Hydrogen fuel-cell vehicle stations dispense gaseous H2 at 700 bar. H2 has LFL 4 percent, MIE 0.02 mJ (static discharge ignites) and nearly invisible flame. NFPA 2 Hydrogen Technologies Code specifies setback, ventilation, detection and canopy rules.
H2 Physical Properties
- LFL: 4 percent (40,000 ppm)
- UFL: 75 percent (very wide)
- MIE: 0.02 mJ (static electricity)
- Auto-ignition: 500 C
- Flame visibility: invisible in daylight (UV/IR detector needed)
- Buoyancy: 14x lighter than air - rises fast
NFPA 2 Setback Distances
- Building opening: 3-10 m (per capacity)
- Ignition source: 8 m
- Public way: 5-15 m
- Other gas tank (LPG, NG): 10 m
- Dispenser-tank: 3 m min
- Overhead clearance: 6 m open sky
700-bar Storage
- Type IV tank: polymer liner + carbon-fiber composite
- Working pressure: 875 bar (nominal 700)
- TPRD (Thermal Pressure Relief): 110 C fusible plug
- Burst pressure: 1,600 bar min
- Impact test: bullet, bonfire, drop
Leak Detection
- H2 sensor: catalytic bead or electrochemical
- Setpoint: 25 percent LFL alarm, 50 percent ESD
- UV/IR flame detector: H2 flame visible in UV
- Acoustic detector: ultrasonic gas leak (40 kHz)
- Dispenser ESD: hose breakaway, auto-shutoff
Deflagration and Jet Fire
- Jet fire: 1-2 m flame, 300 kW/m2 radiation
- Deflagration: risky under canopy - venting required
- DDT (Def-Det Transition): possible in long pipes
- Emergency plan: isolation, 30 m evacuation
- Firewater: cannot extinguish H2 - radiation control only
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Sources & Further Reading
NFPA 2, NFPA 55, ISO 19880-1, HyARC, CSA HGV. NFPA 2 Hydrogen code.