Flare Gas Recovery

A flare gas recovery system (FGRS) captures, compresses, and redirects gas that would otherwise be flared, returning it to the fuel gas system or process. Properly designed FGRS can reduce routine flaring by 80–98%, leaving the flare available only for genuine upset and emergency events.

How it works:

• Liquid ring or screw compressors draw gas off the flare header below the water seal
• Recovered gas is cooled, dehydrated, and conditioned to fuel-gas quality
• Gas is returned to the plant fuel gas system, refinery gas plant, or process
• Flare remains immediately available for any flow exceeding compressor capacity

The result is a major reduction in routine flaring, lower CO₂ and methane emissions, and recovery of valuable hydrocarbons that would otherwise be destroyed.

FGRS preserves flare safety by controlling flare header pressure rather than blocking flow to the flare. Compressors draw gas at a setpoint slightly above the flare seal water level; any flow that exceeds compressor capacity passes immediately to the flare without delay or pressure buildup.

Safety design principles:

• Water seal between flare header and compressor isolates the systems
• Pressure control modulates compressor capacity, never restricts header flow
• Compressor failure trips the system to free-flare mode automatically
• Bypass and shutdown logic ensure relief always reaches the flare during upsets
• Header pressure stays within design range during all operating modes

Result: routine flaring is recovered, but the flare's emergency relief function remains uncompromised. Process safety case is preserved.

Typical FGRS payback is 1–4 years, depending on baseline flare gas volumes, fuel gas value, and applicable carbon credit or regulatory drivers. Sites with high routine flaring and high fuel gas value see the fastest payback; sites with low baseline flaring may need regulatory drivers (carbon pricing, flaring restrictions) to justify investment.

Payback drivers:

• Baseline flare gas volume — higher flow = faster payback
• Fuel gas displacement value — recovered gas offsets imported or produced fuel
• Avoided steam consumption (where flare uses steam-assist)
• Carbon credits or compliance with flaring reduction regulations
• Avoided emissions penalties under regional regulations

For continuous-operation refineries with significant routine flaring, FGRS is often among the highest-ROI environmental projects on the asset.

FGRS units handle gas flows from a few hundred kg/hr to 50,000+ kg/hr at suction pressures typically 5–50 mbarg above atmospheric, matched to the flare header design pressure. Compressors are sized for the routine flare gas profile, not the peak relief case.

Sizing inputs:

• Routine flare gas flow — typical 95th-percentile of historical data
• Composition variability — affects compressor selection and discharge cleanup
• Suction pressure range — matched to flare header low-pressure operation
• Discharge pressure — matched to fuel gas or process tie-in target
• Turndown requirement — multiple compressors or VFD drives provide flexibility

Multiple parallel compressors are common for redundancy and to match variable load profiles, with one or two units cycling on and off as flow varies.

FGRS integrates with an existing flare header through a tie-in upstream of the flare's water seal, with the recovery compressor maintaining a small positive suction. The flare and its header remain unchanged; only the gas path before the seal is modified to allow recovery.

Typical retrofit scope:

• New tie-in point on the flare header (above water seal)
• Compressor skid with knockout drum, cooler, dehydration, and controls
• Discharge piping to fuel gas system or process tie-in
• Pressure control logic integrated with the plant DCS
• Bypass and shutdown logic to ensure flare availability during compressor outage
• Updated relief case analysis and process safety review

Retrofits typically require a brief plant shutdown for tie-in, scheduled with planned maintenance to minimize impact.

During upset or emergency conditions, FGRS automatically relinquishes flow to the flare: when relief flow exceeds compressor capacity, header pressure rises briefly, the water seal breaks, and gas flows directly to the flare. The compressor continues recovering whatever fits within its capacity.

Operating modes:

• Normal: 100% of routine flare gas recovered; flare pilot only
• Minor upset: compressor at full capacity; small excess goes to flare
• Major upset/emergency: water seal breaks; full relief flows to flare; compressor continues recovering background flow
• Compressor trip: all flow goes to flare; recovery resumes when compressor restarts

The flare's emergency relief function is never compromised — FGRS is a parallel recovery path, not a series device.

FGRS designs follow API 521, API 537, API 11P (compressors), and applicable pressure-vessel codes (ASME). Emissions reductions are significant: a properly designed FGRS reduces routine flaring by 80–98%, cutting CO₂, methane, and hydrocarbon emissions proportionally.

Typical emissions impact:

• CO₂ reduction proportional to recovered gas volume and carbon content
• Methane emission reduction (if any methane slip from flare combustion)
• Reduced thermal NOx from lower flare burn rates
• Lower visible flare activity, reducing community impact
• Potential carbon credit eligibility under voluntary or regulatory frameworks

Performance is verified through pre/post flare gas metering, with compliance reporting tied to local environmental regulations and corporate sustainability targets.