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API Bull 2516-2006 pdf free download

API Bull 2516-2006 pdf free download.Evaporation Loss From Low- Pressure Tanks .
The term low-pressure tank, as used in this evapora- tion loss bulletin, refers to vessels having a maximum pressure vent setting in the range from just above atmospheric pressure to 15 psig and a vacuum vent setting normally I to 2 oz per sq in. The tanks are used for the storage of products, such as motor gaso- line, pentanes, and natural gasolines, having a Reid vapor pressure up to 30 lb. Although a storage pressure of less than 2.5 psig may be used for some the type of vessel construction does not permit appre- ciable economy by using lower design pressures. The loss principles applying to 2.5-psig to 15-psig pressure will also appl!’. for higher or lower working pressures than the specified range. Low-pressure tanks are con- structed in many sizes and shapes, depending upon the operating pressure range. Fig. I, 2, 3, and 4 show typical types of construction. Pressure tanks differ from other conservation tanks in that they have neither moving parts nor a variable vapor space. The principle of operation is the same as that for the conservation vented fixed-roof tank. The basic is the ability of low-pressure tanks to withstand higher pressure variations. Because of this, venting loss due to boiling and breathing loss due to daily temperature changes are prevented. By increas- ing the tank pressure, liquids of higher volatility may be stored without breathing loss.
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 conditions. In using the boiling curve shown in Fig. 5, true vapor pressure at 100 F, may be obtained from Fig. 6 for up to 20 lb RVP. For 20 to 30 lb RVP, p1 is approximately 7 per- cent greater than the Reid vapor pressure at 100 F. * 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 filling. The hydro- carbon condenses maintaining nearly a constant partial pressure. A. certain fraction of .vapor space may be filled with liquid before the vent opens, thus decreasing working loss. As filling continues, the total pressure eventually pressure at which the relief valve opens, resulting in venting. Assuming there is no tem- perature change in either the liquid 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 den- sation takes place, the rate of filling and emptying can also 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 dur- ing filling, two assumptions have been made: 1. The vessel is co!11pletely filled, starting with a satu- rated vapor space; i.e., equilibrium exists between the hydroca.rbon in the vapor and the liquid phases under given conditions of temperature and pressure. 2. Filling starts at slightly less than atmospheric pressure.

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