[PSUBS-MAILIST] Overpressure on dome

Wed Aug 3 23:52:30 EDT 2016

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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