[PSUBS-MAILIST] air flow vs. water flow

Fri Mar 30 20:41:30 EDT 2018

Thanks Cliff! It's hard to follow equations when they are written in words,
but a bottom line of 2 seconds sounds like quite fast enough. I was
actually testing the valve itself with this method, checking that it sealed
well, and the speed data point was just an interesting byproduct. But it's
good, it gives me a rough enough idea to be actionable. I think the action
will be to protect those valve handles against accidental activation. I
love the idea of diving fast, but I don't want a passenger coming aboard to
hit a valve with his toes and dive the sub in two seconds!

Best,
Alec

On Fri, Mar 30, 2018 at 8:32 PM, Cliff Redus via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> To get a first order approximation, assume turbulent pressure drop in a
> pipe nipple.  Write two cases, one for air and one for water at same
> temperature and pressure near surface and same pressure drop .  This will
> give you that the Vair is equal to Vwat times the square root of ratio of
> water over density over air  and ratio of  friction factor of water over
> the friction factor of air.  Assume friction factors are the same for first
> order approximation.   Va=Vw*(density_w/densityair * fw/fa)^0.5 At the
> surface this reduces to Va=29*Vw for the same pressure drop.  So if took 60
> sections for water, it would take 2 seconds for air.  This assumes the
> limiting factor is the turbulent pressure drop through the MBT vent valve.
> If the MBT flood valve was to small, then, it would start to impact how
> quickly the MBT would fill with water.  In the real world it is a
> combination of the turbulent frictional resisence through both the MBT vent
> and flood valves.   For better approximation use CFD code to model the MBT
> including the MBT vent and flood ports.
>
> What hank would have already done.  Find a 55 gal oil drum from behind his
> shop.  Remove the 2" cap from the top and turn the drum upside down. Cut
> out a hole to fit the new MBT valve.  Put the drum and attached MBT valve
> in the back of his truck and head to the nearest lake.  Suspend a 20 lb
> weight below the drum so it floats.  Open the MBT and measure how long this
> MBT takes to completely flood.  Prorate this time based on volume of Psub
> MBT compared to 55 gals.
>
> Cliff
>
> On Thu, Mar 29, 2018 at 11:35 AM, Alec Smyth via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>> Hi everyone,
>>
>> I was testing one of my new MBTs yesterday, removed from the sub, by
>> turning it upside down and filling it with water, then opening the mushroom
>> valve to let the water out. It takes 60 seconds to empty. In actual
>> operation it will be air instead of water coming out that valve. The
>> pressure will be the same, but because it will be air instead of water the
>> tank should empty much faster. The question is, does anyone know more or
>> less *how much* faster? One should be able to calculate this with
>> Bernoulli's formula, but this is very turbulent flow so not sure if it
>> would really hold. Any rules of thumb?
>>
>>
>> Thanks,
>> Alec
>>
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>
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