Flare Gas Recovery

A flare gas recovery system (FGRS) captures gas that would otherwise be flared and returns it to the plant fuel gas system or process, leaving the flare for genuine upset and emergency events. A properly designed FGRS can reduce routine flaring by 80 to 98%, depending on the routine flare gas profile. Recovery is by liquid ring or screw compressors for higher sustained flows, or by an eductor that uses high-pressure gas as the motive fluid where no rotating gas-handling equipment is wanted. Recovered gas is cooled, dehydrated and conditioned before it is returned. CRA supplies both routes as skid-mounted packages.

Flare gas recovery preserves flare safety by controlling flare header pressure instead of blocking flow to the flare. The recovery unit draws gas from the header at a set point slightly above the water seal, and any flow beyond its capacity passes straight to the flare with no delay or pressure build-up. The water seal separates the recovery unit from the flare, pressure control modulates recovery capacity and does not restrict header flow, and a trip of the recovery unit returns the system to free flaring automatically. The flare's relief function stays as it was.

Payback on a flare gas recovery system depends on fuel price, run hours and the local carbon or flaring penalty regime, together with the baseline flare gas volume and the value of the fuel gas it displaces. Sites with high routine flaring and a high fuel gas value see the fastest payback. Sites with low baseline flaring usually need a regulatory driver to justify the investment. Avoided steam consumption on steam-assisted flares and avoided emissions penalties also contribute. CRA sizes the recovery unit for the routine flare gas profile, which keeps the capital cost matched to the gas that is actually recoverable.

A flare gas recovery unit handles gas flows from a few hundred kg/h to more than 50,000 kg/h, at suction pressures typically 5 to 50 mbarg above atmospheric to match the flare header design pressure. The compressors are sized for the routine flare gas profile, normally the 95th percentile of the historical flow data, and the peak relief case stays with the flare. Composition variability affects the compressor selection and the discharge gas clean-up, and the discharge pressure is matched to the fuel gas or process tie-in. Parallel compressors or variable speed drives provide turndown as flow varies. CRA sizes the package from the site's flare gas data.

A flare gas recovery unit ties into an existing flare header through a connection upstream of the water seal, so the flare and its header stay unchanged and only the gas path before the seal is modified. The retrofit scope normally covers the new tie-in, the recovery skid with knock-out drum, cooler, dehydration and controls, discharge piping to the fuel gas system or process, and pressure control logic integrated with the plant DCS. The plant's relief case analysis and process safety review are updated for the new arrangement. A retrofit usually needs a short plant shutdown for the tie-in, which is scheduled with planned maintenance.

During an upset or emergency the flare gas recovery unit gives way to the flare automatically. When the relief flow exceeds the compressor capacity, the header pressure rises briefly, the water seal breaks and gas flows directly to the flare, while the compressor keeps recovering what it can. In normal operation the routine flare gas is recovered and the flare runs on pilot only. In a minor upset the small excess goes to the flare, and in a major upset the full relief flow does. If the compressor trips, all flow goes to the flare. The recovery unit is a parallel path and the flare's relief function is unchanged.

CRA flare gas recovery systems are designed to API 521 for the relief and flare system, API 537 for the flare, API 619, API 618 and API 11P for the compressors as applicable, and ASME pressure vessel codes. Recovering routine flare gas reduces CO₂, methane and hydrocarbon emissions in proportion to the gas recovered, lowers thermal NOx from the flare, reduces visible flare activity and may qualify for carbon credits under voluntary or regulatory frameworks. The emissions outcome for a specific site depends on its flare gas volume, composition and the fuel it displaces.