The baseline 8.4 RVP gasoline was used to represent summer gasoline. This is different than the fuel used in the inventory that reflects year round fueling. The effective temperature for the inventory was first calculated from the average RVP. An effective summer and winter temperatures were then determined. The summertime temperatures for vehicle fueling were based on ARB fuel tank measurements. The wintertime temperature is consistent with the overall inventory. This approach, while somewhat arbitrary is based on reasonable temperature values and provides a basis for understanding the differences in emission factors used in this study and those in the State inventory.
Table 4-28 shows the vapor space NMOG for diesel and M100. Vapor emissions from reformulated diesel and FTD were assumed to be the same as those for diesel.
Table 4-28 also shows tank truck distribution emissions for liquid fuels. These emissions take into account vapor recovery effectiveness and a 5 percent defect rate for Stage II emission controls as indicated in Table 4-28. This combination of control effectiveness and defect rates will yield an emission factor lower than that used in State inventories. However, the inventories include uncontrolled fuel dispensing which is not relevant to this study.
4.9.2 LPG Distribution
LPG is stored and distributed in pressurized tanks. The fuel is stored in a liquid state at ambient temperature and the pressure in the tank is in equilibrium. At 70F the storage pressure is 105 psig. When LPG is transferred from a storage tank to a tank truck, or to a vehicle fuel tank, a transfer pump provides about 50 psi of differential pressure. When fueling vehicle tanks, the fuel enters the tank and the LPG ullage condenses. This process can be accelerated with top loaded tanks where the liquid spray can absorb some of the heat from condensing the vapors.
The tank trucks are filled at refineries with a two hose system with one hose acting as a vapor return. Hoses are evacuated after fuel transfer operations at the refinery. Tank trucks can be filled to a safe fraction of its water capacity by weighing the truck during fueling (Lowi 1994), although this is not the current practice. However, current regulations require the use of an "outage" valve that indicates when the tank is full. Some LPG also enters the atmosphere from the fuel transfer fitting.
Table 4-29 shows the emissions associated with LPG storage and distribution. The LPG emissions correspond to the volume of liquid that escapes from the fuel transfer fitting divided by the amount of fuel transferred. Currently, LPG vehicles in California are equipped with an "outage" valve that indicates the 80 percent fill level by spilling LPG to the atmosphere. During vehicle fueling, the outage valve is opened and vapors pass through a 0.060-inch orifice and through the valve. When LPG reaches the 80 percent level in the vehicle tank, liquid enters the fill level line and exits into the atmosphere. A puff of white liquid is visible to the fueler that provides an additional signal that the tank is full. California's vehicle code requires use of the outage valve. As indicated in