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Homework Help: Dimensional Analysis Problem

  1. Feb 27, 2017 #1
    1. The problem statement, all variables and given/known data
    The American Petroleum Institute has published a correlation for determining the hydrocarbon emissions from fixed-roof storage tanks

    Ly = (24/1000) * (p/(14.7-p))^0.68 * D^1.73 * H^0.51 * T^0.5 * Fp * C

    where: Ly is breathing emissions, bbl/yr; p is the true vapor pressure at the bulk temperature, psia; D is the tank diameter, ft; H is the height in ft; T is the average tank outage corrected for roof volume, ft; Fp is the dimensionless paint factor; and C is the dimensionless adjustment factor.

    Is this equation dimensionally consistent?

    2. Relevant equations
    Unit conversions

    3. The attempt at a solution
    I have a doubt with the (p/(14.7-p))^0.68 term. I think it should be dimensionless, but I am not totally sure as there is a 14.7 with no units minus a pressure in psia...
  2. jcsd
  3. Feb 27, 2017 #2


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    What is atmospheric pressure in psi?
  4. Feb 27, 2017 #3


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    Staff: Mentor

    It's terms like that that indicate that this is not an equation, but a formula. There are many such formulas for different disciplines, typically compiled into handbooks, where you must enter the variable values as dimensionless magnitudes of quantities that are given in specific units. A formula is "true" only so long as you specify all quantities in the required units and don't entry the units :smile:

    A formula's constants typically have no units so you can't trivially "translate" the formula to work with other units, and since you have to interpret the result of the formula in particular units you can't just treat the expression as an equation and "solve" for any of the variables in terms of the others without putting some thought into what you're doing.
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