Alternatively, the observed gravity can be calculated using the below formula, in instances where table 5A (or 5B if the tank content is a petroleum product) is not accessible (round the answer to 1 decimal place):
12.1.2 Floating Roof Correction (FRC)
Many floating roof tanks outside of the U.S. have capacity tables that make no correction of any kind for the floating roof. In this situation, the floating roof correction is the entire volume that the weight of the floating roof will displace. The roof deduction is calculated by dividing the weight of the floating roof by the density (weight per unit volume at standard temperature) of the liquid multiplied by the VCF (Volume Correction Factor) to bring this to observed conditions.
Step 2 : Calculate the difference between the observed gravity and the referenced observed gravity as follows:
Referenced observed API gravity 35.0 For each 1.0 API below 35.0 API For each 1.0 API above 35.0 API Referenced observed API gravity Observed API gravity at 84 ° F
No adjustment Add 24.59 barrels
The density units must be consistent to both the VCF and the units of the roof weight, in addition to being a density in air. For example, if the density is kg/l @ 15 ° C then the roof weight must be in kilograms and the VCF applicable to a standard temperature of 15 ° C. Also, the density must be that of the fluid in contact with the roof.
When the API gravity @ 60 ° F does not fall on an exact gravity, the user must interpolate between gravities to arrive at the correct observed gravity, as per the above example.
Subtract 24.59 barrels
35.0
42.4
In this example, the API gravity @ 60 ° F to be corrected to observed gravity is 40.3.
Difference
7.4
For example:
Looking up the API gravity @ 60 ° F on the above table indicates the gravity falls between 40.0 and 40.4. The API of 40.3 falls between the observed gravities of 42.0 and 42.5, which have a difference of 0.5 gravity. To determine what 40.3 API gravity @ 60 ° F equates to as an observed gravity, calculate ¾ of 0.5 gravity. The calculation equates to 0.4 (rounded to 1 decimal place).
Commodity
Crude Oil 0.87520 0.87413 24.5 ° C 0.99236 477602
kg/l @ 15 ° C (vac) kg/l @ 15 ° C (air)
Step 3 : Calculate the floating roof adjustment:
For each 1.0 API above 35.0 API, subtract 24.59 barrels.
Temperature of Liquid
VCF (Table 54A)
Weight of floating roof (kilograms)
(7.4) x (-24.59) = - 181.97 barrels
Floating roof adjustment = subtract 181.97 barrels.
Add this to 42.0, and the observed gravity that equates to 40.3 @ 60 ° F is 42.4 API.
77
Made with FlippingBook Annual report maker