Geothermal Plant H2S and Fire Risk: NFPA 55 and Degasser Design

In geothermal plants — both flash and binary cycle — the well fluid carries dissolved H₂S, CO₂ and methane. H₂S can be lethal even at 50–100 ppm, and it is also flammable, with a lower flammable limit (LFL) of 4.3%. In geothermal fields, the NFPA 55 (compressed gases) approach and a properly designed degasser tower are now non-negotiable requirements.

H2S: Health and Fire Hazards

• TLV-TWA: 1 ppm (ACGIH threshold limit value)
• IDLH: 100 ppm — immediately dangerous to life or health
• LFL: 4.3% by volume
• UFL: 46% by volume
• Autoignition temperature: 260 °C
• Odor threshold: 0.01 ppm — but above 150 ppm the sense of smell is paralyzed, so the nose is no longer a reliable warning

Degasser (NCG Removal)

The main methods used to remove non-condensable gases (NCG) from geothermal fluid:

• Degasser tower: NCG separation under vacuum
• H2S absorption: Iron oxide-based (Selectox) or LO-CAT process
• Reinjection: NCG pumped back underground
• Stack: Stack height is set according to the residual H2S content

Gas Detection and Ventilation

• Fixed H2S detectors: Electrochemical sensors; alarm at 10 ppm, emergency shutdown at 50 ppm
• LEL gas detectors: Catalytic type, alarm at 20% LEL
• Personal gas monitors: Worn by every operator entering the site, without exception
• Forced ventilation: 12 air changes per hour in the turbine hall, with explosion-protected (Ex) equipment
• Positive-pressure control room: Air balance set to prevent H2S from entering

Wellhead Control

• Blowout preventer (BOP): To API 6A, with NACE MR0175-compliant materials for H2S service
• Master valve and wing valve: Both remote and manual opening/closing
• Flowline: Corrosion-resistant duplex steel against the aggressive geothermal fluid
• Emergency shutdown (ESD) system: Able to shut in the well within 30 seconds
• Warning signs on site: A rotten-egg odor at low concentrations; because the sense of smell is paralyzed at high concentrations, leak detection must rely on detectors

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.

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