Geothermal plants (flash or binary) carry dissolved H2S, CO2 and methane in the well fluid. H2S is lethal at 50-100 ppm and flammable with a 4.3 percent LFL. In Turkey's Denizli and Aydin geothermal basins, NFPA 55 (Compressed Gases) and degasser tower design are mandatory.
H2S Health and Fire
- TLV-TWA: 1 ppm (OSHA)
- IDLH: 100 ppm — immediate fatality
- LFL: 4.3 percent vol
- UFL: 46 percent vol
- Autoignition: 260 C
- Odor threshold: 0.01 ppm — but olfactory fatigue above 150 ppm
Degasser (NCG Removal)
NCG (Non-Condensable Gas) from geothermal fluid:
- Degasser tower: vacuum NCG separation
- H2S absorption: ferric oxide (Selectox) or LO-CAT process
- Re-injection: NCG re-injected underground (Kizildere field)
- Stack: height per residual H2S (local limits: max 5 ppm ground level)
Gas Detection and Ventilation
- Fixed H2S detector: electrochemical, 10 ppm alarm + 50 ppm ESD
- LEL gas detector: catalytic, 20 percent LEL alarm
- Personal monitor: worn by all operators
- Forced ventilation: turbine hall 12 ach, explosion-rated
- Positive-pressure control room: excludes H2S ingress
Wellhead Control
- BOP (Blowout Preventer): API 6A, H2S service NACE MR0175
- Master valve + wing valve: remote and manual
- Flowline: corrosion-resistant (duplex steel)
- ESD system: 30-second well shut-in
- Warning signs: foggy plume + garlic odor
Ventilation and gas detection with MEP Calc
MEP Calc sizes 12 ach turbine hall ventilation, H2S/LEL detector placement and degasser stack height per NFPA 55.
Download MEP Calc on the App Store
Sources & Further Reading
NFPA 55, API 6A, NACE MR0175, OSHA 29 CFR 1910.1000, local stack limits. NFPA 55 Compressed Gases standard.