Geothermal Wells: NFPA 850, 500 ppm H2S, Flash Separators and Pentane ORC

With 1,700 MW of installed geothermal capacity, Turkey ranks fourth in the world; the backbone of the sector is two large plants of 242 MW and 162 MW. Geothermal fluid at 200–250 °C carries CO₂, H₂S and CH₄, so what comes out of the wellhead is not just steam but a toxic and flammable mixture. NFPA 850 treats flash separators, pentane ORC units and H₂S abatement systems as separate fire hazards — the design is not complete until each of these three has its own protection.

Geothermal Fluid Composition

• CO₂: 90% of the non-condensable gases
• H₂S: 100–500 ppm — toxic; exposure limits are critical
• CH₄: 1–5% methane
• Temperature: 180–250 °C at the wellhead
• Wellhead pressure: 10–25 bar

Flash Separator Safety

• First-stage flash: 180 °C, 10 bar
• Second-stage flash: 120 °C, 2 bar
• Demister: Captures silica particles before they reach the turbine
• Rupture disc: Burst pressure 1.1 times the maximum allowable working pressure (MAWP)
• Emergency vent: Upward-discharging flare line

Risks of Pentane ORC Units

• Binary cycle: Working fluid is n-pentane or isobutane
• Flash point: -49 °C — Class IA
• LEL: 1.4% — a low explosive threshold
• Sprinklers: Deluge system at 12.2 L/min/m²
• Ex equipment: The surrounding area is Zone 1; electrical equipment is selected accordingly

H2S Abatement

• LO-CAT: Converts H₂S to elemental sulfur (redox process)
• Stretford: Older-generation technology, not preferred in new plants
• Reinjection: Non-condensable gas and water are pumped back into the reservoir
• Monitoring: 1 ppm TLV-TWA threshold (ACGIH)
• SCBA: Mandatory above 100 ppm

Field Examples and Lessons Learned

• Largest plant: 242 MW
• Second-largest plant: 162 MW
• A mid-size plant: 47 MW
• Pentane leak: a binary plant in the US — a reference event for binary cycles
• The Geysers (California): 1,517 MW, the world’s largest geothermal field

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