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Re: [PSUBS-MAILIST] Re Variable Ballast Calculations



joe
 
    the tanks they are refering to are the soft ballast tanks.
which does not effect submerged displacement, they effect surface displacement. soft ballast is normally free flooding
the only time they effect submerged displacement is durring emergency blow. but due to the large quantity of air require to do this and the chance of an uncontrolled accent this is not thee prefered method of operation.
 
    lets look at what is happening in the soft ballast tanks if you want to use them for a controlled accent , for ease of the numbers we will use an accent for 66 ft or 3 atm absolute. fixed factors accent raate is basesd upon drag and the positive displacement. we will use an accent rate of 60 ft /min and a positive displacement of 128 or 2 ft^3.
durring the first thirty three foot rise you will have to vent 1 ft^3 of air thru an oriface/ valve with a differential pressure. in  most tank designs this space would be approx 1 inch allowing for a differential pressure of .03 psi ie a really shitty flow rate.
 
    for the next thirty three feet of rise you will need to vent off 2 cf of air , while the  air density has decreased the flow is not linear based sloely on density
 
    so you would have to throttle the vent valves in order to made any kind of controlled accent. this could be done using control valves and a pid controller incorporated in a plc.but that waywould increase costs dramatically. 
 
    the cheapest way to accomplise this is to use a small trim tank open at the bottom that has only the capacity to give a small quanity of positive displacement. as the air in it expands it will just blow out the bottom. this method will using more air the a sealed trim tank if you are planing multiple accents and decents durring a dive but eliminates the need of a high preesure tank and control system for the water intake valve. although it would require a crew wiegh in to establish basic trim.
 
    a standard scuba tank is 80 cf a 3000 psi/200 bar. assuming a full tank at 1000 ft or 500 psi abient pressure. you will have a reserve lift capacity of
 
tanks size = .4 cf
volume air 80 cf air compressed to five hundred pai ==2.35 cf
an available air supply of 1.95 cf or 125 lbs of lift.
 
the formula is p1v1=p2v2
size of tank
80*14.7=3000x
80*14.7/3000=x
.392= x
 
 
3000*.4=500*x
 
(3000*.4)/500=x
x=2.4 cf total airremember the air that will stay in the scuba tank
2.4-.4 = 2 cf of available air.
 
rick miller
 
 Original Message -----
Sent: Sunday, November 13, 2005 1:28 PM
Subject: Re: [PSUBS-MAILIST] Re Variable Ballast Calculations



Rick,

I had read in the NAVPERS manual for fleet boats, that a submarine does precisely that to submerge, (reduce displacement by flooding the tanks).

As for practicality, well this is all just math for now. Incidentally, I am not using terms correctly in these posts, ie hard, variable, soft ballast.

 

Thanks

Joe


From: "rick miller" <rickm@pegasuscontrols.com>
Reply-To: personal_submersibles@psubs.org
To: <personal_submersibles@psubs.org>
Subject: Re: [PSUBS-MAILIST] Re Variable Ballast Calculations
Date: Sun, 13 Nov 2005 12:16:22 -0800

joe
    you dont reduce displacement by adding water. displacement = the total volume of all non free flooding  spaces.
 
the introduction of large internal ballast tanks  creates a sinificant engineering problem and a possible piont of flooding for the passenger spaces. most hard tanks are hard to inspect for corrosion. not to try to rain on your parade, the kiss pricipal seems to be the way to go here, if you ever want to get in the water.
rick m

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