Posted by Kendall Raine on November 13, 2000 at 18:08:40:
In Reply to: Reg failure questions posted by JRM on November 13, 2000 at 16:45:15:
The failure you're alluding to starts with a failure of the high pressure seat in the first stage. Instead of stepped down intermediate pressure (IP)leaving the first stage (anywhere from 120 psi to 150 psi) the first stage HP seat fails and the second stage receives the whole tank pressure (as much as 3500 psi). Lots of bubbles. With a down stream second, the overpressure blows out the mouth piece. The is why downstreams are said to fail open. With an upstream, the second fails closed. If the Jetstream didn't have the HP relief valve in the hose-the fitting which screws into the body of the second-the high pressure would blow the hose. With the HP relief valve, the HP vents at this point. In either type of reg, upstream or downstream, you're going to switch to your backup pretty quick. If you have a way to isolate the failed reg, you do so really fast before you vent all your gas. If you don't, you find your buddy really fast before you vent all your gas.
BTW, most HP seat failures I've seen happened upon initial pressurization-at the surface. Most underwater free flows are caused not by a blown HP seat but by a freeze up.