The Parker O-Ring Handbook shows Nitrous Oxide with silicone to be 250
times more permeable than Helium with fluorocarbon:
https://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf (see
p. 3-26 for the table)
The reference for the data in the table is "Permeability Data for
Aerospace Applications", IIT Research Institute, March 1968:
http://ntrs.larc.nasa.gov/search.jsp?R=19690004779
-Bob
On 08/19/2015 11:12 AM, Robert Watzlavick wrote:
Interesting. I wasn't planning on leaving the helium tank pressurized very long but I also don't want to be constantly venting it because it can get expensive. I'll look up the permeability numbers for nitrous later to compare with helium. What was the gland design in this case?
-Bob
On Aug 19, 2015, at 09:25, Edward Wranosky <edwardcw@xxxxxxxxx <mailto:edwardcw@xxxxxxxxx>> wrote:
I have had Viton & Buna-N o-rings become permeated with Nitrous Oxide after ~3 days storage and then going from 750 psi to 0 in ~10 seconds. The o-rings bubble and split. Rocket Moonlighting uses polyurethane on the plug valves to last longer.
Edward
On Wed, Aug 19, 2015 at 6:16 AM, Robert Watzlavick <rocket@xxxxxxxxxxxxxx <mailto:rocket@xxxxxxxxxxxxxx>> wrote:
Thanks - that's what I needed to know.
-Bob
On 08/19/2015 01:48 AM, (Redacted sender JMKrell@xxxxxxx
<mailto:JMKrell@xxxxxxx> for DMARC) wrote:
Bob,
Hydrogen and helium permeated o-rings can be damaged when the
pressure is released in <1 second. Surface blisters, cracks, and
extrusion damage are typical signs. Complete failure of the
o-ring is rare, but I have experienced it many times. Increased
mechanical compression or reduced o-ring cross section reduces
the effects.
I would not consider a 30 second blow down even a
rapid decompression.
Krell
In a message dated 8/18/2015 6:52:09 P.M. Pacific Daylight Time,
rocket@xxxxxxxxxxxxxx <mailto:rocket@xxxxxxxxxxxxxx> writes:
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