[PSUBS-MAILIST] air flow vs. water flow

[Cliff Redus via Personal_Submersibles]

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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