[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PSUBS-MAILIST] Soft ballast venting valve
I think you guys are confusing terms. "psi" = pounds per square inch, a
unit of pressure. Pressure (psi, Pa) x Area (in^2, m^2) = Force (lbs_f, N).
The pressure of a water stream leaking into a sub is actually zero (or
rather, equal to the internal ambient pressure) on the inside. The water
gains kinetic energy at the expense of pressure as it flows across the
differential. A small orifice does not restrict pressure from acting
across it at all. 4450 psi acting on a bulkhead with a 1/4" hole in it
will result in 4450 psi developed on the other side of the hole when
equilibrium is reached. 4450 psi acting on a 1/4" plug in that hole will
require ~218 lbs to hold in place (disregarding frictional forces).
I think what you are trying to figure is the force of the water stream -
a function of water density and viscosity, as well as the size and shape
of the aperture and the pressure differential. Effects like turbulence,
cavitation, etc. come into play in determining the mass flow rate of
water through such a leak - you cannot simply assume a lesser
differential pressure based on the aperture area. This mass flow rate is
what determines the force of the water stream, and in the case of damage
control, this force must be overcome in addition to the hydrostatic
pressure to plug a leak. In the simple case above, your 1/4" plug needs
not only 218 lbs to hold it in place, but enough additional force to
insert it through the high velocity water stream (overcoming the
momentum of the water stream).
Your shaft example has an aperture area of ~0.003848 in^2, in which case
the hydrostatic force acting on the interface area is indeed 17.125 lbs
- this would be the force required to hold your leak patch of choice in
place; however, the patch material you use must also be strong enough to
withstand 4450 psi without yielding (or reaching its ultimate tensile
strength) - chewing gum won't cut it. Also, getting a patch installed in
the first place is a different story. A 4450 psi pressure differential
across such an area, once a leak has started, may well result in
supersonic fluid flow across the interface.
-Sean
Brian Cox wrote:
> red that a shaft with a .005 gap in a 1/4"
> bore, that would give you an area of .005 x .78 (circumference of 1/4'
> circle) = .0039 sq in
> 4450 lbs x .0039 sq in = 17.35 psi not bad for 10,000 ft down !
>
> Brian
>
> -----Original Message-----
> From: owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org]On Behalf Of Jon Wallace
> Sent: Tuesday, January 06, 2009 9:29 PM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] Soft ballast venting valve
>
>
>
> Brian,
>
> My guess is that pressure gauges are calibrated for whatever opening is
> physically provided on the measuring unit. So a pressure gauge intended
> to measure PSI using a 1/4 inch hole would certainly indicate 4450 units
> at 10,000 foot depth, but the actual force on the gauge itself would
> only be ~219 pounds. The physics can't be denied. There are 16, 1/4inch
> squares, in one square inch. Divide 4450 by 16, and you have the
> pressure that ONE 1/4inch square tube will allow through.
>
> I know we aren't talking about actual square tubes for this mind
> exercise, but it's somewhat easier to think of in terms of squares.
>
> Jon
>
>
>
> Brian Cox wrote:
>
>> Jay,
>> I saw that figure in the book I quoted, " Man beneath the Sea" , I'm
>> not sure they are refering to the actual pressure coming from the
>> outside sea pressure. Perhaps they are refering to the pressure
>> exerted by the plug outwardly against the wall of the opening. In any
>> case there would seem to be frictional forces that would come into
>> play in order to hold something in place under that pressure. The book
>> did not explain that statement. I think the pressure would still be
>> 4450 psi at that 1/4" hole. If you had a pressure gage on it would
>> surely read 4450 psi.
>> Brian
>>
>
>
>
>
> ************************************************************************
> ************************************************************************
> ************************************************************************
> The personal submersibles mailing list complies with the US Federal
> CAN-SPAM Act of 2003. Your email address appears in our database
> because either you, or someone you know, requested you receive messages
> from our organization.
>
> If you want to be removed from this mailing list simply click on the
> link below or send a blank email message to:
> removeme-personal_submersibles@psubs.org
>
> Removal of your email address from this mailing list occurs by an
> automated process and should be complete within five minutes of
> our server receiving your request.
>
> PSUBS.ORG
> PO Box 53
> Weare, NH 03281
> 603-529-1100
> ************************************************************************
> ************************************************************************
> ************************************************************************
>
>
>
>
> ************************************************************************
> ************************************************************************
> ************************************************************************
> The personal submersibles mailing list complies with the US Federal
> CAN-SPAM Act of 2003. Your email address appears in our database
> because either you, or someone you know, requested you receive messages
> from our organization.
>
> If you want to be removed from this mailing list simply click on the
> link below or send a blank email message to:
> removeme-personal_submersibles@psubs.org
>
> Removal of your email address from this mailing list occurs by an
> automated process and should be complete within five minutes of
> our server receiving your request.
>
> PSUBS.ORG
> PO Box 53
> Weare, NH 03281
> 603-529-1100
> ************************************************************************
> ************************************************************************
> ************************************************************************
>
>
>
************************************************************************
************************************************************************
************************************************************************
The personal submersibles mailing list complies with the US Federal
CAN-SPAM Act of 2003. Your email address appears in our database
because either you, or someone you know, requested you receive messages
from our organization.
If you want to be removed from this mailing list simply click on the
link below or send a blank email message to:
removeme-personal_submersibles@psubs.org
Removal of your email address from this mailing list occurs by an
automated process and should be complete within five minutes of
our server receiving your request.
PSUBS.ORG
PO Box 53
Weare, NH 03281
603-529-1100
************************************************************************
************************************************************************
************************************************************************