Runtime criteria

One of the first steps of designing a gensets fuel oil system is to define runtime specifications in case of power failure (see “Runtime requirements”). Sometimes determined by a combination of relevant codes and owner specifications, the runtime — or how long the genset has to work without refueling during an emergency event — will set the standard for designing and running the oil. Life safety gensets, for example, are usually expected to support emergency loads for a duration of 2 hours following a power failure. Sensitive buildings, such as data centers, are usually required to bear the load for 24 hours or longer, depending on criteria for site resiliency.

Because runtime criteria have a direct bearing on the on-site capacity required for fuel storage, this consideration is critical to the first investigation. Remember that the fuel consumption data are readily available from the manufacturers for gensets at different loads.

Consider the following rule of thumb for preliminary sizing of the fuel storage tanks: 7 gal/hour of No. 2 fuel oil per 100 kW of generator rating is needed (see “Fuel oil design cheat sheet”). Multiplied by the target runtime (hours) the fuel consumption rate (gph) specifies the available fuel requirement (gallons). It is important to remember that depending on the form and design of the tank, usually, about 80 percent to 85 percent of the tank capacity is available. During operation the tank can not be drained completely nor can it be filled fully because it needs head space to handle fuel expansion and avoid overflow.