Filtration / Polishing is used for emergency generator fuel storage to maintain fuel quality during the long storage life of the fuel. Most fuel system used for transportation, turn over their fuel on a regular basis, weekly or monthly. Generator fuel systems can have fuel storage that is present for many years. The large storage quantities needed for the design generator run times, dwarfs the amount of fuel used in testing, maintenance, and minor power interruptions.
As diesel fuel ages it degrades in 2 ways: particulate formation, and biological growth.
Particulates form as fuel ages, when certain components of the fuel start to come out of solutions and form solids. The particulates will increase the wear on engine components, and so filtering is needed. Engines typically have inlet filter units, however it is recommended that the gross particulate formation be removed at the tank, to extend the life of the engine filters, and to prevent the potential for a filter blockage.
Water enters a fuel tank from either (a) water from off-site fuel storage systems and delivery trucks, that are inadvertently introduced in to the tank, (b) water from condensation of air drawn into the tank as the fuel level is lowered.
Any water introduced into the tank will settle to the bottom of the tank. The fuel water interface at this point is a condition that promotes biological growth. This growth can be problematic especially because it can accelerate. Organic material from the fuel tank will very quickly plug the inlet filters at the engine.
Filtration / Polishing systems include water coalescers to remove water from that tank, and in this way minimize the potential for organic growth. Bio-cide additives to the fuel tank are still recommended.
In cold weather climates there is a concern for maintaining the functionality of critical fuel systems in freezing weather. In general a winter blend of #1 and #2 diesel fuel will lower the cloud point to about -10 degrees F and the pour point to minus 30. However this may be borderline for many users, and for that reason exterior piping is often heat traced and insulated.
The problem with cold weather performance is often not the fuel itself, but any water that may be present in trace amounts in the fuel. This water can accumulate within the cavities of valves and other equipment, so it is important that these devices also be heat traced and insulated, or located within heated enclosures.
High temperatures for generator fuel occur because the engine returns 65-95% of the withdrawn fuel back to the day tank. This return fuel flow is at a temperature that is well over 100 degrees F, without cooling measures, and this can cause the temperature in the tank to rise to dangerous levels. The engines often require relatively cool fuel, usually less than 110 degrees F, as a means of cooling certain engine components. And heated fuel can become a safety concern if it is heated above its flash point temperature.
High temperature is not a common problem because most generators include a fuel cooling radiator to treat the return fuel before it is received by the day tank. The fuel cooling radiator is mounted as a slave to the generator engine coolant radiator, and utilizes the same engine fan.
The fuel cooling radiator on the engine may not be practical where the engine utilizes a remote radiator for its coolant system. In this case several methods of fuel cooling may be used:
(a) a dedicated fuel oil radiator and fan are installed at the day tank for return flow fuel from the engine.
(b) fuel return from the engine is directed to a gravity return flow pipe to a bulk storage tank. Since the engine consumption is only 1 / 3 of the engine fuel pump flow, the fule supply to the day tank should be increased by about 3 times to accommodate the higher flow.
(c) A temperature sensor in the day tank activates a return flow pump to pump fuel back to the bulk storage tank, allowing the day tank to be re-filled with fuel at a lower temperature.
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