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Thickness of steel needed

  1. Jul 2, 2013 #1
    What i want to do is have a cylinder that is made of steel (don't know what types there are but i guess regular)
    with a height of 20 ft or so, filled with water. for the bottom of the cylinder i want a piston that would move 6 inches.

    the volume of the water on top of the piston is about 4000 liters
    i was wandering if you could tell me how thick the steel would have to be so that it would support that weight.
  2. jcsd
  3. Jul 2, 2013 #2
    20 feet of water in a pipe is not very heavy.
    4000 liters is about 1,200 gallons.
    Water is close to 8 pounds per gallon. So 1,200x8= 9,600 pounds.
    Almost any pipe wall thickness can handle that. 1/4 inch pipe wall thickness should be more then enough.
    But it depends on a few factors that you haven't stated. Diameter of pipe is one.
  4. Jul 2, 2013 #3
    i am interested in finding out the thickness of a piston that would not bend when it is:

    100 cm in diameter
    holding a column of water 100 cm in diameter and 20 ft high (around 4000 liters, or as stated above about 9,600 pounds)

    hope i gave enough info
    thank you for your time
  5. Jul 2, 2013 #4


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    Science Advisor

    Any thickness will bend, the question is how much can you tolerate. Construction typically allows 1 in 1000. How does your piston seal in the cylinder.

    20ft of water will generate a pressure of about 2/3 atmosphere = 10 psi. The design of the piston will be determined by it's attachments and the way the force 10psi * piston area is opposed. It is highly probable that the construction of the piston should be a circular plate with a cylindrical skirt to house the seals and stop the plate twisting.

    Also is it a steady state situation where head is important or a dynamic situation where an impulse is involved such as in a ram pump, where mass * velocity squared is very important.
  6. Jul 4, 2013 #5
    k well i don't want it to bend to much to lose its seal(the seal will be rubber system that can contract and expand to seal and unseal)
    the design is a bit like this:
    there will be a vertical shaft(in the middle of the piston) that will keep the piston going straight up and down.
    the sealing system will be on the bottom of the piston.
    also in the vertical shaft there would be a smaller shaft that attaches to a smaller piston (i will be using this to transfer the weight to where i need it)
    hope that is enough info thank you.
  7. Jul 4, 2013 #6


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    Science Advisor

    The outer tube will need a wall thickness of between 0.8mm and 12mm. The piston will need to be about as high as the diameter of the thing that keeps it aligned.
    It is impossible to perform a valid engineering analysis without a set of drawings and some idea of what it does and how fast it performs it's functions.
    You could build it using thin materials. If a part bends then double the thickness of that part and test again.
  8. Jul 4, 2013 #7
    k thank you
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