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Homework Help: Pressure vessels problem

  1. Nov 26, 2012 #1
    1. The problem statement, all variables and given/known data
    20 m3 of gas at a pressure of 25 bar is to be stored in a cylindrical
    pressure vessel 2 m long. Given the following information :

    The yield strength of the vessel material is 14,000 psi

    If a factor of safety of 5 is to be used, determine:

    Whether the vessel should be treated as a thin or thick cylinder.

    2. Relevant equations
    iv been given the feed back as follows:
    For this question you need to apply the thin cylinder theory to determine the thickness t, then depending on the answer for r/t, determine whether the cylinder should be treated as a thick cylinder. If it is a thick cylinder, then the thick cylinder theory must be applied to determine the thickness of vessel required.

    The attempt at a solution

    3.1 bar = 100,000 Pa
    factor 5 means that maximumstrength
    yield strength
    =1 /5
    1 psi=6894.7N /m2
    a) We have PV = RT = PSL (L= 2 m long.)
    So the strength of our vessel should be 25⋅105 Pa≈362.6 psi
    From the factor of safety we can find the the maximum strength should be
    14/5⋅103 psi=2.8⋅103 psi
    So, the vessel should be treated as a thick.

    im told that my attempt is incorrect but i dont know how else to solve it useing the feed back im given.
  2. jcsd
  3. Nov 26, 2012 #2
    The length is fixed at 2 m. So, calculate what the diameter needs to be to hold 20 m^3 of gas. From the diameter, calculate what the thickness needs to be and then check the r/t value to determine what set of equations should be used.

    Last edited: Nov 26, 2012
  4. Nov 26, 2012 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Did you work out the radius of the cylinder required to hold the compressed gas?

    The decision on whether to apply thick or thin cylinder theory depends on the ratio r/t, not on what the ratio of the wall stress to yield might be.
  5. Nov 26, 2012 #4
    can you explain to me how to do this, what equations do i need to use.
  6. Nov 26, 2012 #5
    The equation to figure out the volume is straight forward. Just look it up for a cylinder. A basic thickness equation can easily be derived (or looked up) for the stress in the hoop direction. The longitudinal stress is always 1/2 of the hoop stress. So, the hoop stress governs.

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