Day tanks are fuel containment systems designed to be installed close to an engine to provide a reliable supply of diesel fuel. They are often required when the engine mounted fuel pump cannot draw fuel from a remotely mounted fuel tank due to distance or lift issues. For safety and reliability, these fuel systems are configured as suction, which means that the day tank draws fuel from the main tank and the engine draws fuel from the day tank.
There are key differences in the configuration of these systems when used with above ground storage tanks (AST) or underground storage tanks (UST). There are also important considerations that relate to tank accessories.
Integration with AST – In this type of application, attention should be paid to the elevation of the daytime tank relative to the main AST. Due to the possibility of siphoning between the day tank and the AST, certain components must be installed in the fuel line. Usually a solenoid valve and filter are installed at the tank pump inlet. This is an electrically operated valve that will normally be closed. It will only open when the tank level is detected to be low, at which point the solenoid valve opens and the supply pump is energized. When the tank reaches a full level, the solenoid valve and pump are deactivated, which stops the flow of fuel and closes the fuel inlet to the tank. Additional components that are recommended in these systems are manually operated ball valves and fusible link valves that automatically close in the event of fire. These should be installed at the inlet and outlet of the tank to allow isolation of the equipment for emergency or regular scheduled maintenance purposes.
A well-designed system requires that the day tank be able to return any excess fuel to the AST. Failure to stop a fill pump or close a solenoid valve could result in continuous filling of the tank. Without the means to return excess fuel, a spill will be inevitable. The overflow return pump is generally tank mounted and should be sized to exceed the supply pump. Its outlet must be equipped with a check valve to prevent feedback when the return pump stops.
Some applications will have the AST in a remote location that is too far away for the day tank pump to reliably draw fuel. In these cases, the day tank is equipped with an overflow pump and a separate supply pump is mounted near the main tank to draw fuel from the AST and push it into the day tank. The remote pump would continue to interact with the tank level controller to receive start / stop signals.
Integration with UST – In this type of application, consideration should be given to the possibility of the day tank supply pump losing its prime. For this reason, a foot valve is usually installed on the main tank, and a hand pump, check valve, and solenoid valve are installed on the day tank. The hand pump enables emergency filling and also provides the means to restore fuel prime in the line feeding the motor-driven pump. The check valve and solenoid valve act to restrict any backflow to the main UST. In TAS applications, the overflow (return) fuel is typically piped from the day tank to the TAS and gravity is sufficient to prevent overflowing the day tank. As with AST designs, if the day tank is too far from the UST, the supply pump should be located next to the UST.
General configuration recommendations – The following are additional recommendations to consider for a properly configured system:
1. Insist not only on day tanks that meet UL142, but also controls that meet UL508.
2. Size the tank to allow a minimum of 2 hours of engine run time without refilling.
3. The supply pump should be sized to triple the full load fuel consumption of the engine (or engines if the tank powers multiple engines). In the example above (consumption of 120 GPH or 2 GPM), the pump must be a minimum size of 6 GPM.
4. The return pump (if applicable) must be sized that exceeds the capacity of the supply pump.
5. Size all accessories (such as ball valves, solenoid valves, etc.) no smaller than the fuel lines that connect to the accessories.
6. Specify double-walled construction with rupture cell leak detection sensor. This sensor is interconnected to the supply pump to stop its operation in the event of a main containment leak. The controller should also indicate the condition with a visual alarm.
7. Always specify that the proper pressure relief vent caps are included with the tank. These allow emergency ventilation in case normal atmospheric ventilation is obstructed. In indoor installations, the working ventilation must be connected to the outside.
8. Review Local Authorities Having Jurisdiction (AHJ) to understand if controls and accessories should draw power from motor batteries (ie 24 VDC) or if a 120 VAC emergency accessory circuit would be acceptable.
9. Most diesel engines consume more fuel than is necessary for combustion. Excess fuel is generally returned to the tank daily. Depending on the fuel return rate, you may need to consider sending it to the main tank, or you may need to design your system with a larger day tank or fuel cooler to avoid heating up the fresh fuel supply. Heated fuel will reduce engine power rating and can cause engine shutdown if building loads exceed engine power capacity.
10. In a primary service application, increase system reliability by specifying a duplex fuel supply pump assembly with cast iron pump housings and steel rotors. Duplex pumps allow lead / lag or simultaneous operation at the operator’s discretion. The heavy duty cast iron pump will last longer and produce less operating noise.
11. Insist on the following local and remote alarms from your provider:
a) Low level alarm output contact and remote annunciation.
b) high level alarm output contact and remote annunciation.
c) high critical level alarm, pump shutdown and remote annunciation output contact.
d) Critical low level alarm, engine shutdown (prevents loss of fuel prime) and remote annunciation output contact.
e) Tank leak detection alarm and remote announcement output contact.
The above guidelines can assist in selecting the appropriate product for your application. However, we recommend that you contact your supplier to review any options or accessories that may be necessary to ensure safe and reliable operation of a specific fuel system application. For additional resources related to day tanks and related fuel systems, you can visit http://www.hurtado.cc/resources.htm.
