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 subbase 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 it needs to store a substantial amount of fuel (500+ 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.

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.