• Floating Roof Correction (FRC) - In this situation the tank capacity table has not been adjusted for the roof; therefore, a volume, equivalent to the entire weight of the roof, must be calculated and applied as a correction. This correction is always negative and is calculated by dividing the weight of the floating roof by observed density at tank temperature of the fluid in contact with the roof.
Excerpt from a tank capacity table:
12.1 Floating Roof Adjustment & Correction When a tank has a floating roof the roof itself will displace an amount of liquid, equal to the weight of roof, whenever the roof is fully floating. When calculating how much material is in the tank, the amount of this displacement must be taken into consideration. The correction or adjustment for the displacement of a floating roof is volumetric. While the weight of the floating roof remains constant the volume that it displaces is directly proportional to the observed density or API of the liquid that it is floating in. There are two types of floating roofs, internal and external; however, the calculation process to determine the floating roof adjustment or correction is the same for both types of roofs. The determination of the correction, however, can be addressed in one of two ways: • Floating Roof Adjustment (FRA) - This method is used when a correction for the roof has been calculated into the tank capacity table using a reference density. A secondary adjustment must therefore be calculated for any difference between the reference density and the observed density at tank temperature of the fluid in contact with the roof. This correction can be positive or negative.
" A total of 4,088.2662 barrels was deducted from this table between 4 ft 00 in. and 5 ft 00 in. for floating roof displacement based on a weight of 1,215,000 pounds and a gravity of 35.0 API. Gauged quantities above 5 ft 00 in. reflect this deduction but shall be adjusted for varying API gravities at tank temperature according to the following ”:
12.1.1 Floating Roof Adjustment (FRA)
Referenced observed API gravity 35.0 For each 1.0 API below 35.0 API For each 1.0 API above 35.0 API
No adjustment
For most situations in the U.S., the roof correction is calculated into the tank capacity table using a reference Density (API). The floating roof adjustment is a secondary correction for any difference between the reference density and the observed density at tank temperature.
Add 24.59 barrels
Subtract 24.59 barrels
Firstly : Convert the API gravity @ 60 ° F to API gravity @ Observed Temp.
Step 1 : Calculate the observed API gravity for an API gravity @ 60 ° F of 40.3 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) as shown in the example, which is an excerpt from API MPMS Chapter 11.1-1980 / Adjunct to ASTM D1250-80 / Adjunct to IP200/80, Table 5A.
Secondly : Calculate the difference between the API gravity @ Observed Temp. and the referenced API gravity on the capacity table. Thirdly : Multiply the difference by the stated Bbls per degree (which can be positive or negative) to calculate the Floating Roof Adjustment.
For example:
Commodity
Crude Oil
API gravity @ 60 ° F
40.3
84.0 ° F
Temperature of Liquid
76
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