Decisions, decisions… There are many to make in the process of designing a large fuel supply system for emergency generators. A bulk fuel storage system is most common when your clients require large generators, and an extended “run-time” capability. For healthcare and other mission-critical facilities, “bulk storage” can mean tens of thousands of gallons of diesel fuel. With that much fuel on-site, it is normal to have concerns for safety, code-compliance and reliability. Here are some basic steps to ease some of the anxiety over completing such a design!
First up: Fuel Capacity
How much fuel should be stored on-site? Fuel storage capacity is mostly driven by the number of hours that the generators may be required to run (i.e.: “run-time”). Your client’s type of business will be key in defining this point. Whether you are given specific instructions, or simply referred to follow basic NFPA code requirements (see Chapters 99 and 110), be sure to also consult with local Authorities Having Jurisdiction (AHJs) for any special rules. Your run-time calculation should assume worst case conditions and should be tied to your client’s disaster management plan. Once a determination has been made for the required “run-time”, the fuel storage requirement boils down to a simple equation:
SF = (FC x R x N), where:
SF = Stored Fuel (in gallons)
FC = Fuel Consumption (at full load, in gallons per hour)
R = Run-time (in hours)
N = Number of generators
To be precise, some adjustments should be made to account for “usable” fuel. Usable fuel takes into consideration any tank design characteristics, or safety devices that prevent a tank from being filled to its full capacity. There will be some volume of air inside the tank, and also some fuel at the bottom of the tank that will not be accessible to the suction pipe supplying fuel to the generators. And again, be sure to check with local AHJs as they may require a certain percentage “safety factor” over your calculated run-time.
Now on to location…
Where will you place the fuel tanks? Site permitting is an obvious factor that should be looked at early on in the process. Current building codes, site elevation, soil conditions, environmental issues, and impact to neighboring properties can all play a role. Once again, check with State and local AHJs for any specific requirements or limitations.
This question of location may also lead you to another important decision: Will you use above-ground storage tanks (AST), or under-ground storage tanks (UST)?
USTs consume minimal above-grade space, but they face higher installation costs, and potential environmental obstacles. When fuel is stored underground, there is a higher likelihood for fuel degradation due to water contamination (cooler underground temperatures will lead to more condensation of moist air entering through the tank vents). Inspection and servicing of USTs can also be more difficult, and any pumps or valves enclosed in underground sumps are more prone to corrosion damage. These are long-term considerations that the client/owner should understand.
ASTs present their own challenges, mostly related to space and protection. If space is not an issue, the next major concern is how to protect an exposed fuel tank. ASTs can be large and difficult to conceal or protect. Given ideal circumstances, the fuel tanks should be protected by a wall, but most often they are exposed and simply guarded by fencing or bollards. If protection is a major requirement, a “protected” tank should be strongly considered.
For information on the various types of tanks, see this Fuel Storage Tank Guide.
Think of a system, instead of individual components!
Safely transferring the fuel to the generators is just as crucial as the integrity of the fuel storage tanks. Fuel transfer pumps should be selected with sufficient capacity and built-in redundancy. Piping to and from the fuel tanks can present challenges all their own. Underground piping should be listed and approved by the AHJ for the intended use. And be aware of local codes that may require above-ground piping to be protected from wind-blown debris. Finally, any containment sumps, leak detection devices and safety valves should be coordinated with the fuel transfer control system to avoid fuel spills.
Simplify and tune your design…
Once you have made the major decisions, look to simplify your fuel system design by eliminating unnecessary items. Here are some items that often get misinterpreted:
- “Fuel tank vent caps must be installed a minimum of 12′ above grade”. NFPA states that Class I fuels must have vents installed a minimum of 12′ above grade. Diesel fuel, however, is a Class II fuel and this requirement may not be applicable in your jurisdiction.
- NFPA-110 lists a requirement for “fuel solenoid valves to be operated by the engine batteries”. This requirement only applies to the solenoid valve installed immediately upstream of the engine fuel supply connections. Solenoid valves installed elsewhere (i.e.: upstream of a day tank) should operate on AC voltage.
- NFPA limits the size of fuel tanks located inside buildings, but be aware of exceptions allowing larger capacities, when fuel tanks are located in designated mechanical rooms. Also note that only a maximum of 660 gallons can be directly connected to an engine.
- For installations in Florida: the Florida Department of Environmental Protection (FDEP) requires all above-ground storage tanks over 550 gallons to be registered with the State, and listed in their “Approved Equipment List” (approved equipment is identified by an “EQ” number). Some exemptions exist for tanks installed in residences, or entirely within an “industrial occupancy building” (see the Applicability section of the Rule for details).
Fuel systems are a critical part of an emergency generator system. Dedicating some time to the fuel system, early in the design stage, can prevent potentially serious issues during permitting, or during the installation itself. A safe and reliable system can be achieved if all major components are coordinated and designed for the specific project at-hand. When pressed by deadlines, resist the temptation to simply copy a design from a “similar” past project. This can be a costly mistake that I often see consuming valuable time during construction.
Generator fuel systems are a specialized area in design, and I find that few firms have a dedicated team to develop fuel system designs. If you are faced with a project that challenges your experience with fuel systems, I encourage you to reach out to me for assistance. After an initial consultation, you will be in a much better position to ask the right questions, and well on your way to develop the best possible fuel system for your client’s facility.
If you would like to read more on this subject, see “Top 12 Surefire Ways to Improve Your Fuel System Design“.
Thank you for reading, and please post below if you have thoughts or comments on this topic!