[PSUBS-MAILIST] Overpressure on dome

Thu Aug 4 19:18:28 EDT 2016

Yes, it's a big valve. Two and a half inches across if I remember right,
with a bunch of radial holes in it. The thing is, changing the K250 hatch
to take that pressure is just not feasible. You'd basically need a whole
new design for the CT and hatch.

On Thu, Aug 4, 2016 at 4:43 PM, hank pronk via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> I think there is an ABS rule that says you can not have more HP gases than
> the ports or dome can retain.  An over pressure valve is fine if it is big
> and you can hold your depth in the water column.  I think of deflating a
> big inner tube with the valve stem removed.  If you have such a situation
> that requires an over pressure valve to save the day, then there is likely
> a lot of stuff going on that would be stressful.  I say build so your sub
> can handle the over pressure and have an over pressure valve to boot.
> Can't a K250 have a stronger ring, with straps like Emile has and DW's
> I remember Vance saying "copy what the pro's are doing"
> Hank
>
>
> On Thursday, August 4, 2016 6:52 AM, Alec Smyth via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>
> 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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