When Is A Fuel Day Tank Needed?

Should you include a day tank in your emergency generator fuel supply system?  The following three steps will help you answer this question:

  1. Understand the applicable code.
  2. Gather technical information on the proposed engine(s), and
  3. Perform some basic piping calculations.

Let’s begin by identifying the relevant code requirements that will guide you in your decision-making:

Per NFPA-110 7.9.2:  “Fuel tanks shall be close enough to the prime mover for the fuel lift (suction head) of the prime mover fuel pump to meet the fuel system requirements, or a fuel transfer pump and day tank shall be provided.

This is the easiest step.  In a nutshell, if the engine-mounted fuel pump is unable to draw fuel from the fuel source (fuel storage tank), a day tank in close proximity to the engine is required.

Next step, how do you determine the engine’s capacity to draw fuel from a remote fuel tank?

You will need to locate a bit of technical data for the proposed engine.  If the engine is yet to be defined (perhaps the project is open to competitive bidding by multiple engine vendors), you should review the likely candidates, and base your assumptions on the worst-case scenario.  The specific data that you are looking for relates to the engine’s fuel pump limit for fuel intake restriction.  I recommend that you also consider the engine fuel pump’s return line restriction, since an engine unable to return fuel to the source tank raises the same issues as one that cannot draw fuel to itself.  The limiting values are often shown in the generator set data sheets and may appear like this:

Ask your engine/generator vendor for help if you are unable to locate these values.

Once you have the necessary data, you are ready to complete the final step – calculate the expected supply and return pipe restrictions.  These calculations should be performed by a plumbing or mechanical engineer familiar with the fuel piping layout.  Calculations should take into account the piping length, the pipe diameter, existence of any elbows, valves (including foot valves), and the viscosity characteristics of the #2 diesel fuel flowing through the piping.

A quick comparison of the total restriction of the piping system against the limits of the engine should tell you whether a day tank is required for your project.

Oh, and one last thing…  If you find that you do need a day tank, your next decision might involve selecting between a freestanding design or a sub-base (integral) tank design.  Here are my basic recommendations for using one over the other:

I recommend a freestanding day tank if the fuel storage capacity is relatively small (<600 gallons).  Why?  Mainly because of the low cost and the simplicity of design and installation.  At less than 600 gallons, a sub-base tank would be a more expensive proposition.  Beyond the cost of the sub-base tank, you would need a fuel fill pump and possibly also an overflow return pump.  These pumps might need to be in a separate skid or enclosure (instead of top-mounted as in the case of a freestanding day tank).  A freestanding day tank is of standard design, factory-packaged and tested, with all solenoid valves, hand pump, strainer, bypass valves, etc., pre-piped and pre-wired to a UL508 control panel.  You can expect a higher level of reliability and lower cost in a freestanding day tank package.

On the other hand, I would select a sub-base tank design when the application consists of a single generator, in an exterior installation, with the need to store a substantial amount of fuel (600+ gallons).  Sub-base tanks make the most sense when they can provide a fairly-large fuel storage solution within the footprint of the emergency generator. Downsides to a sub-base tank are its flat-bottom and internal baffles, which do not facilitate fuel circulation and can lead to the accumulation of contaminants. Because the “integral tank” sits under the generator, a future tank replacement can also lead to a lengthy disruption of the generator set installation.

Did you find this article useful?  Please comment below or post a question.  If you would you more information on day tanks, head over here for some guidelines on how to specify a day tank.

6 thoughts on “When Is A Fuel Day Tank Needed?”

    • Stephen, I recommend that the supply pump be sized to 3-times the expected fuel consumption (ie: if the engine or boiler draws 60GPH, pump should be sized to 180GPH). The height of the day tank should be such that the engine/boiler remains capable of returning excess fuel to the day tank. A general rule of thumb is to keep the day tank fuel level below the level of the engine’s fuel injectors. Thanks for your question.

      • I am working on a project that has a fuel storage tank above grade and it serves the steam boilers in the basement mech room. We don’t have any day tank or transfer pumps. There are only dedicated Webster D style pumps at each boiler. We have a static head of 10 psi at the inlet of the pump ( Since NFPA requires inlet pressure at the pump not to exceed 3 psi we put a OSV in the supply pipe to the pump.) My question here is, can the pump handle the return static of 10 psi to pump back to the storage tank. please let me know, thanks

        • Thanks for your question Assad. I am not very familiar with the Webster pumps but these specifications on the “R Series” refer to discharge pressures reaching 100psi. If your pump is of similar design, it should be capable of overcoming the 10psi static head. To be sure, I suggest reaching out to the manufacturer with the specific pump part number. Good luck!

  1. Hi David,

    I am having trouble understanding the values of the “maximum suction head of feed pump”.
    They typically give a value for clean and dirty filter. However the value for the dirty filter is higher, eg 2in Hg for clean and 4in Hg for dirty. When I do my pipe losses calcs I have 3.5in Hg. Does that mean that it will only work with a dirty filter since it exceeded the max value w/ a clean filter.

    I hope you understand where my confusion is coming from. Thanks

    • Thank you for your question. The values that you indicate seem to be applicable to the filter itself (a lower pressure drop when the filter is clean; higher pressure drop when the filter is dirty). If you have a suction pump, drawing fuel through this filter, and you also have 3.5″Hg of piping losses, then your total “worst case” restriction is: 3.5″ (pipe losses) + 4″ (dirty filter) = 7.5″Hg. The suction pump’s data sheet should state its maximum suction head, which in this case, should be higher than the calculated 7.5″Hg. I hope this helps.


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