In basic terms a standby generator will supply auxiliary electric power during an outage in the event of a Utility power supply from the local electricity distribution company.

This is done by converting mechanical energy initiated by the combustion of fuel and converting the energy released to electrical energy. If there is no fuel available to the generator during power outages then the purpose of the generator is lost so it is essential to store fuel so that such an eventuality cannot arise with the fuel stored in specially designed fuel tanks. Several factors need to be considered when selecting and installing the appropriate generator fuel tank.

Determining the Fuel Tank Capacity

You should first determine how much fuel you need to store by first estimating the following three parameters

1. Emergfuel tanksency Stock: How much fuel do you require to allow for delays in delivery or excessive consumption?
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 do 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 there are low initial investments in setting up your storage tank and low maintenance costs the unit delivery cost of fuel may 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 quantity. However you will incur a higher initial expenditure (capital costs) in setting up your storage tank. The maintenance costs may also 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 finalise 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 maybe some sort of performance penalty for default/delays should be incorporated in any contract.

Types of Fuel Tanks

Generator fuel tanks are usually of three types –

1. Base Tank
2. Underground storage tank
3. Above ground storage tank

Base Tanks

If you should need to store less than 1,000 liters of fuel then a base tank should suffice. As the name suggests, base tanks are designed to fit above the ground but below the base of the generator set. Base tanks are normally rectangular in cross section and should be of double walled construction using heavy gauge welded steel to help prevent spillage of fuel in the event of leakage. Any piping and fittings should be attached directly to the base tank with 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 should be designed such that there is no spillage during filling and preferable have an inlet overfill prevention valve that automatically closes when the tank is 95% full. Filler necks should be positioned away from the exhaust side of the engine to minimise risk during refueling.

Underground Storage Tanks

Should you need to store more than 1,000 liters of fuel, you can use underground 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 are usually ribbed so as to provide structural strength and 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 double skinned 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 an underground storage tank should be such that any spills or leakages of fuel are contained to within a limited area and are normally surrounded by concrete floor and walls. Usually once the underground storage tank is installed within this area, the external region is packed with sand and gravel.

Above Ground Storage Tanks

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