The purge gas is injected into the relief header at the upstream end and at the major branches to maintain a hydrocarbon-rich atmosphere in each branch, into the off-plot relief system, and into the flare stack. The gas volume typically is enough to maintain the following velocities. The oxygen concentration must not be greater than 6% at 7m inside the tip. When there is enough PSV leakage or process venting to maintain the desired backpressure, no purge gas is injected.