Hospitals, data centers and similar mission-critical buildings rely on their facility’s fuel system to keep their central energy plant running.
Due to their sheer size, facility fuel systems can become complex, as many of them now incorporate multiple fuel storage tanks, redundant fuel transfer systems, and multiple fuel day tanks serving large diesel generators. In an environment where fuel is flowing in such large volumes, fuel flow control should be a primary design task.
This article focuses on fuel day tanks, and the important system design features that allow the safe transfer of fuel to and from a day tank.
With an above-grade fuel storage tank serving one or more day tanks, how is excess fuel returned to the source tank?
Understanding that day tanks are vented to atmosphere, it should be clear that an overfilled day tank will eventually spill fuel via its vent opening. The National Fire Protection Association (NFPA) addresses fuel overflow by stating that any fuel tank filled by a pump shall be equipped with an overflow return line (NFPA 37 6.5.4, 220.127.116.11, 18.104.22.168).
Of key importance, NFPA also requires that this overflow return line be routed “back to the source tank, or to a collection system”. When the day tank (freestanding or generator sub-base mounted) is served by an underground storage tank (below grade), the return of overflow fuel may be possible by gravity alone. But, when the source tank is above-ground and portions of the return piping are elevated beyond the height of the day tank, gravity will not be enough for excess fuel to find its way back to the source tank.
How is this solved? Here are two common methods:
Solution #1: If the system involves a single day tank with a single source tank, the simplest solution is to specify that the day tank be equipped with an overflow return pump. This pump is turned on by the day tank control panel in the event of a “critical high” fuel level, and thereby returns excess fuel back to the source tank.
Solution #2: If there are multiple day tanks with a single source tank, an easier and less costly solution might be the specification of an “overflow fuel day tank”, designed to receive overflow from any of the generator day tanks. This “overflow” day tank would be of relative small size, installed adjacent to the generator day tanks, and would be configured with a fuel return pump activated whenever fuel is present. This system would act as a “collection system” as specified by NFPA, and would also allow for controlled return of excess fuel back to the source tank.
For increased reliability in either of the above solutions, the overflow return pump may be specified to be a “duplex” type, meaning two return pumps with one acting as a backup to the other. Also note that any overflow return pump must be sized to overcome the maximum (total) fuel flow rate entering the day tank (or overflow return tank).
A large facility requires multiple day tanks to be filled from multiple source tanks. How do I control which source tank receives potential overflow fuel returning from the day tanks?
This installation requires careful attention to ensure that any potential fuel overflow be returned back to the proper “source tank” (as NFPA requires). For example, consider that day tank #1 requires fuel and its day tank-mounted fill pump begins to draw fuel from a common fuel supply manifold connected to multiple above-ground storage tanks (ASTs). Without any flow controls, greater fuel flow may come from the nearest source tank, or that tank which provides the lowest resistance to fuel flow. Let’s now imagine that day tank #1 goes into an overflow condition and begins returning fuel via the common fuel return manifold (which is connected to all source tanks). Will the return fuel be guaranteed to flow into AST#1, or could it flow into AST#2? What if AST#2 is nearly full?
When the source tanks are not adjacent to each other, a “tank selection panel” might be required. This panel would select and control which fuel tank is used for fuel supply and also for any potential fuel return. The tank selection is accomplished via electrically-operated valves installed in each of the tank’s supply and return connections. Although NFPA states that any return line “shall be free of valves or traps”, there are also references in NFPA that require that fuel piping include necessary valves for proper fuel flow control during normal operation and emergency operation (NFPA-37 6.8.3). It might be argued that this reference allows the use of electrically-operated valves on return lines to control fuel flow and prevent a tank overflow condition. If valves are used, they should be equipped with position indicators (limit switches) to positively identify whether they have acted as directed. Any control panel used in this application should incorporate these feedback signals into a failsafe logic scheme (failure of any valve to actuate as directed should immediately signal the condition and trigger emergency procedures).
When the source tanks are adjacent to each other, and they are equipped with submersible pumps for fuel supply to the day tanks, a possible solution might be to establish an “equalization header” between the tanks. This solution relies on a siphon effect being created by the submersible pumps as they draw fuel from the tanks. If choosing this option, it is advisable that you check with an experienced fuel system installer to ensure that the siphon effect will be maintained in your specific application. The manner in which pipes are run, the distance between tanks, special feature 3-way valves, and the size of the equalization header are all critical items to make this work.
I hope this article has shed some light on the issue of “overflow fuel” as it affects diesel fuel day tanks. To learn about other design considerations for day tanks, I recommend that you also read this article.
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