¦ API calculation

[21121956]

Hello everybody:

I would like to know if, for aboveground fuel storage tanks, with steam heater coils, is it necessary to determine daily the degrees API? or it does not matter when this measures are taken.

The fuel is Bunker C (grade Nº 6) with around 12º API.
Any comment will be valuable.

 

[zdas04]

"Degrees API" is just a silly surrogate for specific gravity (which is a useful surrogate for density). Since it is supposed to be measured at a reference temperature, the only reason I see for periodically re-evaluating it is to check for contamination. Contracts (either sales or transportation) may specify a frequency that the tank needs to be tested, otherwise it depends on the sensitivity of the users to contamination.

This is really a site-specific and contract-specific question.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[Ashereng]

In our industry, for some wells, we take a sample once or twice a year. The API measured is then used until the next sample.

I agree with zdas04 that the frequency would depend on sensitivity to the magnitude of the change expected - oh, and the contractual obligations of course.

 

[25362]

Densities are measured over a range of temperatures, usually, and for convenience, at the temperature at which the fuel is stored.

The value is then corrected back to 15^oC, which is the reference temperature used by most excise authorities. The correction is made by use of standard tables, derived from a large number of fuels representing world-wide fuel availability.

Knowledge of the density of fuels is needed for quantity calculations, its value is also needed for the selection of the correct gravity discs (or rings) on conventional purifying centrifuges.

Of all the contaminants in marine fuel oils water is the most common. The actual level is normally very low, 0.1-0.2% volume is typical. Ingress of water can be traced to a number of sources, among them I recall tank condensation and steam heating coils leakage.

Since marine fuel oil is purchased by the shipowner to produce heat for conversion into work, a value of its "specific energy" is needed. This specific energy (gross and net) is estimated from formulas based on the density of the fuel, corrected for any sulfur, water and ash that may be present.

Other characteristics of importance (beside price) are fuel stability, compatibility, sludge formation tendencies, pour point and, of course, viscosity.

 

[zdas04]

25362,
If you took any of the posts above as saying that knowing density is not important then you need to read them again. The method and frequency of determining density (or API Gravity since that is the point of this thread) is the issue and there is no one answer to that question.

For instance none of the contracts and/or regulations that I've ever dealt with have specified correcting density to 15C. So what? Every operation in the world has specific quality/sampling requirements that are imposed by the contracts and/or by regulators and/or by company policy.

David

 

[25362]

zdas04,

You are entirely correct . My intention was to add some background to the importance of density and the need for repeated measurements as you and Ashereng have explained, a fact that I didn't make clear enough.

Depending on the storage tank height and volume, these measurements are sometimes made from "top-middle-bottom" samples to check for any (sludge) settling or contamination and take corrective action.

Anyway, I think that a charter party should clearly define the fuel grade to reduce legal issues surrounding "bad bunkers" by minimizing the different interpretations of quality disclaimers under various jurisdictions where different laws and codes apply.

As for the reference temperature, specific energy, etc, I think the move has been toward a consolidation in SI units.
Otherwise one could select, for example, from the old US Bureau of Standards No. 154:

12^oAPI [→] 0.986 (sp gr.) [→] 8.212 lb/gal [→] 0.1218 gal/lb [→] 6.392 bbl/ton [→] 1.016 m³/ton.

Given the right conditions all things succumb to human error.
Dr Angela F. Summers