API Bull 2516:2006 pdf download

API Bull 2516:2006 pdf download.Evaporation Loss From Low- Pressure Tanks.
For convenience, equation (2), as well as equation (1), is plotted (for the condition when P。= 14.7 psia) in Fig.5. The minimum pressure requirements indicated in Fig. 5 have proved adequate to prevent boiling loss under usual storage conditions. In using the boiling curve shown in Fig. 5, Pr, the true vapor pressure at 100 F, may be obtained from Fig. 6 for up to 20 lb RVP. For 20 to 30 Ib RVP, Pr is approximately 7 percent greater than the Reid vapor pressure at 100 F.*
Working Lo8s
Working loss will occur during filling if the pressure of the vapor space exceeds the vent setting and vapors are expelled. If the pressure at the start of filling is less than the pressure vent setting, the air-hydrocarbon mixture will be compressed during flling. The hydro-
carbon condenses maintaining nearly a constant partial pressure. A. certain fraction of .vapor space may be flled with liquid before the vent opens, thus decreasing working loss. As flling continues, the total pressure eveniuauy equais inc pressure ai wnidn Inc reet vaive opens, resulting in venting. Assuming there is no temperature change in either the liquid or the vapor during the filling period, the remaining liquid entering the tank replaces an equal volume of vapors discharged from the vent. The total amount of loss, therefore, depends on the capacity of the vapor space in the tank. Since the temperature normally does change as con densation takes place, the rate of filling and emptying can also affect the amount of loss. The unpredictable effect of these variables has made it difficult to determine the actual loss resulting from filling low-pressure tanks.
To obtain theoretical values for losses incurred during filling, two assumptions have been made:
1. The vessel is completely filled, starting with a saturated vapor space; i.e., equilibrium exists between the hydrocarbon content in the vapor and the liquid phases under given conditions of temperature and pressure.
2. Filling starts at slightly less than atmospheric pressure.
By use of these assumptions, the following working loss equation is derived in Appendix I.