¦ API calculation (revisited) 1

[21121956]

Hello everybody:

The original post appeared in the thread481-144449 and, at the present, the question remains the same with a couple of details to be added.

The people of the chemical lab daily takes samples from the fuel storage tanks in order to calculate the value of º API referenced to a temperature. With that value, operations personnel calculates the specific energy of the fuel; the specific fuel consumption; the Heat Rate of the engines.

For instance, if the º API of the fuel at the moment of unload from the tanker is 10,5 (995,9 kg/m³), to my astonishment, the value of º API change constantly in a period of a month, ranging from 10,4 today to 11,5 tomorrow and 10,6 three days later.

I wonder, is this correct? Is there something wrong?

Obviously, the chem lab people swears they make their work correctly.

As always, your comments will be highly appreciated.

 

[jmw]

How homogeneous is your fuel in storage?


JMW

http://www.ViscoAnalyser.com

 

[mbeychok]

21121956:

An API value of 10.5 degrees is a specific gravity of 0.9965 and an API value of 11.5 degrees is a specific gravity of 0.9895. That is a difference of less than 1 percent (0.7 percent to be exact).

That could very easily be due to daily changes in the temperature of the fuel oil in your fuel tank ... and I doubt that your lab bothers to place their samples in a constant temperature bath so that the samples are tested at the same temperature every day. I know that when I worked in a refinery laboratory, we never bothered to do that.

In any event, such small differences would result in very insignificant differences of heat content and fuel consumption. I don't know what sort of facility you work in, but it seems to me that you are worrying about something that isn't worth worrying about.

Not only does the tank temperature vary from day to day, the fuel used to replenish the tank will vary from lot to lot. If the tank is not kept mixed, it could also stratify into tiers of varying gravity. Nonetheless, if all it does is bounce around from 10 to 12 degrees API or so, it is insignificant in terms of the fuel heat content.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[25362]

To ratify mbeychok's message I took the following data from my old notes on the calorific values (kcal/kg) for an Oficina (venezuelan) type of fuel:

HCV = 9,940 + 32 (^oAPI)
LCV = 9,380 + 26 (^oAPI)

You may notice that for this fuel a change of 1 ^oAPI unit affects the calorific values by about 0.3 %.

 

[21121956]

Hello everybody:

Thanks to all for your comments. Everything regarding with the calorific value and the fuel consumption is O.K. but, how about the periodicity? Is it really necessary to perform these analysis in a daily basis?

Thanks. Gracias.

 

[jmw]

The question of testing may well depend in part on how well the fuels are managed in storage.
It has been usual to consolidate different batches of fuel in one tank. That is, as the level drops and new fuel is received it is simply added to the storage tank. If the fuels are of different densities and the tank contents are not homogenised, then stratification can occur. This is true of heavy fuels oils and can be aggravated if they have been blended from incompatible components.

If the remaining fuel is of lower density than the added fuel it will float on top and over time, dependent on fuel type, may begin to breakdown with an even lighter fraction remaining indefinitely on top (if the tank is never completely emptied) while the heavy fraction sinks and is drawn out first (unless it layers below the pump suction).

Of course, it may also then depend on if this is a heavy fuel oil which is heated as some circulation may be generated by heating and as this may also drive off some of the volatiles.

It now remains to ask how samples are collected and from where.

How significant any of this is? I wonder what others more knowledgeable will say?

With Power station applications with heavy fuel oils sampling and analysis is often necessary because of viscosity variation if they are still using temperature to control fuel heating
in which case it is viscosity that is important. Density?

JMW

http://www.ViscoAnalyser.com