[PSUBS-MAILIST] Overpressure on dome

[Sean T. Stevenson via Personal_Submersibles]

I should add to this that the dome wants to rise as a result of its own buoyancy (~150 lbf for a 24" ID hemisphere), and needs to be held to its seat with enough force to counter that, in addition to countering any positive pressure load from the inside. The former effect is likely more important than the latter.

Sean


On August 3, 2016 8:49:26 PM MDT, "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 2D 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 Kitrigewas one
>of
>>them. So I wanted a bit of analysis on this scenario in general.What
>>sort of overpressure can you expect at a maximum on a dive?Doug's
>>overpressure valve operates at .5psi but if you were 3psi
>>overpressurethe valve wouldn't operate till you were 5ft from the
>>surface & you would have a shorttime 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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