What is the purpose of a fuel day tank?

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.

It doesn’t take a generator technician to know that an engine needs fuel to operate. If you’ve ever run out of gas in your vehicle, you can appreciate the importance of remembering this fundamental detail, and you may understand how easily it can be overlooked. The same is true with fueling standby generators. During an extended power outage, keeping fuel in the tank quickly becomes a priority. The following details are intended to provide a general ‘101’ overview of diesel fuel tanks for standby generators; to provide guidance to select the proper tank, and to ensure your generator’s fuel source will be ready when the next power outage occurs.

1. Tank Style:   The most common fuel tank style for standby generators is the sub-base type – with the generator mounted directly on top of the fuel tank. These can range from a shallow 8 inch height to roughly 40 inches, and the length of the tank may extend beyond the length of the generator-set if necessary to accommodate the desired operation before the tank needs re-fueling.
2. Run-time: The fuel tank run-time is calculated by the generator fuel consumption measurement at 100% load. This anticipates a worst-case scenario, assuming the potential for the generator to be fully loaded during a power outage. The 100% fuel consumption multiplied by 24 = a 24-hour tank. When selecting a fuel tank size keep in mind that generators don’t normally run at 100% load, so the actual run-time will likely be greater than 24 hours.
3. Tank Height: As mentioned, the height of the fuel tank depends on desired run-time. A 12-hour tank might be 10-12” tall, where a 72-hour tank might be 36”. The 24-inch height variance between the two options could be the difference of checking the generator control panel at a position lower than eye level, or requiring a step ladder or platform to view the control panel at eye level. If the application requires a tall fuel tank to achieve the desired run-time, a permanent platform around the unit (or on the sides) may be preferred for ease of maintenance and operation.
4. Run-time Dictated by Code: Local requirements can vary state-to-state and city-to-city. In some applications, a specific run-time may be required to adhere to the local code. For example, in the health care industry, local codes in some areas dictate that the fuel source for standby generators in critical, life-safety applications should be a minimum of 48 hours. Code requirements in other areas may specify more, or less.
5. Day-Tanks / Extended Run-time: When the physical size limit of a sub-base fuel tank simply isn’t enough, a day-tank could be a viable option. A day-tank functions as the immediate fuel source, receiving fuel from a larger fuel storage tank. It can be a stand-alone tank mounted in proximity to the generator, or it could be a sub-base tank with provisions to be used as a day-tank. In either case, a day-tank is typically designed to contain a small amount of fuel which is automatically replenished through the use of pumps and controls.
6. Additional Code Requirements: In some areas, local code may dictate fuel tank requirements beyond run-time. In Dallas/Fort Worth, for example, certain cities have adopted additional safety and environmental requirements for fuel tanks, such as: extended tank venting, fuel-spill containment at the fill port, on-site pressure testing, and high fuel level shut-off. These requirements, or similar, are being adopted in other regions of the U.S. as well. When in question, the local Fire Marshal typically has jurisdiction and can provide details.
7. UL142 or UL2085: Diesel fuel tanks sold in the U.S. for stationary generators typically comply with one of these standards. UL142 is the most common, and the UL requirements cover the construction, performance, and markings of the tank. Features include venting, fill/withdraw, gauging, secondary containment, and openings for leak-detection monitoring. UL2085 standards include the requirements of UL142, with additional requirements to encase the fuel tank in a concrete material for added spill protection. UL2085 tanks are required to withstand a certain resistance to fire, vehicle impact, and projectiles.
8. Diesel Fuel Types: Standard diesel fuel comes in two grades: #1 and #2. The local climate is an important factor to determine the type of fuel used for a standby generator. #1 is more resistant to gelling in colder temperatures and can be used for colder climates. The energy output of #2 diesel is slightly higher than #1, so it is preferred for warmer climates where the risk of gelling is lower. These fuel grades are often blended to provide the benefits of both, and to provide the suitable viscosity for local weather conditions. Fuel suppliers are usually familiar with the grade (or blend) necessary for the local climate.
9. Fuel Treatment and Polishing: Diesel fuel typically starts to deteriorate and form solids within six months. For preventative maintenance, fuel treatment is available to extend the life and ensure the fuel is up to par and ready to perform. It fights micro-organism growth, prevents gelling and stabilizes the fuel. Further, to counter fuel issues, fuel polishing can be performed which removes water and sediment from the tank and filters contaminants. This is an economical, earth-friendly alternative to replacing fuel, as all of the fuel is recycled with no loss of product.
10. Fuel Quality Testing: Fuel quality issues typically don’t show up until the generator is running under load during a power outage – when the reliability of your standby system is most important. Before fuel issues arise, quality and contamination tests can be performed to check for contaminants as well as the overall fuel quality. Contamination sampling can check for water contamination, bottom sediment, gel, flash, and cloud points. This testing also meets the ASTM standards required to comply with annual fuel testing as required by NFPA (NFPA110 8.3.8).

If you have questions about fuel tank selection, requirements, or if you need any information regarding planned maintenance for your fuel supply, contact your local Clifford Power Systems representative for additional information.

For Additional Information on Generator Maintenance: Selecting the Right Generator Maintenance Plan

Clifford Power Systems, Inc. provides dependable generator equipment, service, and rental. We are solely focused on power generation and offer professional consultation to meet all your project needs. Contact us at any of our locations in Oklahoma, Texas, Missouri, Kansas, or Arkansas.

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