Troy,
Because it’s a solution to mitigate the gas permeation and the decompression
issue I guess. Those O rings are made in solid and hollow configurations and as
mentioned, a variety of metals other than stainless. We’ve never had one fail
in our applications that I’m aware of.
Best.
Anthony J. Cesaroni
President/CEO
Cesaroni Technology/Cesaroni Aerospace
<http://www.cesaronitech.com/> http://www.cesaronitech.com/
(941) 360-3100 x101 Sarasota
(905) 887-2370 x222 Toronto
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Troy Prideaux
Sent: Thursday, August 20, 2015 12:11 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Explosive decompression of o-rings with helium
Why would you use stainless *O’ings* though? Fair enough stainless seals of
some geometry especially for cryo where everything is virtually as hard as
glass, but the key advantage that elastometric *O’rings* provide you with is
their tendency to try and extrude themselves out of the opposing crevice/gap
from the direction of the pressure source. This is what provides them with that
“magical O’ring” sealing quality they’re famous for even when they’re damaged
or the groove containing them is imperfect or even the surface they’re sealing
against is subtly scratched and this is only possible if they’re elastic enough
to conform to that extrusion process from the pressures being applied.
I suppose a stainless-stainless radius-on-flat system is used widely in
industry to seal threaded pipe unions to reasonable effectiveness although they
tend to be more prone to teething issues than unions utilizing elastometric
gaskets.
Troy
_____
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
[mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Anthony Cesaroni
Sent: Thursday, 20 August 2015 1:34 PM
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
Subject: [AR] Re: Explosive decompression of o-rings with helium
Wouldn't designing your system with stainless steel O rings eliminate the
issue? We use them on our oxidizer flanges including cryogenic and they seem to
work quite well.
Anthony.
Sent from my iPhone
On Aug 19, 2015, at 11:19 PM, Troy Prideaux <GEORDI@xxxxxxxxxxxxxxxx
<mailto:GEORDI@xxxxxxxxxxxxxxxx> > wrote:
Interesting, nearly all the really high numbers are for silicone.
Regards,
Troy
_____
From: arocket-bounce@xxxxxxxxxxxxx <mailto:arocket-bounce@xxxxxxxxxxxxx>
[mailto:arocket-bounce@xxxxxxxxxxxxx] On Behalf Of Robert Watzlavick
Sent: Thursday, 20 August 2015 1:03 PM
To: arocket@xxxxxxxxxxxxx <mailto:arocket@xxxxxxxxxxxxx>
Subject: [AR] Re: Explosive decompression of o-rings with helium
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