Vinyl chloride monomer (VCM, CH2=CHCl) is a flammable carcinogenic gas/liquid used in PVC manufacture. LEL 3.6 percent, UEL 33 percent, flash point -78 C. Closed tank leaks quickly form explosive atmospheres. ATEX 2014/34/EU and NFPA 30 require explosion-protected design for VCM tanks, transfer pumps and piping.
VCM Properties
- Molecular weight: 62.5 g/mol
- Boiling point: -13.4 C (atmospheric)
- LEL: 3.6 percent vol
- UEL: 33 percent vol
- Autoignition: 472 C
- IARC: Group 1 carcinogen (liver)
ATEX Zone Classification
- Zone 0: Tank ullage - continuous VCM vapor
- Zone 1: Tank vent outlet, pump seal, loading arm (explosive atmosphere likely in normal operation)
- Zone 2: 3 m around Zone 1 (abnormal conditions)
- Equipment category: II 2G Ex db IIA T2 (VCM gas group IIA)
Explosion Pressure
VCM maximum explosion pressure: Pmax = 8.8 bar (1 atm initial).
Kst (deflagration index): 50 bar m/s (St1 class).
NFPA 68 venting: Av = (Kst × V^0.75) / (Pred × 1000) — a 100 m3 tank with Pred = 0.5 bar needs Av ~= 3 m2 vent area.
Fire Protection
- Deluge sprinkler: 10 L/min/m2 tank cooling
- Foam: Alcohol-resistant AFFF (VCM is not polar but polymerizes)
- Gas detection: Catalytic or IR, alarm at 20 percent LEL
- Emergency shutdown: ESD < 30 s for all transfer lines
- Ventilation: 12 ach, explosion-proof fan
ATEX ventilation with MEP Calc
MEP Calc supports ach sizing, ESD flow, explosion pressure and vent area calculations for VCM storage.
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Sources & Further Reading
NFPA 30, ATEX 2014/34/EU, IEC 60079-10-1, NFPA 68, IARC Monographs. NFPA 30 Flammable Liquids code.