Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Square or Rectangle 30 psi storage tank

  1. Aug 21, 2012 #1
    If I want to make a Square or Rectangle storage tank 5 feet deep and 22 feet long and 1 foot wide. The tank will be under 30 psi and also vacuum. That’s not hard to do what I want is the walls not to deflect more then .001 of an inch. The walls can be plastic, plywood with a steel sheet or steel. I want the walls to be flat and smooth. Thanks for any help
  2. jcsd
  3. Aug 22, 2012 #2
    Ummmm....I'm no stress analysis expert, but it seems you want to design a pressure vessel. 30 psi on such a long span of 22 feet and you want deflection to be max of 0.001 inch? Let's see...30 lbf/in^2 X (5 ft X 12 in) X (22 ft X 12 in) = a whole heck of a lot of force. You'll either have to internally/externally reinforce this thing or make the walls as thick as a battleship hull plate. And a cubic-shaped vessel...well, there's all kinds of problems with that concept. Butane tanks, etc., are cylindrical or spherical for a reason.

    Either way, this should be governed by pressure vessel design practices (for example, ASME Pressure Vessel Codes), else you will construct a bomb. Tread lightly there, sport, there be nasty beasties about.
  4. Aug 23, 2012 #3
    Its for water what would the max lenth I should go with then?
  5. Aug 29, 2012 #4
    This sounds more like a homework problem... If this is a real design, you really must use a cylinder... any other shape proves you have no clue and should not construct such a thing.
  6. Aug 29, 2012 #5


    User Avatar
    Science Advisor
    Homework Helper

    Start with the basic fact that a pressure of 1 atmosphere is about 1 ton per square foot. So 30 psi is about 2 tons per square foot. That's more that 200 tons pushing out the 5 ft x 22 ft side of your rectangular tank.

    That's why you don't find many rectangular pressure tanks - and certainly not many made out of plywood or plastic.
  7. Aug 29, 2012 #6


    User Avatar
    Science Advisor
    Homework Helper

    I don't see the connection between "a 5 ft depth of water" and "30 psi and also a vacuum".
    Even a 22 ft depth of water is only about 10 psi at the bottom (and zero at the top).
  8. Aug 29, 2012 #7
    It a piston for a pump
  9. Aug 30, 2012 #8


    User Avatar
    Science Advisor

    The real problem- and the real reason you don't see rectangular tanks- is the edges. Along edges there will be a large force pushing the sides away from each other.
  10. Aug 31, 2012 #9
    Yes I know the corners will need to be inside rounded and I will have to make the size 5 to 7 feet deep and 11" x 14" and add them up to the max.
    Last edited: Aug 31, 2012
  11. Aug 31, 2012 #10
    We could certainly design such a thing as a mental exercise, but the result would be completely unreasonable and impractical to build. It sounds like he wants to build a square piston to pump water. If that be a real need, then find a seal that will enable you to loosen up that .001 number as much as possible. Then build a box out of anything you want and cast it in concrete. Use lots of steel in the concrete and arrange it to best take the loads. Prestessing the steel would reduce the amount of material required.

    If you don't understand how to do that analysis, then this project exceeds your ability to do it safely.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook