Ripley Goes Down to Meet Parker and Brett

During the scene where Ripley goes down to check on the damage with Parker and Brett, why is gas shooting out of the pipes? I ask because after Ripley leaves, Parker seems to turn the gas off. But he then turns it back on. I was thinking that Parker was making the damage appear worse than it was by having the gases leaking out. But then why would he turn it back on once Ripley leaves.