The density of #2 and #4 fuel oil isn't really fixed but I can assume you it will be a lot less than a specific gravity of 1.4.

Two of my reference books gives the minimum specific gravity of #2 fuel oil both as 0.88 (30 deg API). For #4 fuel oil, neither has a minimum value listed.

Typical values (as opposed to minimum) for #2 fuel oil is about 0.84 and for #4 fuel oil, 0.86.  The latter seems a little low to me relative to #2.  The reference book I'm using is a little old.

One other reference book (Steam: its generation and use) gives the range for #2 fuel oil as 28 to 40 deg API. For #4, the range is 15 to 30 (this book is older, from the 1960s) but the above numbers fall in this range.

Fuel oil is not a tightly controlled product from a refinery.  There are several parameters that must be met and how they formulate it and what steams are used to make it will affect the final product's properties.

Thanks for the information. In refineries, tanks are normally built to API 650 or 620. I am located in a pharm. plant in Puerto Rico. The associated piping going to the
vessel operates at 50 psi on both sides (feed and supply to burners). I am trying to make sure the vessel is properly designed.

Heavy marine fuel oil will 'just' exceed a specific gravity of 1.0 by a very small margin (perhaps 1.005) but few other hydrocarbons will exceed 1.0.  

Asphalt and from an FCCU, decant/slurry oil, are a couple I can think of (1.02 and perhaps 1.08 respectively) that exceed 1.0 specific gravity.

These specific gravities are at 60F but unless you chill the oil significantly (and then for these materials you have gel or a solid, not a liquid) you should always be less than these values.

Thanks again. I believe that I have solved my problem.

For future use, since you have solved your problem - The FUEL OIL Calculation Program

http://steamesteem.com/index.html?tables