Large fuel systems use multiple tanks for several reasons: (a) to provide redundancy against a single tank failure, (b) to provide total fuel volumes in excess of maximum commercially available tank sizes, (c) to provide total required fuel volumes where space limitations set the single tank maximum volume, or (d) to provide total required fuel volumes where regulatory limitations set the single tank maximum volume.
Tank selection controls are required for several purposes: (a) to fill multiple tanks safely from a common fill station, (b) to allow supply of generator fuel from one tank, and safely return fuel to that same tank, and (c) to allow filtration of one tank with return of fuel to that same tank
Tank Fill Control:
The fill station controller monitor high level sensors in the tank at 85%, 90% and 95% fill. Inlet pipe actuated valves are controlled for 2 functions: (a) shutoff on high level, and (b) open valve to allow filling of a single tank on a multi-tank system and confirm closed state of other tank valves. Inlet valve positions are monitored by limit switch inputs to the controller.
Generator Fuel Supply:
Facility operators want to manually select the lead supply tank in the system to manage fuel levels and even the operating time for equipment. One tank may be operated until its level is reduced to allow for receipt of a full truckload of fuel to maximize purchasing efficiency.
The control system will automatically switch to a tank for fuel supply if (a) the primary supply tank is in low level, or (b) the primary tank dedicated pumps or valves are in a fail condition.
When a tank with submersible pumps is selected, its pumps are activated when the day tanks require fuel. When suction pumps are used, the controller opens an actuated valve in the suction pipe, and confirms that the valves in the other tank suction piping is closed.
A critical aspect of the control is that the tank selected for fuel supply is required to receive any return flow or overflow fuel. This occurs by opening the return flow pipe actuated valve (or directing a return flow 3-way valve) for the selected tank and confirming the closed position of the return flow valve at other tanks.
Filtration Functions:
Filtration functions are the same as the characteristics of selecting a tank for generator fuel supply. An important difference is the risk associated with failure to properly control return flow valve positions. Since filter transfer pumps operate for relatively long durations – and typically automatically, a small transfer rate between tanks can proceed un-noticed until a tank overfill condition occurs.
For this reason it is critical that tank high level sensors are monitored and disable the filtration system on a high level condition.
Note that actuated ball valves are recommended for tank selection systems for generator fuel supply and filtration systems. Actuated butterfly valves or ball valves are used for tank fill systems. Solenoid valves are not recommended because of their tendency to leak fuel through the valve when the seat is damaged, deteriorates with age, or sealing is compromised by particulates.
