10 Things to Know about Diesel Fuel Tanks for Standby Generators

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.