OOPS!! Gravitational acceleration, as with any linear acceleration,
should be in terms of displacement divided by time squared. ( g =
9.80665 [m/s^2] ). Sorry for the typo.
-Sean
On 08/12/2010 6:48 PM, Sean T. Stevenson wrote:
Jinesh - what you are asking is a pretty basic physics question,
and I don't want to make any assumptions about your background,
but I would like to be sure that you are working within your
comfort zone with this stuff - otherwise the results could be
disastrous.
That said, as a good reality check for any calculation, if you are
unsure, check that the units work out. For example, when you
supposed:
>> Is it just, depth = (P.allowable/density of water)
If you look at the dimensions of each quantity, pressure is force
divided by area, and density is mass divided by volume.
Illustrating this with SI units, dividing pressure [N/m^2] by
density [kg/m^3] gives you [N*m/kg] or [J/kg], which is specific
energy - obviously not a depth unit.
To answer your question, the pressure at a given depth is found by
the equation
P = rho * g * h, where rho is the fluid density, g is the
acceleration due to gravity, and h is the height of the water
column. For example, at a depth of 800 meters in salt water:
P = 1025 [kg/m^3] * 9.80665 [m/s^2] * 800 [m] = 8 041 453 [N/m^2]
or [Pa], or 8.04 MPa, relative to the sea surface. Standard
atmospheric pressure is 101.325 kPa which would then be added to
this value to get the absolute pressure.
-Sean
On 08/12/2010 6:02 PM, jinesh gandhi wrote:
Thanks Sean.
I perfectly got your point. I have values for all the
P.allowable ready with me for all the criteria listed in ABS
guidelines. I also have my assumed design depth ready. How can
I co-relate my design depth with minimum P.allowable?
Is it just, depth = (P.allowable/density of water)
or any other equation that co-relates these two variable?
Thanks,
Jinesh
On Wed, Dec 8, 2010 at 5:47 PM, Sean
T. Stevenson <cast55@telus.net>
wrote:
On Wednesday 08 December 2010 17:28:22
you wrote:
> Hello People,
>
> I am trying to carry out design calculations
for the pressure Hull,
> according to the ABS guidelines. However, I
couldn't find a section in the
> Guidelines suggesting how to find design depth
for the sub.
>
> I have been using Hull Calculator (found on psubs
website) as a reference
> for my calculations. Also, I've found different
criteria (equations) to
> find the design depth on different sources.
>
> Can anyone suggest me what equation to use to
find the design depth?
>
> TIA,
> Jinesh
Design depth is an input which governs your design (i.e.
you determine the
depth capability you need and design to meet it). If you
look at the ABS
guide, each of the metallic pressure boundary component
sections (i.e.
cylindrical sections, cones, spheres) has equations which
allow you to
calculate a P_cr, which is the short term critical
pressure (minimum possible
failure pressure) for the dominant mode of failure for the
particular
component. In addition, you will find a usage factor
which is some multiplier
of P_cr appropriate to the failure condition. The usage
factor multiplied by
P_cr gives the maximum allowable working pressure for the
component under
consideration. Typically, you want to design your sub
such that P_allowable
is equivalent for each hull component - otherwise, you are
making components
heavier or more complicated than they need to be, because
the lowest
P_allowable will fail first. The only caveat to this
process is that you want
your first failure mode (lowest P_allowable) to be a
strength failure and not
a buckling failure, since this is deterministic.
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