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Membranes, Molecules and the Science of Permeation |
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Evaporative losses masked by volume expansion |
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| Marketers using manual "sticking" or electronic tank gauges to perform inventory reconciliation are basing their calculations on gross gallons. Gross gallons are measured at the prevailing temperature of the liquid gasoline. As seen in Table 2, gasoline density and therefore volume is a function of temperature. | |||||||||||||||||||||||||||||||||
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As gasoline is warmed, the density decreases and the volume occupied by a fixed mass therefore must increase. As seen in Table 2, the volume expansion for this gasoline blend (California RFG) is about 0.7 percent for every 10 degree Fahrenheit rise. Thus, if a petroleum marketer takes delivery of 60 degrees F gasoline and if the average storage tank temperature is 80 degrees F, the marketer will gain about 1.4 percent in salable product inventory. The worst case evaporative losses from Table 1 are 10.76 tons a year for a station pumping 100,000 gallons per month (1.2 million gallons a year). The loss computes to 0.34 percent. This would still leave a net gain of 1.06 percent for the marketer (1.4 less 0.34). Therefore, the evaporative losses are masked by the volume expansion of the gasoline due to heat gain. To accurately measure the inventory loss due to evaporation, net temperature corrected volumes should be used in the inventory reconciliation calculation. Since most newer electronic tank gauges can incorporate temperature into their algorithms, a proper inventory reconciliation is possible. |
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