, mist, and decomposition products

SensorLocationAlarm 1Alarm 2Notes
LEL (IR preferred)Above quench tank, 300–500 mm from rim10% LEL25% LELLower setpoints than standard – oil flash fires escalate rapidly
PID (VOC)1.5 m breathing zone, downwind of tank5 ppm (as isobutylene)25 ppmDetects oil vapour/decomposition products. Apply correction factor for quench oil (~0.8)
CONear quench tank cover seal20 ppm100 ppmEndothermic atmosphere escapes when charge is transferred to quench

Additional controls: Local exhaust ventilation (LEV) over quench tank is mandatory. Oil temperature monitoring with high-temp alarm (typically 80°C for fast quench oils, 120°C for marquench). Water contamination of quench oil causes explosive foaming – moisture sensors in oil circuit recommended.

Alarm Levels & Response Procedures

Gas detection alarm setpoints must be set below dangerous concentrations with sufficient margin for response time. Two-stage alarms are standard: Alarm 1 (warning) and Alarm 2 (action/evacuate).

Gas Alarm 1 (Warning) Alarm 2 (Action) Dangerous Level Alarm 1 Response Alarm 2 Response
Oxygen (O2) <19.5% or >23.5% <18% or >25% <16% impairment, <6% fatal Investigate source of O2 depletion/enrichment. Check ventilation. Increase air flow. Evacuate immediately. Do not re-enter without SCBA. Isolate inert gas supplies. Emergency ventilation.
LEL (flammable gas) 20% LEL 40% LEL 100% LEL = lower explosive limit Warning alarm. Investigate leak source. Increase ventilation. Remove ignition sources. Evacuate area. Isolate gas supply. Do not operate electrical switches. Emergency services if uncontrolled.
Carbon monoxide (CO) 20 ppm 100 ppm IDLH 1,200 ppm Investigate source (furnace seal, flame curtain, exhaust). Increase ventilation. Evacuate to fresh air. Isolate furnace atmosphere. Anyone with symptoms to medical attention (CO blood test). Do not re-enter without portable CO monitor.
Hydrogen sulphide (H2S) 5 ppm 10 ppm IDLH 100 ppm. >100 ppm olfactory paralysis Investigate source. Check quench oil system. Increase ventilation. Evacuate immediately. H2S causes rapid incapacitation at high concentrations. SCBA for re-entry.
Ammonia (NH3) 25 ppm 35 ppm IDLH 300 ppm. Corrosive to eyes/lungs Check nitriding furnace seals and exhaust. PPE (ammonia respirator cartridge, goggles). Evacuate. Isolate NH3 supply. Water spray to knock down ammonia cloud (NH3 is water-soluble). Medical attention for eye/respiratory exposure.
Hydrogen (H2) 0.5% vol (12.5% LEL) 1.6% vol (40% LEL) LEL 4% vol. Wide flammable range 4–75% Investigate leak. Check furnace seals, gas supply connections. Increase ventilation at ceiling level. Evacuate. Eliminate all ignition sources (static, electrical). Isolate H2 supply. Ventilate at ceiling level. H2 flames are invisible – approach with caution.
General emergency response principles:
  • Never assume a single gas – where one gas leaks, others may be present
  • Buddy system for all emergency response – never enter a gas alarm zone alone
  • SCBA (self-contained breathing apparatus) for any atmosphere with O2 <19.5% or unknown composition
  • Gas-free certificate required before any maintenance work in areas that have been under alarm
  • Treat all CO exposures as medical emergencies – symptoms may be delayed
Alarm setpoint selection: Setpoints must account for sensor response time (T90), ventilation lag time, and personnel evacuation time. As a rule of thumb, Alarm 1 should be set at no more than 50% of the WEL (toxic) or 20% LEL (flammable) to provide adequate warning margin. Alarm 2 should be at the WEL/STEL or 40% LEL.

LEL / UEL Reference

The Lower Explosive Limit (LEL) is the minimum concentration of a gas in air that can sustain a flame. The Upper Explosive Limit (UEL) is the maximum. Between LEL and UEL, the mixture is explosive if an ignition source is present.

Key Concepts

  • Below LEL: Too lean to burn – insufficient fuel
  • LEL to UEL: Flammable/explosive range
  • Above UEL: Too rich to burn – insufficient oxygen
  • Stoichiometric: Ideal combustion ratio (maximum explosion pressure)
  • Gas detectors read 0–100% LEL (not 0–100% gas volume)
  • 20% LEL alarm = 20% of the way to the explosive limit

Factors Affecting LEL/UEL

  • Temperature: LEL decreases ~0.5% per 100°C rise (wider flammable range)
  • Pressure: LEL relatively unaffected; UEL increases with pressure
  • Oxygen enrichment: LEL decreases, UEL increases dramatically
  • Inert dilution: Both LEL and UEL converge (narrower range)
  • Mixtures: Use Le Chatelier’s mixing rule for multi-gas LEL calculation

Complete LEL/UEL Table – Furnace-Relevant Gases

Gas Formula LEL (% vol) UEL (% vol) Flammable Range Auto-Ignition Temp (°C) Min. Ignition Energy (mJ) Flame Speed (m/s) Vapour Density (Air=1)
Methane CH4 5.0 15.0 10.0% 537 0.28 0.40 0.55
Propane C3H8 2.1 9.5 7.4% 450 0.25 0.46 1.52
Hydrogen H2 4.0 75.0 71.0% 500 0.017 3.46 0.07
Carbon monoxide CO 12.5 74.0 61.5% 609 0.30 0.47 0.97
Ammonia NH3 15.0 28.0 13.0% 651 680 0.15 0.59
Methanol CH3OH 6.0 36.0 30.0% 464 0.14 0.52 1.11
Acetylene C2H2 2.5 100.0 97.5% 305 0.017 1.53 0.90
Hydrogen sulphide H2S 4.3 46.0 41.7% 260 0.077 0.45 1.19
Ethylene C2H4 2.7 36.0 33.3% 490 0.07 0.80 0.97
Butane C4H10 1.8 8.4 6.6% 405 0.25 0.45 2.01
Hydrogen cyanide HCN 5.6 40.0 34.4% 538 — 0.52 0.93
Most dangerous gases by ignition energy: Hydrogen and acetylene have the lowest minimum ignition energies (0.017 mJ). For comparison, a static spark from walking on carpet is ~20 mJ – over 1,000 times the energy needed to ignite H2. In hydrogen furnace areas, anti-static flooring, conductive footwear, and earthed equipment are mandatory.
Le Chatelier’s mixing rule for calculating LEL of gas mixtures:

LELmix = 100 / (C1/LEL1 + C2/LEL2 + ... + Cn/LELn)

Where Cn is the percentage of each component in the fuel mixture (fuel only, excluding air). Example: Endothermic gas (20% CO + 40% H2 + 1% CH4, rest is N2/CO2). Fuel fraction: CO 33%, H2 66%, CH4 1.6%. LELmix = 100 / (33/12.5 + 66/4 + 1.6/5) = 100 / (2.64 + 16.5 + 0.32) = 5.1% vol in air.