Designing Generator Fuel Systems (2)

Per "Designing Generator Fuel Systems (1)", we have discussed (1) Runtime Criteria; and (2) Fuel Storage. We are going to consider:-


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:

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 and suitable when pressure requirements exceed 40 psi.

Submersible pumps: Used for high-flow, low-pressure applications. It require adequate clearance above the fuel tank for accessibility and maintenance, even though the majority of the pump assembly is within the tank.

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.


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 12 degree Celsius rise in fuel temperature above 37 degree Celsius, 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 60 - 65 degree Celsius), 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 reduced.

This basic fuel oil system flow schematic reveals the main fuel storage and auxiliary tanks.


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.


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.

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).


Continue Reading: Designing Generator Fuel Systems (3)



About us

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE) UK. The team exists to provide an interactive environment and opportunities for members of ICT industry and facilities' engineers to exchange professional views and experience.

SMA connects IT, Facilities and Design. For the other design considerations, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection,
(8) Raised Floor,
(9) Code & Standards,
(10) Transformers and Harmonic Distortion,
(11) Multi-mode UPS Systems,

(12) Electrical Rooms,

(13) Generator Systems,
(14) Generator Fuel Systems, etc.

All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.

Designing Generator Fuel Systems (1)

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.

There are 9 Key Considerations when designing fuel oil systems for gensets:-

(1) Runtime Criteria
(2) Fuel Storage
(3) Fuel Pumping
(4) Fuel Cooling
(5) Fuel Piping
(6) Fuel Maintenance
(7) Fuel Filling
(8) System Controls
(9) 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.


Runtime Criteria


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.

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.


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. Both UST and AST have different internation codes specifies the requirements for protected tanks (fire, impact and and associated corrosion 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. However, 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. They 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.


Continue Reading: Designing Generator Fuel Systems (2)



About us

Strategic Media Asia (SMA) is one of the approved CPD course providers of the Chartered Institution of Building Services Engineers (CIBSE) UK. The team exists to provide an interactive environment and opportunities for members of ICT industry and facilities' engineers to exchange professional views and experience.

SMA connects IT, Facilities and Design. For the other design considerations, please visit 

(1) Site Selection,
(2) Space Planning,
(3) Cooling,
(4) Redundancy,
(5) Fire Suppression,
(6) Meet Me Rooms,
(7) UPS Selection,
(8) Raised Floor,
(9) Code & Standards,
(10) Transformers and Harmonic Distortion,
(11) Multi-mode UPS Systems,

(12) Electrical Rooms,

(13) Generator Systems,
(14) Generator Fuel Systems, etc.

All topics focus on key components and provide technical advice and recommendations for designing a data center and critical facilities.