How to Prevent a Fuel Overflow Condition in a Fuel Day Tank.

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.

Design Issue:
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,,

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).

Design issue:
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?Fuel Flow Control

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?

Solution #1:
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).

Solution #2:
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.

If you have questions or a comment on this subject, please use the “reply” option below!

14 thoughts on “How to Prevent a Fuel Overflow Condition in a Fuel Day Tank.”

  1. Thank you for this very informative article. I have a question related to daytank overflow:
    We have two, existing gensets with non-vented daytanks located on the 4th floor of a building served by a bulk fuel tank and pump located in the basement, complete with an overflow/return line back to the bulk tank. A new genset and daytank is to be located on the roof above level 4 and will be served by the bulk tank’s supply/return piping located at level 4.
    Question: Since the new daytank is planned to have the typical normal and emergency venting that will terminate above the roof of the enclosure, would it be overkill to provide some type of fuel sensor in the normal vent piping from the daytank wired to shut off the bulk supply upon sensing fuel in the vent piping even though the new daytank’s overflow will be piped to the existing overflow piping?

    • You’re welcome, and no, I don’t think it is overkill. I would provide some means for shutting off the fuel supply to this day tank in the event of an overfill. You probably already have planned for a high level switch to close the inlet solenoid valve. Redundant high level switches and/or a fuel sensor in the vent line could provide additional reliablity. As a general rule, you should configure any of these “critical high level” signals to trigger the closing of a solenoid valve on the day tank’s inlet, and to also remove the “run” signal from the fuel supply pump. If the generators on the 4th floor need to continue running while the roof-mounted generator is in a “critical high level” condition, be sure to wire the high level alarm in a way that does not prevent the remaining generators from keeping the fuel pump running. If you are concerned that a solenoid valve is not sufficient, consider adding a second one, in series. One N/C valve would be energized to open when the day tank needs fuel, de-energized to close when the day tank is full, and a N/O valve that would only close on a “critical high” level. I hope this helps!

      • Yes, it helps tremendously – thanks! Excellent suggestions regarding redundancy methods.
        One additional item for this installation regards the short run of exposed fuel piping from the roof penetration to the new genset enclosure (since it is not physically possible to locate the new genset directly above). We plan on routing it within containment piping from the level 4 space to the inside of the enclosure. Is the containment piping required to be sealed at one or both ends or would it be allowed to act as a basic “sleeve” between the two areas where the exposed piping is acceptable?

        • From a national code perspective, above-ground piping usually consists of single-wall black iron pipe, as long as it can be visually monitored, and it is extended over an impervious surface. If you decide to use secondary containment, I would still allow for inspection (one end open). Whenever possible, check with your local AHJ for specific requirements based on your project’s location.

  2. Thank you for your useful article

    We have vented day tanks for Prim Power Plant generators. There is fuel fill line to fill the day tank, and fuel supply line and fuel return line for generator. The fuel fill line have solenoid valve to close the line at 90% of the rated tank capacity with over fill alarm and level switch indicator but doesn’t have fuel return line to main/bulk storage. Do we need fuel return line to bulk storage for day tanks?

    • Thank you for your comment. You should consider what would happen if the solenoid valve failed to close when the tank reaches the 90% full level. Yes, I would recommend a fuel return line back to the main/bulk storage tank. Also, is the return of excess fuel possible by gravity alone? If not, you should also consider installing a “return pump” to return any excess fuel back to the main/bulk storage tank.

  3. I have an emergency generator & diesel driven fire pump inside a new building. My emergency generator is located on a upper floor and no problem with the day tank overflow because drain by gravity. Of course we are specifying all the alarmas, etc. The Fire Pump is diferent. Come with it`s own diesel tank, is located in the third basement. I have a main 2000 gallons onderground diesel storage tanks complete with the transfer pump & cleanig system. I would like to use the the transfer pumps also to sypply diesel to the fire pump`s tank. That means I will have to install an overflow pump. It`s this arrangement normal or do you suggest keep the fire pump arrangement as per manufacturer`s supply? What i would not like to see is people moving diesel tanks by car to the third basement or walking by the stairs.

    Thanks for your help


    • Thanks for your post. If pumping fuel from the 2000 gal tank to the fire-pump (FP) tank, yes, you should provide a pump at the FP tank to return any excess fuel to the main tank. This is a common practice. The FP’s fuel tank will need to be equipped with a level controller to automatically start/stop the return pump. This level controller relies on a fuel level sensor that will need to be installed in the FP’s tank (make sure that you have a port available for this sensor). Also, be sure to size the return pump to have sufficient capacity to overcome the supply pump’s flow rate.

  4. I really appreciate the information you provide on this site…really great stuff. Our company specializes in fueling related compliance here in California. We are in the middle of a small (250gal) aboveground tank installation feeding a 55gal day tank that supplies fuel to an emergency generator.

    My question relates to the size of the supply & return piping from the AST to the day tank. The tanks were custom built for this particular install and the manufacturer has designed the tanks with 1/2″ supply AND return inputs on both the AST and the day tank. I had always been under the assumption that the return piping should be one size larger than the supply. Now, this day tank has an electric return pump to push fuel back to the AST in the event of day tank overfill (not a gravity return). Would this pump allow for the use of 1/2″ return piping, or should it be 3/4″ regardless of this pump? Are there specific codes that would require larger size return piping? I’m concerned about installing this system with both 1/2″ supply and 1/2″ return lines. Any information you can provide will be very helpful.

    • Thank you for your feedback and also for the great question. From a national code point of view, NFPA-37 (Ch. requires that the capacity of the day tank’s overflow (return) line exceed the delivery capacity of the supply line. The size of the return line (and the supply line for that matter) should be calculated based on the expected max flow rate and with consideration to the pipe system friction losses. Here is a link to a handy calculator for pipe friction losses. FYI, viscosity of diesel fuel is 2.6 cP, specific gravity is 0.89. Good luck with your installation!

  5. We have a fuel storage tank of capacity 10000 liters which are connected through pipes to the base tank of Gen Sets. The valves are operated manually by the operators while re-filling the Gen Set’s base tanks. It happened few times that the operator turned ON the valve and then forgot to close it resulting in over flowing of the Diesel. Please let me know how it can be controlled.

    Irfan Khan

  6. Hello David,
    … I have an AST tank that feeds and 300 gal day tank on an emergency generator. We have to replace the fuel supply and return piping between them. Originally they did not installed an overflow fuel return pump on the day tank (I don’t know what makes the fuel to go back to the AST, as the day tank elevation is below the AST). Do you have any recommendations for a fuel return pump that can be installed on the day tank? The fuel supply pump is 10 GPM at 25 feet of head. I have looked online but I was not able to fine a pump that can be installed on a day tank that is approximately 20″ deep.
    Any input you can provide would be highly appreciated.

    • Thank you for your message Cris. I would recommend a pre-packaged 15GPM return pump similar to this one. If the installation does not currently have the necessary controls to start/stop a new return pump, the return pump may be specified to include a fuel level controller. If there are space restrictions or you need more detailed help with specifications/selection of a pump package, please see this Basic Design Development package.


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