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Homework Help: Pressure vessel question

  1. Nov 15, 2014 #1
    1. The problem statement, all variables and given/known data
    The strength of longitudinal joint in Fig. 1-17 is 33 kips/ft, whereas for the girth is 16 kips/ft. Calculate the maximum diameter of the cylinder tank if the internal pressure is 150 psi. - See more at:


    2. Relevant equations
    σlongitudinal = Pr / 2t

    3. The attempt at a solution
    I have no way of calculating this since I have 2 unknowns. If you go to the link you can see the steps they've done but it doesnt make sense. Also, where does that 21600 come from?
  2. jcsd
  3. Nov 15, 2014 #2
    Look at their solution. The 21600 is in the place p in an expression of the form
    pD/(2t). This is the 150 psi expressed in lb/ft^2 to be dimensionally consistent
  4. Nov 16, 2014 #3
    isnt p =125 psi? and why is there a second t under 33000
  5. Nov 16, 2014 #4


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    p = 150 psi, the internal pressure, as is stated clearly in the problem statement.

    The longitudinal joint can withstand a maximum loading of 33 kip/ft according to the problem statement. The stress on this joint is then the loading divided by the thickness of the pressure vessel, t, so

    σ = w / t = 33,000 / t

    But also σ = pD/2t, so these equations can be combined thus

    33,000 / t = σ = pD / 2t

    The t's cancel, leaving pD/2 = 33,000, or D = 2 * 33,000 / (150 * 144) = 3.06 ft. = 36.67 in.
  6. Nov 16, 2014 #5
    Check your units.
  7. Nov 16, 2014 #6
    There are 2 formulas to calculate pressure in a vessel. Pr / 2t and Pr/t
    when it doesn't say calculate the circumferential or the longitudinal stress how do I know which one to use?
  8. Nov 16, 2014 #7


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    p = 150 lbf/in2*144 in2/ft2 = 21600 lbf/ft2

    w = 33 kip / ft = 33000 lbf / ft

    D = 2 w / p = 2 * (33000 lbf / ft) / 21600 lbf / ft2 = 3.06 ft = 36.67 in

    Units checked.
  9. Nov 16, 2014 #8
    SteamKing, the comment to check units was intended for the OP. No one doubts that you know how to work this simple problem. I was under the perhaps mistaken impression that our purpose here was to help the OP learn, to be able to think through the problem, rather than to just display our own brilliance.

    I see in this case, that even given a full solution, the OP is not inclined work through it, but rather simply comes back with more simple questions. I think that, in cases like that, it is better not to give full answers, but only to offer hints.
  10. Nov 16, 2014 #9
    Dr. D, this is not my homework. I have a test this week and I look for other sources to help me. I was not given much help at school from the teacher so I have to find my own way of learning and practicing. These could be simple to you but they aren't to me. SteamKing has been nice enough, and the only one that has helped me understand a lot, and I feel that without his help I'd still be stuck.
  11. Nov 16, 2014 #10
    1. tsukuba, you may think that SteamKing has done you a great favor, but I don't think so. It makes no difference whether this is homework, review for a test, or what; it is clearly an academic sort of problem. You started this whole discussion with a fully worked solution available to you, one that any well prepared engineering student should have been able to follow. The whole point of a learning exercise, whether for HW or test review, is for you to learn how to think through the problem. With all the help you had available to you (full solution with the original problem, tips I offered trying to lead you to think through the problem), and you still did not get it until SteamKing spelled out every minute detail for you, I doubt that you have learned very much at all. If this exact problem appears on the test, you may be able to get an answer. If a similar, but slightly different problem appears, I doubt greatly that you will be able to do much with it. That's why I don't think SteamKing has done you any great favor. You have not been pushed to think the problem through as you needed. Good luck with your test.
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