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Storage Tanks should be constructed in accordance with:
Welded Tanks for Oil Storage – Design and Construction of Large, Welded, Low-Pressure Storage Tanks (horizontal tanks)
In areas where there is high humidity, large diurnal temperature variation (high condensation) or marine supply locations, tank lining to reduce particulate (rust) is highly recommended.
All tankage should be designed with a positive slope to a sump drain point to allow the removal of dirt and water contaminants on a continuous basis.
An example of the preferred type of design is shown for a horizontal tank in the diagram below.
Details of a dewatering system for an underground horizontal tank
Inlet lines should ideally be angled to create movement in the tank that encourages the migration of settled dirt and water to the tank dewatering point where it can be removed. Some indications of preferred designs are shown below for horizontal tanks.
Ideally, the inlet and outlet pipework should be well separated. Ideally,a floating suction design should be employed to assure tank bottom contamination is not transferred forward. Where a floating suction cannot be employed, the outlet line should be drawn from the high end of the floor profile and at least 150mm from the tank floor. Where this height represents an unacceptable dead volume in the tank, slotted suctions may be fitted with lower draw points.
Last but not least, the tank vent/ breather design should be carefully considered. The atmospheric environment that the fuel system will be installed should be carefully considered when specifying this component. Inadequately specified tank vents or breathers can lead to catastrophic tank collapse including fuel breaching containment at worst or cavitation effects around the tank outlet leading to higher levels of effluent contamination from the tank floor.
Open tank breathers as shown above are the most common design as they are cheap and represent very little risk of tank collapse, however, they also present no real barrier to airborne contaminant entering the tank when the tank is in service. In areas with low rainfall and minimal airborne dust levels, this type of vent can be satisfactory. However, even in the scenario just described, care must be taken with the mesh screen covering the vent to prevent insect/ bird ingestion. Too fine a mesh screen can get blocked by ice in low temperature climate areas.
Increasingly, the tank vent/ breather of diesel tanks supplying power and vehicle equipment is being fitted with an air quality filter. This type of air filter controls the quality of air entering the tank and will more effectively maintain fuel quality. A breather filter is strongly recommended in environments where high airborne contamination can be expected. The user must pay special attention to the vacuum rating for any tank where this is being considered and seriously consider installing a by-pass function within the system to prevent the tank reaching a critical vacuum level which would cause catastrophic tank collapse.
Open vent type tank breatherWell-designed tankage discussed in this section is not the whole story. It is only part of the fuel management system. In the next section, the fuel system focus will turn to filtration and how this technology can clean up poor fuel quality upon delivery, maintain the fuel quality during storage and assure fuel is dispensed at the proper cleanliness level.
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