EVAPORATION LOSS FROM TANK CARS, TANK TRUCKS, AND MARINE VESSELS
|Publication Date:||1 November 1959|
The Evaporation Loss Committee of the American Petroleum Institute has developed general correlations for estimating evaporation loss from transportation vehicles. These correlations apply to crude oil and products with true vapor pressures up to approximately 9 psia. Whereas loss correlations for storage tanks usually give the average loss for a year, those for transportation vehicles must give loss for each phase of the transportation cycle. Thus, three different losses were considered: unloading, loading, and transit.
Unloading loss is the volume of stock that evaporates into the air drawn into the compartment during unloading, even though this vapor is not expelled until the compartment is reloaded. This vapor, existing in the compartment before loading, will hereafter be referred to as the existent vapor.
Loading loss is the volume of stock that evaporates as the compartment is being filled; loss of the existent vapor is not included.
Transit loss is the breathing loss that occurs between points of receipt and delivery.
Evaporation that occurs in the shipping tank during withdrawal, that is, occurring between the opening and closing gages on the tank, is called withdrawal loss. Because this loss does not occur from the transportation vehicle, it is beyond the scope of this bulletin. However, it may affect the final correlations, depending upon the test method used, and is considered only to that extent
Tank-car and tank-truck shipments, each shipment usually is less than 250 _bbl, normally require only a short period of time to load and unload. Subsurface loading is the common filling method, but splash loading is still used in some areas. The storage compartments are seldom freed from hydrocarbon vapor between shipments, and this vapor retards evaporation during loading. Data on transit loss are not reported; however, such loss for the usual short haul (less than two days) is thought to be too small to be measured by present test methods.
Tankers and barges, large-capacity carriers, may require a day or more for loading and unloading. The fill pipes are integral parts of the carriers and permit loading with a minimum of splashing. The storage compartments sometimes are freed from hydrocarbon vapor after unloading. Under such a condition, maximum evaporation loss occurs during loading. Because voyages frequently last one week or more, the transit Joss usually can be measured.