The best way to “recover” relief gas is to not send it to the flare in the first place. Run the plant to minimize venting and PSV operation. Source control is the first and most important step.
Leaking PSVs, a common source of normal venting, can be detected by ultrasonic probes (such as the UE Systems Inc., “Trouble Shooter”) or infrared imaging devices (such as the “Probe Eye”).
Many refineries have installed flare gas recovery systems that have given good paybacks. The systems are quite simple. A compressor takes suction on the relief header and returns the compressed gas to the process or to the fuel system. A flow history of the relief header is needed to properly size the compressors.
The relief gas recovery system includes a pressure controlled purge gas bleed line working against the water seal to maintain a minimum pressure in the relief header at all times to prevent oxygen entry. This avoids the need for upstream gas purges.