Generators supply auxiliary electric power during outages of power supply from the local electricity distribution company. This is done by using input mechanical energy to initiate combustion of fuel and converting the energy released to electrical energy. Please read this article for more information on how a generator creates electricity.  If no fuel is available to the generator during power outages then the purpose of the generator is lost. Hence it is essential to store fuel so that such an eventuality does not arise. The fuel is stored in specially designed fuel tanks. Several factors need to be considered while selecting and installing the appropriate generator fuel tank.

Determining the Fuel Tank Capacity

You will first need to determine 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

What level of fuel storage capacity is best suited for you?

In the case of power outages that are infrequent or of short durations, a smaller storage tank would suffice for your fuel requirements. However, you will need to purchase fuel more frequently in smaller lots to refill your tank. While you incur low initial investments (capital costs) in setting up your storage tank and low maintenance costs, the per unit delivery cost of fuel would be higher. 

Large storage tanks are required when the generator is used to support large commercial establishments or where power outages are frequent and last for long periods of time. In this case, you can purchase the fuel less frequently and in larger lots. However, you will incur higher initial expenditure (capital costs) in setting up your storage tank. The maintenance costs will also be high 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 finalizing the storage capacity it is essential to finalize the 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.

Types of Fuel Tanks

Generator fuel tanks are usually of three types –
(1) Sub base tanks
(2) Underground storage tanks
(3) Above ground storage tanks

Sub Base Tanks

If you need to store less than 1,000 gallons of fuel, you would need sub base tanks. As the name suggests, sub base tanks are designed to fit above the ground but below the base of the generator set.

Sub base tanks are rectangular in cross section and are double walled tanks. This helps to prevent spillage of fuel in case of leakage. Both tanks should be constructed using heavy gauge welded steel. Several piping and fittings are attached to the primary tank, the important ones being fuel supply and return, air vent, emergency pressure relief valve and high and low level fuel alarms. The tank fill system has to be designed such that there is no spillage during filling and the inlet valve automatically closes when the tank is 95% full. After installation the primary tank is tested at 5 psig and the secondary tank is tested at 3 psig.

Underground Storage Tanks

If you need to store more than 1,000 gallons of fuel, you can opt for underground storage tanks or above ground storage tanks. Underground storage tanks are more costly to install but have a longer life because they are shielded from the environment. Underground storage tanks can be constructed from fiberglass-reinforced plastic. Such tanks are usually ribbed so as to provide structural strength. Alternately, underground storage tanks can be fabricated from steel but with appropriate cathodic protection against corrosion from ground water. Likewise, piping from the underground storage tank to the generator can be of fiberglass-reinforced plastic or cathodically protected steel.

Leakages and spillages in underground storage tank systems can be costly and difficult to rectify. Such systems must be fitted with overfill and spill prevention equipment and procedures. In the worst-case scenario, the installation of underground storage tanks should be such that spills or leakages of fuel are contained to a limited area. Hence, the underground area is surrounded by concrete floor and walls. After the underground storage tanks are installed within this area, the external region is packed with sand and gravel.

Above Ground Storage Tanks

As the name suggests these tanks are installed above the ground. While these are similar in construction to underground storage tanks, there is a lot of difference in the installation procedures of the two kinds of fuel storage tanks. This is due to the different factors that need to be considered for minimizing the hazards. Above ground storage tanks pose a fire hazard with the risk of fire spreading to other facilities in the vicinity. Therefore these tanks have to be installed at a minimum specified distance from other facilities. In order to contain spills and leakages dikes have to be built around above ground storage tanks. The free volume enclosed by the dikes should generally be 110% of the volume of the tanks. Above ground storage tanks also need to be shielded from weather conditions by suitable protective structures.

Approvals and Codes 

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 approvals. You are required to submit a proposal to the State Fire Marshall. The details that have to be furnished include manufacturing drawings, product data sheets for all components purchased additionally, proposed controls and wiring diagram, proposed piping layout and manufacturers’ 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.