[PSUBS-MAILIST] Overpressure on dome

Thu Aug 4 08:51:59 EDT 2016

Hi Hank,

I don't think there's any way a K boat hatch, 250 or 350, would hold that
in. Tanks on the outside with shutoff valves are a mitigating solution, but
an OP valve I'd argue is best because it's automatic and kicks in
instantaneously. The 250 hatch is particularly hard to strengthen because
the ring that receives the acrylic is pretty lightweight, meaning if you
were to put enormous dogs on it the ring itself would probably bend. In
short, I don't think it's feasible to harden the K250 hatch against
over-pressure, so the way to protect against that is with the OP valve.

Best,

Alec

On Thu, Aug 4, 2016 at 8:22 AM, hank pronk via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Alan,
> It seems to me you should have sufficient dome retaining to hold the dome
> in place with all Hp products fully vented into the sub.  I am sure that
> the retainers on Snoopy's dome can do that.  I prefer straps,not saying it
> is a must, but when in doubt, go big.
> Hank
>
>
> On Wednesday, August 3, 2016 10:43 PM, Alan James via
> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>
> I was thinking that if air temperature was above water temperature,
> (as you would get in Canada) then you could potentially get overpressure.
> One guy on the internet was saying that his lake temperature was
> 26C & the air temperature was 10C. In this case the pressure in a
> sub could increase by a couple of psi through the higher water
> temperature,
> body & electrical heat.
> Alan
>
>
> ------------------------------
> *From:* Alan via Personal_Submersibles <personal_submersibles at psubs.org>
> *To:* Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>
> *Sent:* Thursday, August 4, 2016 4:04 PM
> *Subject:* Re: [PSUBS-MAILIST] Overpressure on dome
>
> Sorry, the air temperature in the bellows probably
> wouldn't lag that far behind the air temperature in the sub & so
> wouldn't increase the pressure much.
> Yes there would need to be some major leak.
> However there has been a couple of incidents!
> Alan
>
>
>
> Sent from my iPad
>
> On 4/08/2016, at 3:52 pm, Alan via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
> Sean,
> thanks for the maths.
> I was thinking that the scenario could arise where you dive with an
> internal
> temperature of say 30C & set a bellows add system for the pressure at that
> temperature. Then as you dived down to cool water the temperature in the
> sub dropped to 15C. The air pressure in the sub would gradually halve but
> the
> bellows add system would build the pressure up with added O2. If you rose
>  to near the surface & cruised around letting the sub warm up, you
> could potentially raise the pressure by quite a bit. Hopefully you would
> notice
> that the O2 % was high in the first place, but if you didn't & the dome
> retainers
> weren't substantial, it could contribute to a catastrophic situation.
> Hmmm think I'll put in an overpressure alarm!
> Alan
>
>
>
> Sent from my iPad
>
> On 4/08/2016, at 2:49 pm, Sean T. Stevenson via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
> The force acting on the dome from the inside is the internal pressure
> multiplied by the 222D projected area encapsulated by the innermost seal.
> For example, if your dome is 24" ID and 26" OD (1" thick), if it seats
> against a single o-ring at the mid diameter, the area would be A =
> π(12.5)^2 = 491 in. ^2 . Conversely, if the entire window thickness sits
> against a bearing gasket, the seal extends to the ID, so A = π(12)^2 = 452
> in. ^2 .
> The internal cabin pressure acts uniformly across the entire window, but
> the external sea pressure does not. Across a 26" OD window, the pressure at
> the bottom of the window is about 1 psi greater than at the top, so in the
> absence of proper retention, it would preferentially pull away at the top
> in the event of overpressure.
> As far as what to expect, that number should be zero. The only way you
> will encounter overpressure is if something is leaking gas into the cabin,
> or if there is a dramatic temperature increase.
> Your dome retainers need only be strong enough to hold the dome against
> its seal at whatever delta-P activates the OPV, or some margin above that
> if it is a slow valve. As a thought, I might be inclined to spring load the
> retainer arrangement, so if you have a full bottle dump or some other
> (otherwise) catastrophic event increasing cabin pressure, the dome could
> vent that gas until it dropped below the spring load, and then the OPV or
> manual equalizing arrangements would deal with the rest. Of course, you
> should avoid that possibility in design, but I'm just throwing that out
> there.
> Sean
>
>
> On August 3, 2016 8:10:56 PM MDT, Alan James via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>   Hi all,
> need some group input here.
> I was looking at a picture of Snoopy's dome that Doug posted on Facebook.
> (below)
> The 6 dome retainers that are made of plastic were making me nervous.
> There have been a couple of cases of domes blowing off. I think  George
> Kitrige
> was one of them. So I wanted a bit of analysis on this scenario in general.
> What sort of overpressure can you expect at a max! imum on a dive?
> Doug's overpressure valve operates at .5psi but if you were 3psi
> overpressure
> the valve wouldn't operate till you were 5ft from the surface & you would
> have a short
> time to get the pressure down. Also wave movement would factor in &
> fluctuate the pressure quickly at that depth.
> I think the K250 dome is 24" diameter. I calculated out that there would
> be 452lb
> pressure on the dome retainers for every 1psi overpressure. I based this
> on the area
> of a 24" disc, or should I be basing it on the area of the dome? (Sean)
> At 3psi that would ! be 226lb lifting force on each of those 6 plastic
> retainers!
> What is a good safety factor here? (Sorry for picking on Snoopy Alec)
> Cheers Alan
>
>
>
> ------------------------------
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