Good point. It's weird to think of it that way since in normal use it
acts more as an ablator than a fuel, but silicone is still a mediocre
fuel. Polydimethylsiloxane has a net heat of combustion of 17.75 kJ/g,
compared to 33.1 for NBR (Buna-N).
On Wed, Aug 19, 2015 at 6:56 PM, Troy Prideaux <GEORDI@xxxxxxxxxxxxxxxx> wrote:
The only material truly compatible with Nitrous that I’m aware of is
fluorosilicone. Some sources say that *some* grades of Viton are compatible
which I trust, and some say that silicone is compatible, but it’s also
probably *some* grades also. Once again, I remind you to read the Scaled
Composites study on N2O compatibility where they found (to quote):
“Experience has shown that some materials such as silicone (which is
generally considered N2O compatible) absorb N2O. The N2O saturated silicone
exhibits impact sensitivity, whereas the raw material does not.”
http://www.scaled.com/images/uploads/news/N2OSafetyGuidelines.pdf
I’ve also experienced elastomer material explosion from reasonably rapid
depressurization using N2O and CO2. It should be noted however that if the
o’ring is confined to a reasonably snug groove (ie. not wider or deeper than
the recommendations) you can generally avoid material damage from such and
even if there was localized damage, it’s quite unlikely the sealing
characteristics of the seal would be compromised to the point of generating
sealing failure. There has to be pretty serious damage required for things
to get to that stage for a high pressure seal.
Troy
________________________________
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Paul Mueller
Sent: Thursday, 20 August 2015 4:09 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Re: Explosive decompression of o-rings with helium
That's a great link, Ben. It says that silicone rubber elastomers (it
actually says "Silicon (Q)") are the only ones satisfactory for nitrous--are
there common trade names for silicone rubbers that I should be looking for?
Or is asking for "silicone rubber" good enough?
On Wed, Aug 19, 2015 at 11:31 AM, Ben Brockert <wikkit@xxxxxxxxx> wrote:
Buna-N usually isn't recommended for use with oxidizers because it's a
fuel. Viton is known to swell in nitrous, which is likely because it's
a solvent, a different mechanism than the way helium or hydrogen
infiltrate materials.
http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=55#MaterialCompatibility
I've never had a seal failure from helium, but the pressurant tanks on
Zombie, the Stigs, etc depressurized over the course of a minute or
two; 3 for the level 2 vehicles. MTV-1X went from ~5000 psi to ~1000
in about twenty seconds and has over a dozen SAE ORB seals at that
pressure, but was running nitrogen.
On Wed, Aug 19, 2015 at 10:25 AM, Edward Wranosky <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>
wrote:
Thanks - that's what I needed to know.
-Bob
On 08/19/2015 01:48 AM, (Redacted sender 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 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