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).
Cooling the fuel
Excess fuel in the common rail that isn’t injected into the cylinders is sent back to the tank. The return fuel is at an elevated temperature because it absorbs heat from the injectors and water jacket. When it mixes with cooler fuel in the tank, the supply fuel temperature gradually starts to rise.
For every 10°F rise in fuel temperature above 100°F, the engine horsepower reduces by approximately 1%. High fuel temperature also reduces its ability to lubricate the engine fuel system components. If the temperature of fuel being supplied to the engine exceeds a certain limit (typically 140° to 150°F), the genset shuts down because of the safety cutoff. This is especially problematic when the tank volume is relatively small (e.g., auxiliary tanks) and the return fuel temperature is not abated.
Gensets with unit-mounted radiators typically are equipped with fuel coolers. They take advantage of the engine-driven radiator fan to reject fuel heat. Gensets with remote radiators typically require an external fuel cooler to reject fuel heat. Another option is to provide a return pump at the auxiliary tank and exchange fuel with the main tank (return hot fuel and replace it with cold fuel) if fuel temperature exceeds a certain setpoint. The return pumps also can be enabled manually to empty the auxiliary tank for maintenance, or via level sensor to prevent overflow conditions.
Fuel transfer pipes
When designing underground site piping, a nonmetallic material, such as reinforced thermosetting resin pipe is preferred due to its inherent corrosion protection. Underground piping is almost universally double-wall, and is comprised of a carrier pipe and a containment pipe. The interstitial space between the pipes is monitored with a leak detection system.
Fuel transfer pipes located above ground in accessible areas typically are single-wall carbon steel. Note that local jurisdictions and insurance carriers may require double-wall piping for aboveground applications as well.
Suction pipes are sized to minimize the friction loss through pipes and fittings. It is recommended that absolute pressure at the pump inlet be kept above 15 in. Hg (mercury column). Discharge pipes at centrifugal pumps are sized based on the pumps’ dynamic head limitations and 8 to 12 ft/second for positive displacement pumps.
Fuel oil maintenance
Fuel oil is made up of organic compounds and will gradually degrade over time due to biological growth, water accumulation, and particulate formation. This degradation, if uncontrolled, could result in clogged filters, or could negatively impact the combustion process in the generator engine. In a worse-case scenario, the degradation could shut down the genset because of a safety cutoff.
Degradation is not a concern for applications where fuel is used on a consistent basis and a fresh stock of fuel is introduced regularly—for example, gensets used for combined heat and power applications. For standby generator applications, fuel usage is minimal due to limited runtimes as a result of periodic testing. For such applications, a fuel maintenance or polishing system can be provided for treating fuel oil periodically (usually on weekly or biweekly basis).
The polishing system cycles fuel from the tank and pumps it through an array of prefilters, final filters, and a water separator. Biocides can be introduced manually or injected automatically by the polishing system. If site or budget constraints do not allow for a permanent, onsite polishing system, an option is to provide pipe connections at the tank and enlist the services of a contractor who uses a mobile or roll-up polishing system for periodic fuel maintenance.
Filling the tank
During the design process, it is important to determine what type of delivery truck the fuel-oil vendor will use. Fuel-oil trucks are either a gravity or a pump type (i.e., equipped with an integral fill pump).
Both types of trucks can accommodate USTs and ASTs located at an elevation lower than the truck. Pump trucks are ideal for filling ASTs at higher elevations. Gravity trucks are optimal for filling USTs. However, when the fuel storage tank is at a higher elevation than the truck, a gravity type truck alone won’t work. For such applications, an option is a remote fill system with an integral fuel transfer pump that can enable gravity trucks to fill ASTs at a higher elevation. Remote fill systems are equipped with gauges and sensors to aid and alert the operator during the delivery process.
Extreme considerations must be accounted for as well because the need for fuel oil could happen in an emergency state. For example, it is common for a fuel oil vendor to promise one type of truck, but then in the event of a city-wide power outage when everyone needs fuel, the truck isn’t available. Extreme situations must be considered during fuel oil system design.
