Fuel delivery and fuel quality are quickly becoming two of the most discussed issues among critical facility operators. 
This article will focus on fuel delivery, and one of its main components: the fuel day tank. The day tank is the most immediate source of fuel to the engine, and its design can be responsible for a well-running emergency power system, or one that fails to run during an extended outage.
Following are my Top 12 Recommendations for Fuel Day Tank Design:
- Insist not only in UL142-compliant day tanks but also in UL508-compliant day tank controls. The control wiring should be prewired at the factory, with the exception of alarm outputs that get terminated in the field. Any major field wiring of a day tank control panel should be avoided.
- Size the day tank’s fuel capacity to allow for a minimum 2 hours of engine run-time (at full load), without the need to refill. Consult local authorities for possible requirements for larger capacities, and also take into account the negative effects of fuel recirculation when sizing the day tank.
- The day tank’s fill pump should be sized to three-times the full-load fuel consumption of the engine (or engines, if the day tank will be feeding multiple engines). You do not want the fill pump running continuously, as it tries to keep up with the engine’s fuel demand.
- If the day tank’s overflow piping is routed in a way that prevents overflow fuel from returning to the source tank via gravity, NFPA-37 requires that you specify either a return pump, or a containment area for any potential overflow.
- The return pump, if applicable, should have sufficient capacity to overcome the incoming fill flow rate.
- Specify a fuel flow control solenoid valve on the day tank inlet. The solenoid valve should have means for emergency bypass, and an upstream strainer to filter any impurities.
- The day tanks should have double wall construction with a primary containment leak detection sensor. This sensor is interconnected to the fill pump to stop its operation in the event of a leak in the main containment.
- Always specify that the appropriate pressure relief vent caps be included with the day tank. These allow for emergency venting should the normal atmospheric vent become obstructed. In interior installations, the working vent must be piped to the exterior.
- Some local authorities having jurisdiction (AHJs) require a DC-powered solenoid valve between the day tank and the engine. It is to be powered by the engine batteries. This requirement is often missed and only discovered when the local inspector identifies during a field inspection!
- Most diesel engines draw more fuel than is required for combustion. The excess fuel is typically returned to the day tank. Depending on the rate of return fuel, you may need to consider routing it to the main tank, or you may need to design your system with a larger day tank or a fuel cooler to avoid heating of the fresh fuel supply. Heated fuel will diminish the engine’s horsepower rating, and may lead to engine shutdown if the building loads exceed the de-rated power capability of the engine.
- For prime-duty or mission-critical applications, increase reliability by specifying a duplex fuel pump set with cast iron pump housings and steel rotors. Duplex pumps allow for lead/lag or simultaneous operation at the operator’s discretion. The heavier duty cast-iron pump will last longer and produce less operating noise.
- And finally, specify these local and remote alarms for the day tank controls:
- low fuel level to alert operators of an impending “out of fuel” condition.
- high fuel level to alert of an overfilling day tank.
- critical high fuel level alarm, pump shutdown and remote annunciation output contact.
- critical low level alarm, engine shutdown (prevents loss of fuel prime) and remote annunciation output contact.
- tank leak detection alarm and remote annunciation output contact.
Now that you have read these recommendations, could your standard design specifications use a tune-up? Here is a recommended fuel day tank engineering specification to get you started.
Do you have any questions or thoughts that you would like to add to this article? If so, please post them below. I look forward to reading your comments!