API RP 521 recommends a fuel gas purge stream at the upstream end of the relief header and at each major branch. This purge gas maintains a hydrocarbon rich atmosphere in the relief header, the knockout drums, and the flare stack. In a large relief system with a ground flare, purge gas usage of 50 to 100 EFO/day is not unusual (EFO stands for equivalent fuel oil units measured in barrels).
The purge injection station usually includes a rotameter with a globe valve bypass, block valves, and a blind. The injection rate is usually controlled by a fixed orifice to insure a constant supply, even with instrument malfunction.
Often the purge gas is simply injected in addition to a “normal” venting flow that is sufficient to protect the relief system. Considering the value of the purge gas, it makes economic sense to put purge injection on pressure control. Holding the relief header on pressure control against the water seal permits shutting off the upstream restriction-orifice injection points.
This pressure controlled purge does not replace the purge gas injection above the water seal that protects the flare stack. A gas seal just below the combustion tip reduces the purge requirement from 20,000 scfh to 2,000 scfh. Virtually all the Company stacks have gas seals.