Excerpt from tank capacity table: "A total of 4,088.2662 barrels was deducted from this table between 4' 00" and 5' 00" for floating roof displacement based on a weight of 1,215,000 pounds and a gravity of 35.0 API. Gauged quantities above 5' 00" reflect this deduction but shall be adjusted for varying API gravities at tank temperature according to the following table: Referenced Observed API Gravity 35.0: No adjustment For each 1.0 API below 35.0 API: Add 24.59 barrels For each 1.0 API above 35.0 API: Subtract 24.59 barrels Step 1. Calculate the observed API Gravity for an API Gravity at 60 ° F of 40.0 and an observed temperature of the liquid of 84.0 ° F. This is done by working backwards using Table 5A or 5B if the tank content is a petroleum product. Locate the API gravity at 60 ° F on the horizontal line that corresponds to the observed tank temperature. The API gravity at observed temperature can be read at the top of the vertical column. Step 2. Calculate the difference between the observed API Gravity and the referenced observed API Gravity as follows:
8.6.4 Floating Roof Adjustment (FRA) When a tank has a floating roof, either internal or external, the roof itself will displace an amount of liquid, equal to its weight, whenever the roof is fully floating. When calculating the volume of material in the tank, the amount of this displacement must be taken into consideration. The correction for the displacement of a floating roof is volumetric. While the weight of the floating roof remains constant in most situations, the volume that it displaces is directly proportional to the observed density (API Gravity at observed temperature) of the liquid that it is floating in. You should keep in mind that density is also directly proportional to temperature. For most situations in the U.S., the roof correction is calculated into the tank capacity table using a reference density. The floating roof adjustment is a secondary correction for any difference between the reference density and the observed density at tank temperature. An example of such a correction follows: Input Data:
Referenced Observed API Gravity Observed API Gravity at 84 ° F
35.0 42.0
Difference
7.0
For each 1.0 API above 35.0 API, subtract 24.95 bbls. 7 x -24.59 = -172.13 bbls Floating Roof Adjustment = -172.13 bbls
No floating roof correction will be accurate if the liquid level falls inside the floating roof's critical zone regardless of table style. If significant amounts of water, ice or snow are present on a floating roof, it should either be removed, or its weight estimated and calculated into the roof correction. Roof corrections are not applicable for volumes below the Critical Zone.
Crude Oil 40.0 84.0 ° F
Product: API Gravity at 60 ° F: Temperature of liquid:
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