The nine major steps of designing generator fuel systems
Understand the requirements and challenges to navigate the design of a genset fuel oil system.
- Know the nine key considerations for designing a generator set fuel oil system.
- Consult authorities having jurisdiction to review the proposed design early in the project.
- Recall important rules-of-thumb when designing a fuel oil system.
Backup generator sets (gensets) are critical to business continuity and life safety. To ensure their reliable and efficient operation, the design of the associated fuel system must be approached systematically and thoroughly (see Figure 1).
Gensets that use gaseous fuels have gained acceptance over the past decade. However, No. 2 fuel oil is still the preferred choice for gensets intended for a range of commercial and industrial applications. Classified as a combustible liquid by NFPA 30: Flammable and Combustible Liquids Code, properties of No. 2 fuel oil vary slightly depending on the fuel blend (see Table 1). From an application standpoint, No. 2 fuel oil is nearly identical to diesel intended for onroad use. Gensets that are capable of using No. 2 fuel oil can operate on diesel without any issues.
When designing fuel oil systems for gensets, there are nine key considerations:
- Runtime criteria
- Fuel storage
- Fuel pumping
- Fuel cooling
- Fuel piping
- Fuel maintenance
- Fuel filling
- System controls
- Applicable codes and standards.
Understanding the requirements and challenges of each is critical to navigating the design of any fuel system. Note that although there are inherent nuances, some of the same considerations underlying fuel oil design principles can also be applied to systems intended for other applications, such as oil-fired boilers. Design criteria unique to each project will dictate the ultimate application.
Among the first steps of designing a fuel oil system for gensets is to establish runtime criteria in the event of a power outage (see "Runtime requirements"). Often dictated by a combination of applicable codes and owner requirements, the runtime—or how long the genset must operate during an emergency event without refueling-will set the bar for fuel oil design and operations. For example, life safety gensets typically are required to support emergency loads for a period of 2 hours upon loss of power. Critical facilities, such as data centers, typically are expected to support the load for 24 hours or more, depending on site resiliency requirements.
Because runtime criteria have a direct bearing on the fuel storage capacity required onsite, this consideration is critical to explore first. Note that fuel consumption data for gensets at various loads is readily available from the manufacturers.
For preliminary sizing of the fuel storage tanks, consider the following rule of thumb: 7 gal/hour of No. 2 fuel oil are needed per 100 kW of generator rating (see "Fuel oil design cheat sheet"). The fuel consumption rate (gph) multiplied by the desired runtime (hours) establishes the usable fuel requirement (gallons). It is important to note that only 80% to 85% of the tank capacity is typically usable depending on the tank shape and form. The tank cannot be emptied completely during operation nor can it be filled completely because head space is required to accommodate fuel expansion and prevent overflow.
For example, if the usable capacity requirement is 4,000 gal, a 5,000-gal fuel tank should be considered. For applications requiring a high level of resiliency, consider multiple fuel tanks so that isolated tank issues (e.g., contaminated fuel or component failure) do not jeopardize genset operation.
Fuel oil storage
Fuel oil can be stored in aboveground storage tanks (ASTs) or underground storage tanks (USTs). Each has advantages and disadvantages, and specifying the appropriate type is critical to ensure the optimum design.
ASTs typically are made of steel. From an AST benchmarking perspective, UL 142: Standard for Steel Aboveground Tanks for Flammable and Combustible Liquids specifies the requirements for single- and double-wall tanks, UL 2080: Standard for Fire Resistant Tanks for Flammable and Combustible Liquids specifies the requirements for fire resistant tanks, and UL 2085: Standard for Protected Aboveground Tanks for Flammable and Combustible Liquids specifies the requirements for protected tanks (fire and impact resistant). ASTs offer ease of maintenance; typically, lower installation costs and the ability to be installed by the project's mechanical contractor; ease of relocation; and the option of custom sizes to suit site conditions.
Employing an AST may not be appropriate for all projects because they require usable real estate, pose a greater fire hazard, allowable storage capacity typically is restricted by applicable codes and insurance carriers, and fuel heaters may be required in cold weather applications where the tank is exposed to subfreezing ambient temperatures.
USTs are available in fiberglass or steel construction. From a UST benchmarking perspective, UL 58: Standard for Steel Underground Tanks for Flammable and Combustible Liquids and UL 1746: Standard for External Corrosion Protection Systems for Steel Underground Storage Tanks specify the requirements for steel tanks and associated corrosion protection, and UL 1316: Glass-Fiber-Reinforced Plastic Underground Storage Tanks for Petroleum Products, Alcohols, and Alcohol-Gasoline Mixtures specifies the requirements for fiberglass tanks.
USTs are almost always cylindrical and require minimal real estate above ground, offer potentially greater fuel storage capacity, pose a lower fire hazard, and can maintain a relatively stable fuel temperature. Conversely, USTs can be difficult to access, maintain, and relocate; they typically have a higher installation cost; require comprehensive leak detection systems; and often require a specialized contractor to install.
Fuel oil pumping
Gensets are equipped with gear-driven pumps that pressurize fuel in the common rail of the engine. The integral pump draws fuel from the external tank. Excess fuel not injected into the cylinders is returned back to the tank. The pump has limited capability for priming and overcoming friction losses in the fuel distribution system (piping, fittings, and filters).
Usually, two types of electric-driven fuel oil pumps are used external to the genset: gear pumps and centrifugal submersible pumps—each with their own advantages and disadvantages.
Gear pumps: Mounted on a separate skid and typically used for low-flow, high-pressure applications, these pumps can be internal or external gear type. Classified as positive displacement pumps, gear pumps are suitable when pressure requirements exceed 40 psi. Actually, gear pumps are available with pressure capabilities exceeding 2,000 psi. Gear pumps are constant-flow rate devices and their maximum discharge pressure depends on the motor horsepower.
Submersible pumps: Used for high-flow, low-pressure applications, submersible pumps require adequate clearance above the fuel tank for accessibility and maintenance, even though the majority of the pump assembly is within the tank. There are no issues associated with priming or suction lift.
Static lift and friction losses should be reviewed in detail during fuel system design. The design flow rate of the pumping system should be two to four times the peak demand so that pumps operate intermittently to fill the auxiliary tanks instead of operating continuously.
For applications using sub-base, or belly tanks located directly beneath the gensets, the onboard fuel pump usually is adequate to exchange fuel with the tank. However, for applications where the entire stock of fuel cannot be located in close proximity to the gensets or where there is reasonable variation in elevation of main, or bulk storage tanks and gensets, an option is to use auxiliary, or day tanks dedicated to each genset (see "Using auxiliary tanks"). Auxiliary tanks are smaller tanks placed relatively close to the gensets at a similar elevation. The fuel supply and return pipe from the generator engine are connected to the associated auxiliary tank. Fuel is transferred from the main tank to the auxiliary tanks by external pumps that have been sized to meet the system pressure requirements (see Figure 2).