Running on the same panel as fire alarm, gas detection systems combine natural-gas leak, CO, H2S and combustible-gas (LEL) detection. NFPA 72 2022 treats gas detection as a distinct chapter. Often overlooked by mechanical engineers, it is critical for both residential and industrial facilities.
Gas Types and Sensors
- Natural gas (CH4): Catalytic bead or IR; alarm at 20% LEL
- LPG: Propane/butane; heavier than air; sensor near floor
- CO: Electrochemical; 70 ppm/60 min limit (NFPA 720)
- H2S: Electrochemical; 10 ppm alarm (OSHA TLV)
- Ammonia (NH3): Refrigeration plants; 25 ppm alarm
- O2 deficiency: Confined space; alarm <19.5%
Sensor Placement
Based on gas density relative to air:
- Lighter than air (CH4, H2, NH3): Ceiling, within 6 m of potential source
- Heavier than air (LPG, H2S): Floor level, 30 cm height
- CO: Breathing height (150-180 cm), preferred in sleeping zones
- Ventilation: Sensors must not sit in draft; avoid HVAC diffuser proximity
NFPA 72 Fire Alarm Integration
Gas detection should provide via fire alarm panel:
- Audible + visual alarm (tone distinguishable from fire alarm)
- Automatic gas valve shutoff (main inlet valve)
- HVAC shutdown (fan stop)
- Central logging (SCADA, BMS link)
- Fire department notification (optional, as NFPA 72 trouble)
Calibration and ITM
Electrochemical and catalytic elements experience zero drift. NFPA 72 requirements:
- Bump test every 6 months
- Annual full calibration (reference gas)
- Sensor life 5-10 years (type-dependent); replace thereafter
- Cross-sensitivity check: H2S sensors can false-positive on CO
Application Scenarios
- Residential boiler rooms: CH4 + CO mandatory (Turkey MMO code)
- Cold chain / meat facility: NH3 detection for ammonia refrigeration
- Parking garages: CO widespread; LEL rare
- Mall food court: LPG detection in kitchens
- H2 production / storage: UEL/LEL detection mandatory; ATEX zoning
Gas piping design with MEP Calc
Natural gas pipe sizing, LPG tank capacity, ventilation flow calculations.
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Sources & Further Reading
References: NFPA 72 - National Fire Alarm Code, NFPA 55 - Compressed Gases. Official standard: NFPA 72.