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Tank truck unloading spills

Under ground tank filling — working losses

Under ground tank breathing

Vehicle fuel tank filling — working losses

Fuel is unloaded from a tank truck with vapor recover (referred to as Stage I).  Most liquid fuel in California is stored in underground tanks.  During the course of fuel storage, the vapor or ullage space in the tank expands and contracts with as atmospheric pressure changes and fuel temperature changes.  The fuel temperature remains almost constant in underground tanks.  Fuel is dispensed to vehicles with a vapor recover hose system (Stage II vapor recovery).

The different stages of fuel distribution were observed to provide insight for this project.  There are no significant differences in the unloading of gasoline or alcohol fuels.  Fuel unloading at service stations is performed by the tank truck operator who may be an oil company employee or work for an independent company.  Unloading is accomplished with appropriate precautions for safety and minimizing emissions.  Fuel and vapor transfer hoses are connected from the storage tank to the truck.  The truck carries its own fuel transfer hoses and an assortment of fittings for connection to the underground tank.  After verifying the remaining tank volume with a dipstick measurement, the truck operator initiates the gravity fed unloading operation.  When the fuel transfer is completed, the hoses are returned back to the tank truck.  There is still a considerable volume of fuel in the fuel transfer hose (about 4-inch inner diameter).  The truck operator disconnects the hose from the truck tank and drains the remaining fuel in the bottom of the hose into the underground storage tank by lifting the hose into the air and moving the elevated section towards the connection at the underground tank.  The hose is then disconnected and stored on the truck.  During several such fueling operations, about 250 ml of fuel was observed spilling out of the hose as it was placed back into its holding tube on the truck.  It was estimated that the volume from spills is about 180 g for an 8000 gal fuel load or 0.023 g/gal (0.05 lb/1000 gal).  While this quantity is based on a casual observations, it provides some quantification of a small source that is not explicitly counted in the inventory.  It is difficult to spill no fuel during hose transfers since the inner wall of the transfer hose is covered with fuel as indicated by hooks on some tanker trucks for drying clean up rags.  An even smaller amount of fuel may remain on the hose surface and evaporate later.

Truck transfer is intended to be a no spill operation.  Drivers are instructed to drain the hose into the tank before placing it back on the truck.  Catch drains at the top of underground tanks would capture some spilled fuel if it dripped from the tank connection.  However, some wet hose losses are inevitable.  The thin layer of fuel in the hose will results in some drips and evaporation.  It should be pointed out that the volumes used in this study are based on rough estimates and do not reflect a large sample.  Furthermore, liquid spill volumes are difficult to measure.

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