Product

Flash gas is the light hydrocarbon vapour fraction generated when a pressurised process liquid undergoes rapid pressure reduction across a control valve or in a dedicated flash vessel, causing dissolved light gases to vaporise. It occurs at multiple points across a gas processing plant wherever a high-pressure liquid stream is let down to a lower operating pressure.

Unlike stabiliser overhead gas, flash gas is not produced by deliberate distillation but is an incidental by-product of pressure let-down. Its composition is therefore leaner — predominantly methane and ethane — because the heavier C₃–C₄ components preferentially remain in the liquid phase under flash conditions.

Common sources of flash gas within a gas processing plant include:

• Condensate flash drums — pressure let-down of raw condensate from inlet separators
• Glycol flash drum — dissolved hydrocarbons liberated from rich TEG/DEG after the contactor
• Amine flash drum — hydrocarbons co-absorbed with acid gases released from rich amine
• LNG end-flash drum — light ends flashed from LNG after the main cryogenic heat exchanger
• Storage tank vapours — low-pressure flash from condensate or NGL storage tanks

Typical Composition

Properties

Typical Dispositions

• Routed to plant fuel gas system
• Recompressed into sales gas export stream where economically justified
• Routed to flare if composition (e.g. H₂S content from amine flash) precludes fuel use
• Recovered as refrigerant makeup in cryogenic units (LNG end-flash)

Note on H₂S-Containing Flash Gas

Flash gas from the amine flash drum may contain H₂S stripped from the rich amine. This stream requires special handling — it cannot be routed directly to fuel gas or atmosphere and is typically sent to the acid gas flare or recycled to the AGR absorber inlet.