Requirements for fuel storage are based on the type of fuel, where it’s being used, and whether the fuel is “combustible” or “flammable”. Storing fuel may sound simple, however there are detailed storage requirements which is why it is important to follow best practices and install a safe and reliable fuel storage system. It is important that these requirements are understood when operating a fuel storage system for diesel generators.
A diesel-fueled generator (also known as a genset) is used at many facilities as a source or primary power. They’re also relied upon for emergency back-up power in other critical industries such as coal and nuclear power plants, industrial, commercial, healthcare, and educational facilities. This means that anywhere there is a risk or power supply failure, there will also be the need for a secondary/back-up power supply and subsequently fuel storage.
Several factors must be considered when determining the right size for your bulk diesel storage tank, including the classification of the emergency power supply system (EPSS) in some applications. The EPSS classification is the minimum time in the hours for which the system is designed to operate at its rated load, without being refueled. An example of this is a Class 48 emergency power supply system, would be expected to operate for at least 48 hours before the bulk fuel tank requires refueling. This means that if the generator consumes 450L an hour, the diesel tank will need to be able to provide 48 hours x 450L per hour = 21,600L of fuel plus ullage (ullage is the unusable tank volume; the volume by which a tank falls short of being full).
Under the AS1940 Standard for the storage of fuel, there are three approved methods when it comes to storing diesel to feed a generator. Burying the tank underground, installing the tank within a fire-rated room, or installing a tank above the ground in a fire-proof bund.
This method of fuel storage is a great space saver when the tank needs to be installed in a tight location, however there is potential complexity in the installation process, as well as the possibility of hidden services required. The completed solution will need an ongoing monitoring and maintenance program. Inspection and repairs on a tank of this sort can be difficult as leaks are not easily visible. When burying a tank underground, other factors that need to be considered include access pits for servicing, proximity to property boundaries, the water table level, building foundations & corrosion protection. If a longer run-time or higher load rate was required in the future, obviously more capacity of fuel storage would be required. Relocation is difficult, and the underground tank isn’t easily extracted to be used elsewhere, so a second fuel tank would be required.
A fire-rated room is essentially a room that is designed to protect the tank inside it from fire and other major hazards. In the construction phase of this method the walls, roof and all penetrations in and out of the room will need to have an FRL (Fire Resistance Level) of 240/240/240. Fire-rated ventilation/dampers will also need to be installed. A removable roof for tank replacement will need to be designed and installed, as well as a fire-rated door that is designed to stay closed. There are specific separation distances from other tanks that need to be kept to, along with services, foundations of the building and boundaries or other properties to be considered in keeping a safe distance from. In addition, the whole room will require water-tight bunding to be able to contain the volume of the largest tank within the room.
When installing your tank in a fire-proof room, it’s important to ensure that the concrete products used to construct the room have been certified to the FRL. The FRL is the ability of a building element to withstand a fire under test conditions for a certain period and consists of the three criteria being structural adequacy, integrity, and insulation. This means that if a building element was exposed to a standard fire test, it would not be expected to fail for 240 minutes (4 hours) in each of the following three criteria.
– Structural Adequacy: For a period of 240 minutes (4 hours) the product being tested was able to support a load while subject to fire conditions.
– Integrity: The product did not disintegrate or crack so as to see the flames of the fire for gases to escape.
– Insulation: The product being tested did not transfer an average temperature reading that exceeds 180°C above the ambient temperature.
This method of storing diesel for your generator is fast becoming the simplest and most cost-effective way to store fuel. With a fire-rated tank, it is important that the structural adequacy, integrity and insulation are demonstrated on the complete tank. Where the tank is subject to a recognised US standard test, only a 4-hour fire-rated would be deemed equivalent to an FRL of 240/240/240. An example of an appropriate test is the 4-hour liquid hydrocarbon pool fire test to an appropriate SwRI (Southwest Research Institute).
According to the Standard AS1940:2017 written specifically for ‘The Storage and Handling of Flammable and Combustible Liquids’ (Recognised across Australia, New Zealand, and other Pacific Islands), a 4-hour fire-rated tank that complied with AS1940 in regard to the SwRI-93-01 & 95-03 Standards is permitted. Clause 5.9.2(a) and (b) are, by virtue of clause 5.9.4(c) “regarded as complying with the requirements for tanks in chambers” (see clause 5.13.1 and 5.13.2). This means that accordingly, the 4-hour fire-rated tank may store flammable or combustible liquids inside or close to a regular building.
It is important that bulk diesel storage tanks are equipped with a vent to prevent vacuum formation or overpressure in the tank while it is being filled, or due to changes in the atmosphere. The vent pipe needs to discharge safely 4.0m above ground level, and at least 1.5m away from building openings and 4.5m away from powered ventilation air intake devices.
The above-ground tank venting requirements come under the NZ Health & Safety at Work (Hazardous Substances) Regulations 2017, AS1692:2006, steel tanks for flammable and combustible liquids, and AS1940:2017, The Storage and Handling of Flammable and Combustible Liquids. There are two different types of venting on an above-ground fuel storage tank, as detailed below.
Free to air venting
Where the vapor space is in contact with the atmosphere without any intervening valves or other devices, so that the pressure is substantially that or the surrounding atmosphere. The size of any free vent must be such that the pressure resulting from filling, emptying, or atmospheric temperature change will not cause the maximum allowance stress of the tank to be exceeded, nor the tank to collapse.
Emergency venting
Emergency venting is used to supplement the free to air vent where excessive pressure build-up in emergency conditions such as fire (eg. a fire-rated tank). An above-ground stationary tank used to store liquids with a flammable classification will require emergency venting to comply with section 5.4 of AS1940:2017. It is important for emergency venting to be provided for an above-ground tank containing flammable liquid or which, though it contains a combustible liquid, is in the same compound as a tank containing flammable liquid. The capacity of an emergency vent needs to be in accordance with API Standard 2000 or Appendix H of AS1940.
The day tank for your generator will be located between the bulk diesel storage tank and the diesel engine. The fuel is delivered from the diesel tank to the day tank through a bulk storage delivery pump. The fuel from the day tank is then transferred to the generator engine with a transfer pump, which is mounted on the engine assembly.
The diesel generator will required a day tank when the engine-driven fuel pump is unable to take fuel from the diesel tank due to a large distance or increased/decreased elevation. The day tank will also ensure there is an efficient fuel flow to the engine by eliminating any externally induced pressure head due to the location of the diesel tank or from the delivery pump.
In summary, this article covers the best practices when it comes to storing diesel to feed a generator, and will guide you towards deciding on the correct fuel storage solution for your application. Call Fuelchief today if you’d like to know more about the best diesel storage solution for you.
This is the 4-hour fire-rated fuel storage solution...
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