Diesel Fuel Storage

Diesel-fueled generators (gensets) are used for primary power at many electricity generating stations. Furthermore, emergency and standby diesel units are relied upon at many more sites, including coal and nuclear power plants, as well as industrial, commercial, medical, and educational facilities. This means that diesel fuel is stored almost everywhere power is generated.

Diesel Fuel Storage and Supply

According to NFPA 30, storage requirements are based on whether the liquid fuel is “combustible” or “flammable” as assessed by the fuel flash point. The fuel flash point is the lowest temperature at which the fuel will ignite in the presence of an ignition source. NFPA 30 defines combustible liquids as having a flash point equal to or greater than 100F (37.8C) and flammable liquids as having a flash point less than 100F (37.8C).
The flash point of conventional diesel fuel typically ranges between 126F and 204F (52.2C and 95.5C). Therefore, diesel fuel is considered a combustible liquid. It is further classified as Class II if the flash point is less than 140F or Class III if flash point is greater than 140F, depending on the specific fuel.
However, it is important to note that when diesel fuel is blended with ethanol (E-diesel) to reduce emissions, the blended diesel fuel has a low flash point of about 68F (20C). The blended fuel is therefore considered a flammable liquid, requiring management of associated fire and explosion hazards. 

Sizing the Bulk Diesel Storage Tank

The size of a bulk diesel storage tank can depend on a number of factors, including the classification of the emergency power supply system (EPSS) in some applications. The classification is defined in NFPA 110 as the “minimum time in hours, for which the EPSS is designed to operate at its rated load without being refueled or recharged.” For example, a Class 48 EPSS is expected to operate at its rated load for at least 48 hours without refilling the bulk tank. If the rated load consumes 450 liters per hour, the bulk tank should be able to provide 48 hours x 450 liters per hour = 21,600 liters of fuel.
Additionally, NFPA 110 requires the actual size of the bulk tank to be at least 133% of the quantity established by the EPSS class (or the corresponding low-fuel sensor quantity). In addition, the bulk tank should have 5% spare volume above the maximum liquid level, if it is to comply with EN 12285—the European standard.
The bulk fuel storage capacity of 133% provides the ability to test run the emergency diesel for maintenance purposes several times before the tank requires refilling. The tank refilling cycle should be such that the fuel inventory does not drop below the minimum level based on the EPSS classification per NFPA 110.

Bulk Diesel Storage Tank Installation

Bulk tanks can be installed above ground, within a storage tank vault, underground (direct buried), or within a storage tank building.
Above-ground tanks must be provided with emergency relief venting that will release internal pressure, if the tank is exposed to fire. Means for spill control shall also be provided for above-ground tanks.
Tanks listed as “above-ground tanks” can be installed within a vault, and the vault can be either above or below grade. Backfill is not permitted around the tank in a vault and sufficient clearance is required around the tank for inspection and maintenance purposes. Vaults should be provided with a means to admit a fire suppression agent, recover liquid spills from vaults, and sound an alarm in case of vapor/liquid release detection.
Underground tanks and piping require external corrosion protection by means of cathodic protection or use of corrosion-resistant material. The base and fill shall be clean, non-corrosive, compacted sand or compacted gravel.
Storage tank buildings are subject to minimum separation distances from property lines, public ways, and important buildings on the same property. Storage tank buildings and structures shall have at least a two-hour fire resistance rating and be equipped with manual/fixed fire control equipment. Discharge of liquid shall be prevented from public waterways/sewers or adjoining property, and all vents shall terminate outside the building.

Design Standards for Bulk Diesel Storage Tanks 

For diesel gen-sets, the bulk storage tank is generally an atmospheric tank designed and constructed per the American Petroleum Institute (API) Standard 650 Welded Tanks for Oil Storage. Such tanks are permitted to operate from atmospheric pressure to a gauge pressure of 1.0 psi (6.9 kPa) but must be vented to prevent vacuum. Other design standards may be used but the pressure limits should be checked to avoid distortion of the tank shell during service. 

Daily Tank

The day tank is located between the bulk storage tank and the diesel engine. The fuel is delivered from the bulk storage to the day tank by means of the bulk storage deliver pump. The fuel from the day tank is transferred to the diesel engine by the engine-driven fuel transfer pump, which is mounted on the engine assembly.
The day tank is required when the engine-driven fuel pump is unable to draw fuel from the bulk fuel tank due to distance or elevation issues. The day tank is also used to ensure efficient fuel flow to the engine by eliminating any externally induced pressure head generated by the location of the bulk tank or from the bulk storage delivery pump. The day tank also serves as a heat sink to collect the hot, unused fuel returned from the engine via a cooler.
Use of Gear Pumps for Bulk Fuel Transfer to Day Tank. The fuel transfer pump from the bulk tank to the day tank is generally a positive displacement gear pump located above the bulk tank.