Fuel system controls
Fuel systems typically use UL 508: Standard for Industrial Control Equipment-listed programmable logic controllers (PLCs) to control and monitor transfer pumps, storage tanks, auxiliary tanks, polishing systems, fill systems, fuel inventory, leak detection, and other related subsystems and equipment. They offer communication capabilities, such as BACnet, Modbus, and local operating networks (LonWorks) for integration with the building management system.
For critical applications, such as data centers, the control system typically uses dual independent PLCs and dual power inputs to ensure there are no single points of failure. The fuel system control architecture and sequence of operations should be reviewed in detail during the design phase. The entire scope of work associated with fuel systems (equipment, controls, startup, and training) preferably should be provided by a single vendor who specializes in that field.
Codes and standards
Due to its combustible nature and the detrimental impact on the environment upon a leakage, fuel oil storage and system design is regulated by city, state, and federal authorities. Careful consideration to a multitude of factors is essential during design. For example, the Office of the Illinois State Fire Marshal requires a 10,000-gal AST to be at least 25 ft from the adjoining property line, while the Chicago Building Code requires it be at least 30 ft from the property line. Meeting the worst-case scenario is vital to compliance.
There also are code-mandated requirements related to maximum allowable fuel storage on the property, tank construction, spill containment, location relative to buildings and property lines, fire suppression, high-rise building limitations, and much more. The requirements are specifically stringent for applications involving indoor storage of fuel due to inherent fire hazards. Frequently, the requirements listed in various codes and standards differ significantly. Table 2 indicates key requirements related to a 10,000-gal outdoor AST in Chicago. To avoid surprises at a later stage, it is beneficial to approach the authority having jurisdiction to review the proposed design early in the project—specifically with representatives from the fire department to ensure that all bases are covered.
Considering it all
The successful design of a backup generation system is critical to maintaining business continuity, sustaining critical operations, and life safety in the event of a serious power outage (see "Fuel system design checklist"). Without proper design and ongoing maintenance, fuel oil systems cannot meet the needs of the gensets they serve, and therefore, cannot guarantee the assumed reliability of the facility’s backup power.
Fuel system design checklist
When designing fuel oil systems, remember the following:
- Provide foot valves (to maintain pump prime), anti-siphon valves (to prevent accidental leakage), and fusible link shutoff valves (for fire safety).
- When calculating pump suction lift, assume the worst-case scenario (i.e., a nearly empty tank).
- When calculating friction losses through the fuel oil distribution system, assume the worst-case scenario (i.e., viscosity corresponding to the lowest anticipated fuel temperature).
- If fuel temperature is anticipated to fall below its cloud point, provide a means of heating (tank heaters, space heaters, pipe heat trace, etc.)
- Ensure storage tanks are equipped with adequate ports to accommodate pipe connections, sensors, vents, switches, etc.
Using auxiliary tanks
Consider using auxiliary tanks when:
- The main tanks are located more than 50 ft away from the gensets.
- The main tanks are located more than 12 ft below the gensets.
- The main tanks are located above the gensets.
Fuel oil design cheat sheet
Refer to this cheat sheet for important considerations when designing a fuel oil system:
- No. 2 fuel oil NFPA Classification: Class II
- Genset fuel consumption: approximately 7 gph/100 kW rating
- Atmospheric pressure: 30 in. Hg (mercury column)
- Minimum recommended pressure at external pump inlet: 15 in. wc
- Pressure of 2.6 ft of No. 2 fuel oil: 1 psi.
Runtime requirement for emergency power supply systems, according to NFPA 110: Standard for Emergency and Standby Power Systems include:
- Class 0.083: 0.083 hours = 5 minutes
- Class 0.25: 0.25 hours = 15 minutes
- Class 2: 2 hours
- Class 6: 6 hours
- Class 48: 48 hours
- Class X: other times (application, code, or user dictated).
About the authors
Ben Olejniczak is an experienced mechanical engineer at Environmental Systems Design Inc. He performs data center site assessments and is responsible for the design of mechanical infrastructure and fuel systems for data centers and mission critical facilities.
Saahil Tumber is a senior associate, mission critical facilities, at Environmental Systems Design Inc. He is a lead mechanical engineer responsible for the overall design of mechanical infrastructure including fuel systems for data centers, trading areas, and critical facilities.