How Much Fuel Do I Store For My Diesel Backup Generator?

First, you need to work out how much fuel you need to store as this will determine the capacity of your fuel tank. Calculate the minimum storage capacity by first estimating the following three parameters,

1. Emergency Stock: How much fuel would you require to allow for delays in supplies or excessive consumption needs?
2. Lead-time For Supply: What is the lead-time required to purchase fuel from the vendor to the generator site?
3. Lead-time Stock: How much fuel would you require to operate your generator during the lead-time?

Based on the above three parameters, the minimum storage requirement is determined as: Minimum storage capacity = Emergency Stock – Lead-time Stock

In areas where power outages are either infrequent or of short duration, you could get away with a smaller tank for your fuel requirements. However, it is then likely you will need to purchase fuel more frequently and in smaller lots to refill your tank. While your initial investments (capital costs) would be low in setting up your tank and with low maintenance costs, the per litre delivery cost of fuel would be higher.

Larger tanks are required when the generator is used to support large commercial establishments as described at the beginning of this article, or where power supply is for critical infrastructure or where power supply is not guaranteed. In this case, you would purchase the fuel less often and in larger quantities. However, you will incur higher initial expenditure (capital costs) in setting up your storage tank and maintenance costs will be higher in the long run. On the other hand, the per unit delivery cost of fuel is reduced since you can order large quantities of fuel to be delivered in one go. However, you will also need to account for the hidden costs arising from the hazards of storing a larger quantity of fuel.

Before finalising the storage capacity, it is essential to investigate your options of a fuel supply contract for the supply of fuel with the vendor. The vendor’s capacity to supply the required quantity at the required frequency should be assessed and penalties for default should be incorporated in the contract.

Generator fuel tanks are usually of three types:

1. Generator base tanks
2. Underground storage tanks
3. Above ground storage tanks

Generator Base Tanks

If you need to store a smaller amount of fuel, you could utilise what is commonly known as a generator base tank. Generator base tanks are designed to git between the ground and below the base of the generator set, and are specifically designed to support the weight of the generator. Base tanks are rectangular in cross section and are single or double walled, depending on the compliance requirements in your country. This helps to prevent spillage of fuel in case of leakage. Both tanks should be designed to the appropriate standards such as AS1940:2017 and AS1692:2006.

Underground Storage Tanks

If you need to store a larger quantity of fuel, you can opt for underground storage tanks or above ground storage tanks. While this can be an excellent way of saving space in a tight location, it does come with some implications that need to be considered, including the cost and difficult installation process that can cause delays during the construction process. The completed solution will need an ongoing monitoring program and may be difficult to inspect and repair. Leaks aren’t easily visible, and are more expensive to remove at ‘end-of-life’.

The costs associated with installing underground tanks can be higher than those for setting up an above ground tank and require expensive excavation work, including a variety of permits, land use consents and specialised equipment for the installation. Other factors to consider if burying a tank within a building include access pits for servicing, proximity to property boundaries and building foundations, and corrosion protection.

Aboveground Storage Tanks

As the name suggests, these tanks are installed above the ground and come in either round or rectangular shapes to suit your application. Aboveground tanks are either ‘fire-rated’ or ‘non-fire-rated’. We’ll quickly explain the difference and what the advantages are with each style.

Fire-rated/Protected Tanks

These tanks carry a certification to a certain fire rating, either 2 hours or 4 hours, depending on which fire test was carried out on them. They are self-contained – that is, they are double skinned tanks where the outer skin completely envelopes the inner skin or tank. The tank is constructed with a cement based thermal barrier between the two skins to protect the contents in case of fire or any other impact. This thermal barrier is also porous to be able to operate as a ;self-bunded’ tank. Any potential leak of the inner tank is fully captured by the outer skin.

The advantages of using a 4-hour fire-rated tank are:

• the ability to position tanks closer to buildings, people and the public. This is because it enjoys half the separation required by regulations.
• it saves space, time and construction cost by removing the requirement to fire-rate the building itself.
• superior safety and compliance over a standard tank when storing flammable fuels.
• a valuable and secure asset that can be relocated easily.

Non fire-rated tanks:

These are typically double skin, ‘self-contained’ tanks for storage of fuels. The outer tank is required to be at least 110% capacity of the inner tank to meet regulations. They are not normally impact protected neither carry any fire rating. Therefore they are less flexible when it comes to positioning them. Also, some countries are not certified for storing flammable liquids, only combustible.

The advantages of using a non fire-rated tank:

• no requirement to build a separate open bund (as is the case with single skin tanks)
• they’re cost effective and relocatable

Generator fuel tanks and the accompanying piping systems need pre-approval before installation. This is a requirement not only for industrial undertakings, but also for fuel tanks used for domestic or commercial use. Only tanks of very small capacities may be exempt from approval. You are required to submit a proposal to the State Fire Marshall. These details would include manufacturing drawings, product data sheets for all components purchased additionally, proposed piping layout and manufactures’ installation instructions.

Several codes need to be followed during the manufacturing and installation of generator fuel tanks. The primary code in the US is the National Fire Protection Association Codes and Standards (NFPA). The relevant sections of the NFPA for generator fuel tanks are NFPA30 and NFPA37. The Code of Federal Regulations Title 40 Section 112 deals with spill prevention from generator fuel tanks.