How much fuel should I have?

When a client specifies the fuel autonomy of a Generating Set, it is normally in terms of a number of hours for which the Generating Set should run. Once you know how many hours you want it to run for, you can work out how many litres of fuel this equates to using the rough rules of thumb:

  • For sets >150 kVA rating, the number of litres per hour used is approximately 1/5th of the rating of the set in kVA. (e.g. a 500 kVA unit will use approximately 100 L/h).
  • For sets <150 kVA rating, this figures rises to 1/4th of the rating of the set in kVA (e.g. a 100 kVA unit will use approximately 25 L/h).

The difference in figures is due to the lower engine fuel efficiency and generator efficiency of typical smaller machines.

What physical volume does this equate to? A litre is a litre whatever the fluid so 1 m3 is 1 000 L of fuel. However, you should also allow for:

  • No tank can be completely drained so there is normally a dead portion in the bottom of the tank (“ullage”). Similarly, no tank can be completely filled. How much this is will depend on the shape of the tank. This figure is normally the difference between the “useable” capacity of a tank and the “actual”.
  • If the generating set is drawing and returning fuel directly to the tank (most engines return a significant proportion of the fuel that they pull up as this is used for injector cooling) then the fuel will get warmer. The tank should be sized adequately to ensure that the fuel left in the tank when you have used what you want is not at an unacceptable temperature for the engine.
  • All fuel should be in a bunded area whether it is a bunded room or a bunded tank. This bunding should be at least 10% larger than the capacity of the tank.

Other things that you should consider when sizing a fuel supply:

  • Be careful to state the number of hours that you want at what load: A practice sometimes used by manufacturers is to declare the fuel capacity of a set in terms of the rated output. This can mean the rated load allowing for load factor under certain rating conditions and can be 30% smaller as a result. If you want a given number of hours at full load, be sure to say that.
  • Remember that Generating Sets, like all engines, burn lubricating oil as well as fuel. Lube oil usage is typically 1% of fuel flow, but oil consumption rates can rise at lower loads. Engines typically have enough oil in the sump to run between 20 and 100 h depending on the load (as it gets lower, the oil consumption will get higher). It is no good asking for a fuel supply equivalent to 72 h at full load if the engine only has enough oil for 30 h. Manufacturers can supply lubricating oil make up systems to deal with this but make sure that you specify this.
  • If a fuel supply is to go into a building, be aware of possible fire and planning constraints – fire and/or building regulations can limit the amount of fuel that can be kept in a building or the positioning of adjacent fuel tanks. These can also be varied by local fire regulations.
  • Be aware of the risk of Bio-diesel content in fuel supplies. Bio-diesel, or fuel with bio-diesel content, is not appropriate for use in Stand-by Generation equipment as it has a limited shelf life (potentially as short as six months). This shelf life can be maximised by good design practice and husbandry of the fuel. If you believe that your fuel supply may have bio-diesel content (unless you specify otherwise at time of order, it will have), speak to the Generating Set supplier and the Generating Set servicer about appropriate design and management steps.