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Capital cost calculations, Postscripts, corrections

By Chemical Engineering |

Capital cost calculations I recently read the article “Capital Costs Quickly Calculated” [ Chem. Eng., pp.46­–52, April 2009] and find it extremely informative and useful. I have reservations about the example given about the cost comparison of two spherical storage tanks. The authors state that the costs of spherical storage tanks are proportional to surface area, therefore, to the quantity of metal plate used in fabrication. They came out with a size exponent of 2/3 and concluded doubling the size of a spherical storage tank increases its price by about 60%. Here, the authors made a wrong assumption. They assumed the plate thickness remains the same irrespective of the size of tank. As per relevant design codes, such as ASME-VIII, the thickness of a sphere or a cylindrical vessel is proportional to radius. Doubling the diameter means the thickness would be doubled. Therefore the exponent of 2/3 is not valid. Accounting for the fact the thickness is proportional to the diameter, and tank steel weight is proportional to (i) tank area and (ii) tank wall thickness, the exponent would be 1 and not 2/3. In other words, doubling the size of a spherical storage tank increases its price by about 100% and not 60%, as…
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