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    Hydrostatic Pressure Calculation (Pressure on tank walls)

    Discussion in 'Setup and Filtration' started by nolapete, Mar 25, 2009.

    1. nolapete

      nolapete MFK Members

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      Most of us know how to calculate capacity and entire tank weight, but none that I've seen have been able to give us a formula to calculate the hydrostatic or outward pressure exerted on the walls of our tanks. This is of particular interest to those, like myself, who are building big tanks.

      I've taken the mystery out of the calculation and limited it to the following parameters:

      lbs. per cubic ft. water * height of tank in feet

      62 is approx. lbs. per cubic ft. of water, so use that and it'll be pretty accurate.

      In example, my tank is 5' tall. Even though it won't be filled to the rim, I use that in my calculation.

      62 * 5 = 310 lbs/foot^2

      To get the total forward pressure, multiply the length * height * above result:

      12 * 5 * 310 = 18600 lbs total pressure on front wall (and back since same dimensions.


      to do it in meters

      203.41 * height in meters (i.e. if your tank is .5 meters tall it would be 203.41 * .5 = 115.20 lbs/foot^2

      Use the total forward pressure for feet above and convert to sq. meters for a metric version.

      I can figure it out for you if anyone needs the metric one, but too busy to do it right now.
       
    2. Fuzzy Duck

      Fuzzy Duck MFK Members

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      Hello, Just read and double checked you calculations, my findings are thus:-
      The pressure on the bottom of you tank is correct at 311Lb per sq Ft. for a 5 foot tall tank.
      I have a question for you, is your tank 5' front to back?
      Only then can i tell you if your seconed calculation is correct ( about pressure on front and back)
       
    3. nolapete

      nolapete MFK Members

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      The calculation is for height. Front to back does NOT matter it could be 6" or 6 miles and the pressure will be the same.

      http://scubageek.com/articles/wwwparad.html Hydrostatic Paradox explained.

      The pressure on the front and back are the same since they are both 12' long and 5' tall.
       
    4. Fuzzy Duck

      Fuzzy Duck MFK Members

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      If you only have 1 ton of water in your tank how can the pressure on the front glass be 9 tons?
      And why is the hover dam thicker at its base than at its top because concrete has vey high compression strenght so it could be the same from top to btm dont you think?
      To be honest im drunk but i like messing with mathmatics:)
      wait your reply
       
    5. islander671

      islander671 MFK Members

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      Cool, this is exactly what i was looking for the other day. So is this right?

      L X W X H = 6 X 1.5 X 1.5

      62 X 1.5 = 93

      6 X 1.5 X 93 = 837 lbs of forward pressure?
       
    6. nolapete

      nolapete MFK Members

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      This is not related to my tank, but I did use it as an example. If you see the example on the hydrostatic paradox link, you'll understand. Forward thrust increases as the depth of the water increases.

      This is NOT about the weight of the water of the tank on the BOTTOM, so how much water is in it has no bearing on this equation.

      Just for your reference, my tank, which will be 144"x98"x60" internal dimensions and approx. 3665 gallons capacity, has a water weight of approx. 18.5 tons. (About 2.2 lbs. per sq. in.)


      Islander, that's correct.
       
    7. JakeH

      JakeH MFK Members

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      Nolapete- You are correct about the hydrostatic paradox & basic hydraulics (uniform pressure on surfaces of a fluid-filled container). However, I have doubts about the number you used regarding the weight of a cubic foot (ft^3) of water. I'm not trying to call you out or anything, but I've seen this a few times on various fish forums, converting units of volume (ft^3) to weight and/or converting flow rates to water velocity, etc. Using straight volume, my software says 1 ft^3 of water is equal to 7.481 Gallons. If we use 8 lbs/Gallon of water weight, I get 59.848 lbs. per ft^3. Seems like a small difference, but you have a really BIG tank, so it adds up quick. Where did you get your 62 lb/ft^3 number? Obviously I cant take into account any chemicals/treatments, so if this is a measurement you took from your own water, disregard the lower weight. All that said, your order of operations is incorrect (parentheses first) & your final answer (18,600) is essentially the square footage of 310 of your tank sides, or volume (in ft^3) of your tank if it were 310 ft. deep front to back. In addition, shouldn't exerted pressure be in PSI?
      I could be wrong, but I believe your equation should have been:
      (62 lbs * 5 ft) * 12 ft = 3,720 lbs total forward pressure
      Using my (slightly rounded) numbers, I get:
      59.85 lbs * 5 ft = 299.25 lbs per vertical foot of glass (not sq.ft.)
      299.25 lbs * 12 ft = 3,591 lbs total forward pressure
      I'm not sure about this part, but with 10,368 sq.in. of glass per wall, that is only about .35 lbs/sq.in (psi) by my calcs & 1.79 psi by your calcs.

      I'm no scientist, and I'm not 100% sure, so...Let the games begin!!!!!!
       
    8. Fuzzy Duck

      Fuzzy Duck MFK Members

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      NolaPete answer the girl, She has 839 Lb of water in her tank so the pressure on one side is 839 Lb Yes or No??????
      Islander the man says it dont matter about how deep your tank is front to back its just top to Btm and left to right so that would make your pressure on your front glass only 559.8 Lbs !!
      Nola Pete yor answer please:)
       
    9. JakeH

      JakeH MFK Members

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      If it helps, I show 3665 gallons equals 489.939 ft^3 of water. This is indeed over 29,320 lbs. of water!!!!! WOW!
       
    10. JakeH

      JakeH MFK Members

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      Wouldn't this be easier?: total water weight (in lbs) divided by total square inches of glass wall?
       

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