You should know that fuel oil quality is vary variable. What you may think you are burning and what you are actually burning may be very diferent indeed. Density, which you ask about, is one of the variables, viscosity is another and can have a serious impact on your design. Also, different parts of the pipe will be at different temperatures when you are before or after the pre heaters or in the final feed to the burners.
Your assumptions when designing should consider if this is a non-producer of fuels or hydrocarbons who is buying in fuel (where some form of quality control is theoretically possible, some surveys suggest fuel quality can vary significantly e.g. from 150 to 410cst for a fuel nominally of 380cst)or if you are in a refinery or petrochemical plant. In the latter they may say they burn commercial grade fuels but in my experience they often burn processing "mistakes" or the petrochem plant take the out of spec grades from their refining partners.
Density can vary of course, and for heavy fuel oils this varies from 975 to 981 kg/m3 for 10cst oils (density at 15degC and viscosity at 100degC) to between 991 and 1010 kg/m3 for 55cst fuels. Or at least, these are the limits according to the standards.
As 25362 says, these fuels are then heated to obtain the optimum viscosity for atomisation. This is a whole other area and if you need any information here, please say so and i will add some further comments but it would seem to be getting away from your original questions.
You need to discriminate between what the design standard is and what they actually have as a fuel. They should be able to provide you with daily fuel analysis results, or at the very least, with results of samples taken on individual bunkers supplied.
