No suggestions, just commenting that I have seen o rings explode due to
helium permeation. They looked foamy on the inside
On Aug 18, 2015 6:51 PM, "Robert Watzlavick" <rocket@xxxxxxxxxxxxxx> wrote:
I was planning on using a fluorocarbon (Viton) o-ring as the seal for my
flight helium tank (a D-size medical oxygen cylinder). I've been reading a
bit about explosive decompression where the gas permeates into the o-ring
and then can blow out during rapid decompression. I couldn't find any good
examples of what "rapid" means but in my case, the pressure in the tank
will go from 3000 psi to 0 in about 30 seconds. The Parker literature has
data on gas permeability rates for different compounds and it shows that
Nitrile is slightly better (7.4 e-6) than Fluorocarbon (12.7 e-6) at room
temperature but I would probably do better with some field examples so see
if I'm in the danger zone or not. I'm currently working my way through
Research Report 485, Elastomeric seals for rapid gas decompression
applications in high-pressure services.
A couple of recommendations for minimizing explosive decompression is to
reduce the cross section and use a higher durometer compound . Instead of
making a custom fitting (which isn't actually that complicated), I was
thinking of using a -8 AN fitting with a 2-016 o-ring into the cylinder.
The cylinder port has a pretty good countersink so the o-ring would be
completely contained but it is not a standard boss contour (a -908 o-ring
is too large). A 2-016 o-ring is small enough to fit with a bit of stretch
although I haven't tested it under pressure yet. The other option is to go
with as-intended use of a large PTFE o-ring clamped between the fitting and
flat surface on the cylinder. It's just that when I realized the cylinder
thread was the same as a -8 AN fitting, it seemed like a nifty solution.
-Bob